Water 1

Wastewater Treatment

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Contents Articles Introduction

1

Wastewater

1

Sewage treatment

5

Biochemical oxygen demand

20

Effluent

23

Biofilter

24

Trickling filter

27

Chemical oxygen demand

31

Chlorination

34

Ozone

37

Ultraviolet germicidal irradiation

54

Water treatment

58

Settling

61

Flocculation

64

Activated sludge

67

Slow sand filter

72

Aerated lagoon

75

Advanced oxidation process

77

Aerobic treatment system

78

Anaerobic digestion

81

Bioreactor

97

Carbon filtering

100

Constructed wetland

101

Dissolved air flotation

113

Desalination

115

Electrocoagulation

129

Expanded granular sludge bed digestion

133

Fine bubble diffusers

134

Sedimentation

135

Membrane bioreactor

137

Retention basin

145

Reverse osmosis

146

Rotating biological contactor

154

API oil-water separator

158

Septic tank

160

Stabilization pond

164

Ultrafiltration (industrial)

166

Treatment pond

167

Wet oxidation

170

References Article Sources and Contributors

171

Image Sources, Licenses and Contributors

175

Article Licenses License

178

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Introduction Wastewater Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations. In the most common usage, it refers to the municipal wastewater that contains a broad spectrum of contaminants resulting from the mixing of wastewaters from different sources. Sewage is correctly the subset of wastewater that is contaminated with feces or urine, but is often used to mean any waste water. "Sewage" includes domestic, municipal, or industrial liquid waste products disposed of, usually via a pipe or sewer or similar structure, sometimes in a cesspool emptier. The physical infrastructure, including pipes, pumps, screens, channels etc. used to convey sewage from its origin to the point of eventual treatment or disposal is termed sewerage.

Origin Wastewater or sewage can come from (text in brackets indicates likely inclusions or contaminants): • Human waste (fæces, used toilet paper or wipes, urine, or other bodily fluids), also known as blackwater, usually from lavatories; • Cesspit leakage; • Septic tank discharge; • Sewage treatment plant discharge; • Washing water (personal, clothes, floors, dishes, etc.), also known as greywater or sullage; • Rainfall collected on roofs, yards, hard-standings, etc. (generally clean with traces of oils and fuel); • Groundwater infiltrated into sewage; • Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, etc.); • Urban rainfall runoff from roads, carparks, roofs, sidewalks, or pavements (contains oils, animal fæces, litter, fuel or rubber residues, metals from vehicle exhausts, etc.); • Seawater ingress (high volumes of salt and micro-biota); • Direct ingress of river water (high volumes of micro-biota); • Direct ingress of manmade liquids (illegal disposal of pesticides, used oils, etc.); • Highway drainage (oil, de-icing agents, rubber residues); • Storm drains (almost anything, including cars, shopping trolleys, trees, cattle, etc.); • Blackwater (surface water contaminated by sewage); • Industrial waste • industrial site drainage (silt, sand, alkali, oil, chemical residues); • Industrial cooling waters (biocides, heat, slimes, silt); • Industrial process waters; • Organic or bio-degradable waste, including waste from abattoirs, creameries, and ice cream manufacture; • Organic or non bio-degradable/difficult-to-treat waste (pharmaceutical or pesticide manufacturing); • extreme pH waste (from acid/alkali manufacturing, metal plating); • Toxic waste (metal plating, cyanide production, pesticide manufacturing, etc.);

Wastewater • Solids and Emulsions (paper manufacturing, foodstuffs, lubricating and hydraulic oil manufacturing, etc.); • agricultural drainage, direct and diffuse.

Wastewater constituents The composition of wastewater varies widely. This is a partial list of what it may contain: • • • • • • •

Water ( > 95%) which is often added during flushing to carry waste down a drain; Pathogens such as bacteria, viruses, prions and parasitic worms; Non-pathogenic bacteria; Organic particles such as feces, hairs, food, vomit, paper fibers, plant material, humus, etc.; Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.; Inorganic particles such as sand, grit, metal particles, ceramics, etc.; Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates, thiosulfates, etc.; • Animals such as protozoa, insects, arthropods, small fish, etc.; • Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, children's toys, dead animals or plants, etc.; • Gases such as hydrogen sulfide, carbon dioxide, methane, etc.; • Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, etc.; • Toxins such as pesticides, poisons, herbicides, etc. • Pharmaceuticals and other hormones.

Wastewater quality indicators Any oxidizable material present in a natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes. The result is that the oxygen content of the water will be decreased. Basically, the reaction for biochemical oxidation may be written as: Oxidizable material + bacteria + nutrient + O2 → CO2 + H2O + oxidized inorganics such as NO3 or SO4 Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows: S-- + 2 O2 → SO4-NO2- + ½ O2 → NO3Since all natural waterways contain bacteria and nutrients, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above). Those biochemical reactions create what is measured in the laboratory as the Biochemical oxygen demand (BOD). Such chemicals are also liable to be broken down using strong oxidizing agents and these chemical reactions create what is measured in the laboratory as the Chemical oxygen demand (COD). Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of oxidizable pollutants. The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period. The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD. Because the Ultimate BOD is so time consuming, the 5-day BOD has been almost universally adopted as a measure of relative pollution effect. There are also many different COD tests of which the 4-hour COD is probably the most common.

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Wastewater There is no generalized correlation between the 5-day BOD and the ultimate BOD. Similarly there is no generalized correlation between BOD and COD. It is possible to develop such correlations for a specific waste contaminants in a specific waste water stream but such correlations cannot be generalized for use with any other waste contaminants or waste water streams. This is because the composition of any waste water stream is different. As an example and effluent consisting of a solution of simple sugars that might discharge from a confectionery factory is likely to have organic components that degrade very quickly. In such a case the 5 day BOD and the ultimate BOD would be very similar . I.e there would be very little organic material left after 5 days. . However a final effluent of a sewage treatment works serving a large industrialised area might have a discharge where the ultimate BOD was much greater than the 5 day BOD because much of the easily degraded material would have been removed in the sewage treatment process and many industrial processes discharge difficult to degrade organic molecules. The laboratory test procedures for the determining the above oxygen demands are detailed in many standard texts. American versions include the "Standard Methods For the Examination Of Water and Wastewater" [1]

Sewage disposal In some urban areas, sewage is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to either of these is typically through a manhole. During high precipitation periods a sanitary sewer overflow can occur, forcing untreated sewage to flow back into the environment. This can pose a serious threat to public health and the surrounding environment. Sewage may drain directly into major watersheds with minimal or no treatment. When untreated, sewage can have serious impacts on the quality of an environment Industrial wastewater effluent with neutralized pH from tailing runoff. Taken in and on the health of people. Pathogens can Peru. cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.

Treatment There are numerous processes that can be used to clean up waste waters depending on the type and extent of contamination. Most wastewater is treated in industrial-scale wastewater treatment plants (WWTPs) which may include physical, chemical and biological treatment processes. However, the use of septic tanks and other On-Site Sewage Facilities (OSSF) is widespread in rural areas, serving up to one quarter of the homes in the U.S.[2] The most important aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed by micro-organisms able to absorb and adsorb the organic matter carried in the wastewater. Anaerobic processes are widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. For some waste waters ecological approaches using reed bed systems such as constructed wetlands may be appropriate. Modern systems include tertiary treatment by micro filtration or synthetic membranes. After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality. Nitrates can be removed from wastewater by

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Wastewater microbial denitrification, for which a small amount of methanol is typically added to provide the bacteria with a source of carbon. Ozone Waste Water Treatment is also growing in popularity, and requires the use of an ozone generator, which decontaminates the water as Ozone bubbles percolate through the tank. Disposal of wastewaters from an industrial plant is a difficult and costly problem. Most petroleum refineries, chemical and petrochemical plants[3] [4] have onsite facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the local and/or national regulations regarding disposal of wastewaters into community treatment plants or into rivers, lakes or oceans. Other Industrial processes that produce a lot of waste-waters such as paper and pulp production has created environmental concern leading to development of processes to recycle water use within plants before they have to be cleaned and disposed of.[5]

Reuse Treated wastewater can be reused as drinking water, in industry (cooling towers), in artificial recharge of aquifers, in agriculture (70% of Israel's irrigated agriculture is based on highly purified wastewater) and in the rehabilitation of natural ecosystems (Florida's Everglades).

Algal fuel Woods Hole Oceanographic Institution and Harbor Branch Oceanographic Institution, following the conclusions of the USDOE´s Aquatic Species Program, use wastewater for breeding algae. The wastewater from domestic and industrial sources contain rich organic compounds, which accelerate the growth of algae. This algae can be used to produce algal fuels[6] Algaewheel, based in Indianapolis, Indiana, presented a proposal to build a new wastewater treatment facility in Cedar Lake, Indiana that uses algae to treat municipal wastewater and uses the sludge byproduct to produce biofuel.[7] [8]

Etymology The words "sewage" and "sewer" came from Old French essouier = "to drain", which came from Latin exaquāre. Their formal Latin antecedents are exaquāticum and exaquārium.

Legislation European Union Council Directive 91/271/EEC on Urban Waste Water Treatment was adopted on 21 May 1991,[9] amended by the Commission Directive 98/15/EC.[10] Commission Decision 93/481/EEC defines the information that Member States should provide the Commission on the state of implementation of the Directive.[11]

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References [1] [2] [3] [4]

Standard Methods of the Examination of Water and Wastewater (http:/ / www. standardmethods. org) "Septic Systems" US EPA. 2011 (http:/ / cfpub. epa. gov/ owm/ septic/ septic. cfm?page_id=261) Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834. Tchobanoglous, G., Burton, F.L., and Stensel, H.D. (2003). Wastewater Engineering (Treatment Disposal Reuse) / Metcalf & Eddy, Inc. (4th ed.). McGraw-Hill Book Company. ISBN 0-07-041878-0. [5] J. F. Byrd, M. D. Ehrke, J. I. Whitfield. (1984) "New Bleached Kraft Pulp Plant in Georgia: State of the Art Environmental Control" (http:/ / www. jstor. org/ stable/ 25042250) Water pollution control federation 56(4): 378–385. [6] Biofuels from industrial/domestic wastewater (http:/ / www. merinews. com/ catFull. jsp?articleID=135399) [7] "Algaewheel — Wastewater Treatment Specialists" (http:/ / www. algaewheel. com). . Retrieved 2008-06-18. [8] "Indiana Company to Submit Proposal to Utilize Algae to Treat Wastewater and Create Renewable Energy" (http:/ / www. ewire. com/ display. cfm/ Wire_ID/ 4808). E-Wire. 2008-06-12. . Retrieved 2008-06-18. [9] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CELEX:31991L0271:EN:NOT [10] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CELEX:31998L0015:EN:NOT [11] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CELEX:31993D0481:EN:NOT

Sewage treatment Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff (effluents) and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce an environmentally-safe fluid waste stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or reuse (usually as farm fertilizer). Using advanced technology it is now possible to re-use sewage effluent for drinking water, although Singapore is the only country to implement such technology on a production scale in its production of NEWater.[2]

The objective of sewage treatment is to produce a disposable effluent without causing harm to the surrounding environment, and also [1] prevent pollution.

Origins of sewage Sewage is created by residential, institutional, and commercial and industrial establishments and includes household waste liquid from toilets, baths, showers, kitchens, sinks and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world, with greywater being permitted to be used for watering plants or recycled for flushing toilets. Sewage may include stormwater runoff. Sewerage systems capable of handling stormwater are known as combined systems. Combined sewer systems are usually avoided now because precipitation causes widely varying flows reducing sewage treatment plant efficiency. Combined sewers require much larger, more expensive, treatment facilities than sanitary sewers. Heavy storm runoff may overwhelm the sewage treatment system, causing a spill or overflow. Sanitary sewers are typically much smaller than combined sewers, and they are not designed to transport stormwater. Backups of raw sewage can occur if excessive Infiltration/Inflow is allowed into a sanitary sewer system. Modern sewered developments tend to be provided with separate storm drain systems for rainwater.[3] As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment,

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heavy metals, organic compounds, animal waste, and oil and grease. (See urban runoff.)[4] Some jurisdictions require stormwater to receive some level of treatment before being discharged directly into waterways. Examples of treatment processes used for stormwater include retention basins, wetlands, buried vaults with various kinds of media filters, and vortex separators (to remove coarse solids).

Process overview Sewage can be treated close to where it is created, a decentralised system, (in septic tanks, biofilters or aerobic treatment systems), or be collected and transported via a network of pipes and pump stations to a municipal treatment plant, a centralised system, (see sewerage and pipes and infrastructure). Sewage collection and treatment is typically subject to local, state and federal regulations and standards. Industrial sources of wastewater often require specialized treatment processes (see Industrial wastewater treatment). Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment. • Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment. • Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment. • Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow rejection into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs,...). Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

Process Flow Diagram for a typical large-scale treatment plant

Process Flow Diagram for a typical treatment plant via Subsurface Flow Constructed Wetlands (SFCW)

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Pre-treatment Pre-treatment removes materials that can be easily collected from the raw waste water before they damage or clog the pumps and skimmers of primary treatment clarifiers (trash, tree limbs, leaves, etc.). Screening The influent sewage water is screened to remove all large objects like cans, rags, sticks, plastic packets etc. carried in the sewage stream.[5] This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, whilst in smaller or less modern plants a manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The solids are collected and later disposed in a landfill or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed they become entrained in pipes and moving parts of the treatment plant and can cause substantial damage and inefficiency in the process.[6] :9 Grit removal Pre-treatment may include a sand or grit channel or chamber where the velocity of the incoming wastewater is adjusted to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other equipment. For small sanitary sewer systems, the grit chambers may not be necessary, but grit removal is desirable at larger plants.[6] :10 Fat and grease removal In some larger plants, fat and grease is removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat as a froth. Many plants, however, use primary clarifiers with mechanical surface skimmers for fat and grease removal.

Primary treatment In the primary sedimentation stage, sewage flows through large tanks, commonly called "primary clarifiers" or

An empty sedimentation tank at the treatment plant in Merchtem, Belgium.

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"primary sedimentation tanks." The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities.[6] :9-11 Grease and oil from the floating material can sometimes be recovered for saponification. The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. A typical sedimentation tank may remove from 50 to 70 percent of suspended solids, and from 30 to 35 percent of biochemical oxygen demand (BOD) from the sewage.

Secondary treatment Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed-film or suspended-growth systems. • Fixed-film or attached growth systems include trickling filters, Moving Bed Biofilm Reactors (MBBR [7]), and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface. • Suspended-growth systems include activated sludge, where the biomass is mixed with the sewage and can be operated in a smaller space than fixed-film systems that treat the same amount of water. However, fixed-film systems are more able to cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems.[6] :11-13 Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow them to then be treated by conventional secondary treatment processes. Characteristics include filters filled with media to which wastewater is applied. They are designed to allow high hydraulic loading and a high level of aeration. On larger installations, air is forced through the media using blowers. The resultant wastewater is usually within the normal range for conventional treatment processes. A filter removes a small percentage of the suspended organic matter, while the majority of the organic matter undergoes a change of character, only due to the biological oxidation and nitrification taking place in the filter. With this aerobic oxidation and nitrification, the organic solids are converted into coagulated suspended mass, which is heavier and bulkier, and can settle to the bottom of a tank. The effluent of the filter is therefore passed through a sedimentation tank, called a secondary clarifier, secondary settling tank or humus tank. Activated sludge

A generalized, schematic diagram of an activated sludge process.

In general, activated sludge plants encompass a variety of mechanisms and processes that use dissolved oxygen to promote the growth of biological floc that substantially removes organic material.[6] :12-13 The process traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate ultimately to nitrogen gas. (See also denitrification).

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Surface-aerated basins (Lagoons) Many small municipal sewage systems in the United States (1 million gal./day or less) use aerated lagoons.[8] Most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or air) and microbial action. Surface-aerated basins achieve 80 to 90 percent removal of BOD with retention times of 1 to 10 days.[9] The basins may range in depth from 1.5 to 5.0 metres and use motor-driven aerators floating on the surface of the wastewater.[9]

A Typical Surface-Aerated Basin (using motor-driven floating aerators)

In an aerated basin system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). Typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kW·h. However, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units.[9] Biological oxidation processes are sensitive to temperature and, between 0 °C and 40 °C, the rate of biological reactions increase with temperature. Most surface aerated vessels operate at between 4 °C and 32 °C.[9] Constructed wetlands Constructed wetlands (can either be surface flow or subsurface flow, horizontal or vertical flow), include engineered reedbeds and belong to the family of phytorestoration and ecotechnologies; they provide a high degree of biological improvement and depending on design, act as a primary, secondary and sometimes tertiary treatment, also see phytoremediation. One example is a small reedbed used to clean the drainage from the elephants' enclosure at Chester Zoo in England; numerous CWs are used to recycle the water of the city of Honfleur in France and numerous other towns in Europe, the US, Asia and Australia. They are known to be highly productive systems as they copy natural wetlands, called the "Kidneys of the earth" for their fundamental recycling capacity of the hydrological cycle in the biosphere. Robust and reliable, their treatment capacities improve as time go by, at the opposite of conventional treatment plants whose machinery age with time. They are being increasingly used, although adequate and experienced design are more fundamental than for other systems and space limitation may impede their use. Filter beds (oxidizing beds) In older plants and those receiving variable loadings, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a bed made up of coke (carbonized coal), limestone chips or specially fabricated plastic media. Such media must have large surface areas to support the biofilms that form. The liquor is typically distributed through perforated spray arms. The distributed liquor trickles through the bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the media’s surfaces and eat or otherwise reduce the organic content.[6] :12 This biofilm is often grazed by insect larvae, snails, and worms which help maintain an optimal thickness. Overloading of beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface. Recent

Sewage treatment advances in media and process micro-biology design overcome many issues with Trickling filter designs. Soil Bio-Technology A new process called Soil Bio-Technology (SBT) developed at IIT Bombay has shown tremendous improvements in process efficiency enabling total water reuse, due to extremely low operating power requirements of less than 50 joules per kg of treated water.[10] Typically SBT systems can achieve chemical oxygen demand (COD) levels less than 10 mg/L from sewage input of COD 400 mg/L.[11] SBT plants exhibit high reductions in COD values and bacterial counts as a result of the very high microbial densities available in the media. Unlike conventional treatment plants, SBT plants produce insignificant amounts of sludge, precluding the need for sludge disposal areas that are required by other technologies.[12] In the Indian context, conventional sewage treatment plants fall into systemic disrepair due to 1) high operating costs, 2) equipment corrosion due to methanogenesis and hydrogen sulphide, 3) non-reusability of treated water due to high COD (>30 mg/L) and high fecal coliform (>3000 NFU) counts, 4) lack of skilled operating personnel and 5) equipment replacement issues. Examples of such systemic failures has been documented by Sankat Mochan Foundation at the Ganges basin after a massive cleanup effort by the Indian government in 1986 by setting up sewage treatment plants under the Ganga Action Plan failed to improve river water quality. Biological aerated filters Biological Aerated (or Anoxic) Filter (BAF) or Biofilters combine filtration with biological carbon reduction, nitrification or denitrification. BAF usually includes a reactor filled with a filter media. The media is either in suspension or supported by a gravel layer at the foot of the filter. The dual purpose of this media is to support highly active biomass that is attached to it and to filter suspended solids. Carbon reduction and ammonia conversion occurs in aerobic mode and sometime achieved in a single reactor while nitrate conversion occurs in anoxic mode. BAF is operated either in upflow or downflow configuration depending on design specified by manufacturer. Rotating biological contactors Rotating biological contactors (RBCs) are mechanical secondary treatment systems, which are robust and capable of withstanding surges in organic load. RBCs were first installed in Germany in 1960 and have since been developed and refined into a reliable operating unit. The rotating disks support the growth of bacteria and micro-organisms present in the Schematic diagram of a typical rotating biological contactor (RBC). The treated effluent sewage, which break down and clarifier/settler is not included in the diagram. stabilise organic pollutants. To be successful, micro-organisms need both oxygen to live and food to grow. Oxygen is obtained from the atmosphere as the disks rotate. As the micro-organisms grow, they build up on the media until they are sloughed off due to shear forces provided by the rotating discs in the sewage. Effluent from the RBC is then passed through final clarifiers where the micro-organisms in suspension settle as a sludge. The sludge is withdrawn from the clarifier for further treatment. A functionally similar biological filtering system has become popular as part of home aquarium filtration and purification. The aquarium water is drawn up out of the tank and then cascaded over a freely spinning corrugated fiber-mesh wheel before passing through a media filter and back into the aquarium. The spinning mesh wheel

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develops a biofilm coating of microorganisms that feed on the suspended wastes in the aquarium water and are also exposed to the atmosphere as the wheel rotates. This is especially good at removing waste urea and ammonia urinated into the aquarium water by the fish and other animals. Membrane bioreactors Membrane bioreactors (MBR) combine activated sludge treatment with a membrane liquid-solid separation process. The membrane component uses low pressure microfiltration or ultra filtration membranes and eliminates the need for clarification and tertiary filtration. The membranes are typically immersed in the aeration tank; however, some applications utilize a separate membrane tank. One of the key benefits of an MBR system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge (CAS) processes. The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling. The process is typically operated at MLSS in the range of 8,000–12,000 mg/L, while CAS are operated in the range of 2,000–3,000 mg/L. The elevated biomass concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable materials at higher loading rates. Thus increased sludge retention times, usually exceeding 15 days, ensure complete nitrification even in extremely cold weather. The cost of building and operating an MBR is usually higher than conventional wastewater treatment. Membrane filters can be blinded with grease or abraded by suspended grit and lack a clarifier's flexibility to pass peak flows. The technology has become increasingly popular for reliably pretreated waste streams and has gained wider acceptance where infiltration and inflow have been controlled, however, and the life-cycle costs have been steadily decreasing. The small footprint of MBR systems, and the high quality effluent produced, make them particularly useful for water reuse applications.[13] Secondary sedimentation The final step in the secondary treatment stage is to settle out the biological floc or filter material through a secondary clarifier and to produce sewage water containing low levels of organic material and suspended matter.

Tertiary treatment The purpose of tertiary treatment is to provide a final treatment stage to raise the efluennt quality before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called "effluent polishing."

Secondary Sedimentation tank at a rural treatment plant.

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Filtration Sand filtration removes much of the residual suspended matter.[6] carbon adsorption, removes residual toxins.[6] :19

:22-23

Filtration over activated carbon, also called

Lagooning Lagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates. Nutrient removal Wastewater may contain high levels of the nutrients nitrogen and A sewage treatment plant and lagoon in Everett, phosphorus. Excessive release to the environment can lead to a build Washington, United States. up of nutrients, called eutrophication, which can in turn encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the algae by bacteria uses up so much of oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and phosphorus. Nitrogen removal The removal of nitrogen is effected through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water. Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonia (NH3) to nitrite (NO2−) is most often facilitated by Nitrosomonas spp. (nitroso referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3−), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp. Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process (if designed well) can do the job the most easily.[6] :17-18 Since denitrification is the reduction of nitrate to dinitrogen gas, an electron donor is needed. This can be, depending on the wastewater, organic matter (from faeces), sulfide, or an added donor like methanol. The sludge in the anoxic tanks (denitrification tanks) must be mixed well (mixture of recirculated mixed liquor, return activated sludge [RAS], and raw influent) e.g. by using submersible mixers in order to achieve the desired denitrification. Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment. Many sewage treatment plants use axial flow pumps to transfer the nitrified mixed liquor from the aeration zone to the anoxic zone for denitrification. These pumps are often referred to as Internal Mixed Liquor Recycle (IMLR) pumps.

Sewage treatment Phosphorus removal Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water systems. (For a description of the negative effects of algae, see Nutrient removal). It is also particularly important for water reuse systems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverse osmosis. Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass). When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value. Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminum (e.g. alum), or lime.[6] :18 This may lead to excessive sludge production as hydroxides precipitates and the added chemicals can be expensive. Chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is easier to operate and is often more reliable than biological phosphorus removal. Another method for phosphorus removal is to use granular laterite. Once removed, phosphorus, in the form of a phosphate-rich sludge, may be stored in a land fill or resold for use in fertilizer.

Disinfection The purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganisms in the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy water will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite.[6] :16 Chloramine, which is used for drinking water, is not used in waste water treatment because of its persistence. After multiple steps of disinfection, the treated water is ready to be released back into the water cycle by means of the nearest body of water or agriculture. Afterwards, the water can be transferred to reserves for everyday human uses. Chlorination remains the most common form of waste water disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment. Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the United Kingdom, UV light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Some sewage treatment systems in Canada and the US also use UV light for their effluent water disinfection.[14] [15]

13

Sewage treatment Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated onsite as needed. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators.

Odour Control Odours emitted by sewage treatment are typically an indication of an anaerobic or "septic" condition.[16] Early stages of processing will tend to produce smelly gases, with hydrogen sulfide being most common in generating complaints. Large process plants in urban areas will often treat the odours with carbon reactors, a contact media with bio-slimes, small doses of chlorine, or circulating fluids to biologically capture and metabolize the obnoxious gases.[17] Other methods of odour control exist, including addition of iron salts, hydrogen peroxide, calcium nitrate, etc. to manage hydrogen sulfide levels.

Package plants and batch reactors To use less space, treat difficult waste and intermittent flows, a number of designs of hybrid treatment plants have been produced. Such plants often combine at least two stages of the three main treatment stages into one combined stage. In the UK, where a large number of wastewater treatment plants serve small populations, package plants are a viable alternative to building a large structure for each process stage. In the US, package plants are typically used in rural areas, highway rest stops and trailer parks.[18] One type of system that combines secondary treatment and settlement is the sequencing batch reactor (SBR). Typically, activated sludge is mixed with raw incoming sewage, and then mixed and aerated. The settled sludge is run off and re-aerated before a proportion is returned to the headworks.[19] SBR plants are now being deployed in many parts of the world. The disadvantage of the SBR process is that it requires a precise control of timing, mixing and aeration. This precision is typically achieved with computer controls linked to sensors. Such a complex, fragile system is unsuited to places where controls may be unreliable, poorly maintained, or where the power supply may be intermittent. Extended aeration package plants use separate basins for aeration and settling, and are somewhat larger than SBR plants with reduced timing sensitivity.[20] Package plants may be referred to as high charged or low charged. This refers to the way the biological load is processed. In high charged systems, the biological stage is presented with a high organic load and the combined floc and organic material is then oxygenated for a few hours before being charged again with a new load. In the low charged system the biological stage contains a low organic load and is combined with flocculate for longer times.

14

Sewage treatment

Sludge treatment and disposal The sludges accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Incineration is also used albeit to a much lesser degree.[6] :19-21 Sludge treatment depends on the amount of solids generated and other site-specific conditions. Composting is most often applied to small-scale plants with aerobic digestion for mid sized operations, and anaerobic digestion for the larger-scale operations.

Anaerobic digestion Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at a temperature of around 36°C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for heating the sludge. Anaerobic digestion is the most common (mesophilic) treatment of domestic sewage in septic tanks, which normally retain the sewage from one day to two days, reducing the BOD by about 35 to 40 percent. This reduction can be increased with a combination of anaerobic and aerobic treatment by installing Aerobic Treatment Units (ATUs) in the septic tank. One major feature of anaerobic digestion is the production of biogas (with the most useful component being methane), which can be used in generators for electricity production and/or in boilers for heating purposes.

Aerobic digestion Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. The operating costs used to be characteristically much greater for aerobic digestion because of the energy used by the blowers, pumps and motors needed to add oxygen to the process. Aerobic digestion can also be achieved by using diffuser systems or jet aerators to oxidize the sludge. Fine bubble diffusers are typically the more cost-efficient diffusion method, however, plugging is typically a problem due to sediment settling into the smaller air holes. Coarse bubble diffusers are more commonly used in activated sludge tanks (generally a side process in waste water management) or in the flocculation stages. A key component for selecting diffuser type is to ensure it will produce the required oxygen transfer rate.

Composting Composting is also an aerobic process that involves mixing the sludge with sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat.[6] :20

Incineration Incineration of sludge is less common because of air emissions concerns and the supplemental fuel (typically natural gases or fuel oil) required to burn the low calorific value sludge and vaporize residual water. Stepped multiple hearth incinerators with high residence time and fluidized bed incinerators are the most common systems used to combust wastewater sludge. Co-firing in municipal waste-to-energy plants is occasionally done, this option being less expensive assuming the facilities already exist for solid waste and there is no need for auxiliary fuel.[6] :20-21

15

Sewage treatment

Sludge disposal When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. Typically, sludges are thickened (dewatered) to reduce the volumes transported off-site for disposal. There is no process which completely eliminates the need to dispose of biosolids. There is, however, an additional step some cities are taking to superheat sludge and convert it into small pelletized granules that are high in nitrogen and other organic materials. In New York City, for example, several sewage treatment plants have dewatering facilities that use large centrifuges along with the addition of chemicals such as polymer to further remove liquid from the sludge. The removed fluid, called centrate, is typically reintroduced into the wastewater process. The product which is left is called "cake" and that is picked up by companies which turn it into fertilizer pellets. This product is then sold to local farmers and turf farms as a soil amendment or fertilizer, reducing the amount of space required to dispose of sludge in landfills. Much sludge originating from commercial or industrial areas is contaminated with toxic materials that are released into the sewers from the industrial processes.[21] Elevated concentrations of such materials may make the sludge unsuitable for agricultural use and it may then have to be incinerated or disposed of to landfill.

Treatment in the receiving environment Many processes in a wastewater treatment plant are designed to mimic the natural treatment processes that occur in the environment, whether that environment is a natural water body or the ground. If not overloaded, bacteria in the environment will consume organic contaminants, although this will reduce the levels of oxygen in the water and may significantly change the overall ecology of the receiving water. Native bacterial populations feed on the organic contaminants, and the numbers of disease-causing microorganisms are reduced by natural environmental conditions such as predation or The outlet of the Karlsruhe sewage treatment exposure to ultraviolet radiation. Consequently, in cases where the plant flows into the Alb. receiving environment provides a high level of dilution, a high degree of wastewater treatment may not be required. However, recent evidence has demonstrated that very low levels of specific contaminants in wastewater, including hormones (from animal husbandry and residue from human hormonal contraception methods) and synthetic materials such as phthalates that mimic hormones in their action, can have an unpredictable adverse impact on the natural biota and potentially on humans if the water is re-used for drinking water.[22] In the US and EU, uncontrolled discharges of wastewater to the environment are not permitted under law, and strict water quality requirements are to be met, as clean drinking water is essential. (For requirements in the US, see Clean Water Act.) A significant threat in the coming decades will be the increasing uncontrolled discharges of wastewater within rapidly developing countries.

Effects on Biology Sewage treatment plants can have multiple effects on nutrient levels in the water that the treated sewage flows into. These effects on nutrients can have large effects on the biological life in the water in contact with the effluent. Stabilization ponds (or treatment ponds) can include any of the following: • Oxidation ponds, which are aerobic bodies of water usually 1–2 meters in depth that receive effluent from sedimentation tanks or other forms of primary treatment. • Dominated by algae • Polishing ponds are similar to oxidation ponds but receive effluent from an oxidation pond or from a plant with an extended mechanical treatment. • Dominated by zooplankton

16

Sewage treatment • Facultative lagoons, raw sewage lagoons, or sewage lagoons are ponds where sewage is added with no primary treatment other than coarse screening. These ponds provide effective treatment when the surface remains aerobic; although anaerobic conditions may develop near the layer of settled sludge on the bottom of the pond.[23] • Anaerobic lagoons are heavily loaded ponds. • Dominated by bacteria • Sludge lagoons are aerobic ponds, usually 2–5 meters in depth, that receive anaerobically digested primary sludge, or activated secondary sludge under water. • Upper layers are dominated by algae [24] Phosphorus limitation is a possible result from sewage treatment and results in flagellate-dominated plankton, particularly in summer and fall.[25] At the same time a different study found high nutrient concentrations linked to sewage effluents. High nutrient concentration leads to high chlorophyll a concentrations, which is a proxy for primary production in marine environments. High primary production means high phytoplankton populations and most likely high zooplankton populations because zooplankton feed on phytoplankton. However, effluent released into marine systems also leads to greater population instability.[26] A study done in Britain found that the quality of effluent affected the planktonic life in the water in direct contact with the wastewater effluent. Turbid, low-quality effluents either did not contain ciliated protozoa or contained only a few species in small numbers. On the other hand, high-quality effluents contained a wide variety of ciliated protozoa in large numbers. Due to these findings, it seems unlikely that any particular component of the industrial effluent has, by itself, any harmful effects on the protozoan populations of activated sludge plants.[27] The planktonic trends of high populations close to input of treated sewage is contrasted by the bacterial trend. In a study of Aeromonas spp. in increasing distance from a wastewater source, greater change in seasonal cycles was found the furthest from the effluent. This trend is so strong that the furthest location studied actually had an inversion of the Aeromonas spp. cycle in comparison to that of fecal coliforms. Since there is a main pattern in the cycles that occurred simultaneously at all stations it indicates seasonal factors (temperature, solar radiation, phytoplankton) control of the bacterial population. The effluent dominant species changes from Aeromonas caviae in winter to Aeromonas sobria in the spring and fall while the inflow dominant species is Aeromonas caviae, which is constant throughout the seasons.[28]

Sewage treatment in developing countries Few reliable figures on the share of the wastewater collected in sewers that is being treated in the world exist. In many developing countries the bulk of domestic and industrial wastewater is discharged without any treatment or after primary treatment only. In Latin America about 15% of collected wastewater passes through treatment plants (with varying levels of actual treatment). In Venezuela, a below average country in South America with respect to wastewater treatment, 97 percent of the country’s sewage is discharged raw into the environment.[29] In a relatively developed Middle Eastern country such as Iran, the majority of Tehran's population has totally untreated sewage injected to the city’s groundwater.[30] However now the construction of major parts of the sewage system, collection and treatment, in Tehran is almost complete, and under development, due to be fully completed by the end of 2012. In Israel, about 50 percent of agricultural water usage (total use was 1 billion cubic metres in 2008) is provided through reclaimed sewer water. Future plans call for increased use of treated sewer water as well as more desalination plants.[31] Most of sub-Saharan Africa is without wastewater treatment.

17

Sewage treatment

References [1] Khopkar, S. M. (2004). Environmental Pollution Monitoring And Control (http:/ / books. google. com/ ?id=TAk21grzDZgC). New Delhi: New Age International. p. 299. ISBN 8122415075. . Retrieved 2009-06-28. [2] History of the NEWater (http:/ / www. pub. gov. sg/ about/ historyfuture/ Pages/ NEWater. aspx) [3] Burrian, Steven J., et al. (1999). "The Historical Development of Wet-Weather Flow Management." (http:/ / www. epa. gov/ nrmrl/ pubs/ 600ja99275/ 600ja99275. pdf) US Environmental Protection Agency (EPA). National Risk Management Research Laboratory, Cincinnati, OH. Document No. EPA/600/JA-99/275. [4] Stormwater Effects Handbook: A Toolbox for Watershed Managers, Scientists, and Engineers (http:/ / unix. eng. ua. edu/ ~rpitt/ Publications/ BooksandReports/ Stormwater Effects Handbook by Burton and Pitt book/ MainEDFS_Book. html). New York: CRC/Lewis Publishers. 2001. ISBN 0-87371-924-7. . Chapter 2. [5] Water and Environmental Health at London and Loughborough (1999). "Waste water Treatment Options." (http:/ / www. lut. ac. uk/ well/ resources/ technical-briefs/ 64-wastewater-treatment-options. pdf) Technical brief no. 64. London School of Hygiene & Tropical Medicine and Loughborough University. [6] EPA. Washington, DC (2004). "Primer for Municipal Waste water Treatment Systems." (http:/ / www. epa. gov/ owm/ primer. pdf) Document no. EPA 832-R-04-001. [7] http:/ / www. waterworld. com/ index/ webcasts/ webcast-display/ 5792747027/ webcasts/ waterworld/ live-events/ evaluation_-application. html [8] Maine Department of Environmental Protection. Augusta, ME. "Aerated Lagoons - Wastewater Treatment." (http:/ / www. lagoonsonline. com) Maine Lagoon Systems Task Force. Accessed 2010-07-11. [9] Beychok, M.R. (1971). "Performance of surface-aerated basins". Chemical Engineering Progress Symposium Series 67 (107): 322–339. Available at CSA Illumina website (http:/ / md1. csa. com/ partners/ viewrecord. php?requester=gs& collection=ENV& recid=7112203& q=& uid=788301038& setcookie=yes) [10] Kadam, A.; Ozaa, G.; Nemadea, P.; Duttaa, S.; Shankar, H. (2008). "Municipal wastewater treatment using novel constructed soil filter system". Chemosphere (Elsevier) 71 (5): 975–981. doi:10.1016/j.chemosphere.2007.11.048. PMID 18207216. [11] Nemade, P.D.; Kadam, A.M.; Shankar, H.S. (2009). "Wastewater renovation using constructed soil filter (CSF): A novel approach" (http:/ / www. che. iitb. ac. in/ online/ bibliography/ wastewater-renovation-using-constructed-soil-filter-csf-a-novel-approach). Journal of Hazardous Materials (Elsevier) 170 (2-3): 657–665. doi:10.1016/j.jhazmat.2009.05.015. PMID 19501460. . [12] A documentary video detailing a 3 MLD SBT plant deployed at the Brihanmumbai Municipal Corporation for Mumbai city can be seen at "SBT at BMC Mumbai." (http:/ / www. youtube. com/ watch?v=dKWVtZ81mY0) [13] EPA. Washington, DC (2007). "Membrane Bioreactors." (http:/ / www. epa. gov/ owm/ mtb/ etfs_membrane-bioreactors. pdf) Wastewater Management Fact Sheet. [14] Das, Tapas K. (08 2001). "Ultraviolet disinfection application to a wastewater treatment plant". Clean Technologies and Environmental Policy (Springer Berlin/Heidelberg) 3 (2): 69–80. doi:10.1007/S100980100108. [15] Florida Department of Environmental Protection. Talahassee, FL. "Ultraviolet Disinfection for Domestic Waste water." (http:/ / www. dep. state. fl. us/ water/ wastewater/ dom/ domuv. htm) 2010-03-17. [16] Harshman, Vaughan; Barnette, Tony (05 2000). "Wastewater Odor Control: An Evaluation of Technologies" (http:/ / www. wwdmag. com/ Wastewater-Odor-Control-An-Evaluation-of-Technologies-article1698). Water Engineering & Management. ISSN 0273-2238. . [17] Walker, James D. and Welles Products Corporation (1976). "Tower for removing odors from gases." (http:/ / www. freepatentsonline. com/ 4421534. html) U.S. Patent No. 4421534. [18] EPA. Washington, DC (2000). "Package Plants." (http:/ / www. epa. gov/ owm/ mtb/ package_plant. pdf) Wastewater Technology Fact Sheet. Document no. EPA 832-F-00-016. [19] EPA. Washington, DC (1999). "Sequencing Batch Reactors." (http:/ / www. epa. gov/ owm/ mtb/ sbr_new. pdf) Wastewater Technology Fact Sheet. Document no. EPA 832-F-99-073. [20] Hammer, Mark J. (1975). Water and Waste-Water Technology. John Wiley & Sons. pp. 390–391. ISBN 0-471-34726-4. [21] ORGANIC CONTAMINANTS IN SEWAGE SLUDGE FOR AGRICULTURAL USE, European Commission Joint Research Centre Institute for Environment and Sustainability Soil and Waste Unit H. Langenkamp & P. Part (http:/ / ec. europa. eu/ environment/ waste/ sludge/ pdf/ organics_in_sludge. pdf) [22] Environment-agency.gov.uk (http:/ / www. environment-agency. gov. uk/ business/ 444304/ 1290036/ 1290100/ 1290353/ 1294402/ 1314667/ ) [23] Metcalf & Eddy, Inc. (1972). Wastewater Engineering. McGraw-Hill Book Company. pp. 552–554. ISBN 0-07-041675-3. [24] Haughey, A. (1968) The Planktonic Algae of Auckland Sewage Treatment Ponds, New Zealand Journal of Marine and Freshwater Research [25] Nutrients and Phytoplankton in Lake Washington Edmondson, WT; Nutrients and Eutrophication: The Limiting Nutrient Controversy, American Society of Limnology and Oceanography Special Symposia Vol.1 [26] Caperon, Cattell, and Krasnick (1971) Phytoplankton Kinetics in a Subtropical Estuary: Eutrophication, Limnology and Oceanography [27] Curds and Cockburn (1969) Protozoa in Biological Sewage-Treatment Processes -- I. A Survey of the Protozoan Fauna of British Percolating filters and Activated-Sludge Plants, Water Research [28] Monfort and Baleux (1990) Dynamics of Aeromonas hydrophila, Aeromonas sobria, and Aeromonas caviae in a Sewage Treatment Pond, Applied and Environmental Microbiology

18

Sewage treatment [29] Caribbean Environment Programme (1998). Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region (http:/ / www. cep. unep. org/ publications-and-resources/ technical-reports/ tr40en. pdf). Kingston, Jamaica: United Nations Environment Programme. . Retrieved 2009-10-12. Technical Report No. 40. [30] Massoud Tajrishy and Ahmad Abrishamchi, Integrated Approach to Water and Wastewater Management for Tehran, Iran, Water Conservation, Reuse, and Recycling: Proceedings of the Iranian-American Workshop, National Academies Press (2005) [31] Martin, Andrew (2008-08-10). "Farming in Israel, without a drop to spare" (http:/ / www. iht. com/ articles/ 2008/ 08/ 10/ business/ 10feed. php). New York Times. .

External links • "Anaerobic Industrial Wastewater Treatment: Perspectives for Closing Water and Resource Cycles." (http:// edepot.wur.nl/39480) Jules B. van Lier, Wageningen University, The Netherlands • Arcata, California Constructed Wetland: A Cost-Effective Alternative for Wastewater Treatment (http:// ecotippingpoints.org/our-stories/indepth/usa-california-arcata-constructed-wetland-wastewater.html) • Boston Sewage Tour (http://seagrant.mit.edu/education/resources/bostonsewage/introduction.html) - MIT Sea Grant • Interactive Diagram of Wastewater Treatment - "Go with the Flow" (http://wef.org/apps/gowithflow/theflow. htm) - Water Environment Federation • Phosphorus Recovery (http://www.phosphorus-recovery.tu-darmstadt.de) - Technische Universität Darmstadt & CEEP • Heavy metals recovery (http://enviropark.ru/course/category.php?id=10) - Mendeleev University Science Park • Sewer History (http://www.sewerhistory.org) • The Straight Dope - What happens to all the stuff that goes down the toilet? (http://www.straightdope.com/ mailbag/msolidwaste.html) - Syndicated column by Cecil Adams • Tour of a Washington state sewage plant written by an employee (http://www.poopreport.com/Consumer/ poop_plant.html) • National Water Engineering of Pakistan - Wastewater Treatment Plants in Pakistan (http://www.nwepk.com/)

19

Biochemical oxygen demand

Biochemical oxygen demand Biochemical oxygen demand or BOD is a chemical procedure for determining the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. It is not a precise quantitative test, although it is widely used as an indication of the organic quality of water.[1] It is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 °C and is often used as a robust surrogate of the degree of organic pollution of water. BOD can be used as a gauge of the effectiveness of wastewater treatment plants. It is listed as a conventional pollutant in the U.S. Clean Water Act.

The BOD5 test There are two commonly recognized methods for the measurement of BOD.

Dilution method To ensure that all other conditions are equal, a very small amount of micro-organism seed is added to each sample being tested. This seed is typically generated by diluting activated sludge with de-ionized water. The BOD test is carried out by diluting the sample with oxygen saturated de-ionized water, inoculating it with a fixed aliquot of seed, measuring the dissolved oxygen (DO) and then sealing the sample to prevent further oxygen dissolving in. The sample is kept at 20 °C in the dark to prevent photosynthesis (and thereby the addition of oxygen) for five days, and the dissolved oxygen is measured again. The difference between the final DO and initial DO is the BOD. The loss of dissolved oxygen in the sample, once corrections have been made for the degree of dilution, is called the BOD5. For measurement of carbonaceous BOD (cBOD), a nitrification inhibitor is added after the dilution water has been added to the sample. The inhibitor hinders the oxidation of nitrogen. BOD can be calculated by: • Undiluted: Initial DO - Final DO = BOD • Diluted: ((Initial DO - Final DO)- BOD of Seed) x Dilution Factor BOD is similar in function to chemical oxygen demand (COD), in that both measure the amount of organic compounds in water. However, COD is less specific, since it measures everything that can be chemically oxidized, rather than just levels of biologically active organic matter.

Manometric method This method is limited to the measurement of the oxygen consumption due only to carbonaceous oxidation. Ammonia oxidation is inhibited. The sample is kept in a sealed container fitted with a pressure sensor. A substance that absorbs carbon dioxide (typically lithium hydroxide) is added in the container above the sample level. The sample is stored in conditions identical to the dilution method. Oxygen is consumed and, as ammonia oxidation is inhibited, carbon dioxide is released. The total amount of gas, and thus the pressure, decreases because carbon dioxide is absorbed. From the drop of pressure, the sensor electronics computes and displays the consumed quantity of oxygen. The main advantages of this method compared to the dilution method are: • simplicity: no dilution of sample required, no seeding, no blank sample. • direct reading of BOD value. • continuous display of BOD value at the current incubation time.

20

Biochemical oxygen demand

Test Limitations The test method involves variables limiting reproducibility. Tests normally show observations varying plus or minus ten to twenty percent around the mean.[2] :82

Toxicity Some wastes contain chemicals capable of suppressing microbiological growth or activity. Potential sources include industrial wastes, antibiotics in pharmaceutical or medical wastes, sanitizers in food processing or commercial cleaning facilities, chlorination disinfection used following conventional sewage treatment, and odor-control formulations used in sanitary waste holding tanks in passenger vehicles or portable toilets. Suppression of the microbial community oxidizing the waste will lower the test result.[2] :85

Appropriate Microbial Population The test relies upon a microbial ecosystem with enzymes capable of oxidizing the available organic material. Some waste waters, such as those from biological secondary sewage treatment, will already contain a large population of microorganisms acclimated to the water being tested. An appreciable portion of the waste may be utilized during the holding period prior to commencement of the test procedure. On the other hand, organic wastes from industrial sources may require specialized enzymes. Microbial populations from standard seed sources may take some time to produce those enzymes. A specialized seed culture may be appropriate to reflect conditions of an evolved ecosystem in the receiving waters.[2] :85-87

History of the use of BOD The Royal Commission on River Pollution, which was established in 1865 and the formation of the Royal Commission on Sewage Disposal in 1898 led to the selection in 1908 of BOD5 as the definitive test for organic pollution of rivers. Five days was chosen as an appropriate test period because this is supposedly the longest time that river water takes to travel from source to estuary in the U.K. In 1912, the commission also set a standard of 20 ppm BOD5 as the maximum concentration permitted in sewage works discharging to rivers, provided that there was at least an 8:1 dilution available at dry weather flow. This was contained in the famous 20:30 (BOD:Suspended Solids) + full nitrification standard which was used as a yardstick in the U.K. up to the 1970s for sewage works effluent quality. The United States includes BOD effluent limitations in its secondary treatment regulations. Secondary sewage treatment is generally expected to remove 85 percent of the BOD measured in sewage and produce effluent BOD concentrations with a 30-day average of less than 30 mg/L and a 7-day average of less than 45 mg/L. The regulations also describe "treatment equivalent to secondary treatment" as removing 65 percent of the BOD and producing effluent BOD concentrations with a 30-day average less than 45 mg/L and a 7-day average less than 65 mg/L.[3]

Typical BOD values Most pristine rivers will have a 5-day carbonaceous BOD below 1 mg/L. Moderately polluted rivers may have a BOD value in the range of 2 to 8 mg/L. Municipal sewage that is efficiently treated by a three-stage process would have a value of about 20 mg/L or less. Untreated sewage varies, but averages around 600 mg/L in Europe and as low as 200 mg/L in the U.S., or where there is severe groundwater or surface water Infiltration/Inflow. (The generally lower values in the U.S. derive from the much greater water use per capita than in other parts of the world.)[1]

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Biochemical oxygen demand

BOD Biosensor An alternative to measure BOD is the development of biosensors, which are devices for the detection of an analyte that combines a biological component with a physicochemical detector component. Biosensors can be used to indirectly measure BOD via a fast (usually biofilm, but over the long term phosphorus storage was located in macrophyte> substratum>biofilm components. Medium iron-oxide adsorption provides additional removal for some years.[44]

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A comparison of phosphorus removal efficiency of two large-scale, surface flow wetland systems in Australia which had a gravel substratum to laboratory phosphorus adsorption indicated that for the first two months of wetland operation, the mean phosphorus removal efficiency of system 1 and 2 was 38% and 22%, respectively. Over the first year a decline in removal efficiencies occurred. During the second year of operation more phosphorus came out than was put in. This release was attributed to the saturation of phosphorus binding sites. Close agreement was found between the phosphorus adsorption capacity of the gravel as determined in the laboratory and the adsorption capacity recorded in the field. The phosphorus adsorption capacity of a subsurface flow constructed wetland system containing a predominantly quartz gravel in the laboratory using the Langmuir adsorption isotherm was 25 mg P/g gravel.[23] Close agreement between calculated and realized phosphorus adsorption was found. The poor adsorption capacity of the quartz gravel implied that plant uptake and subsequent harvesting were the major phosphorus removal mechanism.[45]

Metals removal Constructed wetlands have been used extensively for the removal of dissolved metals and metalloids. Although these contaminants are prevalent in mine drainage, they are also found in stormwater, landfill leachate and other sources (e.g., leachate or FDG washwater at coal-fired power plants), for which treatment wetlands have been constructed for mines,[46] and other applications.[47]

Mine water - Acid drainage removal A seminal publication was a 1994 report from the US Bureau of Mines [48] described the design of wetlands for treatment of acid mine drainage from coal mines. This report replaced the existing trial-and-error process with a strong scientific approach. This legitimized this technology and was followed in treating other contaminated waters.

Combined treatment ponds - commercial systems Three types, using reed beds, are used. All these systems are used commercially, usually together with septic tanks.[49] An other way is the combination Constructed wetland- Composting toilet. System types are: • Surface flow (SF) reed beds • Sub Surface Flow (SSF) reed beds • Vertical Flow (VF) reed beds All three types are placed in a closed basin with a substrate. Also, for most commercial undertakings (e.g. agricultural enterprises), the bottom is covered with a rubber foil (to completely waterproof the whole, which is essential in urban areas). The substrate can be either gravel, sand or lavastone.

The 3 treatment set-ups mostly employed in combined treatment ponds

Constructed wetland Design characteristics - commercial systems • Surface flow reed beds - characterized by the horizontal flow of wastewater across the roots of the plants. They are no longer used as much due to the large land-area requirements to purify water—20 square metres (220 sq ft) per person—and the increased smell and poor purification in winter.[49] • Subsurface flow reed beds - the flow of wastewater occurs between the roots of the plants (and not at the water surface). As a result the system is more efficient, less odorous and less sensitive to winter conditions. Also, less area is needed to purify A commercial water-purifying pond, planted with Iris pseudacorus water—5–10 square metres (54–110 sq ft). A downside to the system are the intakes, which can clog easily.[49] • Vertical flow reed beds - these are very similar to subsurface flow reed beds (subsurface wastewater flow is present here as well), according comparable advantages in efficiency and winter hardiness. The wastewater is divided at the bottom with the assistance of a pump. Unlike the 2 previous systems, this system makes almost exclusive use of fine sand to increase bacteria counts. Intake of oxygen into the water is also better, and pumping is pulsed to reduce obstructions within the intakes. The increased efficiency requires only 3 square metres (32 sq ft) of space per person.[49] Plants and other organisms - commercial systems Plants In North America, cattails (Typha latifolia) are common in constructed wetlands because of their widespread abundance, ability to grow at different water depths, ease of transport and transplantation, and broad tolerance of water composition (including pH, salinity, dissolved oxygen and contaminant concentrations). Elsewhere, Common Reed (Phragmites australis) are common (e.g. in greywater treatment systems to purify wastewater). In self-purifying water reservoirs (used to purify rainwater) however, certain other plants are used as well. These reservoirs firstly need to be dimensioned to be filled with 1/4 of lavastone and water-purifying plants to purify a certain water quantity.[50] They include a wide variety of plants, depending on the local climate and location. Plants are usually indigenous in that location for ecological reasons and optimum workings. Plants that supply oxygen and shade are also added in to complete the ecosystem. The plants used (placed on an area 1/4 of the water mass) are divided in 4 separate water depth-zones: 1. 0–20 cm; Yellow Iris (Iris pseudacorus), Simplestem Bur-reed (Sparganium erectum), ... may be placed here (temperate climates) 2. 40–60 cm; Water Soldier (Stratiotes aloides), European Frogbit (Hydrocharis morsus-ranae), ... may be placed here (temperate climates) 3. 60–120 cm; European White Waterlily (Nymphaea alba), ... my be placed here (temperate climates) 4. Below 120 cm; Eurasian Water-milfoil (Myriophyllum spicatum), may be placed here (temperate climates) The plants are usually grown on Coco Peat.[51] At the time of implantation to water-purifying ponds, de-nutrified soil is used to prevent unwanted algae and other organisms from taking over.

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Constructed wetland Fish and bacteria Finally, locally grown bacteria and non-predatory fish are added to eliminate or reduce pests, such as mosquitos. The bacteria are usually grown locally by submerging straw to support bacteria arriving from the surroundings. Three types of (non-predatory) fish are chosen to ensure that the fish can coexist: 1. surface; 2. middle-ground swimmers, and 3. bottom. Examples of three types (for temperate climates) are: 1. Surface swimming fish: Common dace (Leuciscus leuciscus), Ide (Leuciscus idus), common rudd (Scardinius erythrophthalmus), ... 2. Middle-swimmers: Common roach (Rutilus rutilus), ... 3. Bottom-swimming fish: Tench (Tinca tinca), ...

Hybrid systems Hybrid systems for example aerate the water after it exits the final reedbed using cascades A hybrid system using Flowforms in a treatment pond, in Norway. such as Flowforms before holding the water in a shallow pond.[52] Also, primary treatments as septic tanks, and different types of pumps as grinder pumps may also be added.[53]

References Literature citations • Bernard, J.M.; Solsky, B.A. (1976). "Nutrient cycling in a Carex lacustris wetland". Canadian Journal of Botany 55: 630–638. doi:10.1139/b77-077. • Bhamidimarri, R; Shilton, A.; Armstrong, I.; Jacobsen, P.; Scarlet, D. (1991). "Constructed wetlands for wastewater treatment: the New Zealand experience.". Water Science Technology 24: 247–253. • Bowmer, K.H. (1987). "Nutrient removal from effluents by an artificial wetland: influence of rhizosphere aeration and preferential flow studied using bromide and dye tracers". Water Research: 591–599. • Breen, P.F. (1990). "A mass balance method for assessing the potential of artificial wetlands for wastewater treatment". Water Research 24: 689–697. doi:10.1016/0043-1354(90)90024-Z. • Brix, Hans (1994). "Use of constructed wetlands in water pollution control: Historical development, present status, and future perspectives". Water Science & Technology 30 (8): 209–223. • Burgoon, P.S.; Reddy, T.A. DeBusk. "Domestic wastewater treatment using emergent plants cultured in gravel and plastic substrates". In Hammer 1989, pp. 536–541 • Burgoon, P.S.; Reddy, K.R.; DeBusk, T.A. (1991). "Vegetated submerged beds with artificial substrates II: N and P removal". Journal of Environmental Engineering 117 (4): 408–422. doi:10.1061/(ASCE)0733-9372(1991)117:4(408). • Cole, C.V.; Olsen, S.R.; Scott, C.O. (1953). "The nature of phosphate sorption by calcium carbonate". Soil Science Society of America Proceedings 410. • Cole, Stephen (1998). "The emergence of treatment wetlands". Environmental Science & Technology 32 (9): 218–223. • Conway, T.E.; Murtha, J. M. (1989). The Iselin marsh pond meadow In Hammer 1989, pp. 139–140.

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Constructed wetland • Davies, T.H.; Hart, B.T. (1990). "Use of aeration to promote nitrification in reed beds treating wastewater". Advanced Water Pollution Control 11: 77–84. • Finlayson, M.C.; Chick, A.J. (1983). "Testing the significance of aquatic plants to treat abattoir effluent". Water Research 17: 15–422. • Fried, M.; Dean, L.A. (1955). "Phosphate retention by iron and aluminum in cation exchange systems". Soil Science Society American Proceedings: 143–47. • Good, R.E.; Whigham, D.F.; Simpson, R.L., eds (1978). Freshwater wetlands, ecological processes and management potential. New York: Academic Press. • Gelt, Joe (1997). "Constructed Wetlands: Using Human Ingenuity, Natural Processes to Treat Water, Build Habitat" [54]. ARROYO 9 (4). • Guntensbergen, G.R.; Stearns, F.; Kadlec, J.A. (1989). "Wetland vegetation". In Hammer 1989, pp. 73–88 • Hammer, D.A. (1992). Creating freshwater wetlands Lewis Publishers. Chelsea, MI. • Hammer, D.A., ed (1989). Constructed wetlands for wastewater treatment. Chelsea, Michigan: Lewis publishers. • Hammer, D.A.; Bastion, R.K. (1989). Wetlands ecosystems: Natural water purifiers?. In Hammer 1989, pp. 5–20 • Hedin, R.S.; Nairn, R.W.; Kleinmann, R.L.P. (1994). "Passive treatment of coal mine drainage". Information Circular (Pittsburgh, PA.: U.S. Bureau of Mines). • Herskowitz, J. (1986). Listowell artificial marsh project report. Ontario Ministry of the Environment project. p. 253. • Hsu, P.H. (1964). "Adsorption of phosphate by aluminum and iron in soils". Soil Science Society Proceedings 9: 474–478. • Jenssen, P.D., T.; Maehlum, T. Zhu; Warner, W.S. (1992). Cold-climate constructed wetlands. Aas, Norway: JORDFORSK Centre for Soil and Environmental Research, N-1432. • Kadlec, R.H. (1989). Hydrologic factors in wetland water treatment. In Hammer 1989, pp. 21– 40 • Kadlec, R. H. (1995). Wetland treatment at Listowel (revisited) unpublished. • Klopatek, J.M. (1978). Nutrient dynamics of Freshwater Riverine marshes and the role of emergent macrophytes. In Good, Whigham & Simpson 1978, pp. 195–217 • Kotz, J.C.; Purcell, K.F. (1987). Chemistry and chemical reactivity. New York, N.Y.: CBS College Publishing. • Kramer, J.R.; Allen, H.E., eds (1972). Nutrients in natural waters John. Toronto: Wiley and Sons. • Lantzke, I.R.; Mitchell, D.S.; Heritage, A.D.; Sharma, K.P. (1999). "A model controlling orthophosphate removal in planted vertical flow wetlands". Ecological Engineering 12: 93–105. doi:10.1016/S0925-8574(98)00056-1. • Lemon, E. R.; Smith, I.D. (October 1993 Unpublished). Sewage waste amendment marsh process (SWAMP) Interim report,. • Lemon, E.R., G.; Bis.; Braybrook, T.; Rozema, L.; Smith, I. (1997). Sewage waste amendment marsh process (SWAMP) Final report. • Mann, R.A. (1990). "Phosphorus removal by constructed wetlands: substratum adsorption". Advanced Water Pollution Control 11 h. • Mitsch, J.W.; Gosselink, J.G. (1986). Wetlands. New York: Van Nostrand Reinhold Company. • Moss, B. (1988). Ecology of freshwater Blackball Scientific Publishers. London. • Nichols, D.S.; Boelter, D.H. (1982). "Treatment of secondary sewage with a peat-sand filter bed". Journal Environmental Quality 11 (1). • Niering, W.A. (1988). Wetlands: Audubon society nature guide.. Toronto: Random House of Canada Limited. p. 638. • Ontario Ministry of the Environment (1994). 23, Part 8, Sewage Systems. "Storm water management practices planning and design manual". O.B.C.-Ontario Building Code Act (Queen's Printer for Ontario): 8–14. • Patrick, W.H., Jr.; Reddy, K.R. (1976). "Nitrification-denitrification in flooded soils and water bottoms: dependence on oxygen supply and ammonium diffusion". Journal of Environmental Quality 5.

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Constructed wetland • Reddy, K.R.; DeBusk, W.F. (1987). Reddy and, K.R.; Smith, W.H.. eds. "Nutrient storage capabilities of aquatic and wetland plants". Aquatic plants for water treatment and resource recovery (Magnolia Publishing Inc). • Reed, S.C. (1986). "Wetlands as effluent treatment systems". Tech Press (Halifax, N.S): 207–219. • Reed, S.C. (1991). "Constructed Wetlands for Wastewater Treatment". BioCycle (January): 44–49. • Reed, S.C. (1995). Natural systems for waste management and treatment. McGraw Hill, Inc. • Reed, S.C.; Brown, D. (1995). "Subsurface flow wetlands-a performance evaluation". Water Environmental Research 67: 244–248. doi:10.2175/106143095X131420. • Rogers, K.H.; Breen, P.F.; Chick, A.J. (1991). "Nitrogen removal in experimental wetland treatment systems: evidence for the role of aquatic plants". Research Journal Water Political Control Fed 63: 934–941. • Rozema, L.R. , G.N.; Bis, T. Braybrook, E, R, Lemon; Smith, I. (1996). Retention of phosphorus in a Sub-surface flow constructed wetland Presented at: The 31st central Canadian symposium on water pollution research, Burlington, Ontario. • Sah, R.N.; Mikkelson, D. (1986). "Transformations of inorganic phosphorus during the flooding and draining cycles of soil". American Journal Soil Science 50: 62–67. doi:10.2136/sssaj1986.03615995005000010012x. • Smith, I.; Bis, G.N.; Lemon, E.R.; Rozema, L.R. (1997). "A thermal analysis of a vertical flow constructed wetland". Water Science Technology 35: 55–62. doi:10.1016/S0273-1223(97)00052-8. • Snell, D. (1990). Port Perry artificial marsh sewage treatment system unpublished report. • Steiner, R.S.; Freeman Configuration and substrate design considerations for constructed wetlands wastewater treatment, R.J.. In Hammer 1989, pp. 363–377 • Tanner, C. C., J. S.; Clayton; Upsdell, M.P. (199). "Effect of loading rate and planting on treatment of dairy farm wastewater’s in constructed wetlands-II removal of nitrogen phosphorus". Water Research 29: 27–34. doi:10.1016/0043-1354(94)00140-3. • Thut, N.R. (1989). Utilisation of artificial marshes for treatment of pulp mill effluents. In Hammer 1989, pp. 239–251 • United States environmental protection agency. (1988). Design manual: constructed wetlands and aquatic plant systems for municipal wastewater treatment EPA/625/1- 88/022. p. 83. • van Oirschot, Dion; Zaakvoerder; Rietland; Poppel (2002) (in German). Certificering van plantenwaterzuiveringssystemen [55]. Retrieved 2008-06-18. • Watson, J.T.; Reed, S.C.; Kadlec, R.H.; Knight, R.L.; Whitehouse, A.E.. Performance expectations and loading rates for constructed wetlands. In Hammer 1989, pp. 319–353 • Weber, L.R. (1990). Ontario soils Physical, chemical and biological properties and soil management practices—A reprint of Ontario Soils. Guelph, Ontario: Faculty and Staff of the Department of Land Resources Science Ontario Agricultural College University of Guelph. • Wetzel, R.G. (1983). Limnology. Orlando, Florida: Saunders college publishing. • University of Alaska Agriculture and Forestry Station (2005). "Wetlands and wastewater treatment in Alaska". Agroborealis 36 (2).

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Constructed wetland Footnotes [1] Hammer 1989 [2] Hammer 1989, pp. 565–573 Brix, H.; Schierup, H.. "Danish experience with sewage treatment in constructed wetlands". [3] Davies & Hart 1990 [4] Fried & Dean 1955 [5] Sah & Mikkelson 1986 [6] Patrick & Reddy 1976, pp. 469–472 [7] Mitsch & Gosselink 1993 [8] Gray, N.F. (1989). Biology of wastewater treatment. New York: Oxford University Press. p. 828. [9] Mitsch & Gosselink 1986, p. 536 [10] Keeney 1973 [11] Brock & Madigan 1991 [12] Klopatek 1978 [13] Wetzel 1983, pp. 255–297 [14] Bandurski 1965 [15] Nielson et al. 1990 [16] Richardson, et. al. 1978 [17] US EPA, 1988 [18] Reed 1995 [19] Smith et al. 1997 [20] (http:/ / www. oma. on. ca/ environment/ resources/ oma_towards_greener_footprints. pdf) [21] Guntensbergen, Stearns & Kadlec 1989 [22] Bernard & Solsky 1976 [23] Breen 1990 [24] Rogers, Breen & Chick 1991 [25] Sloey, W.E.; Spangler, F.L.; Fetter, Jr, C.W.. "Management of freshwater wetlands For nutrient assimilation". pp. 321–340. In Good, Whigham & Simpson 1978 [26] Thut 1989 [27] Moss 1988 [28] Swindell 1990 [29] Richardson 1985 [30] Pride et al. 1990 [31] Conley et al. 1991 [32] Kramer, J.R.; Herbes, S.E.; Allen, H.E. (1972). Phosphorus: analysis of water, biomass, and sediment. In Kramer & Allen 1972, pp. 51–101 [33] Simpson, R.L.; Whigham, D.F.. Seasonal patterns of nutrient movement in a freshwater tidal marsh. In Good, Whigham & Simpson 1978, pp. 243–257 [34] Hsu 1964 [35] Faulkner, S.P.; Richardson, C.J. (1989). Physical and chemical characteristics of freshwater wetland soils. In Hammer 1989, pp. 41–131 [36] Cole, Olsen & Scott 1953, pp. 352–356 [37] Mann 1990, pp. 97–105 [38] Hammer 1992, pp. 298 [39] Burns, N.M.; Ross, C. (1972). Oxygen-nutrient relationships within the central basin of lake Erie. In Kramer & Allen 1972, pp. 193–250 [40] Williams, J.D.H.; Mayer, T. (1972). Effects of sediment diagenesis and regeneration of phosphorus with special reference to lakes Erie and Ontario. In Kramer & Allen 1972, pp. 281–315 [41] Gosselink, J.G.; Turner, R.E.. The role of hydrology in freshwater wetland ecosystems. In Good, Whigham & Simpson 1978, pp. 63–78 [42] Kramer & Allen 1972 [43] Snell 1990 [44] Lantzke et al. 1999 [45] Lloyd R. Rozema, M.Sc. (excerpt from Master of Science thesis, Brock University, St. Catharines, ON, 2000) [46] (http:/ / technology. infomine. com/ enviromine/ wetlands/ Welcome. htm) [47] (http:/ / www. natural-resources. org/ minerals/ europe/ docs/ PIRAMID_Guidelines_v1. 0. pdf) [48] Hedin, Nairn & Kleinmann 1994 [49] van Oirschot et al. 2002 [50] "LavFilters" (http:/ / www. stowa-selectedtechnologies. nl/ Sheets/ Sheets/ Lava. Filters. html). . Retrieved 2008-06-18. [51] Coconut growing medium used for water purifying plants (http:/ / www. lukmertens. be/ kwekerij. html) [52] (http:/ / www. sheepdrove. com/ article. asp?art_id=115) Reedbed and Flowform cascade polishing, Sheepdrove Organic Farm, England [53] Pictures of hybrid reed bed systems (http:/ / www. pure-milieutechniek. be/ Page22. htm) [54] http:/ / ag. arizona. edu/ AZWATER/ arroyo/ 094wet. html [55] http:/ / www. certipro. be/ docs/ Certificering%20van%20plantenwaterzuiveringssystemen. pdf

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External links • American Society of Professional Wetland Engineers website (http://www.aspwe.org) - a wetland restoration for habitat and treatment 'wiki' • On-line constructed wetlands workshop (http://www.olawai.org) moderated by Greg Gearheart and Bob Gearheart. • U.S.EPA: Constructed Wetlands resources website (http://www.epa.gov/owow/wetlands/watersheds/ cwetlands.html) - United States Environmental Protection Agency • Google Books: "Creating Freshwater Wetlands" - by Donald A. Hammer (http://books.google.com/ books?id=t9WDhaX__KYC&printsec=frontcover) • Constructed wetlands in Lake Macquarie, Australia (http://www.lakemac.com.au/page.aspx?pid=109& vid=10&fid=196&ftype=True) • Federal Park Wetlands, Australia (http://www.ramin.com.au/annandale/wetlands.shtml) • Whites Creek Wetland, Australia (http://www.ramin.com.au/creekcare/whitescreek.shtml) • Wetpark: Water treatment systems website (http://www.holon.se/folke/projects/vatpark/concept.shtml) • WATER REPORT: Compost Toilets and Constructed Wetlands (http://www.a-spi.org/tp/tp58.htm)

Dissolved air flotation Dissolved air flotation (DAF) is a water treatment process that clarifies wastewaters (or other waters) by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank or basin. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it may then be removed by a skimming device.[1] [2] [3] Dissolved air flotation is very widely used in treating the industrial wastewater effluents from oil refineries, petrochemical and chemical plants, natural gas processing plants, paper mills, general water treatment and similar industrial facilities. A very similar process known as induced gas flotation is also used for wastewater treatment. Froth flotation is commonly used in the processing of mineral ores. In the oil industry, dissolved gas flotation (DGF) units do not use air as the flotation medium due to the explosion risk. Natural gas is used instead to create the bubbles.

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Dissolved air flotation

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Process description The feed water to the DAF float tank is often (but not always) dosed with a coagulant (such as ferric chloride or aluminum sulfate) to flocculate the suspended matter. A portion of the clarified effluent water leaving the DAF tank is pumped into a small pressure vessel (called the air drum) into which compressed air is also introduced. This results in saturating the pressurized effluent water with air. The air-saturated water stream is recycled to the front of the float tank and flows through a pressure reduction valve just as it enters the front of the float tank, which results in the air being released in the form of tiny bubbles. The bubbles adhere to the suspended matter, causing the suspended matter to float to the surface and form a froth layer which is then removed by a skimmer. The froth-free water exits the float tank as the clarified effluent from the DAF unit.[1]

A typical dissolved air flotation unit (DAF)

Modern DAF units using parallel plate technology are quite compact. Picture shows a 225 m³/h DAF.

Some DAF unit designs utilize parallel plate packing material, lamellas, to provide more separation surface and therefore to enhance the separation efficiency of the unit.

References [1] Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (1st ed.). John Wiley & Sons. LCCN 67019834. [2] Lawrence K. Wang, Yung-Tse Hung, Howard H. Lo and Constantine Yapijakis (2004). Handbook of Industrial and Hazardous Wastes Treatment (2nd ed.). CRC Press. ISBN 0-8247-4114-5. [3] Kiuru, H.; Vahala, R., eds (2000). "Dissolved air flotation in water and waste water treatment". International conference on DAF in water and waste water treatment No. 4, Helsinki, Finland. IWA Publishing, London. ISBN 1-900222-81-7.

External links • Treatment and Disposal of Ship-Generated Solid and Liquid Wastes (http://www.rempec.org/admin/store/ wyswigImg/file/Information resources/Other Meetings-Activities/Port reception facilities/Technical Reports/ Activity B - Final Report Consolidated.pdf) (REMPEC Regional Marine Pollution Emergency Response Centre for the Mediterranean Sea, Project MED.B4.4100.97.0415.8, April 2004) • htm http://www.kroftaswiss.com (http://www.kroftaswiss.com)

Desalination

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Desalination Water desalination

Methods •

Distillation • • •

Multi-stage flash distillation (MSF) Multiple-effect distillation (MED|ME) Vapor-compression (VC)

•

Ion exchange

•

Membrane processes • • • •

Electrodialysis reversal (EDR) Reverse osmosis (RO) Nanofiltration (NF) Membrane distillation (MD)

•

Freezing desalination

•

Geothermal desalination

•

Solar desalination • •

Solar humidification-Dehumidification (HDH) Multiple-effect humidification (MEH)

•

Methane hydrate crystallization

•

High grade water recycling

•

Seawater greenhouse

Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from water. More generally, desalination may also refer to the removal of salts and minerals,[1] as in soil desalination.[2] Water is desalinated in order to convert salt water to fresh water so it is suitable for human consumption or irrigation. Sometimes the process produces table salt as a by-product. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use in regions where the availability of fresh water is, or is becoming, limited. Large-scale desalination typically uses extremely large amounts of energy as well as specialized, expensive infrastructure, making it very costly compared to the use of fresh water from rivers or groundwater.[3] However, along with recycled water this is one of the only non-rainfall dependent water sources particularly relevant to countries like Australia which traditionally have relied on rainfall in dams to provide their drinking water supplies. The world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates. It is a dual-purpose facility that uses multi-stage flash distillation and is capable of producing 300 million cubic metres of water per year. By comparison the largest desalination plant in the United States is located in Tampa Bay, Florida and operated by Tampa Bay Water, which began desalinating 34.7 million cubic meters of water per year in December 2007.[4] The Tampa Bay plant runs at around 12% the output of the Jebel Ali Desalination Plants. A January 17, 2008, article in the Wall Street Journal states, "World-wide, 13,080 desalination plants produce more than 12 billion gallons of water a day, according to the International Desalination Association."[5]

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116

Methods The traditional process used in these operations is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. This is because the boiling of a liquid occurs when the vapor pressure equals the ambient pressure and vapor pressure increases with temperature. Thus, because of the reduced temperature, energy is saved. A leading distillation method is multi-stage flash distillation accounting for 85% of production worldwide in 2004.[6]

Schematic of a multi-stage flash desalinator A – Steam in B – Seawater in C – Potable water out D – Waste out E – Steam out F – Heat exchange G – Condensation collection H – Brine heater

Plan of a typical reverse osmosis desalination plant

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The principal competing processes use membranes to desalinate, principally applying reverse osmosis technology.[7] Membrane processes use semi-permeable membranes and pressure to separate salts from water. Reverse osmosis plant membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. Desalination remains energy intensive, however, and future costs will continue to depend on the price of both energy and desalination technology.

Considerations and criticism Cogeneration Cogeneration is the process of using excess heat from power production to accomplish another task. For desalination, cogeneration Reverse osmosis desalination plant in Barcelona, is the production of potable water from seawater or brackish Spain groundwater in an integrated, or "dual-purpose", facility in which a power plant is used as the source of energy for the desalination process. The facility’s energy production may be dedicated entirely to the production of potable water (a stand-alone facility), or excess energy may be produced and incorporated into the energy grid (a true cogeneration facility). There are various forms of cogeneration, and theoretically any form of energy production could be used. However, the majority of current and planned cogeneration desalination plants use either fossil fuels or nuclear power as their source of energy. Most plants are located in the Middle East or North Africa, due to their petroleum resources and subsidies. The advantage of dual-purpose facilities is that they can be more efficient in energy consumption, thus making desalination a more viable option for drinking water in areas of scarce water resources.[8] [9] In a December 26, 2007, opinion column in the The Atlanta Journal-Constitution, Nolan Hertel, a professor of nuclear and radiological engineering at Georgia Tech, wrote, "... nuclear reactors can be used ... to produce large amounts of potable water. The process is already in use in a number of places around the world, from India to Japan and Russia. Eight nuclear reactors coupled to desalination plants are operating in Japan alone ... nuclear desalination plants could be a source of large amounts of potable water transported by pipelines hundreds of miles inland..."[10]

Shevchenko BN350, a nuclear-heated desalination unit

Additionally, the current trend in dual-purpose facilities is hybrid configurations, in which the permeate from an RO desalination component is mixed with distillate from thermal desalination. Basically, two or more desalination processes are combined along with power production. Such facilities have already been implemented in Saudi Arabia at Jeddah and Yanbu.[11] A typical aircraft carrier in the U.S. military uses nuclear power to desalinate 400000 US gallons ( l;  imp gal) of water per day.[12]

Desalination

Economics A number of factors determine the capital and operating costs for desalination: capacity and type of facility, location, feed water, labor, energy, financing, and concentrate disposal. Desalination stills now control pressure, temperature and brine concentrations to optimize the water extraction efficiency. Nuclear-powered desalination might be economical on a large scale.[13] [14] While noting that costs are falling, and generally positive about the technology for affluent areas that are proximate to oceans, one study argues that "Desalinated water may be a solution for some water-stress regions, but not for places that are poor, deep in the interior of a continent, or at high elevation. Unfortunately, that includes some of the places with biggest water problems." and "Indeed, one needs to lift the water by 2000 metres (6600 ft), or transport it over more than 1600 kilometres (990 mi) to get transport costs equal to the desalination costs. Thus, it may be more economical to transport fresh water from somewhere else than to desalinate it. In places far from the sea, like New Delhi, or in high places, like Mexico City, high transport costs would add to the high desalination costs. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh and Harare. In many places, the dominant cost is desalination, not transport; the process would therefore be relatively less expensive in places like Beijing, Bangkok, Zaragoza, Phoenix, and, of course, coastal cities like Tripoli."[15] After being desalinated at Jubail, Saudi Arabia, water is pumped 200 miles (320 km) inland through a pipeline to the capital city of Riyadh.[16] For cities on the coast, desalination is being increasingly viewed as an untapped and unlimited water source. Desalination makes sense only after less expensive options are exhausted, including recycling water and fixing broken infrastructure. Water is reused in Las Vegas NV, Fountain Valley CA, Fairfax VA, El Paso TX and Scottsdale AZ. Compared to desalinated sea water, recycling requires 50% less energy due to the significantly lower salt content and produces new water at 30% less cost to the consumer, without the damage to marine life and ecosystems common to desalination plants. Israel is now desalinating water at a cost of US$0.53 per cubic meter.[17] Singapore is desalinating water for US$0.49 per cubic meter.[18] Many large coastal cities in developed countries are considering the feasibility of seawater desalination, due to its cost effectiveness compared with other water supply options, which can include mandatory installation of rainwater tanks or stormwater harvesting infrastructure. Studies have shown that the desalination option is more cost-effective than large-scale recycled water for drinking, and more cost-effective in Sydney than the vastly expensive option of mandatory installation of rainwater tanks or stormwater harvesting infrastructure. The city of Perth has been successfully [19] operating a reverse osmosis seawater desalination plant since 2006, and the Western Australian government have announced that a second plant will be built to serve the city's needs. A desalination plant is now operating in Australia's largest city of Sydney,[20] and the Wonthaggi desalination plant under construction in Wonthaggi, Victoria. The Perth desalination plant is powered partially by renewable energy from the Emu Downs Wind Farm.[21] A wind farm at Bungendore in NSW has been purpose-built to generate enough renewable energy to offset the energy use of the Sydney plant,[22] mitigating concerns about harmful greenhouse gas emissions, a common argument used against seawater desalination due to the energy requirements of the technology. The purchase or production of renewable energy to power desalination plants naturally adds to the capital and/or operating costs of desalination. However, recent experience in Perth and Sydney indicates that the additional cost is acceptable to communities, as a city may then augment its water supply without doing environmental harm to the atmosphere. The Queensland state government also purchased renewable energy certificates on behalf of its Gold Coast plant which will see the plant offset its carbon emissions for the initial 18 to 20 months of operations, bringing its environmental footprint down, in line with the other major plants that will be operating around the same time, in Perth and Sydney. In December 2007, the South Australian government announced that it would build a seawater desalination plant for the city of Adelaide, Australia, located at Port Stanvac. The desalination plant is to be funded by raising water rates to achieve full cost recovery.[23] [24] An online, unscientific poll showed that nearly 60% of votes cast were in favor

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Desalination of raising water rates to pay for desalination.[25] A January 17, 2008, article in the Wall Street Journal states, "In November, Connecticut-based Poseidon Resources Corp. won a key regulatory approval to build the US$300 million water-desalination plant in Carlsbad, north of San Diego. The facility would produce 50000000 US gallons ( l;  imp gal) of drinking water per day, enough to supply about 100,000 homes ... Improved technology has cut the cost of desalination in half in the past decade, making it more competitive ... Poseidon plans to sell the water for about US $950 per acre-foot [1200 cubic metres (42000 cu ft)]. That compares with an average US$700 an acre-foot [1200 m³] that local agencies now pay for water." [26] $1,000 per acre-foot works out to $3.06 for 1,000 gallons, or $.81 for 1 cubic meter, which is the unit of water measurement that residential water users are accustomed to being billed in.[27] While this regulatory hurdle was met, Poseidon Resources is not able to break ground until the final approval of a mitigation project for the damage done to marine life through the intake pipe, as is required by California law. Poseidon Resources has made progress in Carlsbad, CA, despite its unsuccessful attempt to complete construction of Tampa Bay Desal, a desalination plant in Tampa Bay, FL, in 2001. The Board of Directors of Tampa Bay Water were forced to buy Tampa Bay Desal from Poseidon Resources in 2001 to prevent a third failure of the project. Tampa Bay Water faced five years of engineering problems and operation at 20% capacity due to marine life and growth captured and stuck to reverse osmosis filters prior to fully utilizing this facility in 2007.[28] According to a May 9, 2008, article in Forbes, a San Leandro, California, company called Energy Recovery Inc. has been desalinating water for US $0.46 per cubic meter.[29] According to a June 5, 2008, article in the Globe and Mail, a Jordanian-born chemical engineering doctoral student at the University of Ottawa, named Mohammed Rasool Qtaisha, has invented a new desalination technology that is alleged to be between 600% and 700% more water output per square meter of membrane than current technology. According to the article, General Electric is looking into similar technology, and the U.S. National Science Foundation announced a grant to the University of Michigan to study it as well. Because the patents were still being worked out, the article was very vague about the details of this alleged technology.[30] While desalinating 1000 US gallons (3800 l; 830 imp gal) of water can cost as much as $3, the same amount of bottled water costs $7,945.[31]

Environmental Intake One of the main environmental considerations of ocean water desalination plants is the impact of the open ocean water intakes, especially when co-located with power plants. Many proposed ocean desalination plants' initial plans relied on these intakes despite perpetuating ongoing impacts on marine life. In the United States, due to a recent court ruling under the Clean Water Act, these intakes are no longer viable without reducing mortality, by 90%, of the life in the ocean; the plankton, fish eggs and fish larvae.[32] There are alternatives, including beach wells that eliminate this concern, but require more energy and higher costs while limiting output.[33] Other environmental concerns include air pollution and greenhouse gas emissions from the power plants. Outflow To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean, such as the outfall of a waste water treatment plant or power plant. While seawater power plant cooling water outfalls are not freshwater like waste water treatment plant outfalls, the salinity of the brine will still be reduced. If the power plant is medium- to large-sized and the desalination plant is not enormous, the flow of the power plant's cooling water is likely to be at least several times larger than that of the desalination plant. Another method to reduce the increase in salinity is to spread the brine via a diffuser to mix in a mixing zone so that there is only a slight increase in salinity. For example, once the pipeline containing the brine reaches the sea floor, it can split

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Desalination off into many branches, each one releasing the brine gradually along its length through small holes. This method can be used in combination with the joining of the brine with power plant or waste water plant outfalls. There are methods of desalination, particularly in combination with open pond evaporation (solar desalination), that do not discharge brine back into the ocean at all. The concentrated seawater has the potential to harm ecosystems, especially marine environments in regions with low turbidity and high evaporation that already have elevated salinity. Examples of such locations are the Persian Gulf, the Red Sea and, in particular, coral lagoons of atolls and other tropical islands around the world. The UAE, Qatar, Bahrain, Saudi Arabia, Kuwait and Iran have 120 desalination plants between them. These plants flush nearly 24 tons of chlorine, 65 tons of algae-harming antiscalants used to descale pipes, and around 300 kg of copper into the Persian Gulf every day. [34] Because the brine is denser than the surrounding sea water due to the higher solute concentration, discharge into water bodies means that the ecosystems on the bed of the water body are most at risk because the brine sinks and remains there long enough to damage the ecosystems. Careful re-introduction can minimize this problem. For example, for the desalination plant and ocean outlet structures to be built in Sydney from late 2007, the water authority states that the ocean outlets will be placed in locations at the seabed that will maximize the dispersal of the concentrated seawater, such that it will be indistinguishable from normal seawater between 50 and 75 metres (160 and 246 ft) from the outlet points. Sydney is fortunate to have typical oceanographic conditions off the coast that allow for such rapid dilution of the concentrated byproduct, thereby minimizing harm to the environment. In Perth, Australia, in 2007, the Kwinana Desalination Plant was opened. The water is sucked in from the ocean at only 0.1 metres per second (0.33 ft/s), which is slow enough to let fish escape. The plant provides nearly 140000 cubic metres ( cu ft) of clean water per day.[35] This is the same at Queensland's Gold Coast Desalination Plant and Sydney's Desalination Plant. Desalination compared to other water supply options Increased water conservation and water use efficiency remain the most cost-effective priorities in areas of the world where there is a large potential to improve the efficiency of water use practices.[36] While comparing ocean water desalination to waste water reclamation for drinking water shows desalination as the first option, using reclamation for irrigation and industrial use provides multiple benefits.[37] Urban runoff and storm water capture also provide benefits in treating, restoring and recharging groundwater.[38] A proposed alternative to desalinization in the state of California and other areas in the American Southwest is the commercial importation of bulk water either by very large crude carriers converted to water carriers, or via pipelines. The idea is politically unpopular in Canada, where governments have been scrambling to impose trade barriers to bulk water exports as a result of a claim filed in 1999 under Chapter 11 of the North American Free Trade Agreement (NAFTA) by Sun Belt Water Inc. a company established in 1990 in Santa Barbara, California, to address pressing local needs due to a severe drought in that area. Sun Belt maintains a web site where documents relating to their dispute are posted online.[39]

Experimental techniques and other developments In the past, many novel desalination techniques have been researched with varying degrees of success. One such process which has recently been commercialised by Modern Water plc is a forward osmosis based process for desalinated water, with a number of plants reported in operation.[40] [41] Other techniques have also attracted research funding. For example, to offset the energy requirements of desalination, the U.S. government is working to develop practical solar desalination. As an example of newer theoretical approaches for desalination, focusing specifically on maximizing energy efficiency and cost effectiveness, the Passarell Process may be considered.[42]

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Desalination Other approaches involve the use of geothermal energy. From an environmental and economic point of view, in most locations geothermal desalination can be preferable to using fossil groundwater or surface water for human needs, as in many regions the available surface and groundwater resources already have long been under severe stress. Recent research in the U.S. indicates that nanotube membranes may prove to be extremely effective for water filtration and may produce a viable water desalination process that would require substantially less energy than reverse osmosis.[43] Another method being looked into for water desalination is the use of biomimetic membranes [44] On June 23, 2008, it was reported that Siemens Water Technologies had developed a new technology, based on applying electric field on seawater, that desalinates one cubic meter of water while using only 1.5 kWh of energy, which, according to the report, is one half the energy that other processes use.[45] Fresh water can also be produced by freezing seawater, as happens naturally in the polar regions, and is known as freeze-thaw desalination. According to MSNBC, a report by Lux Research estimated that the worldwide desalinated water supply will triple between 2008 and 2020.[46]

Low-temperature thermal desalination Low-temperature thermal desalination (LTTD) takes advantage of the fact that water boils at low pressures, even as low as ambient temperature. The system uses vacuum pumps to create a low pressure, low-temperature environment in which water boils at a temperature gradient of 8 to 10 °C between two volumes of water. Cooling water is supplied from sea depths of as much as 600 metres (2000 ft). This cold water is pumped through coils to condense the evaporated water vapor. The resulting condensate is purified water. The LTTD process may also take advantage of the temperature gradient available at power plants, where large quantities of warm waste water are discharged from the plant, reducing the energy input needed to create a temperature gradient.[47] The principle of LTTD is known for a long time, originally stemming from ocean thermal energy conversion research. Some experiments were conducted in U.S. and Japan to test the low-temperature driven desalination technology. In Japan, a spray flash evaporation system was tested by Saga University.[48] In US, at Hawaii Islands, the National Energy Laboratory tested an open-cycle OTEC plant with fresh water and power production using a temperature of 20 °C between surface water and water at a depth of around 500 m. LTTD was studied by India's National Institute of Ocean Technology (NIOT) from 2004. Their first LTTD plant was opened in 2005 at Kavaratti in the Lakshadweep islands. The plant's capacity is 100000 litres (22000 imp gal; 26000 US gal)/day, at a capital cost of INR 50 million (€922,000). The plant uses deep water at a temperature of 7 to 15 °C (45 to 59 °F).[49] In 2007, NIOT opened an experimental floating LTTD plant off the coast of Chennai with a capacity of 1000000 litres ( imp gal;  US gal)/day. A smaller plant was established in 2009 at the North Chennai Thermal Power Station to prove the LTTD application where power plant cooling water is available.[47] [50] [51]

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Thermo-ionic process In October 2009, Saltworks Technologies, a Canadian firm, announced a process that uses solar or other thermal heat to drive an ionic current that empties all the sodium and chlorine ions from the water.[52]

Existing facilities and facilities under construction Abu Dhabi, United Arab Emirates • Taweelah A1 Power and Desalination Plant has an output 385000000 litres ( imp gal;  US gal) per day of clean water. • Umm Al Nar Desalination Plant has an output of 394000000 litres ( imp gal;  US gal) per day of clean water. • Fujairah F2 is to be completed by July 2010 will have a water production capacity of 492000000 litres ( imp gal;  US gal) per day.[53]

Aruba The island of Aruba has a large (world’s largest at the time of its inauguration) desalination plant with the total installed capacity of 42,000 metric tons (11.1 million gallons or 42 × 103 m3) per day.[54]

Australia A combination of increased water usage and lower rainfall/drought in Australia has caused State governments to build a number of desalination plants, including the recently commissioned Kurnell Desalination Plant serving the Sydney area. While desalination has been adopted by state governments to secure water supply, it is highly energy intensive (~$140 energy demand/ML) and has a high carbon footprint due to continued reliance on Australia's coal-based energy generation.

Bahrain • The Al Hidd Desalination Plant on Muharraq island treats seawater through a multistage flash process, and produces 30 million gallons per day. This project was completed in 2000. The Al Hidd distillate forwarding station, comprises of a 410 million litres distillate water storage in 45 million litres steel tanks. A 135 million litres/day forwarding pumping station sends flows to the Hidd blending station, Muharraq blending station, Hoora blending station, Sanabis blending station and Seef blending station and which has an option for gravity supply for low flows to blending pumps and pumps which forward to Janusan, Budiya and Saar. [55] • When completed in three phases, the Durrat Al Bahrain sea water reverse osmosis (SWRO) desalination plant will have a capacity of 36,000 cubic meters of potable water per day which will serve the irrigation needs of the entire Durrat Al Bahrain development.[56] The Bahrain-based utility company, Energy Central Co (ECC) will provide the plant a 25-year design, build and operate contract.[57]

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123

China China operates the Beijiang Desalination Plant in Tianjin, a combination desalination and coal-fired power plant designed to alleviate Tianjin's critical water shortage. Though the facility has the capacity to produce 200,000 cubic meters of potable water per day, it has never operated at more than one quarter capacity due to difficulties with local utility companies and an inadequate local infrastructure.[58]

Cyprus There are also desalination plants in Cyprus, like the one near the town of Larnaca.[59] This is called the Dhekelia Desalination Plant, which utilises the reverse osmosis system.[60]

Gibraltar The fresh water supply in Gibraltar is supplied by a number of reverse osmosis and multi-stage flash desalination plants.[61] . There is also a demonstation forward osmosis desalination plant operational.[62]

Israel The Hadera seawater reverse osmosis (SWRO) desalination plant in Israel is the largest of its kind in the world.[63] [64] The project was developed as a Build-Operate-Transfer (BOT) by a consortium of three international companies: Veolia water, IDE Technologies and Elran.[65]

Existing Israeli water desalination facilities[66] Location

Ashkelon

Opened

August 2005

Palmachim May 2007 Hadera

Capacity (mln m3/year)

Cost of water (per m3)

Notes

120 (as of 2010) NIS 2.60

[67]

45

NIS 2.90

[68]

NIS 2.60

[69]

December 2009 127

Israeli water desalination facilities under construction Location

Opening

Ashdod

2012

100 (expansion up to 150 possible)

Soreq

2013

150 (expansion up to 300 approved) NIS 2.01 – 2.19 [72]

Capacity (mln m3/year)

Cost of water (per m3) NIS 2.40

Notes

[70] [71]

Maldives Maldives is a small island nation and most of the islands depend on desalination as a source of water.

Saudi Arabia The Saline Water Conversion Corporation of Saudi Arabia provides 50% of the municipal water in the Kingdom, operates a number of desalination plants, and has contracted $1892 million [73] to a Japanese-South Korean consortium to build one capable of producing a billion litres a day, opening at the end of 2013. They currently operate approximately 14 plants in the Kingdom;[74] one example at Shoaiba cost $1060 million and produces 450

Desalination million litres a day.

United Kingdom Beckton Desalination Plant The first large scale water desalination plant in the United Kingdom, the Thames Water Desalination Plant,[75] has been built in Beckton, east London for Thames Water by Acciona Agua

United States El Paso (Texas) Desalination Plant Brackish groundwater has been treated at the El Paso plant since around 2004. Producing 27500000 US gallons ( l;  imp gal) of fresh water daily (about 25% of total freshwater deliveries) by reverse osmosis, it is a crucial contribution to water supplies in this water-stressed city.[76] Tampa Bay Water Desalination Project The Tampa Bay Water Desalination project was originally a private venture led by Poseidon Resources. This project was delayed by the bankruptcy of Poseidon Resources' successive partners in the venture, Stone & Webster, then Covanta (formerly Ogden) and its principal subcontractor Hydranautics. Poseidon's relationship with Stone & Webster through S & W Water LLC ended in June 2000 when Stone & Webster declared bankruptcy and Poseidon Resources purchased Stone & Webster's stake in S & W Water LLC. Poseidon Resources partnered with Covanta and Hydranautics in 2001, changing the consortium name to Tampa Bay Desal. Through the inability of Covanta to complete construction bonding of the project, the Tampa Bay Water agency was forced to purchase the project from Poseidon on May 15, 2002, and underwrite the project financing under its own credit rating. Tampa Bay Water then contracted with Covanta Tampa Construction, which produced a project that did not meet required performance tests. Covanta Tampa Construction's parent company filed bankruptcy in October 2003 to prevent losing the contract with Tampa Bay Water. Then, Covanta Tampa Construction filed bankruptcy prior to performing renovations that would have satisfied contractual agreements. This resulted in nearly six months of litigation between Covanta Tampa Construction and Tampa Bay Water. In 2004, Tampa Bay Water hired a renovation team, American Water/Acciona Aqua, to bring the plant to its original, anticipated design. The plant was deemed fully operational in 2007[28] and is designed to run at a maximum capacity of 25 million gallons per day.[77] Nevertheless, the plant continues to be set with problems limiting it to producing only about half that amount (14 million gallons per day or 42 af/day in 2009.[78] Yuma Desalting Plant (Arizona) The Yuma Desalting Plant was constructed under authority of the Colorado River Basin Salinity Control Act of 1974 to treat saline agricultural return flows from the Wellton-Mohawk Irrigation and Drainage District. The treated water is intended for inclusion in water deliveries to Mexico thereby preserving the like amount of water in Lake Mead. Construction of the plant was completed in 1992 and it has operated on two occasions since then. The plant has been maintained, but largely not operated due to surplus and then normal water supply conditions on the Colorado River.[79] An agreement was reached in April 2010 between the Southern Nevada Water Authority, the Metropolitan Water District of Southern California, the Central Arizona Project and the U.S. Bureau of Reclamation to underwrite the cost of running the plant in a year long pilot project.[80]

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Trinidad and Tobago The Republic of Trinidad and Tobago is using desalination to free up more of the island's water supply for drinking purposes. The desalination facility, opened in March 2003, is considered to be the first of its kind. It is the largest desalination facility in the Americas and will process 28800000 US gallons ( l;  imp gal) of water a day and sell water at the price of $2.67 per 1000 US gallons (3800 l; 830 imp gal).[81] This facility will be located at Trinidad's Point Lisas Industrial Estate, a park of more than 12 companies in various manufacturing and processing functions and will allow for easy access to water for both factories and residents in the country.[82]

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[29] Hydro-Alchemy (http:/ / www. forbes. com/ technology/ 2008/ 05/ 08/ mitra-energy-recovery-tech-science-cx_sm_0509mitra. html), Forbes, May 9, 2008 [30] Ottawa student may hold secret to Water For All (http:/ / www. theglobeandmail. com/ servlet/ story/ RTGAM. 20080605. wgtwater0605/ BNStory/ Technology/ home?cid=al_gam_mostemail), Globe and Mail, June 5, 2008 [31] The Arid West—Where Water Is Scarce – Desalination—a Growing Watersupply Source (http:/ / www. libraryindex. com/ pages/ 2644/ Arid-West-Where-Water-Scarce-DESALINATION-GROWING-WATERSUPPLY-SOURCE. html), Library Index [32] UNITED STATES COURT OF APPEALS FOR THE SECOND CIRCUIT August Term, 2005 (http:/ / www. desalresponsegroup. org/ files/ RiverkeepervEPA1-25-07_decision. pdf). (PDF) . Retrieved on 2011-05-29. [33] Heather Cooley, Peter H. Gleick, and Gary Wolff DESALINATION, WITH A GRAIN OF SALT. A California Perspective (http:/ / www. pacinst. org/ reports/ desalination/ desalination_report. pdf), Pacific Institute for Studies in Development, Environment, and Security, June 2006 ISBN 1-893790-13-4 [34] Emmanuelle Landais (2009-06-14). "Waste dump threatens Arabian Gulf" (http:/ / gulfnews. com/ news/ gulf/ uae/ environment/ waste-dump-threatens-arabian-gulf-1. 72058). Gulf News. . [35] Australia Turns to Desalination Amid Water Shortage (http:/ / www. npr. org/ templates/ story/ story. php?storyId=11134967). NPR. Retrieved on 2011-03-20. [36] Gleick, Peter H., Dana Haasz, Christine Henges-Jeck, Veena Srinivasan, Gary Wolff, Katherine Kao Cushing, and Amardip Mann. (November 2003.) "Waste not, want not: The potential for urban water conservation in California." (http:/ / www. pacinst. org/ reports/ urban_usage/ waste_not_want_not_full_report. pdf) (Website). Pacific Institute. Retrieved on 2007-09-20. [37] Cooley, Heather, Peter H. Gleick, and Gary Wolff. (June 2006.) "Desalination, With a Grain of Salt – A California Perspective." (http:/ / www. pacinst. org/ reports/ desalination/ index. htm) (Website). Pacific Institute. Retrieved on 2007-09-20. [38] Gleick, Peter H., Heather Cooley, David Groves. (September 2005.) "California water 2030: An efficient future." (http:/ / pacinst. org/ reports/ california_water_2030/ ca_water_2030. pdf). Pacific Institute. Retrieved on 2007-09-20. [39] Sun Belt Inc. Legal Documents (http:/ / www. sunbeltwater. com/ docs. shtml). Sunbeltwater.com. Retrieved on 2011-05-29. [40] "FO plant completes 1-year of operation" (http:/ / www. modernwater. co. uk/ files/ files/ WDR - 44. pdf). Water Desalination Report: 2–3. 15 Nov. 2010. . Retrieved 28 May 2011. [41] "Modern Water taps demand in Middle East" (http:/ / www. modernwater. co. uk/ files/ files/ demand_mdeast_n. pdf). The Independent. 23 Nov. 2009. . Retrieved 28 May 2011. [42] The "Passarell" Process (http:/ / www. waterdesalination. com/ theory. htm) [43] Lawrence Livermore National Laboratory Public Affairs (2006-05-18). "Nanotube membranes offer possibility of cheaper desalination" (http:/ / www. llnl. gov/ pao/ news/ news_releases/ 2006/ NR-06-05-06. html). Press release. . Retrieved 2007-09-07. [44] Sandia National Labs: Desalination and Water Purification: Research and Development (http:/ / www. sandia. gov/ water/ desal/ research-dev/ membrane-tech. html). Sandia.gov. Retrieved on 2011-03-20. [45] Team wins $4m grant for breakthrough technology in seawater desalination (http:/ / news. asiaone. com/ News/ AsiaOne+ News/ Singapore/ Story/ A1Story20080623-72473. html), The Straits Times, June 23, 2008 [46] A Rising Tide for New Desalinated Water Technologies (http:/ / www. msnbc. msn. com/ id/ 29735521/ ), MSNBC, March. 17, 2009 [47] Sistla, Phanikumar V.S.; et al. "Low Temperature Thermal DesalinbationPLants" (http:/ / www. isope. org/ publications/ proceedings/ ISOPE_OMS/ OMS 2009/ papers/ M09-83Sistla. pdf). International Society of Offshore and Polar Engineers. . Retrieved 22 June 2010. [48] Haruo Uehara and Tsutomu Nakaoka Development and Prospective of Ocean Thermal Energy Conversion and Spray Flash Evaporator Desalination (http:/ / www. ioes. saga-u. ac. jp/ VWF/ general-review_e. html) [49] Desalination: India opens world’s first low temperature thermal desalination plant – IRC International Water and Sanitation Centre (http:/ / www. irc. nl/ page/ 24010). Irc.nl (2005-05-31). Retrieved on 2011-03-20. [50] Floating plant, India (http:/ / www. headlinesindia. com/ archive_html/ 18April2007_35210. html). Headlinesindia.com (2007-04-18). Retrieved on 2011-05-29. [51] Tamil Nadu / Chennai News : Low temperature thermal desalination plants mooted (http:/ / www. hindu. com/ 2007/ 04/ 21/ stories/ 2007042109200400. htm). The Hindu (2007-04-21). Retrieved on 2011-03-20. [52] Current thinking (http:/ / www. economist. com/ sciencetechnology/ displayStory. cfm?story_id=14743791), Oct 29th 2009, The Economist [53] Abu Dhabi to Build Three Power and Water Desalination Plants by 2016 to Meet Demand (http:/ / www. industrialinfo. com/ showAbstract. jsp?newsitemID=152606). industrialinfo.com (2009-11-18). Retrieved on 2011-03-20. [54] W.E.B. Aruba N.V. – Water Plant (http:/ / www. webaruba. com/ index. php?option=com_content& task=view& id=44& Itemid=159). Webaruba.com. Retrieved on 2011-05-29. [55] Al Hidd Desalination Plant (http:/ / www. water-technology. net/ projects/ hidd/ ). Water Technology. Retrieved on 2011-05-29.

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Desalination [56] Durrat Al Bahrain desalination plant (http:/ / www. water-technology. net/ projects/ durrat-desalination/ ). Water Technology. Retrieved on 2011-05-29. [57] Construction starts on Durrat Al Bahrain desalination plant (http:/ / www. desalination. biz/ news/ news_story. asp?id=4775). Desalination.biz. Retrieved on 2011-05-29. [58] Watts, Jonathan (2011-01-24). "Can the sea solve China's water crisis?" (http:/ / www. guardian. co. uk/ environment/ 2011/ jan/ 24/ china-water-crisis?INTCMP=ILCNETTXT3487). The Guardian. . Retrieved 2011-04-19. [59] Larnaca SWRO Water Desalination Plant (http:/ / www. water-technology. net/ projects/ larnaca/ ). Water Technology. Retrieved on 2011-03-20. [60] Marangou, V; Savvides, K (2001). "First desalination plant in Cyprus — product water aggresivity and corrosion control1" (http:/ / www. cyprus. gov. cy/ moa/ wdd/ wdd. nsf/ All/ E59112ED2B3034B2C22571C4001BAFE6/ $file/ Page1_8. pdf?OpenElement). Desalination 138: 251. doi:10.1016/S0011-9164(01)00271-5. . [61] AquaGib: Gibraltar – Present Plant (http:/ / www. aquagib. gi/ present_plant. html). Aquagib.gi. Retrieved on 2011-03-20. [62] "GIBRALTAR PROVING PLANT EXCEEDING EXPECTATIONS" (http:/ / www. modernwater. co. uk/ files/ files/ 2009-06-29. pdf). . Retrieved 29 May 2011. [63] Israel is No. 5 on Top 10 Cleantech List (http:/ / www. israel21c. org/ briefs/ israel-is-no-5-on-top-10-cleantech-list) in Israel 21c A Focus Beyond (http:/ / www. israel21c. org/ technology/ archive) Retrieved 2009-12-21 [64] Ashkelon Desalination Plant Seawater Reverse Osmosis (SWRO) Plant (http:/ / www. water-technology. net/ projects/ israel/ ). Water-technology.net. Retrieved on 2011-05-29. [65] Sauvetgoichon, B (2007). "Ashkelon desalination plant — A successful challenge". Desalination 203: 75–81. doi:10.1016/j.desal.2006.03.525. [66] Public-Private Partnership Projects (http:/ / ppp. mof. gov. il/ Mof/ PPP/ MofPPPTopNavEnglish/ MofPPPProjectsEnglish/ ), Accountant General, Ministry of Finance [67] water-technology.net: "Ashkelon Desalination Plant Seawater Reverse Osmosis (SWRO) Plant, Israel" (http:/ / www. water-technology. net/ projects/ israel/ ) [68] Globes Business and Technology News: "Palmachim desalination plant inaugurates expansion" (http:/ / www. globes. co. il/ serveen/ globes/ docview. asp?did=1000601526), November 17, 2010 [69] Globes Business and Technology News: "Funding agreed for expanding Hadera desalination plant" (http:/ / archive. globes. co. il/ searchgl/ Production at the plants in Hadera, Palmachim and_s_hd_2L34nD3aqCbmnC30mD3KtE3GsBcXqRMm0. html), November 6, 2009 [70] Desalination & Water Reuse (http:/ / www. desalination. biz/ about. asp?channel=0): "Spanish/Israeli JV awarded Ashdod desalination contract" (http:/ / www. desalination. biz/ news/ news_story. asp?id=5133), 24 November 2009 [71] Globes Business and Technology News: "Mekorot wins battle to build Ashdod desalination plant" (http:/ / www. globes. co. il/ serveen/ globes/ docview. asp?did=1000625564& fid=1725), February 22, 2011 [72] Desalination & Water Reuse (http:/ / www. desalination. biz/ about. asp?channel=0): "IDE reported winner of Soreq desalination contract" (http:/ / www. desalination. biz/ news/ news_story. asp?id=5163), 15 December 2009 [73] Sasakura, Samsung $1.89bn bid lowest for Saudi plant (http:/ / www. reuters. com/ article/ idUSLDE64A0WL20100511). Reuters.com. Retrieved on 2011-05-29. [74] Map on this page (http:/ / www. water-technology. net/ projects/ shuaiba/ shuaiba2. html). Water-technology.net. Retrieved on 2011-05-29. [75] Thames Water Desalination Plant (http:/ / www. water-technology. net/ projects/ water-desalination/ ). water-technology.net. Retrieved on 2011-05-29. [76] El Paso Water Utilities – Public Service Board|Desalination Plant (http:/ / www. epwu. org/ water/ desal_info. html). Epwu.org. Retrieved on 2011-03-20. [77] Tampa Bay Seawater Desalination Plant (http:/ / www. tampabaywater. org/ facilities/ desalination_plant/ index. aspx). Tampabaywater.org. Retrieved on 2011-03-20. [78] More problems for Tampa Bay Water desalination plant – St. Petersburg Times (http:/ / www. tampabay. com/ news/ environment/ water/ article984409. ece). Tampabay.com. Retrieved on 2011-03-20. [79] "Yuma Desalting Plant" (http:/ / www. usbr. gov/ lc/ yuma/ facilities/ ydp/ yao_ydp. html) U.S. Bureau of Reclamation, retrieved May 1, 2010 [80] "A fresh start for Yuma desalting plant" (http:/ / www. latimes. com/ news/ custom/ topofthetimes/ topstories/ la-me-water-20100501-15,0,1233621. story) Los Angeles Times, May 1, 2010 [81] Ionics to build $120M desalination plant in Trinidad|Boston Business Journal (http:/ / www. bizjournals. com/ boston/ stories/ 1999/ 10/ 04/ story7. html). Bizjournals.com. Retrieved on 2011-03-20. [82] Trinidad Desalination Plant (http:/ / www. waterindustry. org/ New Projects/ ionics-2. htm). Waterindustry.org (2000-10-26). Retrieved on 2011-03-20.

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Further reading • Committee on Advancing Desalination Technology, National Research Council. (2008). Desalination: A National Perspective (http://www.nap.edu/catalog.php?record_id=12184). National Academies Press.

Articles • Desalination: The next wave in global water consumption (http://www.tlvinsider.com/tlvinsider/nl3/ interviews?name=nl3_interview2) from TLVInsider (http://www.tlvinsider.com)

External links • International Desalination Association (http://www.idadesal.org) • Examples of sea water desalination plants by the WWWS AG (http://wwws-ag.com/Sea-water-treatment.732. 0.html) • GeoNoria Solar Desalination Process (http://geonoria.org) • National Academies Press|Desalination: A National Perspective (http://books.nap.edu/openbook. php?record_id=12184&page=R1) • World Wildlife Fund|Desalination: option or distraction? (http://assets.panda.org/downloads/ desalinationreportjune2007.pdf) • European Desalination Society (http://www.edsoc.com) • IAEA – Nuclear Desalination (http://www.iaea.org/nucleardesalination/) • DME – German Desalination Society (http://www.dme-ev.de) • Large scale desalination of sea water using solar energy (http://citeseerx.ist.psu.edu/viewdoc/ summary?doi=10.1.1.142.5296) • Desalination by humidification and dehumidification of air: state of the art (http://www.desline.com/articoli/ 4107.pdf) • Zonnewater – optimized solar thermal desalination (distillation) (http://www.zonnewater.net) • SOLAR TOWER Project – Clean Electricity Generation for Desalination. (http://www.enviromission.com.au) • Desalination bibliography Library of Congress (http://www.loc.gov/rr/scitech/tracer-bullets/desalinationtb. html) • Water-Technology (http://www.water-technology.net/projects/) • Cheap Drinking Water from the Ocean (http://www.technologyreview.com/read_article.aspx?ch=nanotech& sc=&id=16977&pg=1) – Carbon nanotube-based membranes will dramatically cut the cost of desalination • Solar thermal-driven desalination plants based on membrane distillation (http://www.desline.com/articoli/ 5140.pdf) • Encyclopedia of Water Sciences, Engineering and Technology Resources (http://www.eolss.net/) • wind-powered desalinization plant in Perth, Australia, is an example of how technology is insulating rich countries from impacts of climate change, while poor countries remain particularly vulnerable. (http://www. nytimes.com/2007/04/03/science/earth/03clim.html/The) • The Desal Response Group (http://www.desalresponsegroup.org) • Encyclopedia of Desalination and water and Water Resources (http://www.desware.net/) • Desalination & Water Reuse – Desalination news (http://www.desalination.biz/) • Desalination: The Cyprus Experience (http://www.ewra.net/ew/pdf/EW_2004_7-8_04.pdf)

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Electrocoagulation

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Electrocoagulation Electrocoagulation, also known as Radio Frequency Diathermy or Short Wave Electrolysis, is a technique used for medical treatment and wastewater treatment.

Medical treatment Electrocoagulation Intervention MeSH D004564 [1]

A fine wire probe or other delivery mechanism is used to transmit radio waves to tissues near the probe. Molecules within the tissue are caused to vibrate which lead to a rapid increase of the temperature, causing coagulation of the proteins within the tissue, effectively killing the tissue. At higher powered applications, full desiccation of tissue is possible.

Electrocoagulation in water treatment Although electrocoagulation (EC) is an evolving technology that has for the past 100 years been effectively applied in industrial wastewater treatment, the paucity of scientific understanding of the complex chemical and physical processes involved as well as the limitations (in terms of size and cost) of the needed power supplies in the past, have curbed large scale applications and hindered progress.[2] In addition, the powerful manufacturers of chemicals have been able to restrict the market penetration of this effective, environmentally friendly non-chemical procedure. With the latest technologies, reduction of electricity requirements, and miniaturization of the needed power supplies, EC systems have now become within reach of water treatment plants and industrial processes worldwide.

Background The need for clean water is particularly critical in developing countries. Rivers, canals, estuaries and other water bodies are being constantly polluted due to indiscriminate discharge of industrial effluents as well as other anthropogenic activities and natural processes. In the latter, unknown geochemical processes have contaminated groundwater with arsenic in many counties. Highly developed countries are also experiencing a critical need for wastewater cleaning because of an ever-increasing population, urbanization and climatic changes. Both the treatment of wastewater prior to discharge and the reuse of wastewater have become absolute necessities. There is, therefore, an urgent need to develop innovative, more effective and inexpensive techniques for treatment of wastewater. A wide range of wastewater treatment techniques are known, which includes biological processes for nitrification, denitrification and phosphorus removal, as well as a range of physico-chemical processes that require chemical addition. The commonly used physico-chemical treatment processes are filtration, air stripping, ion exchange, chemical precipitation, chemical oxidation, carbon adsorption, ultrafiltration, reverse osmosis, electrodialysis, volatilization, and gas stripping.

Electrocoagulation

Technology Treatment of wastewater by EC has been practiced for most of the 20th century with limited success and popularity. In the last decade, this technology has been increasingly used in South America and Europe for treatment of industrial wastewater containing metals.[3] It has also been noted that in North America EC has been used primarily to treat wastewater from pulp and paper industries, mining and metal-processing industries. A large one-thousand gallon per minute cooling tower application in El Paso Texas illustrates electrocoagulations growing recognition and acceptance to the industrial community. In addition, EC has been applied to treat water containing foodstuff waste, oil wastes, dyes, suspended particles, chemical and mechanical polishing waste, organic matter from landfill leachates, defluorination of water, synthetic detergent effluents, and solutions containing heavy metals.[4] Coagulation process Coagulation is one of the most important physio-chemical reactions used in water treatment. The precipitation of ions (heavy metals) and colloids (organic and inorganic) are mostly held in solution by electrical charges. By the addition of ions with opposite charges, these colloids can be destabilized; coagulation can be achieved by chemical or electrical methods. The coagulant is added in the form of suitable chemical substances. Alum [Al2(SO4)3.18H2O] is such a chemical substance, which has been widely used for ages for wastewater treatment. The mechanism of coagulation has been the subject of continual review. It is generally accepted that coagulation is brought about primarily by the reduction of the net surface charge to a point where the colloidal particles, previously stabilized by electrostatic repulsion, can approach closely enough for van der Waals forces to hold them together and allow aggregation. The reduction of the surface charge is a consequence of the decrease of the repulsive potential of the electrical double layer by the presence of an electrolyte having opposite charge. In the EC process, the coagulant is generated in situ by electrolytic oxidation of an appropriate anode material. In this process, charged ionic species metals or otherwise - are removed from wastewater by allowing it to react with an ion having an opposite charge, or with floc of metallic hydroxides generated within the effluent. Electrocoagulation offers an alternative to the use of metal salts or polymers and polyelectrolyte addition for breaking stable emulsions and suspensions. The technology removes metals, colloidal solids and particles, and soluble inorganic pollutants from aqueous media by introducing highly charged polymeric metal hydroxide species. These species neutralize the electrostatic charges on suspended solids and oil droplets to facilitate agglomeration or coagulation and resultant separation from the aqueous phase. The treatment prompts the precipitation of certain metals and salts. "Chemical coagulation has been used for decades to destabilize suspensions and to effect precipitation of soluble metals species, as well as other inorganic species from aqueous streams, thereby permitting their removal through sedimentation or filtration. Alum, lime and/or polymers have been the chemical coagulants used. These processes, however, tend to generate large volumes of sludge with high bound water content that can be slow to filter and difficult to dewater. These treatment processes also tend to increase the total dissolved solids (TDS) content of the effluent, making it unacceptable for reuse within industrial applications."[5] "Although the electrocoagulation mechanism resembles chemical coagulation in that the cationic species are responsible for the neutralization of surface charges, the characteristics of the electrocoagulated flock differ dramatically from those generated by chemical coagulation. An electrocogulated flock tends to contain less bound water, is more shear resistant and is more readily filterable" [6]

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Electrocoagulation Description of the technology In its simplest form, an electrocoagulation reactor is made up of an electrolytic cell with one anode and one cathode. When connected to an external power source, the anode material will electrochemically corrode due to oxidation, while the cathode will be subjected to passivation. An EC system essentially consists of pairs of conductive metal plates in parallel, which act as monopolar electrodes. It furthermore requires a direct current power source, a resistance box to regulate the current density and a multimeter to read the current values. The conductive metal plates are commonly known as "sacrificial electrodes." The sacrificial anode lowers the dissolution potential of the anode and minimizes the passivation of the cathode. The sacrificial anodes and cathodes can be of the same or of different materials. The arrangement of monopolar electrodes with cells in series is electrically similar to a single cell with many electrodes and interconnections. In series cell arrangement, a higher potential difference is required for a given current to flow because the cells connected in series have higher resistance. The same current would, however, flow through all the electrodes. On the other hand, in parallel or bipolar arrangement the electric current is divided between all the electrodes in relation to the resistance of the individual cells, and each face on the electrode has a different polarity. During electrolysis, the positive side undergoes anodic reactions, while on the negative side, cathodic reactions are encountered. Consumable metal plates, such as iron or aluminum, are usually used as sacrificial electrodes to continuously produce ions in the water. The released ions neutralize the charges of the particles and thereby initiate coagulation. The released ions remove undesirable contaminants either by chemical reaction and precipitation, or by causing the colloidal materials to coalesce, which can then be removed by flotation. In addition, as water containing colloidal particulates, oils, or other contaminants move through the applied electric field, there may be ionization, electrolysis, hydrolysis, and free-radical formation which can alter the physical and chemical properties of water and contaminants. As a result, the reactive and excited state causes contaminants to be released from the water and destroyed or made less soluble. It is important to note that electrocoagulation technology cannot remove infinitely soluble matter. Therefore ions with molecular weights smaller than Ca+2 or Mg+2 cannot be dissociated from the aqueous medium.

Reactions within the electrocoagulation reactor Within the electrocoagulation reactor, several distinct electrochemical reactions are produced independently. These are: • Seeding, resulting from the anode reduction of metal ions that become new centers for larger, stable, insoluble complexes that precipitate as complex metal ions. • Emulsion Breaking, resulting from the oxygen and hydrogen ions that bond into the water receptor sites of oil molecules creating a water-insoluble complex separating water from oil, driller's mud, dyes, inks, etc. • Halogen Complexing, as the metal ions bind themselves to chlorines in a chlorinated hydrocarbon molecule resulting in a large insoluble complex separating water from pesticides, herbicides, chlorinated PCBs, etc. • Bleaching by the oxygen ions produced in the reaction chamber oxidizes dyes, cyanides, bacteria, viruses, biohazards, etc. Electron Flooding of the water eliminates the polar effect of the water complex, allowing colloidal materials to precipitate and the increase of electrons creates an osmotic pressure that ruptures bacteria, cysts, and viruses. • Oxidation Reduction reactions are forced to their natural end point within the reaction tank which speeds up the natural process of nature that occurs in wet chemistry. • Electrocoagulation Induced pH swings toward neutral.

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Optimizing EC reactions Careful selection of the reaction tank material is essential along with control of the current, flow rate and pH. Electrodes can be made of iron, aluminum, titanium, graphite or other materials, depending upon the wastewater to be treated and the contaminants to be removed. Temperature and pressure have little effect on the process. In the EC process the water-contaminant mixture separates into a floating layer, a mineral-rich sediment, and clear water. The floating layer is removed by means of a patented overflow/removal method, and moved to a sludge collection tank. The aggregated mass settles down due to gravitational force, and is subsequently removed through a drainage valve at the bottom of the EC reaction tank, and moved to a sludge collection tank. The clear, treated water is pumped to a buffer tank for later disposal and/or reuse in the plant’s designated process.

Advantages of EC • EC requires simple equipment and is easy to operate with sufficient operational latitude to handle most problems encountered on running. • Wastewater treated by EC gives palatable, clear, colorless and odorless water. • Sludge formed by EC tends to be readily settable and easy to de-water, because it is composed of mainly metallic oxides/hydroxides. • Flocs formed by EC are similar to chemical floc, except that EC floc tends to be much larger, contains less bound water, is acid-resistant and more stable, and therefore, can be separated faster by filtration. • EC produces effluent with less TDS content as compared with chemical treatments. If this water is reused, the low TDS level contributes to a lower water recovery cost. • The EC process has the advantage of removing the smallest colloidal particles, because the applied electric field sets them in faster motion, thereby facilitating the coagulation. • The EC process avoids uses of chemicals and so there is no problem of neutralizing excess chemicals and no possibility of secondary pollution caused by chemical substances added at high concentration as when chemical coagulation of wastewater is used. • The gas bubbles produced during electrolysis can carry the pollutant to the top of the solution where it can be more easily concentrated, collected and removed by a motorised skimmer. • The electrolytic processes in the EC cell are controlled electrically and with no moving parts, thus requiring less maintenance. • Dosing the incoming sewage waste water with sodium hypochlorite helps in tremendous reduction of biochemical oxygen demand (BOD) and consequent chemical oxygen demand (COD). Sodium hypochlorite can be generated using electrochlorinators.[7] • Due to the excellent EC removal of suspended solids and the simplicity of the EC operation ... tests conducted for the Office of Naval research concluded that ... the most promising application of EC in a membrane system was found to be as pretreatment to a multi-membrane system of UF / RO or MF / RO. In this function the EC provides protection of the low-pressure membrane that is more general than that provided by chemical coagulation and more effective. EC is more effective at removing species that chemical coagulation and other alternatives can remove and it removes many species that chemical coagulation cannot remove. This has since been adopted in the industrial arena with the use of a 1,000-gpm Powell Water / Quantum-ionics EC / UF / RO system at El Paso Electric, in El Paso Texas.>

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References [1] http:/ / www. nlm. nih. gov/ cgi/ mesh/ 2011/ MB_cgi?field=uid& term=D004564 [2] Holt, Peter K.; Barton, Geoffrey W.; Mitchell, Cynthia A. (2004-12-08). "The future for electrocoagulation as a localised water treatment technology". Chemoshpere (Elsevier) 59 (3): 355–67. doi:10.1016/j.chemosphere.2004.10.023. ISSN 0045-6535. [3] Rodriguez J, Stopić S, Krause G, Friedrich B (2007). "Feasibility Assessment of Electrocoagulation Towards a New Sustainable Wastewater Treatment." (http:/ / www. springerlink. com/ content/ 28801600066879m8/ fulltext. pdf) Environmental Science and Pollution Research 14 (7), pp. 477–482. [4] Lai, C. L., Lin, S. H. 2003. "Treatment of chemical mechanical polishing wastewater by electrocoagulation: system performances and sludge settling characteristics." (http:/ / dx. doi. org/ 10. 1016/ j. chemosphere. 2003. 08. 014) Chemosphere (http:/ / www. elsevier. com/ wps/ product/ cws_home/ 362) 54 (3), January 2004, pp. 235-242. [5] Benefield, Larry D.; Judkins, Joseph F.; Weand, Barron L. (1982). Process Chemistry for Water and Wastewater Treatment. Englewood Cliffs, NJ: Prentice-Hall. p. 212. ISBN 0137229755. [6] Woytowich, David L.; Dalrymple, C.W.; Britton, M.G. (Spring 1993). "Electrocoagulation (CURE) Treatment of Ship Bilge Water for the US Coast Guard in Alaska" (http:/ / www. mtsociety. org). Marine Technology Society Journal (Columbia, MD: Marine Technology Society, Inc.) 27 (1): 92. ISSN 0025-3324. . [7] United States Bureau of Reclamation. Yuma, AZ. "Research Facilities and Test Equipment - Chemistry Research Units." (http:/ / www. usbr. gov/ lc/ yuma/ facilities/ wqic/ yao_wqic_research_chemical. html) Updated August 2009.

Expanded granular sludge bed digestion An expanded granular sludge bed (EGSB) reactor is a variant of the UASB concept.[1] The distinguishing feature is that a faster rate of upward-flow velocity is designed for the wastewater passing through the sludge bed. The increased flux permits partial expansion (fluidisation) of the granular sludge bed, improving wastewater-sludge contact as well as enhancing segregation of small inactive suspended particle from the sludge bed. The increased flow velocity is either accomplished by utilizing tall reactors, or by incorporating an effluent recycle (or both). A scheme depicting the EGSB design concept is shown in this EGSB diagram [2]. The EGSB design is appropriate for low strength soluble wastewaters (less than 1 to 2 g soluble COD/l) or for wastewaters that contain inert or poorly biodegradable suspended particles which should not be allowed to accumulate in the sludge bed.

External links • UASB & EGSB Website [3]

References [1] UASB and EGSB (http:/ / www. uasb. org/ discover/ agsb. htm#egsb) Field, J. (2002) Anaerobic granular sludge bed technology pages, anaerobic granular sludge bed reactor technology [2] http:/ / www. uasb. org/ discover/ agsb. htm#egsb [3] http:/ / www. uasb. org

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Fine bubble diffusers

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Fine bubble diffusers Fine bubble diffusers are a pollution control technology used to aerate wastewater for sewage treatment. They produce a plethora of very small air bubbles which rise slowly from the floor of a wastewater treatment plant or sewage treatment plant aeration tank and provide substantial and efficient mass transfer of oxygen to the water. The oxygen, combined with the food source, sewage, allows the bacteria to produce enzymes which help break down the waste so that it can settle in the secondary clarifiers or be filtered by membranes. A fine bubble diffuser is commonly manufactured in various forms: tube, disc, plate, and dome.[1]

Bubble Size The subject of bubble size is important because the aeration system in a wastewater or sewage treatment plant consumes an average of 50 to 70% of the energy of the entire plant.[2] Increasing the oxygen transfer efficiency decreases the power the plant requires to provide the same quality of effluent water. Furthermore, fine bubble diffusers evenly spread out (often referred to as a 'grid arrangement') on the floor of a tank, provide the operator of the plant a great deal of operational flexibility. This can be used to create zones with high oxygen concentrations (oxic or aerobic), zones with minimal oxygen concentration (anaerobic) and zones with no oxygen (anoxic). This allows for more precise targeting and removal of specific contaminants. The importance of achieving ever smaller bubble sizes has been a hotly debated subject in the industry as ultra fine bubbles (micrometre size) are generally perceived to rise too slowly and provide too little "pumpage" to provide adequate mixing of sewage in an aeration tank. On the other hand, the industry standard "fine bubble" with a typical discharge diameter of 2 mm is probably larger than it needs to be for many plants. Average bubble diameters of 0.9 mm are possible nowadays, using special polyurethane (PUR) or special recently developed EPDM membranes. Fine bubble diffusers have largely replaced coarse bubble diffusers and mechanical aerators in most of the developed world and in much of the developing world. The exception would be in secondary treatment phases, such as activated sludge processing tanks, where 85%-90% of any remaining solid materials (floating on the surface) are removed through settling or biological processes. The biological process uses air to encourage bacterial growth that would consume many of these waste materials, such as phosphorus and nitrogen that are dissolved in the wastewater. The larger air release openings of a coarse bubble diffuser helps to facilitate a higher oxygen transfer rate and bacterial growth. One disadvantage of using fine bubble diffusers in activated sludge tanks is the tendency of floc (particle) clogging the small air release holes.

A Fine Bubble Diffuser in a Tank, courtesy of SSI Aeration, Inc..

Aerating water by means of fine pore membrane diffuser. Compliments of Environmental Dynamics Inc.

Aerating water by means of fine pore 9" Disc membrane diffuser. Compliments of Environmental Dynamics Inc

Fine bubble diffusers

References [1] http:/ / www. epa. gov/ owm/ mtb/ fine. pdf [2] http:/ / www. scipub. org/ fulltext/ ajeas/ ajeas22260-267. pdf

Sedimentation Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration or electromagnetism. In geology sedimentation is often used as the polar opposite of erosion, i.e., the terminal end of sediment transport. In that sense it includes the termination of transport by saltation or true bedload transport. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the termination of the settling process. Sedimentation may pertain to objects of various sizes, ranging from large rocks in flowing water to suspensions of dust and pollen particles to cellular suspensions to solutions of single molecules such as proteins and peptides. Even small molecules such as aspirin can be sedimented, although it can be difficult to apply a sufficiently strong force to produce significant sedimentation. The term is typically used in geology, to describe the deposition of sediment which results in the formation of sedimentary rock, and in various chemical and environmental fields to describe the motions of often-smaller particles and molecules. Process is also used in biotech industry to separate out cells from the culture media.

Experiments In a sedimentation experiment called tripothsis, the applied force accelerates the particles to a terminal velocity at which the applied force is exactly canceled by an opposing drag force. For small enough particles (low Reynolds number), the drag force varies linearly with the terminal velocity, i.e., (Stokes flow) where f depends only on the properties of the particle and the surrounding fluid. Similarly, the applied force generally varies linearly with some coupling constant (denoted here as q) that depends only on the properties of the particle, . Hence, it is generally possible to define a sedimentation coefficient that depends only on the properties of the particle and the surrounding fluid. Thus, measuring s can reveal underlying properties of the particle. In many cases, the motion of the particles is blocked by a hard boundary; the resulting accumulation of particles at the boundary is called a sediment. The concentration of particles at the boundary is opposed by the diffusion of the particles. The sedimentation of particles under gravity is described by the Mason–Weaver equation, which has a simple exact solution. The sedimentation coefficient s in this case equals , where is the buoyant mass. The sedimentation of particles under the centrifugal force is described by the Lamm equation, which likewise has an exact solution. The sedimentation coefficient s also equals , where is the buoyant mass. However, the Lamm equation differs from the Mason–Weaver equation because the centrifugal force depends on radius from the origin of rotation, whereas gravity is presumed constant. The Lamm equation also has extra terms, since it pertains to sector-shaped cells, whereas the Mason–Weaver equation pertains to box-shaped cells (i.e., cells whose walls are aligned with the three Cartesian axes). Particles with a charge or dipole moment can be sedimented by an electric field or electric field gradient, respectively. These processes are called electrophoresis and dielectrophoresis, respectively. For electrophoresis, the sedimentation coefficient corresponds to the particle charge divided by its drag (the electrophoretic mobility). Similarly, for dielectrophoresis, the sedimentation coefficient equals the particle's electric dipole moment divided by

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its drag. Classification of sedimentation: • Type 1 sedimentation is characterized by particles that settle discretely at a constant settling velocity. They settle as individual particles and do not flocculate or stick to other during settling. Example: sand and grit material • Type 2 sedimentation is characterized by particles that flocculate during sedimentation and because of this their size is constantly changing and therefore their settling velocity is changing. Example: alum or iron coagulation • Type 3 sedimentation is also known as zone sedimentation. In this process the particles are at a high concentration (greater than 1000 mg/L) such that the particles tend to settle as a mass and a distinct clear zone and sludge zone are present. Zone settling occurs in lime-softening, sedimentation, active sludge sedimentation and sludge thickeners.

Geology In geology, sedimentation is the deposition of particles carried by a fluid flow. For suspended load, this can be expressed mathematically by the Exner equation, and results in the formation of depositional landforms and the rocks that constitute sedimentary record. An undesired increased transport and sedimentation of suspended material is called siltation, and it is a major source of pollution in waterways in some parts of the world.[1] [2] Climate change also affect siltation rates.[3] Siltation

Chemistry In chemistry, sedimentation has been used to measure the size of large molecules (macromolecule), where the force of gravity is augmented with centrifugal force in a centrifuge.

Biology In biology, the sedimentation of organisms is a critical issue for planktonic organisms, as sinking under gravity moves them away from the surface, where sunlight provides energy.[4]

Notes [1] "Siltation & Sedimentation" (http:/ / blackwarriorriver. org/ siltation-sedimentation. html). blackwarriorriver.org. . Retrieved 2009-11-16. [2] "Siltation killed fish at Batang Rajang - Digest on Malaysian News" (http:/ / malaysiadigest. blogspot. com/ 2009/ 02/ siltation-killed-fish-at-batang-rajang. html). malaysiadigest.blogspot.com. . Retrieved 2009-11-16. [3] U.D. Kulkarni, et al. "The International Journal of Climate Change: Impacts and Responses » Rate of Siltation in Wular Lake, (Jammu and Kashmir, India) with Special Emphasis on its Climate & Tectonics" (http:/ / ijc. cgpublisher. com/ product/ pub. 185/ prod. 38). The International Journal of Climate Change: Impacts and Responses. . Retrieved 2009-11-16. [4] Dusenbery, David B. (2009). Living at Micro Scale, Chapter 12. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.

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Membrane bioreactor Membrane bioreactor (MBR) is the combination of a membrane process like microfiltration or ultrafiltration with a suspended growth bioreactor, and is now widely used for municipal and industrial wastewater treatment with plant sizes up to 80,000 population equivalent (i.e. 48 MLD).[1]

Overview

Simple schematic describing the MBR process

When used with domestic wastewater, MBR processes could produce effluent of high quality enough to be discharged to coastal, surface or brackish waterways or to be reclaimed for urban irrigation. Other advantages of MBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatment plants. It is possible to operate MBR processes at higher mixed liquor suspended solids (MLSS) concentrations compared to conventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate. Two MBR configurations exist: internal/submerged, where the membranes are immersed in and integral to the biological reactor; and external/sidestream, where membranes are a separate unit process requiring an intermediate pumping step.

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Schematic of conventional activated sludge process (top) and membrane bioreactor (bottom)

Recent technical innovation and significant membrane cost reduction have pushed MBRs to become an established process option to treat wastewaters.[1] As a result, the MBR process has now become an attractive option for the treatment and reuse of industrial and municipal wastewaters, as evidenced by their constantly rising numbers and capacity. The current MBR market has been estimated to value around US$216 million in 2006 and to rise to US$363 million by 2010.[2]

Schematic of a submerged MBR

MBR history and basic operating parameters The MBR process was introduced by the late 1960s, as soon as commercial scale ultrafiltration (UF) and microfiltration (MF) membranes were available. The original process was introduced by Dorr-Olivier Inc. and combined the use of an activated sludge bioreactor with a crossflow membrane filtration loop. The flat sheet membranes used in this process were polymeric and featured pore sizes ranging from 0.003 to 0.01 μm. Although the idea of replacing the settling tank of the conventional activated sludge process was attractive, it was difficult to justify the use of such a process because of the high cost of membranes, low economic value of the product (tertiary effluent) and the potential rapid loss of performance due to membrane fouling. As a result, the focus was on the attainment of high fluxes, and it was therefore necessary to pump the mixed liquor suspended solids (MLSS) at high crossflow velocity at significant energy penalty (of the order 10 kWh/m3 product) to reduce fouling. Due to the poor economics of the first generation MBRs, they only found applications in niche areas with special needs like isolated trailer parks or ski resorts for example.

Membrane bioreactor The breakthrough for the MBR came in 1989 with the idea of Yamamoto and co-workers to submerge the membranes in the bioreactor. Until then, MBRs were designed with the separation device located external to the reactor (sidestream MBR) and relied on high transmembrane pressure (TMP) to maintain filtration. With the membrane directly immersed into the bioreactor, submerged MBR systems are usually preferred to sidestream configuration, especially for domestic wastewater treatment. The submerged configuration relies on coarse bubble aeration to produce mixing and limit fouling. The energy demand of the submerged system can be up to 2 orders of magnitude lower than that of the sidestream systems and submerged systems operate at a lower flux, demanding more membrane area. In submerged configurations, aeration is considered as one of the major parameter on process performances both hydraulic and biological. Aeration maintains solids in suspension, scours the membrane surface and provides oxygen to the biomass, leading to a better biodegradability and cell synthesis. The other key steps in the recent MBR development were the acceptance of modest fluxes (25% or less of those in the first generation), and the idea to use two-phase bubbly flow to control fouling. The lower operating cost obtained with the submerged configuration along with the steady decrease in the membrane cost encouraged an exponential increase in MBR plant installations from the mid 90s. Since then, further improvements in the MBR design and operation have been introduced and incorporated into larger plants. While early MBRs were operated at solid retention times (SRT) as high as 100 days with mixed liquor suspended solids up to 30 g/L, the recent trend is to apply lower solid retention times (around 10–20 days), resulting in more manageable mixed liquor suspended solids (MLSS) levels (10-15 g/L). Thanks to these new operating conditions, the oxygen transfer and the pumping cost in the MBR have tended to decrease and overall maintenance has been simplified. There is now a range of MBR systems commercially available, most of which use submerged membranes although some external modules are available; these external systems also use two-phase flow for fouling control. Typical hydraulic retention times (HRT) range between 3 and 10 hours. In terms of membrane configurations, mainly hollow fibre and flat sheet membranes are applied for MBR applications.[3] Despite the more favourable energy usage of submerged membranes, there continued to be a market for the side stream configuration, particularly in industrial applications. For ease of maintenance the side stream configuration can be installed at low level in a plant building. Membrane replacement can be undertaken without specialist equipment, and intensive cleaning of individual banks can be undertaken during normal operation of the other banks and without removing the membranes modules from the installation. As a result research continued with the side stream configuration, during which time it was found that full scale plants could be operated with higher fluxes. This has culminated in recent years with the development of low energy systems which incorporate more sophisticated control of the operating parameters coupled with periodic back washes, which enable sustainable operation at energy usage as low as 0.3 kWh/m3 product.

MBR configurations Internal/submerged The filtration element is installed in either the main bioreactor vessel or in a separate tank. The membranes can be flat sheet or tubular or combination of both, and can incorporate an online backwash system which reduces membrane surface fouling by pumping membrane permeate back through the membrane. In systems where the membranes are in a separate tank to the bioreactor individual trains of membranes can be isolated to undertake cleaning regimes incorporating membrane soaks, however the biomass must be continuously pumped back to the main reactor to limit MLSS concentration increase. Additional aeration is also required to provide air scour to reduce fouling. Where the membranes are installed in the main reactor, membrane modules are removed from the vessel and transferred to an offline cleaning tank.

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External/sidestream The filtration elements are installed externally to the reactor, often in a plant room. The biomass is either pumped directly through a number of membrane modules in series and back to the bioreactor, or the biomass is pumped to a bank of modules, from which a second pump circulates the biomass through the modules in series. Cleaning and soaking of the membranes can be undertaken in place with use of an installed cleaning tank, pump and pipework.

Major considerations in MBR Fouling and fouling control The MBR filtration performance inevitably decreases with filtration time. This is due to the deposition of soluble and particulate materials onto and into the membrane, attributed to the interactions between activated sludge components and the membrane. This major drawback and process limitation has been under investigation since the early MBRs, and remains one of the most challenging issues facing further MBR development,.[4] [5]

Illustration of membrane fouling

In recent reviews covering membrane applications to bioreactors, it has been shown that, as with other membrane separation processes, membrane fouling is the most serious problem affecting system performance. Fouling leads to a significant increase in hydraulic resistance, manifested as permeate flux decline or transmembrane pressure (TMP) increase when the process is operated under constant-TMP or constant-flux conditions respectively. In systems where flux is maintained by increasing TMP, the energy required to achieve filtration increases. Alternatively frequent membrane cleaning is therefore required, increasing significantly the operating costs as a result of cleaning agents and production downtime. More frequent membrane replacement is also expected. Membrane fouling results from interaction between the membrane material and the components of the activated sludge liquor, which include biological flocs formed by a large range of living or dead microorganisms along with soluble and colloidal compounds. The suspended biomass has no fixed composition and varies both with feed water composition and MBR operating conditions employed. Thus though many investigations of membrane fouling have been published, the diverse range of operating conditions and feedwater matrices employed, the different analytical methods used and the limited information reported in most studies on the suspended biomass composition, has made it difficult to establish any generic behaviour pertaining to membrane fouling in MBRs specifically.

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Factors influencing fouling (interactions in red)

The air-induced cross flow obtained in submerged MBR can efficiently remove or at least reduce the fouling layer on the membrane surface. A recent review reports the latest findings on applications of aeration in submerged membrane configuration and describes the enhancement of performances offered by gas bubbling.[5] As an optimal air flow-rate has been identified behind which further increases in aeration have no effect on fouling removal, the choice of aeration rate is a key parameter in MBR design. Many other anti-fouling strategies can be applied to MBR applications. They comprise, for example: • Intermittent permeation, where the filtration is stopped at regular time interval for a couple of minutes before being resumed. Particles deposited on the membrane surface tend to diffuse back to the reactor; this phenomena being increased by the continuous aeration applied during this resting period. • Membrane backwashing, where permeate water is pumped back to the membrane, and flow through the pores to the feed channel, dislodging internal and external foulants. • Air backwashing, where pressurized air in the permeate side of the membrane build up and release a significant pressure within a very short period of time. Membrane modules therefore need to be in a pressurised vessel coupled to a vent system. Air usually does not go through the membrane. If it did, the air would dry the membrane and a rewet step would be necessary, by pressurizing the feed side of the membrane. • Proprietary anti-fouling products, such as Nalco's Membrane Performance Enhancer Technology [6]. In addition, different types/intensities of chemical cleaning may also be recommended: • Chemically enhanced backwash (daily); • Maintenance cleaning with higher chemical concentration (weekly); • Intensive chemical cleaning (once or twice a year). Intensive cleaning is also carried out when further filtration cannot be sustained because of an elevated transmembrane pressure (TMP). Each of the four main MBR suppliers (Kubota, Memcor, Mitsubishi and Zenon) have their own chemical cleaning recipes, which differ mainly in terms of concentration and methods (see Table 1). Under normal conditions, the prevalent cleaning agents remain NaOCl (Sodium Hypochlorite) and citric acid. It is common for MBR suppliers to adapt specific protocols for chemical cleanings (i.e. chemical concentrations and cleaning frequencies) for individual facilities.[3]

Membrane bioreactor

Intensive chemical cleaning protocols for four MBR suppliers (the exact protocol for chemical cleaning can vary from a plant to another)

Biological performances/kinetics COD removal and sludge yield Simply due to the high number of microorganism in MBRs, the pollutants uptake rate can be increased. This leads to better degradation in a given time span or to smaller required reactor volumes. In comparison to the conventional activated sludge process (ASP) which typically achieves 95%, COD removal can be increased to 96-99% in MBRs (see table,[7] ). COD and BOD5 removal are found to increase with MLSS concentration. Above 15g/L COD removal becomes almost independent of biomass concentration at >96%.[8] Arbitrary high MLSS concentrations are not employed, however, as oxygen transfer is impeded due to higher and Non-Newtonian fluid viscosity. Kinetics may also differ due to easier substrate access. In ASP, flocs may reach several 100 μm in size. This means that the substrate can reach the active sites only by diffusion which causes an additional resistance and limits the overall reaction rate (diffusion controlled). Hydrodynamic stress in MBRs reduces floc size (to 3.5 μm in sidestream MBRs) and thereby increases the apparent reaction rate. Like in the conventional ASP, sludge yield is decreased at higher SRT or biomass concentration. Little or no sludge is produced at sludge loading rates of 0.01 kgCOD/(kgMLSS d).[9] Due to the biomass concentration limit imposed, such low loading rates would result in enormous tank sizes or long HRTs in conventional ASP. Nutrient removal Nutrient removal is one of the main concerns in modern wastewater treatment especially in areas that are sensitive to eutrophication. Like in the conventional ASP, currently, the most widely applied technology for N-removal from municipal wastewater is nitrification combined with denitrification. Besides phosphorus precipitation, enhanced biological phosphorus removal (EBPR) can be implemented which requires an additional anaerobic process step. Some characteristics of MBR technology render EBPR in combination with post-denitrification an attractive alternative that achieves very low nutrient effluent concentrations.[8]

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[7]

Nutrients Removal in MBRs for Municipal Wastewater Treatment

Anaerobic MBRs Anaerobic MBRs were introduced in the 1980s in South Africa and currently see a renaissance in research. However, anaerobic processes are normally used when a low cost treatment is required that enables energy recovery but does not achieve advanced treatment (low carbon removal, no nutrients removal). In contrast, membrane-based technologies enable advanced treatment (disinfection), but at high energy cost. Therefore, the combination of both can only be economically viable if a compact process for energy recovery is desired, or when disinfection is required after anaerobic treatment (cases of water reuse with nutrients). If maximal energy recovery is desired, a single anaerobic process will be always superior to a combination with a membrane process.

Mixing/Hydrodynamics Like in any other reactors, the hydrodynamics (or mixing) within an MBR plays an important role in determining the pollutant removal and fouling control within an MBR. It has a substantial effect on the energy usage and size requirements of an MBR, therefore the whole life cost of an MBR. The removal of pollutants is greatly influenced by the length of time fluid elements spend in the MBR (i.e. the residence time distribution or RTD). The residence time distribution is a description of the hydrodynamics/mixing in the system and is determined by the design of the MBR (e.g. MBR size, inlet/recycle flowrates, wall/baffle/mixer/aerator positioning, mixing energy input). An example of the effect of mixing is that a continuous stirred-tank reactor will not have as high pollutant conversion per unit volume of reactor as a plug flow reactor. The control of fouling, as previously mentioned, is primarily undertaken using coarse bubble aeration. The distribution of bubbles around the membranes, the shear at the membrane surface for cake removal and the size of the bubble are greatly influenced by the mixing/hydrodynamics of the system. The mixing within the system can also influence the production of possible foulants. For example, vessels not completely mixed (i.e. plug flow reactors) are

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more susceptible to the effects of shock loads which may cause cell lysis and release of soluble microbial products. Many factors affect the hydrodynamics of wastewater processes and hence MBRs. These range from physical properties (e.g. mixture rheology and gas/liquid/solid density etc.) to the fluid boundary conditions (e.g. inlet/outlet/recycle flowrates, baffle/mixer position etc.). However, many factors are peculiar to MBRs, these cover the filtration tank design (e.g. membrane type, multiple outlets attributed to membranes, membrane packing density, membrane orientation etc.) and it’s operation (e.g. membrane relaxation, membrane back flush etc.). Example of computational fluid dynamic (CFD) modelling results (streamlines) for a full

The mixing modelling and design scale MBR (Adapted from the Project AMEDEUS – Australian Node Newsletter August [10] techniques applied to MBRs are very 2007 ). similar to those used for conventional activated sludge systems. They include the relatively quick and easy compartmental modelling technique which will only derive the RTD of a process (e.g. the MBR) or the process unit (e.g. membrane filtration vessel) and relies on broad assumptions of the mixing properties of each sub unit. Computational fluid dynamics modelling (CFD) on the other hand does not rely on broad assumptions of the mixing characteristics and attempts to predict the hydrodynamics from a fundamental level. It is applicable to all scales of fluid flow and can reveal much information about the mixing in a process, ranging from the RTD to the shear profile on a membrane surface. Visualisation of MBR CFD modelling results is shown below. Investigations of MBR hydrodynamics have occurred at many different scales, ranging from examination of shear stress at the membrane surface to RTD analysis of the whole MBR. Cui et al. (2003) [5] investigated the movement of Taylor bubbles through tubular membranes, Khosravi, M. (2007) [11] examined the entire membrane filtration vessel using CFD and velocity measurements, while Brannock et al. (2007) [12] examined the entire MBR using tracer study experiments and RTD analysis.

References [1] S. Judd, The MBR book (2006) Principles and applications of membrane bioreactors in water and wastewater treatment, Elsevier, Oxford [2] S. Atkinson, research studies predict strong growth for MBR markets. Membrane Technology (2006) 8-10 [3] P. Le-Clech, V. Chen, A.G. Fane, Fouling in membrane bioreactors used for wastewater treatment – A review. J. Memb. Sci. 284 (2006) 17-53 [4] Membrane Bioreactors (http:/ / www. membrane. unsw. edu. au/ research/ mbr. htm) [5] Z.F. Cui, S. Chang, A.G. Fane, The use of gas bubbling to enhance membrane process, J. Memb. Sci. 2211 (2003) 1-35 [6] http:/ / www. nalco. com/ ASP/ applications/ membrane_tech/ products/ mpe. asp [7] M. Kraume, U. Bracklow, M. Vocks, A. Drews, Nutrients Removal in MBRs for Municipal Wastewater Treatment. Wat. Sci. Tech. 51 (2005), 391-402 [8] A. Drews, H. Evenblij, S. Rosenberger, Potential and drawbacks of microbiology-membrane interaction in membrane bioreactors, Environmental Progress 24 (4) (2005) 426-433 [9] T. Stephenson, S. Judd, B. Jefferson, K. Brindle, Membrane bioreactors for wastewater treatment, IWA Publishing (2000) [10] MBR-Network (http:/ / www. mbr-network. eu/ mbr-forum/ forum_entry. php?id=194) [11] ., Khosravi, M. and Kraume, M. (2007) Prediction of the circulation velocity in a membrane bioreactor, IWA Harrogate, UK [12] Brannock, M.W.D., Kuechle, B., Wang, Y. and Leslie, G. (2007) Evaluation of membrane bioreactor performance via residence time distribution analysis: effects of membrane configuration in full-scale MBRs, IWA Berlin, Germany

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Retention basin A retention basin is a type of best management practice (BMP) that is used to manage stormwater runoff to prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay. Sometimes called a wet pond or wet detention basin, it is an artificial lake with vegetation around the perimeter, and includes a permanent pool of water in its design.[1] [2] It is distinguished from a detention basin, sometimes called a dry pond, which temporarily stores water after a storm, but eventually empties out at a controlled rate to a downstream water body. It also differs from an infiltration basin which is designed to direct stormwater to groundwater through permeable soils.

Trounce Pond, a retention basin landscaped with natural grassland plants, in Saskatoon, Saskatchewan, Canada

Wet ponds are frequently used for water quality improvement, groundwater recharge, flood protection, aesthetic improvement or any combination of these. Sometimes they act as a replacement for the natural absorption of a forest or other natural process that was lost when an area is developed. As such, these structures are designed to blend into neighborhoods and viewed as an amenity.[3]

Design features Storm water is typically channeled to a retention basin through a system of street and/or parking lot storm drains, and a network of drain channels or underground pipes. The basins are designed to allow relatively large flows of water to enter, but discharges to receiving waters are limited by outlet structures that function only during very large storm events. Retention ponds are often landscaped with a variety of grasses, shrubs and/or wetland plants to provide bank stability and aesthetic benefits. Vegetation also provides water quality benefits by removing soluble nutrients through uptake.[4] In some areas the ponds can attract nuisance types of wildlife like ducks or Canada Geese, particularly where there is minimal landscaping and grasses are mowed. This reduces the ability of foxes, coyotes and other predators to approach their prey unseen. Such predators tend to hide in the cattails and other tall, thick grass surrounding natural water features.

The Corporate Park retention basin in Stafford, Texas

Retention basin in Pinnau, Germany

Other meanings A retention basin can also be a part of a nuclear reactor used to contain a core meltdown.

References [1] Water Environment Federation (http:/ / wef. org), Alexandria, VA; and American Society of Civil Engineers (http:/ / www. asce. org), Reston, VA. "Urban Runoff Quality Management." (http:/ / books. google. com/ books?id=AdU-VXXV_H0C) WEF Manual of Practice No. 23; ASCE Manual and Report on Engineering Practice No. 87. 1998. ISBN 1-57278-039-8. Chapter 5.

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[2] U.S. Environmental Protection Agency. Washington, D.C. "Preliminary Data Summary of Urban Storm Water Best Management Practices." (http:/ / epa. gov/ guide/ stormwater/ ) Chapter 5. August 1999. Document No. EPA-821-R-99-012. [3] Mississippi State University. College of Engineering. Stormwater Retention Basins. Chapter 4, Best Management Practices. (http:/ / www. abe. msstate. edu/ csd/ p-dm/ all-chapters/ chapter4/ chapter4/ srb1. pdf) [4] Urban Drainage & Flood Control District (http:/ / www. udfcd. org), Denver, CO. Urban Storm Drainage Criteria Manual. Volume 3, Structural BMPs. (http:/ / www. udfcd. org/ downloads/ pdf/ critmanual/ Volume 3 PDFs/ 04 Structural BMP 2008-04 rev. pdf)

External links • Virginia retention basin standards (http://www.dcr.virginia.gov/sw/docs/swm/Chapter_3-06.pdf) • Detention vs. retention (http://www.projectbrays.org/detention.html) – Harris County, Texas Flood Control District • Stormwater Ecological Enhancement Project (http://natl.ifas.ufl.edu/seep.htm) – University of Florida • The use of retention ponds in residential settings (http://www.southalabama.edu/geography/fearn/480page/ 02Jordan/Jordan.htm) • International Stormwater BMP Database (http://bmpdatabase.org) – Performance Data on Urban Stormwater BMPs

Reverse osmosis Reverse osmosis (RO) is a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.

Schematics of a reverse osmosis system (desalination) using a pressure exchanger. 1:Sea water inflow, 2: Fresh water flow (40%), 3:Concentrate Flow (60%), 4:Sea water flow (60%), 5: Concentrate (drain), A: High pressure pump flow (40%), B: Circulation pump, C:Osmosis unit with membrane, D: Pressure exchanger

In the normal osmosis process the solvent naturally moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The movement of a pure solvent to equalize solute concentrations on each side of a membrane generates a pressure and this is the "osmotic pressure." Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to membrane filtration. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure, and water flux rate.[1] . Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other substances from the water

Reverse osmosis molecules.

History The process of osmosis through semipermeable membranes was first observed in 1748 by Jean Antoine Nollet. For the following 200 years, osmosis was only a phenomenon observed in the laboratory. In 1949, the University of California at Los Angeles (UCLA) first investigated desalination of seawater using semipermeable membranes. Researchers from both UCLA and the University of Florida successfully produced fresh water from seawater in the mid-1950s, but the flux was too low to be commercially viable[2] . By the end of 2001, about 15,200 desalination plants were in operation or in the planning stages worldwide.[1]

Process Osmosis is a natural process. When two liquids of different concentration are separated by a semi permeable membrane, the fluid has a tendency to move from low to high concentrations for chemical potential equilibrium. Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases, the membrane is designed to allow only water to pass through this dense layer, while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for A semipermeable membrane coil used in desalinization. fresh and brackish water, and 40–82 bar (600–1200 psi) for seawater, [3] which has around 27 bar (390 psi) natural osmotic pressure that must be overcome. This process is best known for its use in desalination (removing the salt and other minerals from sea water to get fresh water), but since the early 1970s it has also been used to purify fresh water for medical, industrial, and domestic applications. Osmosis describes how solvent moves between two solutions separated by a permeable membrane to reduce concentration differences between the solutions. When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential. Osmosis is an example of diffusion. In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the

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difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.

Applications Drinking water purification Around the world, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking. Such systems typically include a number of steps: • a sediment filter to trap particles, including rust and calcium carbonate • optionally, a second sediment filter with smaller pores • an activated carbon filter to trap organic chemicals and chlorine, which will attack and degrade TFC reverse osmosis membranes • a reverse osmosis (RO) filter, which is a thin film composite membrane (TFM or TFC)

Marines from Combat Logistics Battalion 31 operate ROWPUs for relief efforts after the 2006 Southern Leyte mudslide

• optionally, a second carbon filter to capture those chemicals not removed by the RO membrane • optionally an ultra-violet lamp for disinfecting any microbes that may escape filtering by the reverse osmosis membrane In some systems, the carbon prefilter is omitted, and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon postfilter is needed to remove chlorine from the final product, water. Portable reverse osmosis (RO) water processors are sold for personal water purification in various locations. To work effectively, the water feeding to these units should be under some pressure (40 psi or greater is the norm). Portable RO water processors can be used by people who live in rural areas without clean water, far away from the city's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (saline water may need special membranes). Some travelers on long boating, fishing, or island camping trips, or in countries where the local water supply is polluted or substandard, use RO water processors coupled with one or more UV sterilizers. RO systems are also now extensively used by marine aquarium enthusiasts. In the production of bottled mineral water, the water passes through an RO water processor to remove pollutants and microorganisms. In European countries, though, such processing of Natural Mineral Water (as defined by a European Directive[4] ) is not allowed under European law. In practice, a fraction of the living bacteria can and do pass through RO membranes through minor imperfections, or bypass the membrane entirely through tiny leaks in surrounding seals. Thus, complete RO systems may include additional water treatment stages that use ultraviolet light or ozone to prevent microbiological contamination. Membrane pore sizes can vary from 0.1 nanometres (3.9×10−9 in) to 5000 nanometres (0.00020 in) depending on filter type. "Particle filtration" removes particles of 1 micrometre (3.9×10−5 in) or larger. Microfiltration removes particles of 50 nm or larger. "Ultrafiltration" removes particles of roughly 3 nm or larger. "Nanofiltration" removes particles of 1 nm or larger. Reverse osmosis is in the final category of membrane filtration, "hyperfiltration", and removes particles larger than 0.1 nm. In the United States military, Reverse Osmosis Water Purification Units are used on the battlefield and in training. Capacities range from 1500 to 150000 imperial gallons (6800 to l) per day, depending on the need. The most common of these are the 600 and 3,000 gallons per hour units; both are able to purify salt water and water

Reverse osmosis contaminated with chemical, biological, radiological, and nuclear agents from the water. During 24-hour period, at normal operating parameters, one unit can produce 12000 to 60000 imperial gallons (55000 to l) of water, with a required 4-hour maintenance window to check systems, pumps, RO elements and the engine generator. A single ROWPU can sustain a force the size of a battalion, or roughly 1,000 to 6,000 servicemembers.

Water and wastewater purification Rain water collected from storm drains is purified with reverse osmosis water processors and used for landscape irrigation and industrial cooling in Los Angeles and other cities, as a solution to the problem of water shortages. In industry, reverse osmosis removes minerals from boiler water at power plants. The water is boiled and condensed repeatedly. It must be as pure as possible so that it does not leave deposits on the machinery or cause corrosion. The deposits inside or outside the boiler tubes may result in under-performance of the boiler, bringing down its efficiency and resulting in poor steam production, hence poor power production at turbine. It is also used to clean effluent and brackish groundwater. The effluent in larger volumes (more than 500 cu. meter per day) should be treated in an effluent treatment plant first, and then the clear effluent is subjected to reverse osmosis system. Treatment cost is reduced significantly and membrane life of the RO system is increased. The process of reverse osmosis can be used for the production of deionized water. RO process for water purification does not require thermal energy. Flow through RO system can be regulated by high pressure pump. The recovery of purified water depend upon various factor including - membrane sizes, membrane pore size, temperature, operating pressure and membrane surface area. In 2002, Singapore announced that a process named NEWater would be a significant part of its future water plans. It involves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into the reservoirs.

Food Industry In addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids (such as fruit juices) than conventional heat-treatment processes. Research has been done on concentration of orange juice and tomato juice. Its advantages include a lower operating cost and the ability to avoid heat-treatment processes, which makes it suitable for heat-sensitive substances like the protein and enzymes found in most food products. Reverse osmosis is extensively used in the dairy industry for the production of whey protein powders and for the concentration of milk to reduce shipping costs. In whey applications, the whey (liquid remaining after cheese manufacture) is concentrated with RO from 6% total solids to 10–20% total solids before UF (ultrafiltration) processing. The UF retentate can then be used to make various whey powders, including whey protein isolate used in bodybuilding formulations. Additionally, the UF permeate, which contains lactose, is concentrated by RO from 5% total solids to 18–22% total solids to reduce crystallization and drying costs of the lactose powder. Although use of the process was once avoided in the wine industry, it is now widely understood and used. An estimated 60 reverse osmosis machines were in use in Bordeaux, France in 2002. Known users include many of the elite classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux.

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Car Washing Because of its lower mineral content, reverse osmosis water is often used in car washes during the final vehicle rinse to prevent water spotting on the vehicle. Reverse osmosis is often used to conserve and recycle water within the wash/pre-rinse cycles, especially in drought stricken areas where water conservation is important. Reverse osmosis water also enables the car wash operators to reduce the demands on the vehicle drying equipment, such as air blowers.

Maple Syrup Production In 1946, some maple syrup producers started using reverse osmosis to remove water from sap before being further boiled down to syrup. The use of reverse osmosis allows approximately 54-42% of the water to be removed from the sap, reducing energy consumption and exposure of the syrup to high temperatures. Microbial contamination and degradation of the membranes has to be monitored.

Hydrogen production For small scale production of hydrogen, reverse osmosis is sometimes used to prevent formation of minerals on the surface of electrodes.

Reef aquariums Many reef aquarium keepers use reverse osmosis systems for their artificial mixture of seawater. Ordinary tap water can often contain excessive chlorine, chloramines, copper, nitrogen, phosphates, silicates, or many other chemicals detrimental to the sensitive organisms in a reef environment. Contaminants such as nitrogen compounds and phosphates can lead to excessive, and unwanted, algae growth. An effective combination of both reverse osmosis and deionization (RO/DI) is the most popular among reef aquarium keepers, and is preferred above other water purification processes due to the low cost of ownership and minimal operating costs. Where chlorine and chloramines are found in the water, carbon filtration is needed before the membrane, as the common residential membrane used by reef keepers does not cope with these compounds.

Desalination Areas that have either no or limited surface water or groundwater may choose to desalinate seawater or brackish water to obtain drinking water. Reverse osmosis is the most common method of desalination, although 85 percent of desalinated water is produced in multistage flash plants.[5] Large reverse osmosis and multistage flash desalination plants are used in the Middle East, especially Saudi Arabia. The energy requirements of the plants are large, but electricity can be produced relatively cheaply with the abundant oil reserves in the region. The desalination plants are often located adjacent to the power plants, which reduces energy losses in transmission and allows waste heat to be used in the desalination process of multistage flash plants, reducing the amount of energy needed to desalinate the water and providing cooling for the power plant. Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that has been commercially used since the early 1970s. Its first practical use was demonstrated by Sidney Loeb and Srinivasa Sourirajan from UCLA in Coalinga, California. Because no heating or phase changes are needed, energy requirements are low in comparison to other processes of desalination, but are still much higher than those required for other forms of water supply (including reverse osmosis treatment of wastewater). The Ashkelon seawater reverse osmosis (SWRO) desalination plant in Israel is the largest in the world.[6] [7] The project was developed as a BOT (Build-Operate-Transfer) by a consortium of three international companies: Veolia water, IDE Technologies and Elran.[8] The typical single-pass SWRO system consists of the following components:

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Intake Pretreatment High pressure pump Membrane assembly Remineralisation and pH adjustment Disinfection Alarm/control panel

Pretreatment Pretreatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. The material is engineered in such a fashion as to allow only one-way flow through the system. As such, the spiral wound design does not allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems, they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. Pretreatment in SWRO systems has four major components: • Screening of solids: Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pump components. • Cartridge filtration: Generally, string-wound polypropylene filters are used to remove particles between 1 – 5 micrometres. • Dosing: Oxidizing biocides, such as chlorine, are added to kill bacteria, followed by bisulfite dosing to deactivate the chlorine, which can destroy a thin-film composite membrane. There are also biofouling inhibitors, which do not kill bacteria, but simply prevent them from growing slime on the membrane surface and plant walls. • Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater result in a scaling tendency when they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soluble carbonic acid form. CO3−2 + H3O+ = HCO3- + H2O HCO3- + H3O+ = H2CO3 + H2O • Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium carbonate scaling tendency is estimated using the Langelier saturation index. Adding too much sulfuric acid to control carbonate scales may result in calcium sulfate, barium sulfate or strontium sulfate scale formation on the RO membrane. • Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of all scales compared to acid, which can only prevent formation of calcium carbonate and calcium phosphate scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales, disperse colloids and metal oxides. Despite claims that antiscalants can inhibit silica formation, there is no concrete evidence to prove that silica polymerization can be inhibited by antiscalants. Antiscalants can control acid soluble scales at a fraction of the dosage required to control the same scale using sulfuric acid [9] .

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High pressure pump The pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 375 psi (15.5 to 26 bar, or 1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 psi (55 to 81.5 bar or 6 to 8 MPa). This requires a large amount of energy.

Membrane assembly The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. RO membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and hollow-fiber.

Remineralisation and pH adjustment The desalinated water is very corrosive and is "stabilized" to protect The layers of a membrane. downstream pipelines and storages, usually by adding lime or caustic to prevent corrosion of concrete lined surfaces. Liming material is used to adjust pH between 6.8 and 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control.

Disinfection Post-treatment consists of preparing the water for distribution after filtration. Reverse osmosis is an effective barrier to pathogens, however post-treatment provides secondary protection against compromised membranes and downstream problems. Disinfection by means of UV lamps (sometimes called germicidal or bactericidal) may be employed to sterilize pathogens which bypassed the reverse osmosis process. Chlorination or chloramination (chlorine and ammonia) protects against pathogens which may have lodged in the distribution system downstream, such as from new construction, backwash, compromised pipes, etc.

Disadvantages Household reverse osmosis units use a lot of water because they have low back pressure. As a result, they recover only 5 to 15 percent of the water entering the system. The remainder is discharged as waste water. Because waste water carries with it the rejected contaminants, methods to recover this water are not practical for household systems. Wastewater is typically connected to the house drains and will add to the load on the household septic system. An RO unit delivering 5 gallons of treated water per day may discharge 40 to 90 gallons of wastewater per day.[10] Large-scale industrial/municipal systems have a production efficiency closer to 48%, because they can generate the high pressure needed for more efficient RO filtration.

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New developments Prefiltration of high fouling waters with another, larger-pore membrane with less hydraulic energy requirement, has been evaluated and sometimes used, since the 1970s. However, this means the water passes through two membranes and is often repressurized, requiring more energy input in the system, increasing the cost. Other recent development work has focused on integrating RO with electrodialysis to improve recovery of valuable deionized products or minimize concentrate volume requiring discharge or disposal.

Notes [1] [Crittenden, John; Trussell, Rhodes; Hand, David; Howe, Kerry and Tchobanoglous, George. Water Treatment Principles and Design, Edition 2. John Wiley and Sons. New Jersey. 2005.] [2] Glater, J. (1998). "The early history of reverse osmosis membrane development". Desalination 117: 297–309. [3] Lachish, Uri. "Optimizing the Efficiency of Reverse Osmosis Seawater Desalination" (http:/ / urila. tripod. com/ Seawater. htm). . [4] http:/ / eur-lex. europa. eu/ LexUriServ/ LexUriServ. do?uri=CONSLEG:1980L0777:19961213:EN:PDF [5] Water Technology – Shuaiba Desalination Plant (http:/ / www. water-technology. net/ projects/ shuaiba) [6] Israel is No. 5 on Top 10 Cleantech List (http:/ / www. israel21c. org/ briefs/ israel-is-no-5-on-top-10-cleantech-list) in Israel 21c A Focus Beyond (http:/ / www. israel21c. org/ technology/ archive) Retrieved 2009-12-21 [7] Desalination Plant Seawater Reverse Osmosis (SWRO) Plant (http:/ / www. water-technology. net/ projects/ israel/ Ashkelon) [8] Ashkelon desalination plant — A successful challenge (http:/ / www. sciencedirect. com/ science?_ob=ArticleURL& _udi=B6TFX-4MWRDDH-B& _user=10& _rdoc=1& _fmt=& _orig=search& _sort=d& _docanchor=& view=c& _searchStrId=1143068268& _rerunOrigin=google& _acct=C000050221& _version=1& _urlVersion=0& _userid=10& md5=50ff032e4f44023c49838d77d7febfc5) [9] Malki, M.,Optimizing scale inhibition costs in reverse osmosis desalination plants,INTERNATIONAL DESALINATION AND WATER REUSE QUARTERLY,2008, VOL 17; NUMB 4, pages 28-29 http:/ / www. membranechemicals. com/ english/ Optimizing%20Operational%20Costs%20in%20Reverse%20Osmosis%20Desalination%20Plants%20-%202008. pdf [10] Treatment Systems for Household Water Supplies (http:/ / www. ag. ndsu. edu/ pubs/ h2oqual/ watsys/ ae1047w. htm#disadvantage)

References • Kramer, Matt. Making Sense of Wine. Philadelphia: Running Press, 2003.

External links • Sidney Loeb – Co-Inventor of Practical Reverse Osmosis (http://www.weizmann.ac.il/ICS/booklet/8/pdf/ sidney.pdf)

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Rotating biological contactor

Rotating biological contactor A rotating biological contactor or RBC is a biological treatment process used in the treatment of wastewater following primary treatment.[1] [2] [3] [4] [5] The primary treatment process removes the grit and other solids through a screening process followed by a period of settlement. The RBC process involves allowing the wastewater to come in contact with a biological medium in order to remove pollutants in the wastewater before Schematic diagram of a typical rotating biological contactor (RBC). The treated effluent discharge of the treated wastewater to clarifier/settler is not included in the diagram. the environment, usually a body of water (river, lake or ocean). A rotating biological contactor is a type of secondary treatment process. It consists of a series of closely spaced, parallel discs mounted on a rotating shaft which is supported just above the surface of the waste water. Microorganisms grow on the surface of the discs where biological degradation of the wastewater pollutants takes place.

Biotechnology for wastewater Environmental consciousness and concern sustainable society have driven the society to the direction of re-organization of the infrastructures and the urban systems. To build an environmental management system that satisfies various social needs simultaneously in the water environment, it is essential to optimize environmental control technologies by comprehensive and systematic approaches. In this course, we critically discuss several key issues that are important in achieving desirable environmental technology systems. Biochemistry to understand the technology of wastewater treatment technologies using microorganisms is the main topic. The characteristics of complex microbial community and mathematical design modeling for Rotating Biological Contactors are discussed in this project. Biotechnology for wastewater treatment is needed so that we can use our rivers and stream for fishing, swimming and drinking water. For the first half of the 20th century, population in the Nation’s urban waterways resulted in frequent occurrences of low dissolved oxygen, fish kills, algal blooms and bacterial contamination. Early efforts in water pollution control prevented human waste from reaching water supplies or reduced floating debris that obstructed shipping. Pollution problems and their control were primarily local, not national, concerns. Since then, population and industrial growth have increased demand on our natural resources, altering the situation dramatically. Progress in abating pollution has barely kept ahead of population growth, changes in industrial processes, technological developments, and changes in land use, business innovations, and many other factors. Increases in both the quantity and variety of goods produced can greatly alter the amount and complexity of industrial wastes and challenge traditional treatment technology. The application of commercial fertilizers and pesticides, combined with sediment from growing development activities, continue to be source of significant pollution as runoff washes off the land. Water pollution issues now dominate public concerns about national water quality and maintaining healthy ecosystems. Although a large investment in water pollution control has helped to reduce the problems, many miles of streams are still impacted by variety of different pollutants. This, in turn, affects the ability of people to use the water for beneficial purpose. Past approaches used to control must be modified to accommodate current and emerging issues. Hence the appropriate biotechnology should be used for wastewater

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treatment plant.

Operation The rotating packs of disks (known as the media) are contained in a tank or trough and rotate at between 2 and 5 revolutions per minute. Commonly used plastics for the media are polythene, PVC and expanded polystyrene. The shaft is aligned with the flow of wastewater so that the discs rotate at right angles to the flow with several packs usually combined to make up a treatment train. About 40% of the disc area is immersed in the wastewater. RBC’s are closely packed circular discs submerged in wastewater and rotated slowly. Biological growth is attached to the surface of the disc and forms a slime layer. The disc contact wastewater and air for oxidation as it rotates. Helps to slough off excess solids. About one third of the disc is submerged. The disc system can be staged in series to obtain nearly any detention time or degree of removal required. Since the systems are staged, the culture of the later stages can be acclimated to the slowly degraded materials. RBC media in the form of large, flat disc mounted on common shaft are rotated through specially contoured tanks in which waste water flow on a continuous basis. The medium consists of plastic sheets ranging from 2 to 4 m in dia and up to 10 mm thick. Several modules may be arranged in parallel and / or in series to meet the flow and treatment requirements. The discs are submerged in waste water to about 40% of there diameter and are rotated by power supplied to the shaft. Approximately 95% of the surface area is thus alternately immerged in waste water in then exposed to the atmosphere above the liquid under normal operating conditions; carbonaceous substrate is removed in the initial stage of RBC. Carbon conversion may be completed in the first stage of a series of modules, with nitrification being completed after the 5th stage. Most design of RBC systems will include a minimum of 4 or 5 modules in series to obtain nitrification of waste water. Biofilms, which are biological growths that become attached to the discs, assimilate the organic materials in the wastewater. Aeration is provided by the rotating action, which exposes the media to the air after contacting them with the wastewater, facilitating the degradation of the pollutants being removed. The degree of wastewater treatment is related to the amount of media surface area and the quality and volume of the inflowing wastewater. RBC’s were first installed in West Germany in 1960 and were later introduce in U.S and Canada, 70% of the RBC systems installed are used for carbonaceous. BOD removal only,25 % for combine carbonaceous BOD removal and nitrification, and 5% for the nitrification of secondary effluent.

A schematic cross-section of the contact face of the bed media in a rotating biological contactor (RBC)

Construction Rotating Biological contactor is the attached growth process. Rotating biological consist of 3-4m diameter plastic sheet of thickness 10mm attached to a shaft which is connected to a motor power 40kW, rotate at 1-2 rpm. 1 module contains 4-6 discs. And 5-6 module in series to assure complete nitrification Process-in this process the disc rotate in the tank at 1-2 rpm to assure proper growth of bio logical film on the disc. The disc is submerged in the waste water about 45% to 90% of it dia according to the characteristic of waste water. When the disc rotates outside the tank the air enters the voids of the disc and water inside the disc trickles out the surface of the disc on the biological growth. During the submergence period the microbes present in the waste water get attached to the disc and from a bio-logical film. The film is around 3-4mm thick. This film when enter in to the waste water it consumes the organic

Rotating biological contactor waste by breaking the complex organic matter into the compound organic matter. Again when the disc surface faces the open atmosphere to receive enough oxygen to sustain and carry out their metabolic activities. Since the bio film is oxygenated externally from the wastewater, aerobic condition may develop in the liquid. Under normal operating condition the carbonaceous sustain in the initial stage of RBC. The carbon conversion may be completed in the first stage of a series of modules with nitrification being completed after the fifth stage. Nitrification proceeds only after carbon concentration is substantially reduced. Most design of RBC system will include minimum of four to five module in series to obtain nitrification of wastewater. The sloughed bio mass is relatively dense and settles well in secondary clarifier. Since it is continuous process it has no detention time.

Details History The first RBC was installed in Germany in 1960, later it was introduced in U.S.A. In U.S.A Rotating biological contactors are used for industries producing high B.O.D. i.e. for petroleum industry, dairy industries etc. Detail:- size of disc-