A Guide to Waste Water Treatment
Short Description
Descripción: He who need general description of WWM is here.............
Description
A GUIDE TO WASTEWATER TREATMENT
Case studies Included i
CONTENTS PAGE
SECTION 1
INTRODUCTION ......................................................................................................... 0 1.1 Wastewater sources, composition and flow rate estimation ................................................................. 1 1.1.1
Domestic wastewater ........................................................................................................ 1
1.1.2
Industrial Wastewater ....................................................................................................... 2
1.2 Water Quality Standard-Measures of Water Quality- When is water contaminated ............................ 3 1.2.1
Dissolved oxygen .............................................................................................................. 3
1.2.2
Biochemical oxygen demand............................................................................................ 4
1.2.3
Solids ................................................................................................................................. 5
FIGURE1 CLASSIFICATION OF TOTAL SOLIDS (BASED ON FILTRATION) (VESILIND & ROOKE, 2003) ............................................................................................. 5 1.2.4
Nitrogen............................................................................................................................. 5
1.2.5
Phosphorous ...................................................................................................................... 6
1.2.6
Bacteriological measurements .......................................................................................... 6
1.3 Wastewater characteristics .................................................................................................................... 7 1.3.1
Physical characteristics of wastewater ............................................................................. 8
1.3.2
Chemical wastewater characteristics ................................................................................ 9
1.3.3
Biological Characteristics of Wastewaters ..................................................................... 11
1.4 Effects of Untreated liquid effluents ................................................................................................... 12 1.4.1
Health effects .................................................................................................................. 12
1.4.2
Increase in the B.O.D. & C.O.D. content of water bodies ............................................. 13
1.4.3
Increase in nutrient content ............................................................................................. 13
1.4.4
Increase of soil deposition .............................................................................................. 14
1.4.5
Effects of odours ............................................................................................................. 14
1.4.6
Effects of Increased Temperatures ................................................................................. 14
1.5 Wastewater collection systems ........................................................................................................... 15 1.5.1
Sanitary sewer systems ................................................................................................... 16
1.5.2
Storm sewer systems ....................................................................................................... 16
1.5.3
Combined sewer systems ................................................................................................ 17
1.5.4
Collection System Components...................................................................................... 17
FIGURE2 (A) JUNCTION BOXES (B) INTERCEPTOR PIPES (DRINAN & WHITING, 2001) ................................................................................................................. 18
ii
2
WASTE WATER TREATMENT METHODS ........................................................ 18
FIGURE3 TYPICAL STAGES IN THE CONVENTIONAL TREATMENT OF SEWAGE
19
FIGURE4 .............................................................................................................................. 20 constituent.................................................................................................................................................... 20 Unit operation or process ............................................................................................................................. 20 Suspended Solids ......................................................................................................................................... 20 3
INDUSTRIAL WASTE WATER TREATMENT METHODS .............................. 22 3.1 Physical/chemical treatment methods ................................................................................................. 23 3.1.1
Screening ......................................................................................................................... 23
FIGURE5 INCLINED BAR SCREEN ........................................................................... 24 FIGURE6 CURVED BAR SCREEN .............................................................................. 25 FIGURE7 RADIAL BAR SCREEN ............................................................................... 25 FIGURE8 STEP TYPE SCREEN ................................................................................... 25 FIGURE9 BRUSH TYPE SCREEN ............................................................................... 26 3.1.2
Sedimentation.................................................................................................................. 26
FIGURE10
CIRCULAR AND RECTANGULAR SETTLING TANKS .................. 26
FIGURE11
PRIMARY CLARIFIER ELEVATION VIEW ...................................... 27
FIGURE12
PRIMARY CLARIFIER PLAN VIEW ................................................... 27
FIGURE13
SUCTION TUBE CLARIFIER ELEVATION ....................................... 27
FIGURE14
PICKET FENCE SLUDGE THICKENER ............................................. 28
3.1.3
Flotation and Skimming ................................................................................................. 28
FIGURE15 ............................................................................................................................ 29 3.2 Chemical treatment methods............................................................................................................... 29 3.2.1
FIGURE16 3.2.2
FIGURE17
Chlorination .................................................................................................................... 30
CHLORINATOR ....................................................................................... 30 Ozonation ........................................................................................................................ 31
OZONE WATER TREATED AREA ...................................................... 31 iii
FIGURE18
OZONATOR .............................................................................................. 32
3.3 Biological treatment methods ............................................................................................................. 32 3.3.1
Activated-sludge Process ................................................................................................ 34
FIGURE19
AN AERATION BASIN (PEPPER, GERBA, & RUSSEAU, 2006) ...... 35
FIGURE20
DENITRIFICATION SYSTEMS: (A) SINGLE-SLUDGE SYSTEM.
(B) MULTISLUDGE SYSTEM (PEPPER, GERBA, & RUSSEAU, 2006). ................... 36 FIGURE21
DENITRIFICATION SYSTEM: BARDENPHO PROCESS (PEPPER,
GERBA, & RUSSEAU, 2006). ............................................................................................ 37 3.3.2
FIGURE22
Trickling Filters .............................................................................................................. 38
(A) A UNIT OF PLASTIC MATERIAL USED TO CREATE A
BIOFILTER. THE DIAMETER OF EACH HOLE IS APPROXIMATELY 5 CM. (B) A TRICKLING BIOFILTER OR BIOTOWER. THIS IS COMPOSED OF MANY PLASTIC UNITS STACKED UPON EACH OTHER. DIMENSIONS OF THE BIOFILTER MAY BE 20 M DIAMETER BY 10–30 M DEPTH (PEPPER, GERBA, & RUSSEAU, 2006).................................................................................................................. 39 3.3.3
FIGURE23
Oxidation Ponds .............................................................................................................. 40
AN OXIDATION POND. TYPICALLY THESE ARE ONLY 1–2
METERS DEEP AND SMALL IN AREA. ....................................................................... 40 3.3.4
FIGURE24
Aerobic ponds ................................................................................................................. 40
AEROBIC
WASTE
POND
PROFILE
(PEPPER,
GERBA,
&
RUSSEAU, 2006).................................................................................................................. 41 3.3.5
FIGURE25
Anaerobic ponds ............................................................................................................. 41
ANAEROBIC WASTE POND PROFILE (PEPPER, GERBA, &
RUSSEAU, 2006).................................................................................................................. 41 3.3.6
FIGURE26
Facultative ponds ............................................................................................................ 41
MICROBIOLOGY OF FACULTATIVE POND (PEPPER, GERBA, &
RUSSEAU, 2006).................................................................................................................. 42 3.3.7
4
Aerated lagoons or ponds ............................................................................................... 42
INTRODUCTION ....................................................................................................... 44 4.1 Fibers Categorization .......................................................................................................................... 44 iv
FIGURE27
SCHEMATIC DIAGRAM OF DIFFERENT PROCESSING SECTORS
IN TEXTILE INDUSTRY (RAMESH BABU, 2007) ....................................................... 45 4.1.1
Cultivating and harvesting .............................................................................................. 46
4.1.2
Preparatory Processes ..................................................................................................... 46
4.1.3
Spinning- Yarn manufacture .......................................................................................... 46
4.1.4
Weaving- Fabric manufacture ........................................................................................ 47
4.1.5
Finishing- Processing of Textiles ................................................................................... 47
4.2 Textile Industry Chemicals ................................................................................................................. 49 Hydrophobic/ Oleophobic
Agents ............................................................................................ 52
1.5.5 ............................................................................................................................................... 52 1.5.6
Antistatic Agents .......................................................................................................... 52
1.1.1 ............................................................................................................................................... 52 1.5.7
Oxidative compounds ..................................................................................................... 52
4.3 The origin of textile effluents ............................................................................................................. 53 4.3.1
Colour .............................................................................................................................. 53
4.3.2
Persistent Organics.......................................................................................................... 53
4.3.3
AOX and heavy metals ................................................................................................... 54
4.3.4
Toxicants ......................................................................................................................... 54
4.3.5
Surfactants ....................................................................................................................... 54
4.3.6
Temperature .................................................................................................................... 55
4.4 Waste disposed from each section ...................................................................................................... 57 4.5 Treatment Methods ............................................................................................................................. 57 4.5.1
FIGURE28 4.5.2
Primary treatments .......................................................................................................... 58
MECHANICAL WASTEWATER SCREENING (HH AG, 2005) ....... 58 Secondary treatments ...................................................................................................... 60
FIGURE29 ............................................................................................................................ 61 FIGURE30
COMPACT
CHEMICALLY
ENHANCED-TRICKLING
FILTER
SYSTEM (AHMED, 2006) .................................................................................................. 63 FIGURE31
ACTIVATED SLUDGE (BABU B.V., 2008) ........................................... 64
FIGURE32 ............................................................................................................................ 64 FIGURE33 ............................................................................................................................ 67 FIGURE34
SCHEMATIC DIAGRAM OF THE EXPERIMENTAL APPARATUS
FOR PHOTOCATALYTIC REACTION ......................................................................... 70 v
FIGURE35
ADSORPTION COLUMN........................................................................ 71
FIGURE36
SCHEMATICS OF A THERMAL EVAPORATOR ............................. 71
4.6 Example 1 - WASTEWATER CHARACTERISTICS IN TEXTILE FINISHING MILLS ......................................................................................................................................................... 71 FIGURE37
SCHEMATIC DIAGRAM OF THE TEXTILE FINISHING MILL
SHOWING DIFFERENT SECTIONS .............................................................................. 72 4.7 Example 2- Textile Wastewater Treatment Plant ............................................................................... 74 4.7.1
FIGURE38 5
Plant operation ................................................................................................................ 75
AMARAVATHI COMMON EFFLUENT TREATMENT PLANT ..... 78
INTRODUCTION ....................................................................................................... 79 5.1 EFFLUENT SOURCE ........................................................................................................................ 80
FIGURE39
OIL STORAGE TANK ............................................................................. 81
5.3 EFFLUENT PARAMETERS ............................................................................................................. 81 5.4 EFFLUENT TREATMENT ............................................................................................................... 83 5.4.1
PRE TREATMENT ........................................................................................................ 83
5.4.2
PRIMARY TREATMENT ............................................................................................. 84
FIGURE40
PROCESS DIAGRAM OF TREATMENT METHODS
SOURCE:
STEFAN T. O, 2008 ............................................................................................................. 84 5.4.3
SECONDARY TREATMENT....................................................................................... 84
5.4.4
TERTIARY TREATMENT ........................................................................................... 84
5.5 LEGISLATION .................................................................................................................................. 85 FIGURE41
FIG. 1 DISSOLVED AIR FLOTATION SYSTEM................................ 87
FIGURE42
FIG. 2 HYDRO-CYCLONE SEPARATOR............................................ 88
FIGURE43
API OIL-WATER SEPARATOR ............................................................ 89
FIGURE44
A TYPICAL BIOLOGICAL TREATMENT PLANT ........................... 90
5.7 LIQUID EFFLUENT MONITORING ............................................................................................... 90 FIGURE45
WASTE WATER ANALYSIS LABORATORY .................................... 91
6
LEGISLATIONS ON TEXTILE INDUSTRY CASE STUDY:.............................. 95
7
LEGISLATIONS......................................................................................................... 96 vi
8
REFERENCES ............................................................................................................ 99
vii
1
INTRODUCTION
Liquid effluents refer to water discharged from a community after it has been contaminated by various uses. It is often referred to as wastewater and it is combination of water removed from residences, institutions, industrial establishments, and surface and ground waters (Metacalf & Eddy Inc, 1991). It consists of 99.94 percent water by weight and the remaining 0.06 percent is suspended or dissolved material (Shun & Lee, 2000). In the United States in the early 19th century, liquid effluents from residences, commercial premises and industries were generally discharged in to large bodies of water on to land directly without treatment. However, as the cities got larger, population increased and the demand for land became higher. Waste could no longer be dumped into the land untreated. A similar method was also being applied in the UK. Most settlements of old were located where there was easy access to water supply. However, as the clean water was used up, it was replaced by used dirty water. The polluted water was then sent back into the homes for use. Population explosion of urban areas produced massive outbreak of cholera and in 1848 and 14,000 people died of the disease. However, at this time there was no link between polluted water and disease. It was not until 1852 that a link was made between the polluted water and disease. Soon after this, laws were then enacted and certain actions were carried out. For instance in the London, efforts were made to clean up the river Thames which was regarded as biologically dead. Water from the Thames was first treated before being sent to homes for use. The water was also treated after use before being discharged back into the Thames. Today, the river Thames is considered as one of the cleanest rivers to run through a city (Read & Vickridge, 1997). Today, in most modern cities, wastewater is treated before being discharged in to natural water bodies. In the US, 15,000 wastewater treatment plants treat approximately 150 billion liters of wastewater per day (Pepper, Gerba, & russeau, 2006). In this chapter, a brief introduction into wastewater, its sources, water quality standards, its effect and collection mechanism is described. Brief notes are also given to describe some references that can be consulted for more detailed information.
0
Wastewater sources, composition and flow rate estimation Wastewater is usually generated from residences, institutions and commercial houses, industries, farms, and run-offs from storms. Waste waters from residences, institutions, commercial houses, and farms are referred to as domestic wastewaters. However, recently farmers are required to set up on-site treatment systems for animal waste (Ministry for the Environment of Manatu Mo Te Taiao, 2009). Industrial wastewaters are sometimes treated separately but this depends on the type of industry and the size of the community. Most communities collect and treat both domestic and industrial wastewaters together in municipal wastewater treatment plants. Below is a brief introduction into some of the sources of wastewaters, their composition and flow rate. Domestic wastewater The components of domestic wastewater are; wastewater from homes, commercial places,
1.1.1
water from rain runoff and infiltration wastewater (Kiely, 1997). Residential wastewater usually comprises of water from toilets, laundry, washing dishes etc and they are usually referred to as sewage. They can also be divided into two groups; Black water-which is basically water from toilets and Grey water which is water from every other source like kitchen sinks. Humans excrete 100–500 grams wet weight of faeces and 1–1.3 Liters of urine per person per day. The composition and concentration of domestic water varies depending on the time of the day, the day of the week, the month of the year and other conditions (Metacalf & Eddy Inc, 1991). Table 1 gives a data of typical composition of domestic wastewater. The composition refers to the amount of physical, chemical, and biological pollutants present1.
To reduce the load of water in the treatment plant, some countries separate the pipe network for rain runoff from the main sewer water body. However, some countries do not have such and it will be too expensive to embark on the project of creating new sewer networks
1
For details on the composition of domestic wastewater further reading can be carried out in the book by (Metacalf & Eddy Inc, 1991) 1
TABLE 1
Typical composition of untreated domestic wastewater (Pepper, Gerba, &
russeau, 2006). CONTAMINANTS Solids, total 720 1200 Dissolved, total 250 500 850 Volatile 105 200 325 Suspended solids 100 220 350 Volatile 80 164 275 Settleable solids 5 10 20 Biochemical oxygen demanda 110 220 400 Total organic carbon 80 160 290 Chemical oxygen demand 250 500 1000 Nitrogen (total as N) 20 40 85 Organic 8 15 35 Free ammonia 12 25 50 Nitrites 0 0 0 Nitrates 0 0 0 Phosphorous (total as P) 4 8 15 Organic 1 3 5 Inorganic 3 5 10 5-day, 20°C (BOD, 20°C).
1.1.2
CONCENTRATION (mg/l) LOW MODERATE 350 720 250 500
HIGH 1200 850
105 100
200 220
325 350
80 5
164 10
275 20
110
220
400
80
160
290
250
500
1000
20
40
85
8 12
15 25
35 50
0 0
0 0 8
0 0 15
4 1 3
3 5
5 10
Industrial Wastewater
This is wastewater generated from industrial processes. The wastewater generated from industries varies in flow and composition depending on the type of industries. Metacalf & 2
Eddy Inc. (1991) suggest that for industries with little or no wet processes, the estimated flow is about 1000 – 1500 gal/acre.d (9-14 m3/ha . d) for light industries and 1500-3000 gal/acre . d (14-28m3/ha . d) for medium industrial development. Generally, to determine the wastewater flow from an industry, a flow duration curve is created by taking measurements from the wastewater streams continuously using automatic continuous flow recorders. However, as this is expensive and time consuming, adequate measurements can also be obtained by using autosampling-autoanalytical equipments (Kiely, 1997). As mentioned earlier, wastewater composition from industries vary and before treatment processes can be set up, waste flow diagrams or mass balance of waste flows and characteristics have to be carried out. Kiely, (1997) identifies five major steps required in the survey; •
Identifying the unique process from start to finish
•
Identifying the liquid waste streams
•
Calculating flows of all wastewater streams
•
Determining the pollutant load of all wastewater streams
•
Analysing the pollutant load for the most suitable parameter to identify the waste stream
Water Quality Standard-Measures of Water Quality- When is water contaminated The quality of water is relative to its use. What may be considered as a pollutant for a particular water use may be of importance in another application. For example, organics in water help to support plant and animal life. However, organics in water will have an adverse effect if the water were to be used in a cooling tower (Vesilind & Rooke, 2003). The standard reference for water quality based on physical, chemical and biological characteristics is “Standard Methods for Examination of water and wastewater” The book is a compilation of test methods for measuring water quality. Some of the parameters measured are discussed below.
1.1.3
Dissolved oxygen
This is a very important parameter in the determination of water quality. Water devoid of oxygen will have odours and facilitate anaerobic conditions which will also result in odours 3
and loss of aquatic life. The oxygen content can be measured using an oxygen probe and meter (Vesilind & Rooke, 2003). 1.1.4
Biochemical oxygen demand
This is another very important parameter. It is a measures of both the rate at which oxygen is used up by microorganisms to break down organic matter and the amount of organic matter present in the water. In wastewater treatment, removal of BOD is essential as if left untreated and the rate of oxygen consumption is greater than re-oxygenation from the atmosphere unfavourable conditions will develop in the water body the wastewater is being discharged into. The BOD in wastewater can be detected using the standard BOD test known as the 5-day BOD test which is run at 20°C for five days. The test is also carried out in the dark to prevent algae from producing oxygen. However, the test is not accurate as it depends on the use of oxygen by microorganisms. Other tests that have been employed are determination of chemical oxygen demand (COD test) and Total Oraganic Carbon (TOC test). The COD test makes use of strong oxidants to destroy the organic compounds present in the wastewater. It is based on the assumption that all the organics are destroyed. The organics present can then be estimated from stoichiometry. The TOC test measures the total carbon content of the wastewater. This is done by injecting the sample wastewater into a heating coil and measuring the amount of carbon dioxide gas produce and relating it stoichiometrically to the amount of carbon. Since the test does not measure the organic food material alone, the %5 day test is still used for the determination of BOD (Vesilind & Rooke, 2003). The BOD content for most domestic wastewater discharge is approximately between 150 and 250 mg/L but that of industrial wastewater maybe as high as 30,000mg/L (Vesilind & Rooke, 2003). The BOD test carried out to;
•
To determine the amount of oxygen that will be required for biological treatment of the organic matter present in a wastewater
•
To determine the size of the waste treatment facility needed
•
To assess the efficiency of treatment processes, and 4
•
To determine compliance with wastewater discharge permits.
Solids In wastewater, anything other than water or gas is classified as solids. However, the basic
1.1.5
definition for solids is anything that remains after evaporation at 103°C (Vesilind & Rooke, 2003). These solids are often referred to as Total solids. They can be classified into two groups based on filtration; suspendes solids and dissolved solids( as shown on figure 1.1). If left untreated, these solids can serve as serious pollutants leading to several effects which will be discussed later.
Total
Suspended
Dissolved
FIGURE1 CLASSIFICATION OF TOTAL SOLIDS (BASED ON FILTRATION) (VESILIND & ROOKE, 2003) To determine the amount of total solids present, a known volume of wastewater is placed on an evaporating dish until all the water has evaporated. The total solids is expressed in milligrams per liter. As the name implies, dissolved solids are those components that dissolve in the water and will crystallize upon evaporation. Solids can also be classified in another way based on combustion into; volatile suspended solids and fixed suspended solids. Volatile suspended solids are generally organic in nature and a considered to combust at about 600°C.
Nitrogen The presence of Nitrogen in wastewater being discharged untreated into a water body can
1.1.6
cause euthrophication (presence of excess nutrients leading to the increase in microbial life). Nitrogen is an important element in biological reactions and is present in the organic form (i.e as amino acids and amines) and in ammonia form. It oxidises to nitrate reducing the oxygen levels in the stream. Nitrogen presence can be detected analytically by calorimetric techniques. A known sample of wastewater can be reacted with Nessler reagent (a solution of potassium mercuric iodide). A yellow-brown colloid is formed and then it indicates the presence of Nitrogen. The precise amount can’t be determined from photometric analysis of the colloid. 5
1.1.7
Phosphorous
Phosphorous is the limiting nutrient that prevents euthrophication and limits the rate of metabolic activity. If allowed to exceed natural limits can disrupt the ecological balance of the water body (Vesilind & Rooke, 2003).
1.1.8
Bacteriological measurements
Pathogens are organisms that cause illness and their determination is very important. However, the detection of pathogens is challenging for some reasons: Each pathogen has a specific detection procedure. Also, their concentration is so small as to make detection difficult. Yet, the presence of one or two of these organisms in water may be sufficient to cause infection (Vesilind & Rooke, 2003). In the United states pathogens of importance include Salmonella, Shigella, the hepatitis virus, Entamoeba histolytica, Giardia lambilia, Crptosporidium, and Escherichia coli H57 strain2. Some of these pathogens cause gastro intestinal disease and sometimes can lead to death. As mentioned earlier, it is impossible to measure all the pathogens carried by wastewater hus an indicator is used to define the bacteriological water quality. The most commonly used indicators are a group of microbes called coliforms (Vesilind & Rooke, 2003). Coliforms have five important attributes which is why they have become universal indicators; •
They are normal inhabitants of the digestive tracts of warm-blooded animals;
•
They exist in abundance and thus are not difficult to find
•
They are easily detected
•
They are generally harmless except in unusual circumstances
•
The can survive longer than most known pathogens
The amount of coliforms in water can be measured by passing a known amount of water through a sterile filter, then placing the filter in a Petri dish and soaking it with sterile agar solution that promotes the growth of coliforms alone. The number of dark blue-green dots formed after 24 or 48 hours indicates the coliform colonies present and it is expressed as coliforms/100mL. The removal of coliforms has become perfected by most wastewater treatment plant and the US EPA is tending towards the use of enterococci as an indication for 2
For more information on the disease caused by each pathogen consult (Vesilind & Rooke, 2003). 6
contamination. Table 2 below shows some of the pathogenic organisms found in water and their typical concentration. TABLE 2
Types and numbers of microorganisms typically found in untreated
domestic wastewater (Pepper, Gerba, & russeau, 2006)3. ORGANISM
CONCENTRATION (per ml)
Total coliform
105–106
Fecal coliform
104–105
Fecal streptococci
103–104
Enterococci
102–103
Shigella
Present
Salmonella
100–102
Clostridium perfringens
101–103
Giardia cysts
10_1–102
Cryptosporidium cysts
10_1–101
Helminth ova
10_2–101
Enteric virus
101–102
The objective of wastewater treatment is to prevent the receiving water body from being contaminated by reducing; •
Biochemical oxygen demand (BOD)
•
Total suspended solids (TSS)
•
Nitrogen and Phosphorous
•
Faecal coliforms
However, other objectives may be set depending on the country and the legislation set up regarding the disposal of wastewater. Wastewater characteristics A combination of domestic and industrial wastewater is often referred to as municipal wastewater. Some countries have separate sewer networks for domestic and industrial effluents. However, in most countries the sewer systems are combined. In order to ensure that 3
(Pepper, Gerba, & russeau, 2006) this book contains detailed information on test that are used to detect amount of BOD, COD and TOD. It also contains detailed calculations and example. The book generally looks into all forms of environmental pollution with a chapter dedicated to water pollution. 7
toxic substances are not released into the municipal wastewater system, industrial wastewaters have to be pre-treated to a certain standard depending on the country (Hammer, 1986). In order to effectively collect, treat and dispose of wastewater an understanding of the basic characteristics of wastewater is essential. Wastewater is characterised in terms of; •
Physical
•
Chemical
•
Biological
However, some of these characteristics are interrelated. For example, temperature is a physical property but affects both the biological activity and the solubility of gases in wastewater (Metacalf & Eddy Inc, 1991). A summary of the characteristics is shown on Table 1.2. However some properties will be discussed briefly below. Physical characteristics of wastewater 1.1.9.1 Solids in wastewater
1.1.9
Solids can exist in water either as suspended or dissolved solids as mentioned earlier. They are made up of organic or inorganic particles or immiscible liquids like oils and grease. The total solid content in wastewater is regarded as the residue upon evaporation at 103 to 105°C (Metacalf & Eddy Inc, 1991). They are often characterised by their size distribution, state and chemical characteristics. Solids are of importance in wastewater treatment as they serve as adsorption sites for micro organisms and chemicals and thus reduce the efficiency of treatment (Drinan & Whiting, 2001). Domestic wastewaters usually contain suspended solids that are organic in nature while industrial wastewaters contain a diverse variety of both organic and inorganic pollutants. Solids can be removed by primary sedimentation. However for particles of size 0.001 to 1 µm, secondary methods can be used to remove the solids (Metacalf & Eddy Inc, 1991). The TSS standards for primary and secondary effluents are usually set at 30 and 12 mg/L (Shun & Lee, 2000)4 1.1.9.2 Colour The colour of waste water indicates how septic the waste is. At the initial stages, the wastewater is brownish or light grey in colour. As it flows further down the collection system, anaerobic reactions occur and it becomes dark grey or black in colour (Drinan & Whiting, 4
Shun & Lee, 2000 gives more detials on how to measure total suspended solids in wastewater and it also includes detailed calculations and examples. 8
2001).
1.1.9.3 Odour Treatment basins, clarifiers, aeration basins, and contact tanks are some of sources where bad odour is generated in a wastewater treatment plant. Odours are generated from the anaerobic decomposition of organic compounds in wastewater. The units are normally covered to prevent the odours from escaping. However explosive gases may ensue and cause problems. Thus the units are vented to a scrubber to prevent that (Drinan & Whiting, 2001). Odours can be detected using olfactory systems. (Koe & Tan, 1998) in their work came up with a method to quantify wastewater odour strength using an olfactometer and a first order model5. Although there are four independent factors for the characterisation of odours: intensity, character, hedonics and detectability, the only factor commonly used in statutory development is detectability (Metacalf & Eddy Inc, 1991). 1.1.9.4 Temperature The temperature of the wastewater is a very important parameter as it affects the rate of both the chemical and biological treatment. If temperatures are high, the solubility of the chemicals for treatment increases and microbial action is more effective. However if temperatures are low, microbial activity is slow and more chemicals will be required (Drinan & Whiting, 2001). 1.1.10 Chemical wastewater characteristics
The chemical characteristics of wastewater refers to the total dissolved solids (TSD) which comprises majorly of alkaline minerals, organics, PH, chlorides and nutrients. They are related to the solvent capabilities of the wastewater (Drinan & Whiting, 2001).
1.1.10.1 Total dissolved solids These are the solid compounds that remain as residue after the wastewater has been filtered and has undergone evaporation. They can be removed from wastewater by filtration and evaporation, and also by electrodialysis, reverse osmosis, or ion-exchange (Drinan & 5
A method of quantifying the odor strength of wastewater samples has been investigated. Wastewater samples from two locations of a wastewater treatment plant were collected and subjected to air stripping. The off‐gas odor concentration was measured by a dynamic olfactometer at various time intervals. Applying a first order model to the decay of odorous substances in the wastewater under air stripping, the initial odor strength of the wastewater was determined. The model was found to be acceptable under five different air‐stripping rates studied. (Koe & Tan, 1998) 9
Whiting, 2001). As discussed earlier they can be grouped into total suspended solids(TSS) and total dissolved solids(TDS). Each group can then be further divided into volatile and fixed fractions (Shun & Lee, 2000). 1.1.10.2 Metals Metals such as cadmium, copper, lead, zinc,mercury, and others are of great concern in wastewater treatment because they are very toxic. And if discharged untreated can lead to severe complications and even death. Not only are they toxic, but the can greatly reduce the removal efficiency of some biological process (e.g. activated sludge process). The can be removed via chemical treatment and their presence in wastewater streams often increase the cost of the wastewater treatment plant. Their major source are from industrial wastes (Drinan & Whiting, 2001). 1.1.10.3 Organic Matter Wastewater contains organic compounds which have their roots from both the plant and the animal kingdom. According to Metacalf & Eddy Inc, 1991, "In wastewater of medium strength, about 75% of the suspended solids and 40% of the filterable solids are organic in nature," Also organic compounds synthesised by man are found in wastewater. The compounds are usually made up of carbon, hydrogen, and oxygen. Compounds like sulphur, phosphorous,nitrogen and iron. The major organic compounds found in wastewaters are; proteins, carbohydrates, urea, fats and oils. The manmade compounds found are pesticides, surfactants and volatile organic compounds. As a result of industrialisation the amount of synthetic organic compounds in wastewaters are rapidly increasing. However, these compounds are not easily removed from wastewaters by biological treatment. Detailed brief description of some of these compounds can be found in (Metacalf & Eddy Inc, 1991) and (Drinan & Whiting, 2001)6.
1.1.10.4 pH This is an indication of the hydrogen ion concentration present in the wastewater. The PH affects the chemical and biological processes in wastewater treatment. For instance, if the pH is high, the amount of chlorine required for the disinfection process will be greatly increased 6
(Metacalf & Eddy Inc, 1991) contains more details on types of organic compounds and the measurement of these organic constituents. 10
(Drinan & Whiting, 2001). 1.1.10.5 Nutrients These are inorganic compounds that are essential to the growth and reproduction of plants and animals. The nutrients of greatest concern in wastewater treatment are nitrogen and phosphorous. Other nutrients include carbon, sulfur, calcium, iron, potassium, manganese, cobalt, and boron. The presence of nitrogen and phosphorous in surface waters is an indication of wastewater contamination. Their presence can lead to the growth of unwanted plants like algae and euthrophication. Typical ranges of nitrogen concentration in domestic raw wastewater are 25-85 mg/L for total nitrogen (the sum of ammonia, nitrate, nitrite, and organic nitrogen) and for phosphorus its 2 - 20 mg/L, which includes 1-5 mg/L of organic phosphorus and 1-15 mg/L of inorganic phosphorus (Shun & Lee, 2000).
1.1.11 Biological Characteristics of Wastewaters
Wastewater contains millions of microorganisms per milliliter. However many or these organisms are harmless. The water becomes contaminated with dangerous pathogens from waste discharged from people who are infected with them. Although micro organisms are used in various treatment processes, the final effluent discharge most not carry dangerous levels of pathogens. Basic orgfanisms of interest are bacteria, parasitic worms, protozoa, viruses and algae (Drinan & Whiting, 2001). Below is a brief description of these organisms. 1.1.11.1 Bacteria Bacteria is common place in wastewater treatment procrsses. However the presence of some type of bacteria may cause gastrointestinal disorders.
1.1.11.2 Protozoa These are single celled organisms that are widely distributed and highly adaptable. They are active participants of the activated sludge process. However they have to be revoved either by sedimation or filtration. Although most protozoans are harmless, two categories Entamoeba 11
histolytica (amebiasis), and Giardia lamblia (giardiasis) are considered as very harmful. The levels of protozoa should be kept minimal in effluents. 1.1.11.3 Viruses The presence of virus in wastewater is of great concer. This is because viruses are very small and cannot be easiliy removed by filtratio. Also, viruses remain inactive until they find a host and can reenter the water supply further downstream. Finally, testing for viruses is limited as there are limited methods available. 1.1.11.4 Algae Algaes grow in fresh water, saltwater and ppolluted water. They are usually found at the surface as they require light for they metabolism. They are often used in waste water treatments like fluculative and aerobic ponds to generate oxygen. However, their growth is not easy to control, they encourage the formation of suspended solids and die of when the wheather is cold 1.1.11.5 Worms (Helminths) These are organisms that metabolise organic compounds aerobically. They are indicators that a water body has been contaminated by wastewater. Parasitic worms like helminths are transmitted to humans via contact with untreated wastewater.
Effects of Untreated liquid effluents The discharge of untreated effluents into local water may lead to several unwanted situations like the destruction of aquatic life, contaminations of drinking water which will lead to illness or even death, bad odours etc. Below some of the effects of untreated wastewater will be discussed.
1.1.12 Health effects
Of all the effects the health effects of untreated wastewaters are one of the most important ones. The first set of legislations on wastewater effluent discharge was focused towards their effect on human health. Wastewater contains millions of bacteria that originate from human 12
faeces. However most are harmless. The organisms that may cause diseases are called pathogens. Contact with the contaminated water may lead to disease such as typhoid, cholera and gastrointestinal problems. The main class of viruses of concern are enteric viruses, which cause gastro-enteritis; for example, calcivirus (Norwalk virus), rotavirus, enterovirus (polio and meningitis) and hepatitis (Ministry for the Environment of Manatu Mo Te Taiao, 2009). Asides from bacteria and viruses, other substances present in wastewater may lead to bad health or even death. For instance, compounds like mercury, volatile organic compounds, zinc, pesticides and other chemicals. Some of these compounds exist naturally but human activities have increased their concentration in natural water systems. They may not have immediate effects but may bio-accumulate in food and cause complications. According to the Ministry for the Environment of Manatu Mo Te Taiao, some ivestigations are being carried out to investigate the ability of some of these compounds to act as endocrine disruptor. Endocrine distruptors are chemicals that when absorbed into the body mimics or hinders the normal functions of hormones in the body.
1.1.13 Increase in the B.O.D. & C.O.D. content of water bodies
The discharge of untreated wastewater into springs, rivers and lakes will cause the BOD and COD to rise. This will reduce the amount of oxygen available to aerobic (oxygen demanding) aquatic animals like fish. Also this will encourage the growth of plants like algae and other anaerobic organisms. Eventually, this will render the water body septic and biologically dead (Weiner & Matthews, 2003)
1.1.14 Increase in nutrient content
Increased nutrient content (that is, organics from wastewater) will lead to algal bloom and eutrophication. Nitrogen in the form of nitrate (NO3) in surface waters indicates contamination with sewage and is an immediate health threat to both human and animal infants (Drinan & Whiting, 2001). Excessive nitrate concentrations in drinking water can cause death. The limiting factor for accelerated growth or some organisms is the absence of nutrients nitrogen and phosphorus in the water body. These compounds exist in water naturally but in limited quantities. An increased amount of these nutrients will cause the accelerated growth of some toxic organisms like algae which will slowly lead the water body to become septic. 13
1.1.15 Increase of soil deposition
Solids in water can have several harmful effects some of which are listed below •
Solids can cause unsightly floating scum
•
They can sink to the bottom of the stream or river and form potentially hazardous mud banks
•
Most solids are organic in nature and upon decomposition create a demand for oxygen
•
Floating solids serve as sites where pathogenic organism can hide and pose a threat to human health.
1.1.16 Effects of odours
Odours although cause no direct physical harm to humans have great psychological effects that may eventually lead to social and economic collapse in a community. Odours from wastewater treatment plant can cause; loss of appetite, water intake, impaired respiration, nausea, vomiting and mental perturbation. Offensive odours can also discourage capital investment and lower socio-economic status of the community if left untreated. In a paper prepared by Schiffman, et al., 2000, they proposed three paradigm which ambient odors may produce health symptoms in communities with odorous manures and biosolids7. Many communities have opposed the several wastewater treatment plant projects as a result of public perception of odours.
1.1.17 Effects of Increased Temperatures
The temperatures of wastewater is usually higher than the atmospheric temperature and the receiving water. High temperature decreases the solubility of oxygen. This combined with 7
Schiffman, et al., 2000, proposed three paradigm by which ambient odors may produce
health symptoms in communities with odorous manures and biosolids. This site summarises the three paradigms 14
increased biochemical oxygen demand can greatly affect the oxygen content of the receiving water body. Eventually this will affect the aquatic life of the waterbody. Decreasing fish lifand supporting the growth of unwanted organisms.
Below shows a summary of some of the effects of pollutants contained in wastewater. TABLE 3
Effects of pollutants in wastewater (Kiely, 1997) Pollutants
Effects
Soluble organics
Deplete dissolved oxygen
Suspended solids
Deplete
dissolved
oxygen
and
release undesirable gases Trace organics
Affects taste odours and toxicity
Heavy metals
Toxic to aquatic and human life
Colour and turbidity
Affects aesthetics
Nutrients (N and P)
Cause eutrophication
Refractory substances resistant to
Toxic to aquatic life
biodegradation Oil and floating substances
Unsightly
Volatile substances e.g H2S and
Air pollution
VOC
http://www.woodlands-junior.kent.sch.uk/riverthames/pollution.htm dirty river thames http://www.metrovancouver.org/services/wastewater/treatment/Pages/default.aspx
Wastewater collection systems Wastewater collection systems are employed to transport wastewater form source to treatment plant before disposal (Read & Vickridge, 1997). Collection systems are made up of a series of network of pipes and pumping systems. In designing a collection system one must consider; 15
1. Health and Environmental aspect. That is the proposed collection system must not pose a risk to human health and to the environmental. 2. The area the sewer network is going to service. The treatment plant must be adequate in serving the allocated region. 3. The natural topography and drainage. Collection systems must be designed to take advantage of the natural systems and thus reduce cost of installing pumps. There are three main types of sewer networks (Kiely, 1997); 1. Sanitary sewer systems 2. Storm sewers 3. Combined sewer systems Below is a description of each of these systems.
1.1.18 Sanitary sewer systems
For these systems, wastewaters generated from both domestic and industrial sources are carried by separate systems of sewers to treatment plants while surface runoffs are carried of by another set of systems to natural watercourses. This type of system is mostly adopted in newer towns and cities (Read & Vickridge, 1997). Rain water washes contaminants from roofs, streets and other areas, however the contaminant load is considered insignificant compared to wastewater discharges from domestic and industrial sources (Hammer & Hammer, Water and Wastewater Technology, 2008). Sanitary contains majorly human waste as a result of the most important aspect of sanitary sewer design is the prevention of sewage overflow (as they contain pathogens dangerous to human health) (Drinan & Whiting, 2001).
1.1.19 Storm sewer systems
These systems handle wastewaters generated from run-offs as a result of rainfall or melting snow. They are becoming very important in developed and populated areas. This is because in such areas, the ground is paved and this prevents water from naturally percolating and 16
recharging ground water. Instead, heavy run-offs result which carry large amounts of contaminants (Drinan & Whiting, 2001). Storm drains should be designed to handle contaminants like sand, silt and also sudden heavy flows. Since they do not contain sewage, they can be discharged directly into the natural environment although sometimes, primary treatment may be required (Hammer & Hammer, 2008).
1.1.20 Combined sewer systems
These are the oldest and most common type of collection system. For this type of system, both surface runoffs and municipal wastewater are transported by the same pipe networks to sewage treatment plants. This type of system is mostly found in older cities and towns (Read & Vickridge, 1997)8. The systems are designed to accommodate large flows, especially those resulting from heavy rain falls. However during storms, the system overflows and excess flow above the plant capacity is bypassed into natural water bodies. This may become a health hazard especially if water is used as supply for drinking water (Hammer & Hammer, 2008). Typical storm water contains a BOD of 30mg/l while overflow from a combined sewer contains contains 120mg/liter
of BOD. Combined sewer overflows (CSO) are of great
concern. However, it is very expensive to change the entire sewer network and other methods such as storage for later treatment are being explored (Shun & Lee, 2000).
1.1.21 Collection System Components
Most components of collection systems are built under streets easements, and right of way and they are designed to meet considerations of population size, estimated flowrates, minimum and maximum loads, velocity, slope depth, and need for additional system elements 8
This book contains detailed information on the history, construction, hydraulics and design of sewer systems. Focusing mainly on sewer rehabilitation, repair, and management. 17
to ensure adequate sy ystem flows and access tto maintenannce (Drinan & Whiting, 2001). A typicall community y wastewaterr system connsists of: •
vice that carries wastewaater from poiint of generaation to mainns Building serv
•
M Mains that carry the wastee to collectioon sewers
•
C Collectors/sub b-collectors that carry waste w to trunk k lines
•
Trrunk lines thhat carry wasstewater flow ws to interceeptors
•
Innterceptors thhat carry waaste to treatm ment plant
•
O Other elemen nts which maay include; liift stations, manholes, m veents, junctionn boxes and cllean out poin nts. Fig 2 beelow shows interceptors i and junctionn boxes.
(a)
(b))
RE2 (A) JUNCTION J N BOXES (B) INTERCEPTOR R PIPES (DRINAN ( & FIGUR WHITIN NG, 2001)
2
STE WATE ER TREATMENT ME ETHODS WAS
Sewage treatment t options may bee classified into i groups of o processess according to t the functioon they perfo form and theiir complexitty: 18
Preliminary: this includes simple processes such as screening (usually by bar screens) and grit removal. (through constant velocity channels) to remove the gross solid pollution. Primary: usually plain sedimentation; simple settlement of the solid material in sewage can reduce the polluting load by significant amounts. Secondary: for further treatment and removal of common pollutants, usually by a biological process. Tertiary: usually for removal of specific pollutants e.g. nitrogen orphosphorous, or specific industrial pollutants
FIGURE3 TYPICAL STAGES IN THE CONVENTIONAL TREATMENT OF SEWAGE
19
FIGURE4 Shown in table1 are some constituents found in wastewater and conventional water treatment methods used to purify the water.
TABLE 4 CONSTITUENT
Suspended Solids
UNIT OPERATION OR PROCESS 20
Screening
Biodegradable organics
Grit removal
Sedimentation
High-rate clarification
Flotation
Chemical Precipitation
Depth Filtration
Surface Filtration
Aerobic suspended growth variation
Aerobic attached growth variation
Aerobic suspended growth variation
Nitrogen
Aerobic attached growth variation
Lagoon variation
Physical chemical systems
Chemical oxidation
Advanced oxidation
Membrane filtration
Chemical oxidation
Suspended-growth nitrification and denitrification variations
Fixed-film nitrification and denitrification variations
Phosphorous
Nitrogen and phosphorous
Air stripping
Ion exchange
Chemical treatment
Biological phosphorous removal
Biological nutrient removal variations
Pathogens
Colloidal and dissolved solids
Chlorine compounds
Chlorine dioxide
Ozone
Ultraviolet radiation (UV)
Membranes
Chemical Treatment
21
Volatile organic compounds
Odours
3
Carbon adsorption
Ion exchange
Air stripping
Carbon adsorption
Advanced oxidation
Chemical scrubbers
Carbon adsorption
Bio filters
Compost filters
INDUSTRIAL WASTE WATER TREATMENT METHODS
The same way that you would know the steps of the process that you would be running in industry, a critical study should be carried out to familiarize yourself with the wastewater to find ways that the wastewater is generated in the plant. Treatment methods can be divided into three general cases -
Physical/Chemical treatment methods
-
Thermal Treatment methods
-
Biological treatment methods
Waste water treatment methods
Physical
Chemical
Sedimentation
Chlorination
Screening
Ozonation Neutralization Coagulation
Biological
Aerobic
Anaerobic
22
Lagoons Trickling Filtration
Lagoons Septic tanks
Equalization Degassification Flotation & skimming
3.1
Physical/chemical treatment methods
After its biological treatment the waste water is almost clean and fresh again. However, the micro-organisms responsible for cleaning should now be kept in their individual basins and not run off with all the rest. Physical chemical treatment methods encompass a wide variety of technologies, including gravity separation, filtration, chemical precipitation, evaporation, oxidation, reduction, air stripping, carbon adsorption, ion exchange, adsorption on other media, electrolytic recovery and membrane separation. Gravity separation is used to extract clean water when the waste is settled in the bottom of the tank. There are three types of separation methods which uses the same principal. Clarifiers, Oil water separators and catch basins and sumps.
3.1.1
Screening
Mechanical treatment is indispensable as the first process step of preliminary treatment for both municipal and industrial wastewater applications. It removes the bulk of the non biodegradable matter such as plastic, women materials, metallic items so that the subsequent treatment stages are protected against damage/pollution or to relieve them. There are many different types of screens in industry at present designed to suit different needs. Some examples as stated in EPCO, Australia are Inclined bar, curved bar, radial bar, step type, brush type, back-raked and static screens.
23
In operation in all these types, the sewage flows through the screen which approaches it from the upstream side and after passing through exits from the downstream side. A mechanized comb system is attached between the two side chains and is driven through a head shaft and sprockets, to rake the screen periodically and the screenings collected are removed by a doctor blade at the top of the comb travel as stared in [Epco, Australia]. These screenings are dropped onto a skid plate which transports the screening down to a container.
FIGURE5 INCLINED BAR SCREEN
24
FIGURE6 CURVED BAR SCREEN
FIGURE7 RADIAL BAR SCREEN
FIGURE8 STEP TYPE SCREEN
25
FIGURE9 BRUSH TYPE SCREEN 3.1.2
Sedimentation
Sedimentation, a fundamental and widely used unit operation in waste-water treatment, involves the gravitational settling of heavy particles suspended in a mixture, sedimentation tanks are also known as clarifiers in the wastewater treatment industry. This process is used for the removal of grit, particulate matter in the primary settling basin, biological floc in the activated sludge settling basin, and chemical flow when the chemical coagulation process is used.
FIGURE10 CIRCULAR AND RECTANGULAR SETTLING TANKS Circular sedimentation tanks are preferred over rectangular tanks due to the ease of maintenance, faster sludge removal and higher removal efficiencies. There is a scraper mechanism adopted inside the tank which is used to collect the settled solids out of the tank with the use of a pump. As stated by [Hammer 2004, pg 370] the scraper mechanism takes different forms depending on which part of the treatment it is used for, i.e. primary, secondary or tertiary. As further stated in circular sedimentation tanks these sludge scrapers are attached to the rotating arm which scrapes the sludge towards the centre hopper where as in rectangular tanks the scrapers are carried along in the tank bottom which collects the sludge into a hopper which is situated at the influent end of the tank. There are 3 types of clarifiers which are named as primary, secondary and tertiary tanks. Primary tanks The point at which the coarse solids and the grit are removed from the sewage stream is the 26
beginning of the primary process. The scraper mechanism as stated in [Epco] for primary tanks would also be fitted with scum removal equipment to remove the floating matter and in the primary stage as further stated approximately 65% of the organic solids and 35% pr the BOD in the sewage is removed.
FIGURE11 PRIMARY CLARIFIER ELEVATION VIEW
FIGURE12 PRIMARY CLARIFIER PLAN VIEW Secondary Tank The water from the primary stage goes through some biological treatment and then enters the secondary sedimentation which separates the mixed liquids and suspended solids and humus sludge. The secondary clarifiers are fitted with scraper blades like in primary systems but could also adapt a suction tube system as shown in the picture below. These systems also are equipped with scum skimming systems.
FIGURE13 SUCTION TUBE CLARIFIER ELEVATION
Tertiary Tank 27
These systems often adapt simple sweeper chains but could also be fitted with the same mechanisms as the primary and the secondary treatments. The final clarifiers are designed to use with biological aeration. Activated sludge is withdrawn through suction pipes located along the collector arm for rapid return to the aeration basin. Sludge thickeners and fermenters are also used to scrape heavier sludges as shown in the figure.
FIGURE14 PICKET FENCE SLUDGE THICKENER 3.1.3
Flotation and Skimming
Effluent Effluent
Float Discharge Settled solids discharge
28
FIGURE15 Dissolved air flotation is achieved by releasing fine air bubbles that attach to sludge particles and cause them to float. Small units tend to be rectangular for and fabricated using steel. Larger units are circular and manufactured in steel or concrete. Waste activated sludge enters the bottom of the flotation tank, where it is merged with recirculated flow that contains compressed air. A portion of the clarified effluent is pressurized in a separate retention tank under an air pressure of approximately 60 psi to force air into solution. On pressure release, the air dissolved in the recirculated flow forms fine bubbles to the suspended solids. The process underflow is returned to wastewater treatment, and the overflow, discharge by
3.2
Chemical treatment methods
This treatment method uses burning or exposure of wastewater to high temperatures to destroy the waste. Some waste that is burned could be used to recover energy in industrial furnace or cement kiln on site. Treatment facilities such as hazardous waste incinerators are another mean of for wastewater treatment but isn’t cost effective if used in small businesses as they are quite expensive, unless the facility generates a large amount of waste. Some industries tend to use off site facilities to treat wastewater but it is among the last choices to use such means. Wet air oxidation is another method used to treat waste water which is difficult to treat by other means. But this demands a huge amount of energy which in the long run is more cost effective if the wastewater is treated off site.
29
3.2.1 Chlorination Chlorination is basically the process of adding liquid or gaseous chlorine in order to purify water. This process isn’t solely used for disinfection; it is also used for odor control and prevention of septicity, ammonia removal, destroying cyanides and phenols etc as stated by the water quality and health council. The liquid form of chlorine which is generally more expensive comes in the form of soluble salts (hypochlorites) while the gaseous form first needs to be dissolved in water before it is used in the waste water industry. There are a few reactions that occur in chlorination when used in the waste water industry. When the chlorine is dissolved in water it firstly forms hypochlorous acid and hypochlorites. Cl2 + H2O Æ HOCl + H + Cl HOCl Æ OCl + H As chlorine is an active oxidizing agent when added into waste water even in small amounts it would react rapidly firstly with H2S, ferrous iron etc which are all compounds capable of reducing. After all inorganic reducing matter is converted chlorine subsequently reacts with the organic matter, ammonia or other nitrogeneous compounds to produce chloramines. The device used for the control of the chlorine added is called the chlorinator.
FIGURE16 CHLORINATOR http://www.backyardcitypools.com/chemicals/feeders/Hydrotools-Automatic-Chlorinator.htm
30
3.2.2
Ozonation
Before
After
FIGURE17 OZONE WATER TREATED AREA It is one of the modern methods used in wastewater treatment with a growing popularity. A device known as the ozone generator is used to break down pollutants in the wastewater. The ozone generator uses up the oxygen in the environment to produce ozone with the air of ultraviolet radiation which is discharged by an electric field. This ozone which is known to be highly reactive oxidise the bacteria, moulds and other pollutants in the wastewater.
As stated in the water pollution guide there are many advantages and disadvantages in using ozone in wastewater treatment. •
Advantages: o
Effective killing of bacteria.
o
Ease of extracting irons and sulphur compounds as they are oxidised.
o
No nasty odours or residues hence precautions or measures for residue treatment is not needed.
o
As the oxygen to ozone conversion is a reversible reaction and the backward reaction is fast the ozone converts back to oxygen instantly leaving no traces of an oxygen use up.
•
Disadvantages:
31
o
This method is unreliable as it needs electricity to run and would not run during an electric shortage and also costs money due to its power requirement.
o
The treatment cannot remove dissolved minerals and salts.
o
Ozone treatment can sometimes produce by-products such as bromate that can harm human health if they are not controlled.
FIGURE18 OZONATOR
3.3
Biological treatment methods
Biological processes aid in the removal of non-settleable colloidal solids, inorganic compounds and some organic matter with the aid of micro organisms. Biological processes are often referred to as secondary wastewater treatment method as they aid in the removal of biodegradable organic matter that could not be removed during primary treatment (Kiely, 1997). In wastewater treatment, the main objectives is the reduction of organic contents and nutrients like nitrogen and phosphorus and also the removal of toxic organic compounds such that the discharge to a water body should lead to little or no removal of oxygen in it by bacterial action. During biological processes, organic pollutants are converted to less harmful compounds like water and cell tissues. These can then be removed by gravity settling. The commonly used biological treatment processes: •
Activated-sludge process 32
•
Aerated lagoons
•
Trickling filters
•
Rotating biological contactors
•
Stabilization ponds
Fundamentals of Biological wastewater treatment process Biological treatment processes all take place in a vessel called a reactor and designers are interested in the rate at which biodegradable compounds are removed from the inflow and also, the rate of growth of the biomass in the reactor. (Benefield & Randal, 1980). The biomass refers to the microbial body responsible for breaking down the pollutants. In designing a biological process, it is very important to understand the nature and biochemical activities carried out by the micro organism. Below, the types of micro organisms used and their nutritional requirements will be mentioned briefly. Important microorganisms The important microorganisms in biological treatments are; Bacteria, Fungi, Algae, Protozoa and Rotifiers. Bacteria are single- celled organisms that reproduce mainly by binary fission although some species can produce asexually or by budding. They are made up of 80% water and 20% dry material. They also vary widely in size and their growth is greatly affected by the conditions of temperature and pH (Metacalf & Eddy Inc, 1991). Fungi are multicellular organisms. Example of this is yeast. They can reproduce sexually or asexually. They have the ability to withstand lower pH and Nitrogen levels than bacteria and this makes them very important in wastewater treatment. Algaes are unicellular or multicellular compounds. They are very important in wastewater treatment processes because of their ability to generate oxygen from photosynthesis. However, excess algae growth can lead to the biological death of a water body. Protozoa and Rotifiers are single celled motile protists. Most protozoa are aerobic and are generally larger than bacteria and alsoeat bacteria as an energy source. Thus are used to polish effluents from biological waste treatments. Rotifiers generally perform the same duties as protozoas in wastewater treatment. Their 33
presence indicates a highly efficient biological process (Metacalf & Eddy Inc, 1991). Nutritional requirements For micro organisms grow, reproduce and function efficiently they must have; 1. A source of energy 2. Source of carbon for synthesis of new cellular material 3. Source of inorganic nutrients such as Nitrogen, phosphorous, sulphur, potassium calcium and magnesium (Metacalf & Eddy Inc, 1991). The carbon and energy sources are considered as substrate 3.3.1 Activated-sludge Process This is one of the most popular biological treatments adopted in most countries and it is also known as aeration-tank digestion. In this process, wastewater that has undergone primary treatment is pumped into a large tank and mixed with bacteria rich slurry known as activated sludge (Pepper, Gerba, & russeau, 2006). To encourage bacterial growth and decomposition of the organic materials present, air or oxygen is pumped into the tank. The mixture is then sent to a secondary settling tank where water is removed from the top and the bacteria rich sludge is removed from the bottom. About 20 percent of the sludge is recycled back into the primary aeration tank as inoculums while the remainder known as secondary sludge is removed (Kiely, 1997). Fig below shows an aeration basin. The activated sludge culture is made up of bacteria, protozoa, rotifiers and fungi. The bacteria is mostly responsible for the break-down of organic material while the protozoa and rotifiers remove the bacteria.
34
FIGURE19 AN AERATION BASIN (PEPPER, GERBA, & RUSSEAU, 2006) The content of the aeration tank is referred to as the mixed-liquor suspended solids (MLSS) and the organic part of the MLSS is called the mixed-liquor volatile suspended solids (MLVSS), which consists of the non-microbial organic matter as well as dead and living microorganisms and cell debris (Kiely, 1997). The activated sludge process must be controlled to maintain a proper ratio of substrate (organic load) to microorganisms or food-tomicroorganism ratio (F/M) (Pepper, Gerba, & russeau, 2006). This is expressed as BOD per kilogram per day. It is expressed as:
/
[1]
where: Q -flow rate of sewage in million gallons per day (MGD) BOD5 - 5-day biochemical oxygen demand MLSS - mixed-liquor suspended solids (mg/L) V - volume of aeration tank (gallons) It can thus be observed that the higher the wasting rate, the higher the food-micro organism ratio. A low F/M ratio indicates that the micro organisms are starved and will tend to have higher removal efficiencies. In conventional aeration tanks, the F/M ratio is 0.2–0.5 lb BOD5/day/lb MLSS, but it can be higher (up to 1.5) for activated sludge when high-purity oxygen is used (Hammer, 1986). The parameters that controll the operation of an activated sludge process are; •
organic loading rates
•
oxygen supply
•
control and operation of the final settling tank
An important parameter to consider is the sludge settleability in the sludge tank. The biomass must settle well in order for it to be returned to the aeration tank. The best conditions for settling are achieved when carbon and energy sources are limited and the specific microbial growth rate is local. Conditions that hinder effective settleability are sudden changes in temperature, pH, absence of nutrients, and presence of toxic metals and organics. Another 35
important factor is the presence of filamentous bacteria. For effective settling, a residence time of three or four days is required (Metacalf & Eddy Inc, 1991). Removal of Nitrogen and Phosphorous by activated sludge process Activated sludge process can be modified such that they not only remove organic compounds but can also remove nutrients like nitrogen and phosphorous. Nitrogen Removal For nitrogen removal, the sludge is left to age for over four days to encourage nitrification of ammonia to nitrate by nitrifying bacteria. The nitrogen is then removed via denitrification process. Examples of avtivated sludge systems that have been modified for nitrogrn removal are: •
Single sludge system
•
Multisludge system and
•
Bardenpho process
Figure below shows a schematic diagram of these processes9
FIGURE20 DENITRIFICATION SYSTEMS: (A) SINGLE-SLUDGE SYSTEM. (B) MULTISLUDGE SYSTEM (PEPPER, GERBA, & RUSSEAU, 2006).
9
Pepper, Gerba, & russeau, 2006 can be consulted for more detailed information on each process. 36
Phosphorous removal Phosphorous can also be removed by modifying the activated sludge process. The process involves an uptake of the phosphorous during the aerobic stage and the release during the anaerobic stage. Two processes used are; A/O (anaerobic/toxic) process: This process consist of and anaerobic zone upstream the conventional aeration tank. In the aerobic phase, soluble phosphorus is taken up by bacteria and is synthesised to polyphosphates and during the anaerobic stage, the phosphorus is released by the hydrolysis of the polyphosphates formed. Bardenpho process: This process can also be used for the removal of nitrogen A schematic diagramof these process are shown on fig below
FIGURE21 DENITRIFICATION SYSTEM: BARDENPHO PROCESS (PEPPER, GERBA, & RUSSEAU, 2006). Design of an activated sludge process In designing an activated sludge process, the following conditions are considered; •
Mixing regimes
•
Load criteria
•
Sludge viability
•
Oxygen requirement
•
Nutrient requirement
•
Temperature
•
Solid-liquid separation
•
Effluent quality 37
Another very important consideration is the choice of reactor as this will define the geometry of the reactor, the bath of effluent into the reactor and the mixing regime. There are two mixing regimes applicable in the activated sludge process (1) Plug-flow: in this type of regime, the wastewater flows into the reactor (aeration tank) in an orderly fashion with no element of mixing. (2) Complete mixing flow: Here, the reactor is constantly stirred and kept uniform. This type of mixing is often referred to as steady state. However, complete mixing or plugflow is not achieved in the reactor but the design has to ensure that the conditions are almost met (Benefield & Randal, 1980)10.
3.3.2
Trickling Filters
This is one of the oldest biological treatment methods. However the main mechanism is not filtration as the name suggest. Treatment is achieved by diffusion and microbial assimilation. In the process, the effluent from the primary treatment is pumped through an overhead sprayer onto a bed of stones or plastic where bacteria and other organism reside. As the organic materials trickle past, the bacteria intercepts it and decomposes it aerobically. In older trickling filter designs, the beds were made of stones. But these had the disadvantage of limited depth of 3-10 ft, low void space, and requirement for structural design (Benefield & Randal, 1980). However, in modern trickling filter designs, the bed is made up of plastic units. Other materials that can be used are ceramic, hard coal. The most common type of plastic bed used is polyvinyl chloride (PVC) because of their light weight. Other advantages are the greater void space and larger specific area. The PVC are stacked in towers as shown on fig below.
10
Details on the design of activated sludge process and the associated kinetics involved can be found in this text. 38
(a)
(b)
FIGURE22 (A) A UNIT OF PLASTIC MATERIAL USED TO CREATE A BIOFILTER. THE DIAMETER OF EACH HOLE IS APPROXIMATELY 5 CM. (B) A TRICKLING BIOFILTER OR BIOTOWER. THIS IS COMPOSED OF MANY PLASTIC UNITS STACKED UPON EACH OTHER. DIMENSIONS OF THE BIOFILTER MAY BE 20 M DIAMETER BY 10–30 M DEPTH (PEPPER, GERBA, & RUSSEAU, 2006).
As the organic matter passes through the filter, it is converted to a microbial biomass that forms a bio film called zooleal on the filter surface. With time, the film thickens and the lower part has limited access to oxygen and as a result film sloughs off ( also called sloughing) and a new bio film is formed. Effluents from a trickling filter are sent to a clarifier for further removal of solid compounds. A typical trickling filter had a BOD removal efficiency of 85% (Pepper, Gerba, & russeau, 2006). Two important properties of the filter media are; The specific area of the media The percent void space The greater the surface area, the greater the amount of biomass per unit volume. Also, the greater the void space, the higher the hydraulic loading can be without restricting oxygen
39
transfer (Benefield & Randal, 1980)11.
3.3.3
Oxidation Ponds
These are often referred to as sewage lagoons or stabilization ponds. They are the oldest of the wastewater treatment process requiring huge land space. Here, the wastewater is detained for a period of 1-4 weeks( sometimes longer) while microorganisms degrade the organic matter in them. A tyoical oxidation pond is shown below on fig
FIGURE23 AN OXIDATION POND. TYPICALLY THESE ARE ONLY 1–2 METERS DEEP AND SMALL IN AREA. There are four categories of osidation ponds which are often used in series: aerobic ponds, anaerobic ponds, facultative ponds and aerated ponds.
3.3.4
Aerobic ponds
Here, the wastewater is detained for 3-5 days at a depth of about 1.5m to encourage the growth of algae which in turn promotes the generation of oxygen. A section of an aerated pond is shown on fig below.
11
Detail design calculations for tricling filters can be found in Benefield & Randal, (1980) 40
FIGURE24 AEROBIC WASTE POND PROFILE (PEPPER, GERBA, & RUSSEAU, 2006)
3.3.5
Anaerobic ponds
These are about 1-10m deep and have a longer detention time of about 20 – 50 days. They are normally used to treat wastewater with high BOD content and do not requireany form of mechanical aeration. They also generate comparably small amount of sludge. Fig shows the profile of an anaerobic pond.
FIGURE25 ANAEROBIC WASTE POND PROFILE (PEPPER, GERBA, & RUSSEAU, 2006)
3.3.6
Facultative ponds
They are normally used for the treatment of domestic waste and they have a dentention time of 5-30 days. These type of ponds range in depth from 1- 1.25 m and are is made up of three sections: an upper aerated zone, a middle facultative zone, and a lower anaerobic zone as shown on fig . The make use of both aerobic and anaerobic treatment.
41
FIGURE26 MICROBIOLOGY OF FACULTATIVE POND (PEPPER, GERBA, & RUSSEAU, 2006)
3.3.7
Aerated lagoons or ponds
These are usually about 1-2 m deep with a detention time of less than 10 days. The removal efficiency depends on the aeration time and temperature as well as the source of the wastewater. Limitations of oxidation ponds Although oxidation ponds are cheap and require minimum technology, they have several draw backs. Biodegradable organic matter and turbidity are not as effectively removed when compare to the activated sludge process. Also, they have a potential for short circuiting and detectable levels of pathogens can be found in their effluents. This method is used to remove organic compounds from wastewater. It is most suitable for wastewater that contains a relatively constant source of biochemical oxygen demand (BOD) and very low concentrations of toxic metals. A surge tank to equalize wastewater flow and concentration variations can help the treatment system work effectively. This is commonly used to treat domestic sewage.
42
CASE STUDY 1:
43
4
INTRODUCTION
The textile industry is a major industry involved in the manufacture of textiles as the final product. Textiles refer to the finished products of a production process involving the conversion of fibres to fabrics and the material fabricated into clothes or other artifacts. The production process requires the use of several chemicals and a large volume of water at various stages of the production. Textile production is simply based on the conversion of three types of fibres into yarn, yarn into fabric and then fabric into textiles. Fibres can be classified into two major groups, namely natural and artificial fibres (man-made fibres). 4.1
Fibers Categorization
Natural
Synthetic (Artificial)
Vegetable fibres (Plant origin)
Man-made fibres (Artificial)
• • •
Flax Hemp Jute
Nylon Polyester Polyamides
Protein fibres (Animal origin) • • •
Wool Silk Angora
The production stages in a typical textile company includes: fibre production, fibre processing, spinning, yarn production, fabric production and finishing. Due to the nature and applicable technology of the production process, a high amount of water is consumed in manufacturing of textiles that is consequently generating a considerable amount of wastewater (Nemerow, 1978). Textile industries are a major source of effluent in the environment. (Ghoveishi and Haghighi,2003).The major pollutant in textile wastewater are high suspended solid, chemical oxygen demand, heat, colour, acidity and other soluble substances (Venceslau et al. 1999, World Bank, 2007). Most of these pollutants are produced from the finishing section. The impacts resulting from textile industry on the environment have been recognisable for some time both in terms of discharge of pollutants and the consumption of water and energy (Lacasses and Baumann, 2006). Some significant impacts the textile industry has on the 44
environment have been identified as primary water consumption (80 – 100m3/ton of finished textile) and wastewater discharge (115 – 175Kg of COD/ton of finished textile) a large range of organic chemicals, low biogradability, color and salinity. These pollutants resulting from the production process differ greatly in composition due to several factors (Bisschop and spanjer, 2003). Cotton is one of the mostly used fibres in textile manufacturing. It is an important natural fibre which posses unique characteristics. The production process of textile from cotton involves the following processing steps:
FIGURE27 SCHEMATIC DIAGRAM OF DIFFERENT PROCESSING SECTORS IN TEXTILE INDUSTRY (RAMESH BABU, 2007)
45
4.1.1
Cultivating and harvesting
This refers to the preliminary production process carried out to attain the raw material. Cotton plants are cultivated and harvested upon maturity. They are usually grown anywhere in long, hot dry summers with plenty of sunshine and low humidity. Indian cotton, gossypium arboreum is finer but the staple is only suitable for hand processing. American cotton, gossypium hirsutum produces the longer staple needed for machine production. The cotton bolls are harvested by stripper harvesters and spindle pickers that remove the entire boll from the plant. The cotton boll is the seed pods of the cotton plant, attached to each of the thousands of seeds are fibres about 2.5 cm long. 4.1.2 •
Preparatory Processes Opening and cleaning: The stage ensures the cleaning of the cotton bolls. A cotton opener and picker are employed in the stage of preparation.
•
Ginning: This refers to the process whereby the cotton seeds are separated from the fibre and other contaminants such as leaves, in a Gin.
•
Carding: the fibres are separated and then assembled into a loose strand (sliver or tow) at the conclusion of this stage.
•
Combing: this is used to remove the shorter fibres, creating a stronger yarn.
•
Drawing: the fibres are straightened and it sliver form the combing process processed into rovings
4.1.3 •
Spinning- Yarn manufacture
Spinning: This is carried out in a spinning machine; the rovings are thinned, twisted and wound onto the bobbin in preparation for fabric manufacture.
•
Checking: This is the process where each of the bobbins is rewound to give a tighter bobbin.
•
Folding and twisting Plying is done by pulling yarn from two or more bobbins and twisting it together, in the opposite direction that that in which it was spun. Depending on the weight desired, the cotton may or may not be plied, and the number of strands twisted together varies. 46
•
Gassing Gassing is the process of passing yarn, as distinct from fabric very rapidly through a series of Bunsen gas flames in a gassing frame, in order to burn off the projecting fibres and make the thread round and smooth and also brighter.
4.1.4
Weaving- Fabric manufacture
The weaving process uses a loom. The length-way threads are known as the warp, and the cross way threads are known as the weft. The warp which must be strong needs to be presented to loom on a warp beam. The weft, passes across the loom in a shuttle that carries the yarn on a pirn. These pirns are automatically changed by the loom. Thus, the yarn needs to be wrapped onto a beam and onto pirns before weaving can commence. •
Winding After being spun and plied, the cotton thread is taken to a warping room where the winding machine takes the required length of yarn and winds it onto warper’s bobbins •
Sizing This is a process whereby starch is added to the wrap to strengthening it.
•
Drawing in or Looming The process of drawing each end of the warp separately through the dents of the reed and the eyes of the healds.
•
Pirning (Processing the weft) Pirn winding frame was used to transfer the weft from cheeses of yarn onto the pirns that would fit into the shuttle
4.1.5
Finishing- Processing of Textiles
47
The grey cloth, woven cotton fabric in its loom-state, not only contains impurities, including warp size, but requires further treatment in order to develop its full textile potential. Furthermore, it may receive considerable added value by applying one or more finishing processes. •
Desizing Depending on the size that has been used, the cloth may be steeped in a dilute acid and then rinsed, or enzymes may be used to break down the size.
•
Scouring Scouring, is a chemical washing process carried out on cotton fabric to remove natural wax and non-fibrous impurities (eg the remains of seed fragments) from the fibres and any added soiling or dirt.
•
Bleaching Bleaching improves whiteness by removing natural coloration and remaining trace impurities from the cotton; the degree of bleaching necessary is determined by the required whiteness and absorbency.
•
Mercerising This is a further treatment process to improve the quality of the fabric. The fabric is treated with caustic soda solution to cause swelling of the fibres. This results in improved lustre, strength and dye affinity. Singeing Singeing is designed to burn off the surface fibres from the fabric to produce smoothness. The fabric passes over brushes to raise the fibres, and then passes over a plate heated by gas flames.
•
Raising
48
During raising, the fabric surface is treated with sharp teeth to lift the surface fibres, thereby imparting hairiness, softness and warmth, as in flannelette. •
Calendering Calendering is the third important mechanical process, in which the fabric is passed between heated rollers to generate smooth, polished or embossed effects depending on roller surface properties and relative speeds.
•
Shrinking (Sanforizing) Finally, mechanical shrinking (sometimes referred to as sanforizing), whereby the fabric is forced to shrink width and/or lengthwise, creates a fabric in which any residual tendency to shrink after subsequent laundering is minimal.
•
Dyeing Finally, cotton is an absorbent fibre which responds readily to colouration processes. Dyeing, for instance, is commonly carried out with an anionic direct dye by completely immersing the fabric (or yarn) in an aqueous dyebath according to a prescribed procedure. For improved fastness to washing, rubbing and light, other dyes such as vats and reactives are commonly used. These require more complex chemistry during processing and are thus more expensive to apply.
•
Printing Printing, on the other hand, is the application of colour in the form of a paste or ink to the surface of a fabric, in a predetermined pattern. It may be considered as localised dyeing. Printing designs on to already dyed fabric is also possible.
4.2
Textile Industry Chemicals
49
Process
Basic
chemicals
structure
chemicals Cationic Anionic Washing
detergent
Nonionic
Acid Dye
Basic Dye
Direct Dye
Dying & Printing
Dye Vat Dye
Mordant Dye
Reactive Dye
50
Disperse
Azo Dye
Sulphur Dye Dye
Pigments
Dying &
Silicon
printing
antifoam Anti foam
Non-silicon antifoam
Natural
Starch, Alginate, seeds such as acacia
Thickener
CMC, PVA Synthetic Emulsion
Oil in water (O/W) CH2=CHCOO−
Acrylate based Butadiene acetate Binders
Vinyl acetate
Fixing
Melamine-
Agent
formaldehyde
51
Sodium hypochlorite
NaClO3
Calcium Bleaching
hypochlorite
Agent
Hydrogen Peroxide
Ca(ClO2)2 H2O2
Softening
non‐ionic surfactants
fatty acids, fatty esters and fatty amides
cationic surfactants
quaternary ammonium compounds, amido amines, imidazolines
Agent
paraffin and polyethylene waxes oregano-modified silicones Wax-based repellents
zirconium- & aluminium-based salts
Resin-based repellents
condensed fatty compounds (amines,
Hydrophobic / Oleophobic Agents
alcohols or acids) Silicone repellents
polysiloxane-active substances
Fluorochemical repellents
Fluoroalkyl- acrylates /methacrylates copolymers
Finishing
Flame
Inorganic FR agents
Zirconium , Aluminium and Titanium salts
Retardants
Halogenated FR agents
Cl/ Br compounds
Phosphor-organic FR
PO* radicals
agents 1.1.22 1.1.23 Antis
quaternary ammonium compounds
tatic
phosphoric acid ester
derivatives
Agen ts Sizing
Natural
Starch, cellulosic derivatives (CMC), glue,
Agents
gelatine, albumen Synthetic
Poly acrelytes, poly vinyl alcohol (PVA), Styrene/Maleic acid copolymers
Desizing
Enzymes
Amylases
Agents
1.1.24 Oxidative
Sodium per sulphate, sodium bromite
compounds Acidic Agents
Sulphuric/Hydrochloric acids
52
4.3 The origin of textile effluents Based on both magnitude and complexity of the effluents composition, discharged from textile industry, this sector is considered as one of the most polluting industries. Bleaching, washing (or scouring) and those wet procedures undertaken within dyeing and finishing sectors are at most responsible for the disposed effluents. Huge variety of dyes and chemicals disposed in textile wastewater make it hard to be treated in conventional WWTPs. The nature of textile wastewater is studied based on chemicals consumed and in terms of some general parameters such as TS, TSS, BOD, COD, heavy metals, Phosphor and Nitrogen contents. The main troublesome pollutants in textile waste can be categorized to dyes, persistent organics, absorbable organic halogens (AOX), toxicants and surfactants (Vandevivere et al., 1998).
4.3.1
Colour
Chromagen is the central point of every dye that adheres to the fibre and absorbs the visible light. There are about twelve different types of chromagens which are mainly (60-70%) azo type and anthraquinone type. Dyes are basically resistant toward degradation; therefore, majority of them are not biodegraded in aerobic activated sledges. Azo dyes stability under aerated conditions strongly depends on the complexity of their chemical structure. Azo dyes are readily reduced to amines that are amongst most carcinogenic chemical compounds. Decolourization of reactive dyes is highly concerned because of three main reasons. First of all reactive dyes have dominated the market by having about 20-30% of the total share, because they are used for dyeing and printing cotton fibres. Secondly about 30% of reactive dyes used is hydrolyzed and discharged into wastewater. At last the conventional wastewater treatment plants that are mainly functioning based on aerobic degradation and sorption are vulnerable in treating reactive and other anionic soluble dyes.
4.3.2
Persistent Organics
The persistent compounds exist in textile effluents are produced from different types of chemicals; mainly include dyes, dyeing auxiliaries such as deflocculating agents (naphtalenesulfunates or lignins), sequestering agents, phosphonates and polyacrylates, antistatic agents for manmade fibres, preservatives (substituted phenol), fixing agents applied in direct dyeing of cotton, carriers used in disperse dyeing of polyester and great amounts of finishing chemicals applied for water-, moth- and fire- proofing. Although just small portions of these chemicals are used in each stage, their great persistency against degradation makes their treatment highly challenging. 53
4.3.3
AOX and heavy metals
Sodium hypochlorite has some priorities to H2O2 as a bleaching agent. It results in reasonably better whiteness, lower cost and less structural damages caused by H radicals. However all types of hypochlorite bleaching agents produce high amounts of absorbable organic halogens (AOX)(in this case chloroform) that are highly toxic and carcinogenic. Also chemicals used for moth proofing and shrink proofing of wools that contain chlorine in their structure effectively contribute to AOX formation. There are also some reactive dyes that are basically AOX. Heavy metals present in textile wastewater causing another hardship in treating these waste. Cr, Mg, Na, Zn, Cu, Ni and Ca are the most famous heavy metals that mainly exist in metalcomplex dyes and some chemical auxiliaries.
4.3.4
Toxicants
The heterotrophic activities are slowed down very slightly by textile wastewater in the activated sludge; while the functioning of chemoautotrophic nitrifying bacteria is inhibited significantly by these types of effluents. Therefore, inorganic compounds are hardly biodegraded and oxidized and so several types of them remain as hazardous toxic matters in the wastewater.
Moreover, great amounts of azo dyes, which can be easily reduced to aromatic amines, metallic compounds remained from metal complex dyes and numerous finishing agents such as cross linking, water and flame retardants and softeners that constitute aromatic compounds in their chemical structures can be accounted as persistent toxic waste in the final textile effluent.
4.3.5
Surfactants
Majority of textile wet processes such as washing and scouring, weaving, spinning, sizing, desizing, printing, dyeing and most of the chemical finishing procedures consume great amounts of surfactants. Alkyl phenol ethoxylates are the main non-ionic surfactants highly used in different textile processes. These surfactants are biodegraded to alkyl phenols and readily adsorbed to the sewage sludge, accumulate there and increase the regional sludge 54
concentration. The concentrations of up to 1000 ppm have been reported. Alkyl phenols are much more hazardous and toxic than the ethoxylated compounds.
4.3.6
Temperature
The temperature of the wastewater obtained from textile wet processes is unusually higher than what is disposed from other industries. Rinse waters used in dyeing and printing sectors has temperatures of up to 90°c and result in high temperature wastewater of about 40°c. Therefore a prior heat dissipation system is always necessary to reduce the effluent’s temperature to 30°c or less before transferring the waste to the treatment cycle. This step can significantly enhance the treatment efficiency. The final textile wastewaters can be generally classified into three categories based on their colour intensity and their COD content (Lin & Peng, 1993). The high strength wastewater has a dark colour with very low transparency and COD concentration of more than 1600 mg/l. The medium strength wastewater also has dark colour but with higher transparency and COD concentration of between 800 and 1600 mg/l; while the COD concentration of the low strength, light colour wastewater is mainly less than 800 mg/l.
TABLE 5
Characteristics of typical textile wastewater
55
Process
Wastewater
Residual wastes
Fibre Preparation
little or none
Hard waste, packing waste, fibre waste
Yarn Spinning
little or none
Sized yarn, packaging waste, cleaning and processing waste, fibre waste
Slashing/Sizing
Size, metals, cleaning waste,
Un‐used starch based sizes,
BOD, COD
packaging waste, fibre lint, yarn waste
Weaving
little or none
Used‐oil, off‐spec fabric, yarn and fabric scraps, packaging waste
Knitting
little or none
Yarn and fibre scraps, packaging waste, off‐spec fabric
Tufting
little or none
Yarn and fibre scraps, packaging waste, off‐spec fabric
Desizing
BOD from lubricants, synthetic
Cleaning materials such as
size, water‐soluble sizes, anti
filters, rags, wipes; yarn waste,
static compounds and biocides
fibre lint, packaging waste, maintenance and cleaning wastes containing solvents
Scouring
Insecticide and disinfectants,
NaOH, pectin, oil, fats, wax, detergent, knitting lubricants,
Little or none residual waste
spin finishes, spent solvents Bleaching
Sodium silicate or organic
stabilizer, Hydrogen Peroxide, high PH
Little or none residual waste
Singeing
little or none
Little or none residual waste
Mercerizing
NaOH, High PH
Little or none residual waste
Heat setting
little or none
Little or none residual waste
Dyeing
Surfactants, Salts, Metals,
organic processing assistance, toxics, Sulphide, BOD, spent
Little or none residual waste
solvents, acidity/alkalinity. Printing
Urea, metals, colours, foam,
BOD, heat, solvents, suspended solids
Little or none residual waste
Finishing (cross‐linking, water
BOD, COD, spent solvents,
Packaging waste, fabric scraps
proofing, flame retardant
toxics, suspended solids, Urea
and trimmings
56
processing) Product Fabrication
Class
little or none
BOD
COD
(mg/l)
(mg/l)
High Strength
500
1500
Medium
270
100
PH
Fibre scraps
SS
Temperature
Oil (mg/l)
Conductivity
(mg/l)
(˚C)
10
250
28
50
2900
970
9
137
28
21
2500
460
10
91
31
10
2100
(μS cm-1)
Strength Low Strength
4.4
WASTE DISPOSED FROM EACH SECTION
List of the waste materials disposed from each sector in textile industry (Ramesh Babu, et al. 2007), (Yussuf, 2004)
4.5
Treatment Methods
57
The treatment methods applicable to the textile industry can be divided into three main categories which are Primary, Secondary and Tertiary treatment methods. These treatment methods are classified based on their simplicity and application in the industry
4.5.1
Primary treatments
4.5.1.1 Screening Screening is an important but simple primary treatment that is applied for removing Coarse suspended substances such as rib and rag parts, lints, yarns, fibres and pieces of fabric(Das, 2005). Mechanically cleaned fine screens and bar screens eliminate most of the fibres. Preceding to the secondary treatments such as susceptible biological and oxidation processes these suspended matters should be completely removed from the wastewater. Clog trickle filters, carbon beads and seals are mainly used in this system.
FIGURE28 MECHANICAL WASTEWATER SCREENING (HH AG, 2005)
4.5.1.2 Sedimentation The main goal of primary treatment (clarification or sedimentation) is removing floatable solids and settling organics (Das, 2005). Sedimentation is considered as an efficient and economic alternative to remove suspended material in textile effluent. Sedimentation and clarification sectors are mostly capable of removing 25-35% BOD, 40-60% TSS and 90-95% settling solids. Removing these great amounts of floatable, suspended and settling matters in the primary treatment stages reduces the organic loading of wastewater transferred to other treatment steps. 58
In this step the velocity of the wastewater is reduced by 1-2 ft/min rate, sedimentation and floatation processes take place, and enhanced by slowing the wastewater flow (Ramesh Babu, 2008). In the sedimentation tanks floatable foam and grease and the settled sludge material are collected respectively and then pumped to the disposal or transferred for further treatments. Rectangular and centre-feed clarifiers are the most common types of equipments used in this sector. In rectangular clarifiers the effluent flows horizontally from one side to the other side and finally with help of a single bottom scrapper on a channel bridge or sets of flights placed on parallel chains the settled sludge is transferred to a hopper. In the centre-feed clarifiers the effluent is fed from the centre and flows outward. The sludge is collected from a hopper in the middle of the tank bottom. In both types of clarifiers it is a surface skimmer that is responsible for removing floatable matters (mainly oil and grease).
4.5.1.3 Equalization Wastewater streams are gathered in a sump tanker. The rotating agitators and air compressors are responsible for stirring the mixed effluents (Eswaramoorthi, 2009). In the case of using compressed air flow the air is blown in high velocity from below of the sump tanker. The conical bottom of the tanker enhances the efficiency of removing suspended solids which are tiny fibres and accumulated colours, printing pastes and metallic compounds.
4.5.1.4 Neutralisation
This is a process which entails the removal of excess acidity or alkalinity from the wastewater by the treatment with a chemical of the opposite composition. The adjustment of the pH value is the main criterion in the neutralisation process. Acidic wastewater i.e in the region of 0-6.9 can be neutralised with chemicals such as sodium hydroxide (NaOH), Sodium bi-carbonate etc. A detailed table providing alternative chemicals suitable for neutralisation can be found in Wastewater Engineering treatment and Reuse 4th edition, chapter 6) In the textile industry, some of the chemicals used during production which may influence the pH value of the wastewater are: Cationic Surfactant/Blend for Dyed Cellulosic fibres, Polyester Acrylic and Blended fabrics these chemicals impart excellent soft feel on the finished fabric. Anionic Surfactant /Strong Detergent the chemical acts as an emulsifier /scouring and wetting Agent. Poly Vinyl Acetate Emulsion imparts handle stiffness with hard 59
feel on the finished fabric. A detailed source of textile chemicals their composition and applications
can
be
found
in
Textile
processing
chemicals
available
at
http://www.indiamart.com/atulitchemical/chemical.html
4.5.1.5 Mechanical Flocculation & Chemical Coagulation Flocculation refers to the transport step which brings about the cohesion of destabilised particles needed to form larger particles known as ‘flocs’. These are readily removed by settling or filtration. The purpose of wastewater flocculation is the formation of flocs or aggregates from finely divided particles which cannot be removed by simple sedimentation. In the textile industry, the textile wastewater is passed through a tank under gentle stirring addition of a chemical coagulant might be necessary to aid the floc formation. The resultant effluent is usually a clear and free from colloidal particles such as sizing agents; suspended particles etc. 80-90% removal of Total suspended solids can be achieved via this process. 4070% BOD removal can be achieved over a period of five days while COD removal can be up to 60%. Das S. (Textile Effluent Treatment – A Solution to the Environmental Pollution)
4.5.2
Secondary treatments
Secondary treatments are used in succession to the primary treatment methods in the textile industry. These methods are basically suitable for the removal of organics, BOD, COD, which have not the removed by the primary methods
4.5.2.1 Aerated Lagoons Lagoons are relatively shallow earthen basins with varying dept in the range of 2 – 5m. Mechanical aerator provides oxygen for the biological treatment of the wastewater and to keep the biological solids in suspension. These aerators are either floats or fixed platforms. Aerated lagoons are operated on a flow-through basis or with solid recycles and three principle types can be identified based on the manner in which the solids are handled. These are: 1. Facultative partially mixed 2. Aerobic flow through with partial mixing 3. Aerobic with sold recycle and nominal complete mixing The differences in the manner in which the solids are handled affect the treatment efficiency, 60
power requirement, hydraulic solid retention time, sludge disposal and environmental consideration. An in-depth description of the behaviour of each of these lagoons can be accessed from Wastewater Engineering 4th edition chapter 8 table 8-29. In the textile industry, the effluent from the primary treatment methods are collected in these earthen basins and treated for 2-6 days. The mechanical aerators ensure the oxidation of the organics. Up to 99% of the BOD present in the wastewater can be removed via this process. Aerobic lagoon with solid recycle essentially is the same as extended aeration activatedsludge process but in an earthen basin.
Source: http://wpcontent.answers.com/wikipedia/commons/1/1d/Surface-Aerated_Basin.png
FIGURE29 The process of design of lagoon basically consider these factors a. BOD removal b. Effluent characteristics c. Temperature effects d. Oxygen requirement e. Energy requirements for mixing f. Solid separation In depth information of the aforementioned can also be found in Wastewater engineering 4th edition.
4.5.2.2 Trickling filtration Trickling filters typically have a rectangular or circular bed, with 1 to 3 meter depth, that is filled of a well-graded media such as Gravels, Clinkers, Synthetic resins, Coal, PVC or broken stone of sizes between 40 and 150 mm (Babu, 2008 and Das, 2005). The textile 61
wastewater is sprayed uniformly over the solid medium from a slowly rotating distributor equipped by nozzles or orifices (such as rotary sprinkler). As the wastewater seeps through the entire bed the growth of microorganisms (Larvae, worms or helminthes, algae, fungi, bacteria and protozoa) accelerates. The organic compounds inside the wastewater are consumed as the main nutrients for microorganisms and in the mean time oxygen is flowed in a counter-current direction to that of wastewater flow to provide an aerated condition at the outer side of the filter. All these together produce a gelatinous layer of aerobic microorganisms and bacteria called “Zooglea” on the medium surface. By increasing the amounts of nutrients and oxygen supplied, the thickness of the film increases and more organics, nitrates, sulphates, carbon dioxide and other stable by-products are produced. Then these materials coagulate at the feed side of the filter and subsequently removed from that region. The following relationship simply shows the trickling procedure; Organisms + Organics → Solid Wastes + CO + More Organisms When the slime layer becomes extremely thick it blocks the wastewater flow through the solid medium; therefore, it is cleaned up in the final settling tank. In some cases the filtered effluent is circulated back to the main stream, once or twice, in order to enhance the overall efficiency of the filtration process. Trickling filtration is a cost effective method that requires low energy supply, forms small portions of sludge and is suitable for treating biodegradable matters inside the effluents (Ahmed, 2006). Moreover this technique has shown great results in removing ammonia from the wastewater. The hybrid systems of trickling filtration and chemical methods have shown advantageous features in wastewater treatment.
62
FIGURE30 COMPACT CHEMICALLY ENHANCED-TRICKLING FILTER SYSTEM (AHMED, 2006)
4.5.2.3 Activated sludge Biological treatments are all basically natural self purifying processes which are enforced by some artificial bio compounds injected into the system (Ramesh Babu, 2007). Gaining the original quality of the present aquatic environment is the ideal objective of these types of treatments. Activated sludge process is one of the most flexible biological oxidation techniques mainly used for removing coarse solid organic compounds, colloids and dissolved solids from the textile wastewater (Das, 2005). The elimination rate of oxidizable material processed in this method is up to 90% (Babu B. V., 2008). In this process the effluent is aerated by some aerobic microbial flocs that is suspended in a reaction tank. Subsequently the waste is biodegraded to water and carbon dioxide molecules. The fast growing suspended microbial floc is called Activated Sludge. The effluent and the activated sludge are separated from each other by settling their mixture in the reaction tank. A portion of the sludge is reused and injected to the tank to enhance the microbial reaction. The remaining sludge, in addition to what is achieved from the primary sedimentation is digested in a sludge digester. The main problem is that most of the dyes and chemicals used in the textile industry are not highly biodegradable and using the activated sludge individually will not effectively reduce the contamination of the wastewater. Therefore, this method is generally applied in combination with other techniques or by adding adsorbents such as activated carbon or bentonite clay to remove the toxic-organic matters and non-biodegradable components from the textile effluent (Ramesh Babu, 2007). Subsequent oxidative chemical treatments or reaction with organic flocculants are options can be applied in frequently after the biological treatment. Biological aerated filtration (BAF) is a biological treatment that has been developed recently and takes advantage of aeration of a stationary organism as a medium.
63
FIGURE31 ACTIVATED SLUDGE (BABU B.V., 2008)
4.5.2.4 Oxidation ditch A similar method of pre-treatment used in the textile industry in oxidative ditch method. Oxidation ditch is an aerobic process similar to the activated sludge process. However, an oxidation ditch is ring-shaped and is equipped with mechanical aeration devices. It could be classified under biological treatment method and particularly suitable for treating BOD, Alkalinity, TSS polluted wastewater.
http://www.gec.jp/JSIM_DATA/WATER/WATER_2/img/Fig_231-1.jpg
FIGURE32
4.5.2.5 Anaerobic digestion
64
In this step the sludge, which has been formed in the primary sedimentation process, and the waste obtained from the humus tank are both digested and fermented slowly in reaction with anaerobic bacteria in a sludge digester (Das, 2005). The sludge is maintained for 30 days in this tank at pH of 7-8 and temperature of 35 ˚C. CO2, CH4 and small portions of NH3 are the main final products of this process. The textile sludge can be effectively treated by anaerobic digestion process (Asia et al., 2006). Considerable reductions in nitrates, phosphates, COD, BOD and suspended solids have been observed via processing the textile wastewater in this system. Gaining bio-fertilizer and biogas as the final products makes this method more advantageous. Moreover, this method has a relatively low operation costs in comparison with other secondary treatments. There are many bacteria that are capable of decolourizing azo dyes under anaerobic conditions (Georgiou, 2006). In the first stage the highly electrophilic azo bonds are broken via bacterial reactions, the azo dye is decolorized and the aromatic amines are formed. Basically the uncharged azo dyes in anoxic sediment environments tend to reduced to their corresponding amine; Amines which are extremely carcinogenic, toxic and mutagenic. Despite easy reduction of azo dyes under laboratory conditions, the complete molecular mineralisation of these dyes is hard. The main disadvantage of the conventional anaerobic biological techniques is the long hydraulic residence time of sludge in the tank. This weakness leads to provide high volume reactors due to long generation time of anaerobic bacteria (Georgiou, 2003). Therefore some systems and methods are coupled to this method that prevent biomass from accumulation and subsequently reduce the hydraulic residence time.
4.5.2.6 Oxidation techniques Oxidation treatment methods are applicable to textile wastewater for the removal of colours, 65
BOD, COD etc. However, conventional oxidation techniques have been found quite inefficient in the removal of colours which are mainly produced from insoluble dyes and complex organic structure at low concentration. Therefore, advanced oxidation processes have been applied to the treatment of wastewater in the textile industry. These processes generate hydroxyl free radical by different techniques such as combination of ozone (O3) Hydroxyl peroxide (H2O2) and Ultra-violet light The goal is to furnish hydroxyl ions to destroy colours, complex organic pollutants and compounds which cannot be destroy by conventional oxidation methods. A list of possible route of producing hydroxyl radicals are shown in table 3. The Generation of hydroxyl radicals is quickened by combining O3, H2O2, TiO2, UV radiation, electron-beam irradiation and ultrasound. The most promising are O3/H2O2, O3/UV and H2O2/UV which hold efficient routes to oxidize textile wastewater. TABLE 6
Source: Al-kdas A. et al. (2005)Treatment of textile wastewater by Advanced Oxidation Processes
4.5.2.7 Electrolytic precipitation The use of electrolytic precipitation in the treatment of textile wastewater involves the application of electric current to the wastewater in a cell. Electro-precipitation corresponds to the use of an electrochemical reactor with membrane. This facilitates the removal of heavy metals from the wastewater which are usually from dyestuffs. In the cell, the polluted wastewater is maintained at the cathode side. When the system is started up the pH in anodic part decreases by oxidation reaction of water to oxygen gas. On the other hand, in cathodic part, hydrogen is released by reduction reaction of water. The pH in this part is slowly increased until it reaches the precipitation pH of metal contained in the solution leading to the removal of heavy metal from wastewater.
66
Source: http://www.chemistryexplained.com/images/chfa_02_img0277.jpg
FIGURE33 4.5.2.8 Membrane Process Membrane separation, as an in-plant process, is a feasible method due to high water costs and the importance of water profligate re-usage( Ramesh Babu, 2007). Membranes offer a great way of reducing dyeing auxiliaries and hydrolysed dyes. Moreover, they play an essential role in the decolouration of the effluent and descending BOD and COD levels of waste water. Reverse osmosis, Nanofiltration, Ultrafiltration and Microfiltration are the main membrane methods which are used in the textile industry. Qualitative characteristics of the final product define the specific membrane method that should be used. 4.5.2.8.1 Reverse Osmosis Reverse osmosis membranes typically have a retention rate of 90% or higher for ionic compounds and enable superior permeate quality (Ghayeni et al., 1998). Reverse osmosis facilitates the removal of chemical auxiliaries, hydrolyzed reactive dyes and mineral salts. The waste from the dyeing sector can be decolourized via a single pass reverse osmosis. The concentration of dissolved salt is highly important in this method and if it is increased the osmotic pressure role becomes more significant.
4.5.2.8.2 Nanofiltration Nanofiltration membranes retain dyeing auxiliaries, hydrolyzede reactive dyes, large monovalent ions, devalentions and low molecular weight organic compounds (Ramesh Babu, 2007). The combination of a preceding adsorption process joined to a nanofiltration system can be used effectively for treatment of coloured effluents. By decreasing the concentration 67
polarization of the waste in the adsorption process the quality of the final product, after passing through the nanofilter, becomes much higher. 4.5.2.8.3 Ultra filtration Ultra filtration is a good method for eliminating macromolecules and particles but it is a weak technique in terms of removing polluting components such as dyes (only between 32% and 75%) (Ramesh Babu, 2007). The water treated by ultra filtration is rarely re-used as feed water in the textile industry and especially not in those sensitive processes such as dyeing. Ultra filtration can be used for enhancing a biological reactor performance or as a pretreatment, carried out before the reverse osmosis section. 4.5.2.8.4 Microfiltration Microfiltration can be applied effectively for treating effluents containing pigment dyes and the waste of the subsequent rinsing baths (Ghayeni et al., 1998). This method can be considered as a suitable pre-treatment for micro and nanofiltration.
4.5.2.9 Electrochemical method Inorganic salts and elevated levels of toxic colorants, present in textile effluent, are considered as the main threads for the ecosystem (Sundaram, Kupferle), (Esteva, Silva, 2004) Electrochemical oxidation technique, which has been developed in the 90’s, is a relatively effective method in decolourization of the wastewater. No sludge formation, low or no consumption of chemicals in this method and removal of some specific pollutants such as polyaromatic organic compounds like anthraquinones all together make this method much more advantageous to other traditional physical or chemical treatments. This method is considered as an advanced oxidation technique. In both, Direct and Indirect Electrochemical methods, the chemical structure of dyes and chemicals and the residence time of processing the wastewater are amongst the main influential factors on the treatment efficiency (Sanroman, et al., 2004). The effects of some factors, such as the effluent conductivity, PH, addition of polyelectrolytes, and Power requirement on the efficiency of electrochemical method have been investigated (Lin & Peng, 1993). Based on these studies most the effluents obtained from the finishing mill and dyeing sectors have the conductivities within the acceptable range for electrochemical procedure and therefore no extra conductivity adjustment is required. The 68
common PH of 5 to 10, mainly exists in the textile wastewater, also doesn’t show any negative influence on the process efficiency. The addition of 40 mg/l of polyaluminium chloride (PAC) and providing power density of about 92.5 amp/m2 can result in higher efficiency and a greater COD removal. Therefore, it can be concluded that Electrochemical technique is an effective method for treating textile wastewater. However the main problem of this method is the cost of electricity. The Indirect electrochemical oxidation has been observed as an efficient technique to complete colour and COD removal. In indirect Electrochemical method NaCl molecules, present in textile wastewater, are used in order to form active chlorine based oxidants to decolorize highly coloured azo-compounds at the anode (Sundaram, Kupferle).
4.5.2.9.1 Ion-Exchange The main objective of this process is to clean the wastewater from the undesirable cations and anions present in waste (Das, 2005). In this sector textile effluent is passed through the beds which have already enriched by ion-exchange resins. These resins are responsible for absorbing anions and cations inside the waste and exchange them by hydrogen and sodium ions separated from the resins structures. The main Ion exchange resins in use today are those synthetic polymeric materials which contain ionic groups of quaternary ammonium, sulphonyl groups and etc. Ion-exchange treatment can be used for lowering COD, Reducing the concentration of metallic ions such as Fe, reducing Alkalinity, conductivity, total hardness SS and TSS of the textile effluent (Lin, Chen, 1996).
4.5.2.9.2 Photo catalytic Photo catalytic treatment has been applied to textile wastewater polluted with colours, dyestuffs and complex compounds. This is an advanced method to decolourize a broad range of dyes and colour compounds comprising complex structures. In this process, photoactive catalyst illuminates with UV light, generates highly reactive radical, which facilitates the decomposition of pollutants. Titanium dioxide and Zinc oxide are particularly utilised as 69
photo catalysts. The work of Attia A.J. et al (2007) practically investigated the use of TiO2 and ZnO in the textile industry wastewater. Treatment followed screening and pre-treatment to removed TSS and other solids. After which TiO2 and ZnO suspended in the polluted textile wastewater are placed in a photoreaction cell.
. (A) gas container, (B) gas flow meter, (C) circulating water thermostat (D) magnetic stirrer (E) quartz cell, (G) lenses, (H) low pressure mercury lamp, (I) power supply unit. (Attita, et al., 2007)
FIGURE34 SCHEMATIC DIAGRAM OF THE EXPERIMENTAL APPARATUS FOR PHOTOCATALYTIC REACTION
4.5.3.0 Adsorption The use of adsorption treatment method for textile wastewater entails usually the use of granular activated carbon for the exchange of pollutants between the two immiscible phases. Owen’s work (1978) shows that adsorption is an economical and feasible route for eliminating colour from textile wastewater. Also, recent studies have investigated the use of powdered activated carbon in the removal of cationic and anionic dyestuffs such as methylene blue, basic yellow etc from textile wastewater under different experimental conditions
70
FIGU URE35 ADS SORPTION N COLUMN N Source: http://www.frreepatentsonlinne.com/662032 26-0-large.jpg
vaporation 4.5.3.1 Thermal ev The treattment of texttile wastewaater has beenn equally doone with therrmal evaporrators’ sodiuum per sulphhate has beenn reported too be more eco-friendly in the treatm ment processs. The proceess entails thhe heating of o wastewatter in a vaccuum chamb ber by the application of electriciity through a heating fiilament. Thee evaporatedd componennt condensess on the subbstrate and is selectivelly extracted through the vacuum sysstem.
RE36 SCHE EMATICS O OF A THER RMAL EVA APORATOR FIGUR So ource: http://ww ww.icmm.csicc.es/fis/g/evapooracion_termica_in.gif
4.6
E Example 1 -
WASTEWAT W TER CHA ARACTERISTICS IN N TEXTIL LE
FINISHIING MILLS S
The work k of Savin and Butnaru u (2008) shhows the ch haracteristicss of the wasstewater from different sections of a textile finnishing mill and their correspondinng loading cooncentrationns. This workk is thereforre presented in this reporrt as a case study. o poollutants froom a textile factory andd the possibble This casee study illusstrates the obtainable 71
degree of concentrations. The results of the wastewater samples presented in this case study were collected over a period of two months while sampling and analysis were done to obtain a daily average. The daily averages were subsequently subjected to statistically analysis and a general average presented.
FIGURE37 SCHEMATIC DIAGRAM OF THE TEXTILE FINISHING MILL SHOWING DIFFERENT SECTIONS Source: Savin and Butnaru /Environmental Engineering and Management Journal 7 (2008) (1. Burning Sector; 2. CH Station; 3. Bleaching Station; 4. Mercerization Section; 5. Thermo fixing Section, 6. Chemicals Warehouse, 7. Dyestuff Warehouse; 8. Dyeing Sector; 9. Dyeing Gauge; 10. Printing Sector; 11. Printing Warehouse; 12. Dressing Sector, 13. Wastewater Treatment Plant)
TABLE 7
72
Source: Savin and Butnaru /Environmental Engineering and Management Journal 7 (2008)
The tables above show the concentrations of ten pollutants from different sections of the mill. Fixed residue and COD predominately have the highest values. Tables 7, 8, 9, 10, 11 and 12 shows that fixed residues and COD had values of 3840.4 and 2629.3 mg/L, 6590.9 and 2688.5 mg/L, 3877.2 and 2788.2mg/L, 2016 and 1907.8mg/L, 2178.3 and 3606.3mg/L, 358.6 and 73
1097mg/L respectively. This clearly shows that fixed residue was the most obtained pollutant from most of the section of the textile mills. 4.7
EXAMPLE 2- TEXTILE WASTEWATER TREATMENT PLANT
Amaravathi Common Effluent Treatment Plant is an example of a real treatment plant that is studied in this part in order to give a better idea about the more popular methods applied in treating textile effluent (Das, 2005). Amaravathi plant has located in Karur, the north-central of Tamil Nadu in India. The effluents of 43 different textile processing units are collected via pipelines and all transferred to this plant. The following table provides useful information about numbers and dimensions of the components exist in this plant. TABLE 8 Components
Numbers
Dimensions (meter)
Screen Chamber
1
4×1×3.5
Receiving Sump
1
8×5.25
Equalization Tank
1
31×20×3
Flash Mixer
1
1.7×1.7×1.7
Clarriflocculator
1
12×3.3
Aeration Tank
2
23×16×3
2
22×16×3
Clarifier
2
13×4
Sludge well
1
6×3
Sludge Thickener
1
7×3.3
Centrifuge
1
7×4
Generator Room
1
7×3
Office/Lab
1
12×6.2 floors
Transformer Yard
1
14×7.5
Sludge Drying Beds
12
4.9×4.9
In this plant, equalization and neutralization sectors serve as the main preliminary treatments. The next steps are sedimentation, floatation, screening and physical flocculation that are considered as primary treatment methods. In the third step the waste is passed to the secondary treatment section which constitutes of biological oxidation systems and facilitated chemical/physical separation methods. The main objective of this unit is removing organic compounds from the waste. In the final part the effluent is transferred to the tertiary treatment unit which is very important in the means of polishing the waste treatment. 74
4.7.1 Plant operation The collected waste from all those 43 textile factories is treated by passing through the following stages: 1. Screen chamber Large solids are removed by objective screens provided in this chamber to avoid clogging of hydraulic system and abrasion of mechanical equipments 2. Collection tank Effluents collected from the screening chamber are stored for a while in the collection tank and then transferred to the equalization tank. 3. Equalization tank Wastewater is stored for 8 to 12 hours in this tank for a homogeneous mixing. Therefore the concentration of the effluent becomes constant that subsequently results in more constant pH in all parts of the waste stream. The effluent gets neutralized in this system and the shock loading on the next treatment unit is reduced. The settling of solids is also eliminated by continuous mixing of the wastewater in the equalization tank. 4. Flash mixer In this part the following coagulants are added to the effluent; Coagulant
Concentration (ppm)
Effects
Lime
800-1000
Raise the pH 8-9
Ferrous Sulphate
200-300
Remove colour
Poly electrolyte
0.2
Settle the suspended solids
In the Flash mixer the above mentioned flocculates are added to the homogenized waste stream via rapid mixing. This process results in micro flocs production.
5. Clarriflocculator In this part the wastewater is stirred continuously. The overflowed water is passed to the aeration tank to settle down the solid wastes, collect them separately and finally dry them. Flocculation part is responsible for mixing the wastewater slowly, producing maro flocs 75
settling particles in the clarifier zone. Settled solids such as primary sludge are transferred to the sludge dyeing beds. 6. Aeration tank A thin film of wastewater is passed over the staircase arrangement. In this part the direct Aeration of the waste leads to great reduction in BOD which has calculated to be up to 90%. 7. Clarifier This part receives the biological sludge. Subsequently the treated wastewater is accepted by Bureau of Indian standard is disposed to rivers via pipelines. 8. Sludge thickener The input wastewater constitute of 60% water and 40% solids. The centrifugal system provided in this part reduces the water content of the mixture and changes the constitution to 40% of water and 60% of solids. By repeating this process the solids are slowly separated from water. 9. Drying beds In this section the sludge obtained from primary and secondary units are subjected to solar evaporation to be dried. The magnitude amounts of died sludge after treatment is collected and packed in polyethylene bags and covered in water proof sheets. This bulk of sludge should be disposed in an offsite location stated by the State Pollution Control of Tamil Nadu, India.
TABLE 9
Hazardous waste
Hazardous
Quantity
waste
generated
State of waste
Type of Hazard
Mode of storage and disposal
per day Packed in polyethylene bags sludge
TABLE 10
2.5 mT
solid
Chemical
covered with water proof
sludge
sheets
Effluent Quality Management (average for a month)
Quality parameters
Inlet water
76
Outlet water
pH
6-10
6.5-8.5
Biological Oxygen demand
100-150 mg/l
20-30 mg/l
Chemical Oxygen Demand
300-400 mg/l
140-250 mg/l
Total Dissolved solids
2500-3000 mg/l
1800-2100 mg/l
Suspended Solids
70-200 mg/l
50-90 mg/l
Chlorides
1000-1500 mg/l
700-1000 mg/l
Sulphites
1-2 mg/l
Nil
TABLE 11
Regulatory Standards to which effluent needs to be treated Quality parameters
Tolerance limits
pH
5.5-9
Biological Oxygen demand
30 mg/l
Chemical Oxygen Demand
250 mg/l
Total Dissolved solids
2100 mg/l
Suspended Solids
100 mg/l
Chlorides
1000 mg/l
Sulphites
Nil
The following figure provides a schematic diagram of the Amaravathi Common Effluent Treatment Plant.
77
CASE STUDY 2: FIGURE38 AMARAVATHI COMMON EFFLUENT TREATMENT PLANT
78
5
INTRODUCTION
One of the most difficult pollutions to control are those not discharged at point source but those which enters surface water through water that runs over urban or agricultural land or mineral rich areas (Gray,2005). Essentially, these pollutants do not originate from a single point (Diffused) and among which are pollutants from oil and other hydrocarbons. Against this background, the focus of this section is to understudy discharges from a typical oil storage facility in Nigeria with the view to identifying the source, input measuring parameters and standards, treatment technologies, monitoring, management techniques and legislations. The ideas are to overview current practices and recommend ways of improving the system in other to reduce and/or manage liquid effluent to acceptable standards as stipulated by legislation. The oil storage depot uses substantial volume of water in their operations which are discharged as surface water coupled with oil from leakages and handling. To a large degree, wastewater discharge is hazardous to human health, damage aquatic live and alter the environment. On the whole, liquid effluent are treated to assert physical and chemical quality to ensure that degradation standards are not exceeded before it is discharged to the environment or recycled (Mark J. H, 2001). 79
5.1
EFFLUENT SOURCE
The characteristic of an industrial effluent is to a large extent dependent on the diverse activities going in the environment. However not peculiar with diffuse source, there are several environmental problems associated with liquid effluent which can be defined by the toxicity, groundwater contamination (sediment), nuisance in the form of surface water, change in taste of portable water supplies and contamination of urban streams (Gray, 2005). The hot spot of discharge in an oil storage facility are namely; •
Discharge from car maintenance
•
Floating , production, storage and offloading (FPSO) vessels
•
Waste oil disposal
•
Spill from handling
•
Road run-off and industrial run-off
•
Domestic sanitary wastewater from toilets, sinks, showers and laundries. The volume and concentration of which dependent on time, facility occupancy and operational conditions (Nigerian Manual of Petroleum Laws, 1969).
• Substantial amount of discharge also arises from the storage tanks containing oil and process water which is allowed in other balance the vessel as well as provide a platform for removal of any external impurities that may be associated with the oil. This is majorly associated with routine clean up exercise where the process water contaminated with oil and other impurities in the storage tanks are flushed. Other significant causes of wastewater include equipment failure, leakages, malfunctioning oil separators, spills arising from overfilling of tanks and leakages from loading. These contaminants are usually picked by process water and rainwater of which may be routed to sea or river and a subsequent impact on the aquatic environment if unmanaged. Minor leakages within the facility are usually contained using dust or bioremediation techniques which encourage the growth of micro organism by degrading the oil molecules (Harrison, 1995). In a case of major spill, the oil is scooped or skimmed for recovery. However, the remaining part is flushed with pressurised water and further collected via sewer manhole or storm drain and channelled to the settling tank through an interceptor in form of a bar screen to remove large materials such as sticks, plastic material, rock bits and 80
paper. The essence is to capture materials capable of damaging equipment for treatment and properly disposed while other material that floats is skimmed from the surface of the separation tank. the combination of process water and sewage is known as clarified water which is treated to eliminate or reduce the waste content to acceptable levels before it finally discharged as effluent (Obot et al., 2007)
FIGURE39 OIL STORAGE TANK
5.3 EFFLUENT PARAMETERS The nature of pollutants determines potential control options in the sense that there can be variable elemental pollutants from difference sources. The composition of pollutants in wastewater discharge in an oil depot includes; acids, alkalis (pH), oil (free and dissolved), sulphides,
ammonia/nitrates,
cyanides,
heavy metals,
heat,
other organic materials,
nutrients, settle-able solids, colour, toxic compounds, taste and odour producers (Stephan T. O, 2008). Table 12 depicts the composition of effluent, characteristics and exposure limits in an oil 81
storage facility in line with the Federal Environmental Protection Agency (FEPA) and the Department of Petroleum Resources (DPR). Basically, these parameters relates to the oil storage facility for a maximum period of 24hours. It is worthy of that contaminant analysis of oil and wastewater is quite associated with business interest and as such, it is not readily given out as it is regulated by various laws and standards with compliance being a major issue.
TABLE 12
Liquid effluent Parameter
Effluent Characteristics
Exposure Limits, Maximum/day
BOD, mg/l
10
COD, mg/l
40
Total Dissolved Solids (TDS), mg/l
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