Steps in Wastewater Treatment

Steps in Wastewater Treatment Pretreatment Coarse Screening Often the first step in wastewater treatment is coarse screening to remove large materials (rags, logs and other large objects) from the wastewater that could interfere with the treatment process. Coarse screens (also known as bar racks) are made up of vertical or inclined bars with openings one to three inches wide. These screens are raked cleaned from time to time. Some wastewater treatment plants have automated equipment that clean the screens automatically. At other plants, the screens are manually raked. Grit Chambers The next step is to remove smaller objects (such as sand, broken glass, silt and pebbles). If these objects are not removed, they can damage pumps and other mechanical devices. These objects also have a tendency to settle in corners and bends, thus reducing flow capacity and eventually clogging pipes and channels. Many grit chambers are concrete-lined ponds, similar to swimming pools, where the wastewater enters one end and leaves at the opposite end. These grit chambers are typically sized so that the speed of the wastewater flowing through the chamber is kept well below one foot per second. Flows this slow allow sand- and silt-sized particles to settle out of the wastewater. Another design that is sometimes used is an aerated grit chamber. In am aerated grit chamber, air bubbles are injected into a wastewater basin to force a spiral, or rolling, flow. The air bubbles are supposed to strip organic material off the surfaces of the inert grit, and the grit settles in the bottom. Each wastewater treatment plant usually has at least two grit chambers. This allows each one to be closed for cleaning. Mechanical devices are usually installed to clean each grit chamber. Normally, the grit is buried in a sanitary landfill. Comminutors Comminutors are mechanical devices with revolving cutting bars. Comminutors are placed downstream of the grit chambers to cut and shred any remaining solids. Flow Equalization Wastewater seldom flows into wastewater treatment plants at the same rate throughout each day. In many cities, the greatest flows reaching the wastewater treatment plants arrive mid-morning. Such uneven flow volumes reduce wastewater treatment plants' efficiency.

To even out these periods of high and low flow, large basins are constructed at some wastewater treatment plants to store the wastewater flow from peak periods and release it for treatment. These basins require aeration and mixing to prevent odors and deposition of solids.

Primary Treatment After screening the solids and removing the grit, the wastewater still contains light organic suspended solids. Some of these can be removed by gravity in a sedimentation tank. These tanks are typically twelve feet deep and hold the wastewater for two or three hours. What settles out is called sludge. The sludge is removed from the primary treatment tank with mechanical scrapers and pumps. Grease, oil, and other floating substances rise to the top, where they are removed by surface skimming equipment.

Secondary Treatment Secondary treatment removes soluble materials that require oxygen for decay, as well as further removal of suspended solids. BOD Many of the pollutants in city wastewater, if not treated and dumped directly into streams, kill fish and other wildlife by depriving them of oxygen. These pollutants naturally rot, or decompose, on their own; but require oxygen to do so. These pollutants steal dissolved oxygen from the water as they decompose, and fish and other aquatic wildlife die from oxygen starvation. The amount of these oxygen-stealing pollutants present in wastewater is measured in terms of biochemical oxygen demand (BOD, pronounced "bee-oh-dee"). BOD is the amount of oxygen, usually measured in milligrams of oxygen per liter of wastewater, required to decompose the organic pollutants found in the wastewater. Because these organic pollutants decompose on their own in the presence of oxygen and common microorganisms, these pollutants can be decomposed by merely forcing oxygen into the wastewater and giving these pollutants time to decompose by natural processes. This is the procedure used in secondary treatment. The two most common secondary treatment methods are trickling filters and activated sludge. Trickling Filters A trickling filter is a bed of coarse stone or perforated plastic material over which wastewater is sprayed. The most common design is a bed of stones three to ten feet deep inside a large circular concrete tank. Some tanks are more than 200 feet in diameter. The wastewater is sprayed over the filter from rotating arms.

As the wastewater trickles through the bed, microorganisms establish themselves on the stone or plastic surfaces as slime. The wastewater picks up oxygen as it is sprayed over the filter and passes over these microorganisms. These microorganisms, in the presence of high amounts of oxygen, feed on the organic materials in the wastewater. The microbial slime on the filter bed will grow and eventually clog the filter if not washed out. Thus, the flow from the filter is sent to a sedimentation basin to allow these solids to settle out. This sedimentation basin is called a secondary clarifier or a final clarifier to distinguish it from the sedimentation basin used for primary treatment. Activated Sludge Activated sludge is another method of providing secondary treatment to wastewater, whereby a mixture of wastewater and biological sludge (microorganisms) is agitated and aerated. The biological solids are then allowed to settle out. The name "activated sludge" comes from the biological mass formed when oxygen (in the form of air) is continuously injected into the wastewater. In this process, microorganisms are thoroughly mixed with organics under conditions that stimulate their growth. As the microorganisms grow and are mixed by the agitation of the air, the individual microorganisms clump (or flocculate) together to form a mass of microbes called activated sludge. About eight cubic feet of air are required for every cubic foot of wastewater. In the activated sludge process, wastewater flows continuously into an aeration tank where air is injected into the wastewater to mix the wastewater with the activated sludge, and also to provide the oxygen needed for the microorganisms to break down the organic pollutants. The mixture of wastewater and activated sludge is called mixed liquor. The mixed liquor flows to a secondary clarifier (settling tank) where the activated sludge settles out. Some (usually twenty or thirty percent) of the settled sludge is returned to the aeration tank (and hence is called return sludge) to maintain a high population of microbes to break down the organics. Since more activated sludge is produced than is needed for return sludge, the excess sludge is removed and disposed of. Secondary Clarifiers Regardless which method of secondary treatment is used, the end result is a mixture of microorganisms and partially treated wastewater that is essentially free of dissolved and suspended organic material. The organic material that was dissolved or suspended in the wastewater has now been broken down and consumed by these microorganisms. These microorganisms, while very small, are still large enough to settle out of the wastewater. Wastewater leaving trickling filters and activated sludge aeration tanks is then sent to secondary clarifiers (settling tanks) where the microorganisms settle out. At this point, the wastewater treatment process is nearly completed. Disinfection

The next step in secondary wastewater treatment is disinfection. In the United States, the most common method of disinfection is chlorination. Chlorine is injected into the wastewater and the wastewater is held in a basin for about fifteen minutes to allow the chlorine to react with any remaining pathogens. Since chlorine is toxic to fish, the chlorine is often removed from the wastewater as a last step. The treated wastewater can then be released into a stream.

Sludge Both the primary and the secondary treatment processes produce large amounts of sludge. Sludge is commonly disposed of by storing it in a tank (called a digester) where it undergoes anaerobic digestion. Methane gas is one by-product of anaerobic digestion, which can be burned off or used as fuel. Another commonly used method of disposal is incineration.

Advanced Wastewater Treatment Although secondary treatment can remove over eighty-five percent of the BOD, suspended solids and nearly all pathogens, sometimes additional treatment is required. Sometimes a higher level of BOD removal is required. Sometimes other pollutants, such as nitrogen, phosphorus, non-biological chemical oxygen demand (COD), or heavy metals may be present that require removal. Besides removing these other pollutants, advanced wastewater treatment (AWT) can be so effective in removing pollutants that the treated wastewater can be reused. In many parts of the world, water is too valuable a resource to be thrown away. Some processes used in AWT include filtration, carbon adsorption, phosphorus removal and nitrogen removal. Filtration Secondary treatment processes are highly effective in reducing the BOD in wastewater. However, the secondary clarifiers used to settle out microorganisms in the secondary treatment process are not totally effective. Some of these microorganisms remain in the wastewater after it leaves the secondary clarifier, and they add BOD since the decay of these microorganisms will exert its own oxygen demand. Sand filters, similar to those used for producing drinking water, are sometimes used for additional removal of microorganisms and other solids. Unlike water treatment sand filters, however, the filters used in wastewater treatment often use large, lightweight aggregates (such as coal) at the top to improve efficiency and facilitate cleaning. Partially treated wastewater usually contains higher concentrations of solids than does water in a water treatment plant, so these filters must be designed for greater efficiency and for more frequent cleaning. Carbon Adsorption

Even after secondary treatment and filtration, soluble organics may still be present in the wastewater. These remaining materials are called refractory organics. The most practical way to remove refractory organics is by adsorbing them on activated carbon. Adsorption (not to be confused with absorption) is the accumulation of materials on an interface (in this case, the liquid/solid boundary layer). Carbon is activated by heating it in the absence of oxygen. This activation process creates many small pores in each carbon particle. Activation thus increases the surface area of each carbon particle, making it more effective as an adsorption agent. After the adsorption capacity of the carbon has been exhausted, it can be restored by re-heating it in the absence of oxygen. This process drives off the adsorbed organics, which can be consumed in an afterburner. Phosphorus Removal Phosphorus, in wastewater, is considered a pollutant because it encourages the growth of algae. Phosphorus removal usually involves the addtion of ferric chloride, alum or lime to the wastewater, mixing it in a reaction basin, and then sending the mixture to a clarifier to allow the phosphorus-containing precipitate to settle out. Nitrogen Control Nitrogen in any soluble form is a plant nutrient and may need to be removed from the wastewater to control the growth of algae. In addition, nitrogen in the form of ammonia exerts an oxygen demand and can be toxic to fish. Nitrogen can be removed from wastewater by both biological and chemical means. The biological process is called nitrification/denitrification and the chemical process is called ammonia stripping. Nitrification/Denitrification

The natural nitrification process can be forced to occur in the activated sludge process by maintaining a cell detention time of at least fifteen days. Bacteria can convert nitrates into water and the gases nitrogen and carbon dioxide. Small amounts of organic materials (such as methanol, or raw or settled sewage) is added to provide a food source for the bacteria for this denitrification process. Ammonia Stripping

Nitrogen in the form of ammonia can be removed chemically by raising the pH (often, by adding lime) to convert the ammonium ion into ammonia, which can be stripped from the water by blowing large quantities of air through the water. Ref : http://www.bcwater.org/waterfacts/wastewatertreatment.asp