Stages in typical municipal water treatment
Normally there are three principal stages in water purification:-
- Primary treatment- Collecting and screening including if required pumping from rivers and initial storage
- Secondary treatment- removal of fine solids and the majority of contaminants using filters, coagulation, flocculation and membranes
- Tertiary treatment- polishing, pH adjustment, carbon treatment to remove taste and smells, disinfection, and temporary storage to allow the disinfecting agent to work. Here disinfection is most important.
- Pumping and containment– If required water is to be pumped from its source or directed into pipes or holding tanks. To avoid adding contaminants to the water, this physical infrastructure must be made from appropriate materials and constructed so that accidental contamination does not occur.
- Screening - The first step in purifying surface water is to remove large debris such as sticks, leaves, trash and other large particles which may interfere with subsequent purification steps.
- Storage- Water from rivers may also be stored in bank side reservoirs for periods between a few days and many months to allow natural biological purification to take place. This is especially important if treatment is by slow sand filters. Storage reservoirs also provide a buffer against short periods of drought or to allow water supply to be maintained during transitory pollution incidents in the source river.
- Pre-conditioning- Waters rich in hardness salts are treated with soda-ash (Sodium carbonate) to precipitate calcium carbonate out utilizing the common ion effect.
- Pre-chlorination- In many plants the incoming water was chlorinated to minimize the growth of fouling organisms on the pipe-work and tanks.
There are a wide range of techniques that can be used to remove the fine solids, micro-organisms and some dissolved inorganic and organic materials. The choice of method will depend on the quality of the water being treated, the cost of the treatment process and the quality standards expected of the processed water.
1.pH adjustment - If the water is acidic, lime or soda ash is added to raise the pH. Lime is the more common of the two additives because it is cheaper, but it also adds to the resulting water hardness. Making the water slightly alkaline ensures that coagulation and flocculation processes work effectively and also helps to minimize the risk of lead being dissolved from lead pipes and lead solder in pipe fittings.
2.Coagulation - Together, coagulation and flocculation are purification methods that work by using chemicals which effectively "glue" small suspended particles together, so that they settle out of the water or stick to sand or other granules in a granular media filter. The coagulation chemicals are added in a tank (often called a rapid mix tank or flash mixer), which typically has rotating paddles. In most treatment plants, the mixture remains in the tank for 10 to 30 seconds to ensure full mixing. The amount of coagulant that is added to the water varies widely due to the different source water quality.
One of the more common coagulants used is aluminum sulfate, sometimes called filter alum. Aluminum sulfate reacts with water to form flocs of aluminium hydroxide. Iron (II) sulfate or iron (III) chloride is other common coagulants. Iron (III) coagulants work over a larger pH range than aluminum sulfate but are not effective with many source waters. Other benefits of iron (III) are lower costs and in some cases slightly better removal of natural organic contaminants from some waters. Coagulation with iron compounds typically leaves a residue of iron in the finished water. This may impart a slight taste to the water, and may cause brownish stains on porcelain fixtures. The trace levels of iron are not harmful to humans, and indeed provide a needed trace mineral. Because the taste and stains may lead to customer complaints, aluminium tends to be favoured over iron for coagulation.
Now a days inorganic polymer of Aluminium chloride is widely used as a coagulant to control high turbidity in monsoon season. Oftenly called as Poly Aluminium Chloride. Cationic and other polymers can also be used. They are often called coagulant aids used in conjunction with other inorganic coagulants. The main advantages of polymer coagulants and aids are that they do not need the water to be alkaline to work and that they produce less settled waste than other coagulants, which can reduce operating costs. The drawbacks of polymers are that they are expensive, can blind sand filters and that they often have a very narrow range of effective doses.
3 Flocculation- The joining of the particles so that they will form larger settable particles is called flocculation. The larger formed particles are called floc. In flocculation coagulants are used but the resultant floc is settled out rather than filtered through sand filters. The chosen coagulant and the raw water are slowly mixed or water which previously coagulated is directly taken in a large tank called a flocculation basin (Chamber). Unlike a rapid mix tank, the flocculation paddles turn very slowly to minimize turbulence. The principle involved is to allow as many particles to contact other particles as possible generating large and robust floc particles. Generally, the retention time of a flocculation basin is at least 30 minutes with speeds between 0.5 feet and 1.5 feet per minute (15 to 45 cm / minute).
4 Sedimentation/Clarification/Settling -Water exiting the flocculation basin (chamber) enters the sedimentation basin, also called a clarification or settling basin (chamber). It is a large tank with slow flow, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between does not permit settlement or floc break up. Sedimentation basins can be in the shape of a rectangle, where water flows from end to end, or circular where flow is from the center outward. Sedimentation basin outflow is typically over a weir so only a thin top layer-furthest from the sediment-exits. The amount of floc that settles out of the water is dependent on the time the water spends in the basin and the depth of the basin. The retention time of the water must therefore be balanced against the cost of a larger basin. The minimum clarifier retention time should be normally 4 hours. A deep basin will allow more floc to settle out than a shallow basin. This is because large particles settle faster than smaller ones, so large particles bump into and integrate smaller particles as they settle. In effect, large particles sweep vertically though the basin and clean out smaller particles on their way to the bottom. As particles settle to the bottom of the basin a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated. The amount of sludge that is generated is significant, often 3%-5% of the total volume of water that is treated. The tank may be equipped with mechanical cleaning devices (called as bridge units in conventional clarifiers) that continually clean the bottom of the tank or incase of sedimentation tanks, without such cleaning devices, the tank can be taken out of service when the bottom needs to be cleaned.
Recently a new type of settlers called Tube Settlers have been used. Tube settlers offer an inexpensive method of upgrading existing water treatment plant clarifiers and sedimentation basins to improve performance. They can also reduce the tankage/footprint required in new installations or improve the performance of existing settling basins by reducing the solids loading on downstream filters. Made of lightweight PVC, tube settlers can be easily supported with minimal structures that often incorporate the effluent trough supports. They are available in a variety of module sizes and tube lengths to fit any tank geometry, with custom design and engineering offered by the manufacturer.
5.Filtration - After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc. The most common type of filter is a rapid sand filter. Water moves vertically through sand which often has layers of sand. If charcoal is used as topmost layer of a filter media then, it removes organic compounds including taste and odor. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. Effective filtration extends into the depth of the filter. This property of the filter is key to its operation: if the top layer of sand were to block all the particles, the filter would quickly clog. To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called back flushing or backwashing) to remove embedded particles. Prior to this, compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process; this is known as air blowing. This contaminated water can be disposed of, along with the sludge from the sedimentation (clarifiers) basin, or it can be recycled by mixing with the raw water entering the plant. Some water treatment plants may employ pressure filters. These work on the same principle as rapid gravity filters differing in that the filter medium is enclosed in a steel vessel and the water is forced through it under pressure.
Disinfection is normally the last step in purifying drinking water. Water is disinfected to destroy any pathogens which pass through the filters. Possible pathogens include viruses, bacteria, including Escherichia coli, Campylobacter and Shigella, and protozoans, including G. lamblia and other Cryptosporidia. Mostly public water supplies are required to maintain a residual disinfecting agent throughout the distribution system, in which water may remain for days or hours before reaching the consumer. Following the introduction of any chemical disinfecting agent, the water is usually held in temporary storage - often called a contact tank or clear well to allow the disinfecting action to complete.
1.Chlorine- The most common disinfection method is some form of chlorine or its compounds such as chloramines or chlorine dioxide. Chlorine is a strong oxidant that kills many micro-organisms. Because chlorine is a toxic gas, there is a danger of a release associated with its use. This problem is avoided by the use of sodium hypochlorite, which is a relatively inexpensive liquid that releases free chlorine when dissolved in water. We can also use TCP/ Bleaching powder. Handling the solid, however, requires greater routine human contact through opening bags and pouring than the use of gas cylinders which are more easily automated. These disinfectants are widely used despite their respective drawbacks. A major drawback to using chlorine gas or sodium hypochlorite is that they react with organic compounds in the water to form potentially harmful levels of the chemical by-products trihalomethanes (THMs) and halo acetic acids, both of which are carcinogenic and regulated by the U.S. Environmental Protection Agency (EPA). The formation of THMs and haloacetic acids is minimized by effective removal of as many organics from the water as possible before disinfection. Although chlorine is effective in killing bacteria, it has limited effectiveness against protozoans that form cysts in water. (Giardia lamblia and Cryptosporidium, both of which are pathogenic). Hence water purification plants must have a very good filtration system.
2.Chlorine dioxide is another fast-acting disinfectant. It is, however, rarely used, because it may create excessive amounts of chlorate and chlorite, both of which are regulated to low allowable levels. Chlorine dioxide also poses extreme risks in handling: not only is the gas toxic, but it may spontaneously detonate upon release to the atmosphere in an accident.
3.Chloramines are another chlorine-based disinfectant. Although chloramines are not as effective as disinfectants, compared to chlorine gas or sodium hypochlorite, they are less prone to form THMs or haloacetic acids. It is possible to convert chlorine to chloramine by adding ammonia to the water along with the chlorine: The chlorine and ammonia react to form chloramines. Water distribution systems disinfected with chloramines may experience nitrification, wherein ammonia is used a nitrogen source for bacterial growth, with nitrates being generated as a byproduct.
4.Ozone (O3) is a relatively unstable molecule of oxygen which readily gives up one atom of oxygen providing a powerful oxidizing agent which is toxic to most water borne organisms. It is a very strong, broad spectrum disinfectant that is widely used in Europe. It is an effective method to inactivate harmful protozoans that form cysts. It also works well against almost all other pathogens. Ozone is made by passing oxygen through ultraviolet light or a "cold" electrical discharge. To use ozone as a disinfectant, it must be created on site and added to the water by bubble contact. Some of the advantages of ozone include the production of relatively fewer dangerous by-products (in comparison to chlorination) and the lack of taste and odor produced by ozonation. Although fewer by-products are formed by ozonation, it has been discovered that the use of ozone produces a small amount of the suspected carcinogen Bromate. Another one of the main disadvantages of ozone is that it leaves no disinfectant residual in the water. Ozone has been used in drinking water plants since 1906 where the first industrial ozonation plant was built in Nice, France.
5.UV radiation is very effective at inactivating cysts, as long as the water has a low level of colour so the UV can pass through without being absorbed. The main drawback to the use of UV radiation is that, like ozone treatment, it leaves no residual disinfectant in the water. Because neither ozone nor UV radiation leaves a residual disinfectant in the water, it is sometimes necessary to add a residual disinfectant after they are used. This is often done through the addition of chloramines, discussed above as a primary disinfectant. When used in this manner, chloramines provide an effective residual disinfectant with very little of the negative aspects of chlorination.