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water treatments

Water Treatments Methods

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 a layer of activated carbon or anthracite coal above the sand. The top layer removes organic compounds which could include dangerous disinfection by-products as well as those with 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. So not just the top layer of the filter cleans the water, but 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 blocked all particles the filter would quickly clog. To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backflushing 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 scouring. This contaminated water can be disposed of, along with the sludge from the sedimentation basin, or it can be recycled by mixing with the raw water entering the plant.

Some water treatment plants 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.

Where sufficient land and space are available, water may be treated in slow sand filters. These rely on biological treatment processes for their action rather than physical filtration. Slow sand filters are carefully constructed using graded layers of sand with the coarsest at the base and the finest at the top. Drains at the base convey treated water away for disinfection. When bringing a new slow sand filter bed into use, raw water is carefully decanted onto the filter material to a water depth of one to three metres, depending on the size of the filter bed. The water passing through the filter for the first few hours is recirculated and not put into supply. Within a few hours, a film of bacteria, protozoa, fungi, and algae builds on the surface of the sand. This is the Schmutzdecke layer that removes all the impurities. An effective slow sand filter may remain in service for many weeks or even months if the pre-treatment is well designed and produces an excellent quality of water which physical methods of treatment rarely achieve.

Disinfection

Disinfection with aggressive chemicals like chlorine or ozone is normally the last step in purifying drinking water. Water is disinfected to destroy any pathogens which passed through the filters. Possible pathogens include viruses, bacteria including Escherichia coli, Campylobacter and Shigella and protozoans including Giardia lamblia and other Cryptosporidium. Many water treatment systems add sufficient disinfection agent to ensure that an effective concentration remains in the water throughout the distribution system. In many cities, this period can be many days.

The most common disinfection method is some form of chlorine such as chlorine gas, sodium hypochlorite, chloramine or chlorine dioxide. The water and chemical mix are allowed to sit in a large tank, called a clear well. The water must sit in the clear well to ensure that the water is in contact with the disinfectant for a minimum amount of time because it takes time to inactivate the harmful microbes. Chlorine is a strong oxidant that kills many microorganisms and remains in the water to provide continuing disinfection. Other disinfection methods include using ozone, which acts very rapidly, or ultraviolet light, which is almost instantaneous.

Chlorine gas and sodium hypochlorite are the most commonly used disinfectants, because they are inexpensive and easy to manage. They are effective in killing bacteria, but have limited effectiveness against protozoans that form cysts in water (Giardia lamblia and Cryptosporidium, both of which are pathogenic). Chlorine gas and sodium hypochlorite both have strong residuals in the water once it enters the distribution system.

The main drawback in using chlorine gas or sodium hypochlorite is that these react with organic compounds in the water to form potentially harmful levels of the chemical by-products trihalomethanes (THMs) and haloacetic 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 and/or by adding ammonia immediately after chemical disinfection is completed. Formerly, it was common practice to chlorinate the water at the beginning of the purification process, but this practice has mostly been abandoned to minimize the production of THMs.

Chloramines are not as effective disinfectants compared to chlorine gas or sodium hypochlorite, but do not form THMs or haloacetic acids. They are typically used only in stored and distributed treated water. An example of this sort is proceeses using ozone for primary disinfection which is very quickly accomplished then using monochloramine to create a residual level of disinfectant in the water. Chlorine dioxide is another rapid acting disinfectant against bacteria but unlike ozone it leaves a long lasting residual in the water. Despite these beneficial characteristics, it is rarely used because it may creates excessive amounts of chlorate and chlorite, both of which are regulated to low allowable levels.

Ozone is a very strong, broad spectrum disinfectant and is widely used in Europe to disinfect water. It is a most effective method to inactivate harmful protozoans that form cysts and works well against almost all other pathogens. To use ozone as a disinfectant, it must be created on site and added to the water by bubble contact. Other benefits of ozone are that it does not form any dangerous by-products and does not add any taste or odor to the water. One of the main disadvantages of ozone is that it leaves no disinfectant residual in the water.

UV radiation can be used to disinfect water as well. UV radiation is very effective at inactiavitng 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 it, like ozone treatment, leaves no residual disinfectant in the water.

Many environmental and cost considerations affect the location and design of water purification plants. Groundwater is cheaper to treat, but aquifers usually have limited output and can take thousands of years to recharge. Surface water sources should be carefully monitored for the presence of unusual types or levels of microbial/disease causing contaminants. The treatment plant itself must be kept secure from vandalism and terrorism. The large quantities of dangerous chemicals suggests special training for workers and emergency personnel. Facilities typically responsibly dispose of waste and prevent them from contaminating the treatment components and the source water. All facilities disinfect finished water, but the exact method of disinfection can be controversial, and the costs and benefits of different methods weighed.

Information source: Wikipedia

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