Water Purification System

Municipal Water Treatment Process There are typically (5) steps in the municipal water treatment process. 1. Pre-treatment - In this step, several things happen in order to prepare water for purification. Water must first be pumped and contained in holding tanks to await treatment, and where some natural, biological purification takes place. This physical infrastructure must be made from appropriate materials and constructed so that accidental contamination does not occur. Large debris such as sticks, leaves, and trash, which may interfere with subsequent purification steps, are then removed. Many waters rich in hardness salts are "conditioned" with soda-ash (Sodium carbonate) to precipitate the removal of calcium carbonate utilizing the common-ion effect. In many plants the incoming water was chlorinated to minimize the growth of fouling organisms on the pipe-work and tanks. See the diagram below for a typical municipal water treatment process. pH Adjustment If the water is acidic (lower than 7), lime or soda ash is added to raise the pH. Lime is the more common of the two additives because it is cheap, 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. If the water is alkaline, acid (HCl) or carbon dioxide (CO2) may be added in some circumstances to lower the pH. Having an alkaline water does not necessarily mean that lead or copper from the plumbing system will not be dissolved into the water but as a generality, water with a pH above 7 is much less likely to dissolve heavy metals than a water with a pH below 7. 2. Flocculation - This process is taken to clarify the water. Clarifying means removing any turbidity or color so that the water is clear. Clarification is done by causing a precipitate to form in the water, very small particles as when the water is gently stirred, stick together to form bigger particles - this process is sometimes called flocculation. Many of the small particles that were originally present in the raw water absorb onto the surface of these small precipitate particles and so get incorporated into the larger particles that coagulation produces. In this way the coagulated precipitate takes most of the suspended matter out of the water and then generally filtered off through a coarse sand filter. Coagulants / flocculating agents that may be used include Iron, Aluminum Hydroxide, and PolyDADMAC, a synthetic polymer that is now widely used. There are concerns about possible health impacts and accidental poisoning of some of these materials. 3. Sedimentation - Water exiting the flocculation basin may enter a large sedimentation basin, also called a clarifier or settling basin. 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. As particles settle to the bottom of the basin, a layer of sludge is formed on the floor of the tank, which must then be removed and treated. The cost of treating and disposing of the sludge can be a significant part of the operating cost of a water treatment plant. 4. 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, whereas the water moves vertically through sand, often with a layer of activated carbon or anthracite coal to remove organic compounds, which contribute to taste and odor. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough, whereas 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. This contaminated water can be disposed of, along with the sludge from the sedimentation basin, or can be recycled by mixing with the raw water entering the plant. Membrane filters are widely used for filtering both drinking water and sewage (for reuse). For drinking water, membrane filters can remove virtually all particles larger than 0.2 um--including Giardia and cryptosporidium. Membrane filters are an effective form of tertiary treatment when it is desired to reuse the water for industry, for limited domestic purposes, or before discharging the water into a river that is used by towns further downstream. However no filtration can remove substances that are actually dissolved in the water such as phosphorus, nitrates and heavy metal ions. 5. Disinfection - Disinfection is accomplished both by filtering out harmful microbes and also by adding disinfectant chemicals in the last step in purifying drinking water. Water is disinfected to kill any pathogens, which pass through the filters. Possible pathogens include viruses, bacteria, including Escherichia coli, Campylobacter and Shigella, and protozoa, including Giardia lamblia and other cryptosporidia. In most developed countries, public water supplies are required to maintain a residual disinfecting agent throughout the distribution system, in which water may remain for days 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. Chlorination The most common disinfection method in municipal water treatment is some form of chlorine or its compounds such as chloramine or chlorine dioxide. Chlorine is a strong oxidant that rapidly kills many harmful microorganisms. 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 solution that releases free chlorine when dissolved in water. All forms of chlorine are widely used despite their respective drawbacks. One drawback is that chlorine from any source reacts with natural organic compounds in the water to form potentially harmful chemical by-products trihalomethanes (THMs) and haloacetic acids (HAAs), both of which are carcinogenic in large quantities and regulated by the United States Environmental Protection Agency (EPA). The formation of THMs and haloacetic acids may be minimized by effective removal of as many organics from the water as possible prior to chlorine addition.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). Although chloramine is not as strong of an oxidant as Chlorine, it does provide a longer-lasting residual, and it won't form THMs or haloacetic acids. Chlorine is often converted to chloramine by adding ammonia to the water after addition chlorine. Water distribution systems disinfected with chloramines may experience nitrification, wherein ammonia is used a nutrient for bacterial growth, with nitrates being generated as a byproduct. Source: http://pdwtechnologies.com/water_treatment_process.html