Water purification Methods

Author: Peter Young
Date Published: 2005-07-20

Water Treatment Methods

1. Screening - A municipal surface water treatment plant must first screen or sieve out large objects such as trash and leaves. The tighter the mesh of the sieve, the smaller the particles must be to pass through. Filtering is not sufficient to completely purify water, but it is often a necessary first step, since such particles can interfere with the more thorough purification methods.
2. Storage- Water from rivers may also be stored in bankside 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 to be by slow sand filters The filtered water is then treated to remove any microscopic organisms including protozoa and bacteria. This is generally followed by a disinfection stage to eliminate any residual bacteria and viruses. For waters that are particularly difficult to treat such as from catchments with intensive agriculture, both physical and biological treatment methods may be combined.
3. Flocculation - The water is treated with small volumes of appropriate chemicals which will form a chemical floc which entraps particles. The most common flocculent chemicals are metal salts such as aluminium sulfate, ferric sulfate, or ferric chloride. Flocculation can be enhanced by adjusting the pH and alkalinity with acid, lime, and/or carbon dioxide. Polymers are also used in addition to, or in place of, metal salts at some plants.
4. Rapid Sand Filters - The use of rapid sand filters is the most common form of physical treatment of water. Passing flocculated water through a sand filter strains out the floc and the particles trapped within it. Where taste and odour may be a problem (organo-leptic impacts), the sand filter may include a layer of activated carbon to remove the taste and odour. Sand filters become clogged with floc after a period in use and they are then backwashed or pressure washed to remove the floc. This backwash water is run into special settling tanks so that the floc can precipitate out and is then disposed of as waste material. In some countries this may be used as a soil conditioner.
5. Slow Sand Filters - Where land and space are available, water may be treated in slow sand filter beds. 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 buried at the base of the filter convey treated water away for disinfection. When a new slow sand filter bed is brought into use, raw water is carefully decanted onto the filter material until a water depth of 1 to 3m is achieved, dependant on the size of the filter bed. The water passing through the filter for the first few hours is recirculated through the filter and not put into supply. Within a few hours, a biological film comprised of bacteria, protozoa, fungi, and algae builds on the surface of the sand. This is the Schmutzdecke layer and it is this 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.
6. Disinfection - The finished water is then disinfected with chlorine gas, chloramine, sodium hypochlorite, chlorine dioxide, ozone, or ultraviolet light, before it is pumped into the distribution system of water mains and storage tanks on its way to consumers. Some plants also pre-chlorinate their raw water influent after the screening phase to reduce the incidence of biological films in the treatment cycle. They may also pre-chlorinate to oxidize and precipitate out dissolved iron and manganese from the water, in order to prevent unsightly iron (orange) and manganese (black) stains in the consumer's sink. Water utilities may choose to further boost chlorine levels (termed re-chlorinating) in the distribution system to counteract any pathogens that may occur. Bleach may be used for emergency disinfection at the rate of 2 drops of 5% bleach per liter or quart of clear water according to a treatment table in the following US EPA document Emergency Disinfection.

Many environmental and cost considerations affect the siting and design of water purification plants. Groundwater is cheaper to treat, but aquifers once depleted can take thousands of years to recharge. Surface water sources must be carefully monitored for the presence of unusual types or levels of contaminants. The treatment plant itself must be kept secure from vandalism or terrorism and the presence of large quantities of dangerous chemicals mandates special training for workers and emergency personnel. The facility must responsibly dispose of its settled and filtered solids and prevent them from contaminating the treatment components or the source waters. All facilities disinfect finished water, but the exact method of disinfection can be controversial, and the costs and benefits of different methods must be evaluated.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Water Purification ".

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