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The purification of surface water from sources such as reservoirs or rivers usually requires several phases of treatment. A municipal surface water treatment plant must first screen out large objects such as trash and leaves. 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 filter s The filtered water is then treated to remove any microscpic organisms including protozoa and bacteria. This is generally followed by a disinfection stage to eliminate any residual bacteria and viruses. For waters that are particulalrly difficult to treat such as from catchments with intensive agriculture, both physical and biological treatment methods may be combined.
The use of sand filters is the most common form of physical treatment of water. The water is often pre-treated with small volumes of appropriate chemicals which will form a chemical floc which entraps particles. Passing this flocculated water through a sand filter strains out the floc and the particles trapped within it. The most common flocculant chemicals are aluminium salts such as aluminium sulfate which is flocculated by the small addition of lime to raise the pH.
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.
Where land and space are available, water may be treated in slow sand filter beds. These rely on biological treatent 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 Schmutzedecke 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.
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. Water utilities may choose to further boost chlorine levels (termed rechlorination) in the distribution system to counteract any pathogens that may occur.
Groundwater from an aquiferAn aquifer is an underground layer of water-bearing permeable rock, sand, or gravel. see also groundwater). The upper boundary of the topmost (open) aquifer is known as the water table. Some areas have several aquifers, each capped on top by an impervious not immediately influenced by surface runoff is generally considered to require less rigorous treatment, but must meet the same standards of safety and palatability. Soil and rock layers will have naturally filtered the groundwater to a high degree of clarity even before it is pumped to the treatment plant, but the facility may need to reduce the ironThis article is about metallic iron. For the ironing device, see ironing manganese iron cobalt Fe Ru Full table General Name, Symbol, Number iron, Fe, 26 Chemical series transition metal Group, Period, Block 8 (VIIIB), 4 , d Density, Hardness 7874 kg/m3, or manganesechromium manganese iron Mn Tc Full table General Name, Symbol, Numbermanganese, Mn, 25 Chemical series transition metals Group, Period, Block 7, 4 , d Density, Hardness 7470 kg/m3, 6. 0 Appearance silvery metallic Atomic properties Atomic weight 54. 93804 content of this water to make it pleasant for drinking, cooking, and laundry use. Disinfection is also required.
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 vandalismVandalism is an act motivated by hostility to the arts and literature of a culture, or willful destruction or defacement of its built environment, construed to be in the spirit of the Germanic Vandals in their attacks on building of the Roman Empire. or terrorismTerrorism refers to the use of violence against noncombatants for the purpose of achieving a political, religious or socio-economic goal. Terrorist acts can be carried out by individuals or groups, and are sometimes sponsored by governments as an alternat 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.