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The goal of treatment is to remove unwanted components from water, making it safe for drinking or suitable for specific uses in industrial or medical applications.A wide variety of techniques can be used to remove contaminants such as fine solids, microorganisms and some dissolved inorganic and organic materials, or environmentally persistent pharmaceutical pollutants.The choice of method will depend on the quality of the water being treated, the cost of the treatment process and the expected quality standard of the treated water.The following process is commonly used in water purification plants.Depending on the size of the plant and the quality of the raw (source) water, some or most may not be used.
1.Preprocessing
2.Pumping and sealing most of the water must be pumped from the source or go directly into a pipe or storage tank.To avoid adding pollutants to the water, this physical infrastructure must be made of appropriate materials and constructed so that accidental contamination does not occur.
3.Screening (see also Mesh Filters).The first step in purifying surface water is to remove large debris such as twigs, leaves, litter, and other large particles that may interfere with subsequent purification steps.Most deep groundwater does not require screening prior to other purification steps.
4.Storage-River water can also be stored in onshore reservoirs for days to months for natural biological purification. This is especially important if processing through slow sand filters. Cisterns can also buffer short-term droughts, or maintain water supplies during brief pollution events in source rivers.
5.Pre-chlorination-In many plants, the incoming water is chlorinated to minimize the growth of fouling organisms in pipes and tanks.This has largely been discontinued due to potential adverse quality effects.
PH adjustment 
Pure water has a pH close to 7 (neither alkaline nor acidic). Seawater has a pH range of 7.5 to 8.4 (moderately alkaline).Depending on the geology of the catchment basin or aquifer and the effects of pollutant input (acid rain), the pH of freshwater can vary widely. If the water is acidic,lime, soda ash, or sodium hydroxide can be added to the water purification process to raise the pH.Adding lime increases the concentration of calcium ions, which increases the hardness of the water.For highly acidic water, a forced-air deaerator can raise the pH by removing dissolved carbon dioxide from the water.Making the water alkaline helps the coagulation and flocculation process to proceed efficiently and also helps minimize the risk of lead dissolution from lead solder in lead pipes and fittings. Sufficient alkalinity also reduces the corrosiveness of water to iron pipes.In some cases, an acid (carbonic acid, hydrochloric acid, or sulfuric acid) can be added to alkaline water to lower the pH. Alkaline water (above pH 7.0) does not necessarily mean that lead or copper in the plumbing system will not dissolve into the water.The ability of water to precipitate calcium carbonate to protect metal surfaces and reduce the potential for toxic metals to dissolve in the water is a function of pH, mineral content, temperature, alkalinity, and calcium concentration.
Coagulation flocculation
The first step in most traditional water purification processes is the addition of chemicals to help remove suspended particles in the water.Particles can be inorganic, such as clay and silt, or organic, such as algae, bacteria, viruses, protozoa, and natural organic matter. Inorganic and organic particles contribute to water turbidity and color.The addition of inorganic coagulants such as aluminum sulfate (or alum) or iron(III) salts such as iron(III chloride) causes multiple chemical and physical interactions to occur simultaneously on and between particles.Within seconds, the negative charges on the particles are neutralized by the inorganic coagulant.Also within seconds, metal hydroxide precipitates of iron and aluminum ions begin to form.These precipitates are combined into larger particles by natural processes such as Brownian motion and by induced mixing (sometimes called flocculation).Amorphous metal hydroxides are referred to as "floes".Large, amorphous aluminum and iron(III) hydroxides adsorb and coat particles in suspension and facilitate particle removal through subsequent settling and filtration processes.Aluminum hydroxide forms over a fairly narrow pH range, typically: 5.5 to about 7.7. Iron(III) hydroxide can form over a wider pH range, including pH levels below those effective for alum.In the literature, there is much debate and confusion over the use of the terms coagulation and flocculation: where does coagulation end and flocculation begins? In a water purification plant, there is typically a high energy, fast mixing unit process (residence time in seconds) where coagulation chemicals are added, followed by flocculation tanks (residence time range 15 to 45 minutes), and the low energy input will Large paddles or other gentle mixing equipment to enhance floc formation. In fact, once the metal salt coagulant is added, the coagulation and flocculation process continues.Organic polymers were developed in the 1960s as coagulant aids and, in some cases, as replacements for inorganic metal salt coagulants.Synthetic organic polymers are high molecular weight compounds with negative, positive or neutral charges. When organic polymers are added to water together with particles, high molecular weight compounds are adsorbed to the surface of the particles and coalesce with other particles through interparticle bridging to form flocs. PolyDADMAC is a popular cationic (positively charged) organic polymer used in water purification plants.
