1.pH adjustment - If the water is acidic, lime or soda ash is added to raise the pH. Lime is the more common of the two additives because it is cheaper, 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.
2.Coagulation - Together, coagulation and flocculation are purification methods that work by using chemicals which effectively "glue" small suspended particles together, so that they settle out of the water or stick to sand or other granules in a granular media filter. The coagulation chemicals are added in a tank (often called a rapid mix tank or flash mixer), which typically has rotating paddles. In most treatment plants, the mixture remains in the tank for 10 to 30 seconds to ensure full mixing. The amount of coagulant that is added to the water varies widely due to the different source water quality.
One of the more common coagulants used is aluminum sulfate, sometimes called filter alum. Aluminum sulfate reacts with water to form flocs of aluminium hydroxide. Iron (II) sulfate or iron (III) chloride is other common coagulants. Iron (III) coagulants work over a larger pH range than aluminum sulfate but are not effective with many source waters. Other benefits of iron (III) are lower costs and in some cases slightly better removal of natural organic contaminants from some waters. Coagulation with iron compounds typically leaves a residue of iron in the finished water. This may impart a slight taste to the water, and may cause brownish stains on porcelain fixtures. The trace levels of iron are not harmful to humans, and indeed provide a needed trace mineral. Because the taste and stains may lead to customer complaints, aluminium tends to be favoured over iron for coagulation.
Now a days inorganic polymer of Aluminium chloride is widely used as a coagulant to control high turbidity in monsoon season. Oftenly called as Poly Aluminium Chloride. Cationic and other polymers can also be used. They are often called coagulant aids used in conjunction with other inorganic coagulants. The main advantages of polymer coagulants and aids are that they do not need the water to be alkaline to work and that they produce less settled waste than other coagulants, which can reduce operating costs. The drawbacks of polymers are that they are expensive, can blind sand filters and that they often have a very narrow range of effective doses.