Pre-chlorination:

Pre-chlorination refers to the practice of injecting chlorine into the raw water when it is not so turbid but has a high bacteria count. Fairly high dose of chlorine (2-5 mg/1) is used. During the lengthy period spent by water in the settling basins, this oxidizes and precipitates iron and manganese if present in water.

Pre-chlorination also kills algae and bacteria, reduces color and slime formation and assists in settlement. If excessive silt is present in suspension, pre-chlorine is not so effective because silt absorbs chlorine without settling. Hence, in case of heavily turbid water, it is not very effective; in case of clear ground water with high ammonia content, it is effective.