Treatment plants clean water by making dirt clump into floc
Most particles clouding river water carry a negative charge that keeps them drifting apart. Add alum or an iron salt and within seconds those charges are neutralised, metal hydroxides start to form, and the fine muck gathers into fluffy clumps called floc that can finally sink or be filtered out.
What a plant must do depends on where its water comes from. Deep groundwater may have fallen as rain centuries or millennia earlier, filtered by rock to high clarity and usually free of harmful microbes, though it can be loaded with dissolved calcium and magnesium or need iron and manganese removed. Upland reservoirs near river sources carry few pathogens but may be tinted brown by peat and are often acidic. Lowland rivers bring heavy bacterial loads, algae and silt. Looks reveal little: only chemical and microbiological testing shows what treatment is required, and even spring water, trusted in the 19th century, is now analysed first.
Surface water is usually screened first to catch sticks, leaves and rubbish. River water may then sit in bankside reservoirs for days or months, letting natural biological processes begin cleaning it and providing a reserve during droughts or pollution spills. Operators adjust acidity too, adding lime, soda ash or caustic soda to acidic water. Raising the pH helps coagulation work and reduces the risk of lead dissolving from old pipes and solder.
The heart of conventional treatment is coagulation and flocculation. Chemicals go in during a rapid, high-energy mix lasting seconds, and the water then spends 15 to 45 minutes in basins where large paddles stir gently to grow the floc. Aluminium hydroxides form only between a pH of about 5.5 and 7.7, while iron hydroxides work across a wider band. Since the 1960s, synthetic organic polymers have been used to help or replace metal salts, bridging particles together.
Next comes a sedimentation basin, a large, slow tank where floc settles as sludge. The clarified water leaves over a weir, so only the thin top layer farthest from the sludge moves on to filtration and disinfection.
Source: Water purification
