Answer:
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Explanation and working: 5 Marks
Sewage treatment plants or wastewater treatment plants are large plants where wastewater is cleaned before being sent to the nearest water bodies or being reused. The sewage treatment involves physical, chemical and biological processes to remove impurities from the wastewater.
Physical Process
1. Filtration:The wastewater is passed through bar screen. Large objectslike rags, sticks, plastic bags, cans, etc. are removed in this process.
2. Grit and sand removal:The wastewater is slowly passed through the grit and sand removal tank. Grit, sand, and pebbles settle down at the bottom.
3. Sedimentation:The wastewater is then sent to the sedimentation tanks. Solidslike faeces settle at the bottom. Floatable impuritieslike oil and grease float on the surface. A scraper removes the faeces from the bottom of the tank. The impurity thus collected is called sludgewhich is sent to the sludge tank. The sludge can be used to produce biogas or to produce manure. A skimmer removes the floatable impurities. Now, the water is called clarified water.
Biological Process
4. Aeration:Air is pumped into the clarified water so that bacteria can proliferate. Bacteria consume the human waste. It decomposesfood waste, soap and other unwanted materials in the water. The microbes settle down at the bottom after several hours. Water is then removed from the top. This water is fit for irrigation and can be used for that purpose.
Chemical Process
5. Chlorination:Water purified through aeration is not fit for human consumption. It needs to be treated with chlorine. For this, bleaching powder is added to the water. The chlorine kills the rest of the germs. After chlorination, the water is fit for drinking.