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www.GOALias.blogspot.comFig. 18.5 Water clariferThe sludge is transferred to aseparate tank where it is decomposedby the anaerobic bacteria. The biogasproduced in the process can be used asfuel or can be used to produce electricity.4. Air is pumped into the clarified waterto help aerobic bacteria to grow.Bacteria consume human waste,food waste, soaps and otherunwanted matter still remaining inclarified water (Fig. 18.6).After several hours, the suspendedmicrobes settle at the bottom of the tankas activated sludge. The water is thenremoved from the top.The activated sludge is about 97%water. The water is removed by sanddrying beds or machines. Dried sludgeis used as manure, returning organicmatter and nutrients to the soil.The treated water has a very low levelof organic material and suspendedmatter. It is discharged into a sea, a riveror into the ground. Nature cleans it upfurther. Sometimes it may be necessaryto disinfect water with chemicals likechlorine and ozone before releasing itinto the distribution system.Become an active citizenWaste generation is a natural part ofhuman activity. But we can limit thetype of waste and quantity of wasteproduced. Often we have been repelledby offensive smell. The sight of opendrains is disgusting. The situationThe water in a river is cleanednaturally by processes that aresimilar to those adopted in awastewater treatment plant.Did you know ?Fig. 18.6 AeratorIt has been suggested that we shouldplant eucalyptus trees all alongsewage ponds. These trees absorb allsurplus wastewater rapidly andrelease pure water vapour into theatmosphere.224SCIENCE