The primary effluent is passed into aeration tanks where it is constantly agitated and pumped in with air. This induces the growth of aerobic microbes into flocs. These microbes feed on the organic matter in the effluent thereby reducing the Biochemical Oxygen Demand (BOD). BOD is taken as a measure of the organic matter in the effluent.
The effluent is then passed into a settling tank. Here, the flocs are allowed to sediment. This sediment is now called as the activated sludge. Small part of the sludge is pumped back to the aeration tank where it acts an inoculum and the large part of it is pumped into anaerobic sludge digesters. The anaerobic bacteria here digest the flocs in the sludge. During this process of digestion, mixture of gases such as methane, hydrogen sulfide and carbon dioxide are produced. These form the biogas.
Then, the effluent from the secondary treatment plant is released in natural water bodies.
The differences between inbreeding and outbreeding in cattle are as follows:
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