MP Board · Class 12 · Biology · Microbes in Human WelfareHow do sewage treatment plants (STPs) help in reducing water pollution? Explain the primary and secondary treatment stages.

Municipal waste water, also known as sewage, is treated in sewage treatment plants to make it less polluting before it is released into natural water bodies like rivers and streams. This treatment is carried out in two main stages: Primary treatment and Secondary treatment. Primary treatment is a physical process that involves the removal of large and small particles from sewage through sequential filtration and sedimentation. Initially, floating debris is removed by sequential filtration, and then grit consisting of soil and small pebbles is removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. Secondary treatment, also known as biological treatment, involves passing the primary effluent into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs, which consume the major part of the organic matter in the effluent, significantly reducing the Biological Oxygen Demand (BOD) of the waste water before safe disposal.