Q.Explain the steps involved in the process of sewage treatment. OR Explain the uses of microbes as bio control agents.

Sewage treatment purifies wastewater in stages — first removing solid material physically (primary treatment), then using microorganisms to break down dissolved/suspended organic matter (secondary/biological treatment) — so the water can be safely released without harming the receiving river or lake. (Answering the primary question; the OR alternative on microbial biocontrol agents is not separately required.)

Steps of sewage treatment:

1. Primary treatment (physical): raw sewage first passes through sequential filters to remove large floating debris. It then goes to a sedimentation tank where smaller, heavier particles like grit and soil settle out. The organic and inorganic sludge that settles at this stage is called primary sludge, and the remaining liquid, called effluent, is passed on for secondary treatment.
2. Secondary treatment (biological): the primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped in. This encourages the vigorous growth of aerobic microbes (bacteria) within the effluent, which form flocs (masses of fungal filaments entangled with bacteria). These microbes consume the majority of the remaining organic matter in the effluent, which significantly reduces the biochemical oxygen demand (BOD) of the sewage — BOD is a measure of the organic matter present, based on the oxygen microbes would need to break it down; the more polluted the water, the higher its BOD. …