(a) Write the first step the primary effluent undergoes when it enters the secondary treatment plant and state the purpose.

(b) What is the level of B.O.D indicative of in the secondary treatment plant ? Mention its significance.

Correct Answer:
(a) The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows the vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures) which consume the major part of the organic matter in the effluent, thereby significantly reducing the B.O.D. (Biochemical Oxygen Demand) level of the primary effluent.
(b) The level of B.O.D. is indicative of the amount of organic matter present in the water and thus measures its polluting potential. A greater B.O.D. value indicates a higher amount of organic waste and consequently a higher polluting potential of the water.

Step-by-step Explanation:

Part (a): First Step of Secondary Treatment and its Purpose
1. Passing into Aeration Tanks: Once primary treatment (which involves physical removal of particles through filtration and sedimentation) is complete, the supernatant liquid called the primary effluent is transferred into large aeration tanks.
2. Mechanical Agitation and Aeration: In these tanks, the effluent is continuously agitated mechanically, and air is pumped into it. This ensures a constant supply of oxygen and physical mixing.
3. Vigorous Growth of Flocs: The rich availability of oxygen and organic nutrients stimulates the rapid growth of useful aerobic microorganisms. These microbes aggregate along with fungal filaments to form mesh-like structures called flocs.
4. Consumption of Organic Matter: As these flocs grow, they actively consume the organic matter present in the primary effluent. This microbial action reduces the Biochemical Oxygen Demand (B.O.D.) of the effluent before it can be safely released or further treated.

Part (b): B.O.D. Level and its Significance
1. Indicative Value: Biochemical Oxygen Demand (B.O.D.) represents the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria. Therefore, the B.O.D. level directly indicates the concentration of biodegradable organic matter in the wastewater.
2. Significance as a Measure of Pollution: The level of B.O.D. is a direct index of water pollution. A higher B.O.D. indicates that there is a large amount of organic waste present, meaning the water is highly polluted. As the microbes break down the waste, the B.O.D. drops, signaling that the water is becoming cleaner and its polluting potential is reducing.