Microbes Control Bacterial Diseases — NEET Biology

Correct answer: C — S1, S2, and S6 are correct

The secondary treatment stage of sewage involves several key microbial processes. S1: This statement is correct. Primary effluent is passed into aeration tanks where it is constantly agitated and air is pumped, allowing vigorous growth of useful aerobic microbes into flocs. S2: This statement is correct. Flocs are indeed masses of bacteria associated with fungal filaments that consume the major part of the organic matter in the effluent. S3: This statement is incorrect. The purpose of aeration and agitation is to promote *aerobic* conditions, not anaerobic, for the growth of useful aerobic microbes. S4: This statement is incorrect. BOD reduction indicates a decrease in *organic matter*, not inorganic contaminants. S5: This statement is incorrect. Only a *major part* of the activated sludge is pumped into anaerobic sludge digesters; a small part is pumped back into the aeration tank to serve as inoculum. S6: This statement is correct. In anaerobic sludge digesters, anaerobic bacteria digest the sludge, producing a mixture of gases such as methane, hydrogen sulphide, and carbon dioxide, which form biogas and are inflammable. Therefore, statements S1, S2, and S6 are correct. The tested concept is the detailed process and outcomes of secondary sewage treatment.