Comprehension:
Municipal waste water, also called as sewage, contains large amounts of organic matter, pathogens and other pollutants. Before being discharged into the natural water bodies, the sewage is treated in the sewage treatment plants (STPs) to undergo primary (physical removal of large and small particles) and secondary (removal of organic load and pathogens) treatment. During secondary treatment, vigrous growth of useful aerobic microbes consume the major portion of organic matter in the sewage water, thereby significantly reducing the BOD of treated water. The bacterial flocs formed during this process are allowed to settle in the bottom of the sedimentation tank as “activated sludge”. A small portion of activated sludge is used as inoculum and pumped back into the aeration tank, while the rest is anaerobically digested to produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide, which form ‘Biogas’. A similar process of anaerobic digestions using methanogens is used for production of ‘Gober gas’ from cattle dung.
Biological treatment of waste water in a STP is a part of ______
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Secondary treatment
The passage defines secondary treatment as the removal of organic load and pathogens, carried out by the vigorous growth of useful aerobic microbes in the aeration tank. Because living organisms do the work, it is the biological stage of the plant.
Primary treatment is purely physical — screening and sedimentation of large and small particles — while tertiary and advanced treatment use chemical and physico-chemical polishing steps such as nutrient removal, filtration and disinfection.