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.
Discharge of effluent from STPs in a lotic system will most likely _______________.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → increase biochemical oxygen demand
Even treated effluent still carries a residual organic load and nutrients into the river. Micro-organisms in the receiving water set about decomposing that organic matter, and the oxygen they draw for the purpose raises the biochemical oxygen demand of the stream.
The consequences run the other way for the remaining options: dissolved oxygen falls as BOD rises, the water tends towards reducing rather than oxidising conditions, and the added nitrogen and phosphorus promote phytoplankton and algal growth.