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.
‘BOD’ test is a measures of _______ in a water body.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Biodegradable Organic waste
The biochemical oxygen demand is the amount of dissolved oxygen that micro-organisms consume while decomposing organic matter in a water sample, usually measured over five days at 20°C. It is therefore an indirect measure of the biodegradable organic load of the water — the higher the BOD, the more polluted the water is.
It has nothing to do with the oxygen breathed by fish or used by plants, and the demand met by both biological and chemical oxidation is the COD, not the BOD.