Question

Explain the process of secondary treatment given to the primary effluent up to the point it shows a significant change in the level of biological oxygen demand (BOD) in it.

Answer 1

Sewage treatment includes two stages, primary treatment and secondary treatment. In primary treatment, it removes substances which either float or settle out by gravity. In secondary treatment, it removes the organic substances that escape from the primary treatment.

Complete answer:
Primary treatment requires physical removal of large and small substances from the sewage through sedimentation and filtration. Floating detritus (debris) is removed by filtration.
Soil and small pebbles are detached by sedimentation. The outflow from the primary settling tank is taken for secondary treatment. The primary outflow is transferred into a large aeration tank where effluent is continuously agitated automatically and the air is pumped into it.
This allows strong growth of useful aerobic microbes into flocs. Removal of dissolved organic matter at the treatment. The plant helps to save the liquefy oxygen balance of receiving river, lake or stream.
While increasing these microbes occupy the major part of the organic matter in the effluent. This remarkably reduces the BOD (Biological oxygen demand).
BOD test measures the absorption of oxygen by microorganisms selected water thus, BOD measures the organic substances present in the water.
If the increase in the BOD of wastewater then it signs a more polluting potential. Three biological methods involve secondary treatment, trickling filter, activated sludge process, and oxidation pond. Fourth is the less usual method I.e., Rotating biological contactor.

Note:
Biological oxygen demand (BOD) means the quantity of the oxygen that would be absorbed if all the organic matter in one litre of water where the bacteria is oxidised. The sewage water treatment is continued till BOD is refused.