Question

The Biological Oxygen Demand (BOD) of a primary effluent during sewage treatment is reduced. Enlist the process of how this is achieved.

Answer 1

1. Flocs: These are masses of bacteria held together by slime and fungal filaments to form mesh-like structures. These are used during the secondary sewage treatment in the aeration tank to increase the rate of decomposition. The microbes digest a lot of organic matter, converting it into microbial biomass and releasing many minerals. As a result, the BOD of sewage is reduced. The BOD of waste is reduced to 1% of that of raw sewage, and it is passed to the settling tank. In these tanks, flocs are allowed to undergo sedimentation.
2. Activated sludge: The sediment of the settling tank is called activated sludge. A part of it is used as an inoculum in aeration tanks. The remaining part is passed into a large tank called an anaerobic sludge digester. The anaerobic microbes digest the organic matter, while aerobic microbes from activated sludge also play a role in the process.