Microbes in Human Welfare — BIOLOGY STD 12 Science — Question

Primary Sewage Treatment: Large and small particles are physically separated from the sewage by filtration and sedimentation. These are separated in different stages. Initially the floating waste is removed by sequential filtration. The grit (soil and small vesicle pebbles) is then removed by sedimentation. All the solids that are particles settled at the bottom form the primary sludge and the supernatant form the effluent. The effluent is taken from the primary filtration (settling) tank for secondary treatmentd.
Secondary Sewage Treatment: Primary effluent is passed through large pneumatic tanks. Where it is continuously stirred mechanically and the air is pumped into it. Due to this, beneficial aerobic microbes start growing vigorously into flocs.
During growth, this microbe consumes major part of organic substance present in the effluent. It starts reducing BOD (biochemical oxygen demand) of the effluent significantly. BOD refers to the amount of O₂ that the bacteria can consume and oxidize the organic substance present in 1 L of water. Sewage is treated until BOD is reduced.