Test: Microbes in Sewage Treatment (January 10) - NEET MCQ

Flocs are masses of bacteria held together by slime and fungal filaments to form mesh-like structures.

A is Mechanical agitation; B is Reduced BOD; C is Activated sludge; D is Anaerobic sludge digesters.

The Ministry of Environment and Forests has. initiated Ganga Action Plan and Yamuna Action Plan, to save these major rivers of our country from pollution. Under these plans, it is proposed to build a large number of sewage treatment plants so that only treated water may be discharged in the rivers.

Activated sludge, is formed during secondary sewage treatment. It posses flocs of decomposer microbes. Formation of activated sludge requires aeration.

Primary or physical treatment is the process of removal of small and large, floating and suspended solids from sewage through two processes of filtration and sedimentation. First floating and suspended matter is removed through sequential filtration with progressively smaller pore filters. The sediment is called primary sludge while the supernatant is called effluent. The primary sludge traps a lot of microbes and debris. It is subjected to composting, land fill or anaerobic digestion to produce biogas and manure.

BOD (biochemical oxygen demand) refers to the amount of the oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The BOD test measures the rate of update of oxygen by microorganisms in a sample of water and thus, indirectly BOD is the measure of the organic matter present in the water. The greater the BOD of wastewater, more is its polluting potential.

Primary treatment in sewage treatment involves physical removal of particles (large and small) from the sewage through filtration and sedimentation. Initially floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. 

The activated sludge system, a part of secondary treatment is one of the widely used aerobic treatment systems for  waste water in which very vigorous aeration of the sewage is done. The sewage is passed into an aeration tank from the primary settling tank.
The flocs are allowed to settle down in secondary settling tank, the bacterial flocks are allowed to undergo sedimentation. The effluent or supernatant is  generally passed into natural water bodies like rivers and streams. The sediment of the settling tank is called activated sludge. A part of it is used as inoculum in aeration tanks. The remaining part is passed into a large tank called anaerobic sludge digesters.

Secondary treatment of sewage (or biological treatment) deplets 90−95% of the BOD and may pathogens are removed. Reduction of BOD by 90% is achieved through mineralisation of small fraction of organic matter and convertion of large proportion to removable solids.