Explain the role of microbes in sewage treatment.

Explanation

Microbes in sewage treatment.
Sewage. Municipal waste water is called sewage. It contains organic matter and microbes which are pathogenic.
Sewage is treated in STPs (Sewage Treatment Plants)
Primary Treatment.
(i) It involves physical removal of particles by (debris) filtration and sedimentation (grit).
(ii) Settled solids called sludge are discarded and supernatent effluent is taken for secondary treatment.
Secondary treatment/Biological treatment.
(i) Primary effluent is mechanically agitated and aerated in large aeration tanks to allow growth of aerobic microbes into flocs. (Which is inter woven mass of bacteria and fungal filaments).
(ii) Microbes grow and consume organic matter thus reducing BOD (Biochemical Oxygen Demand).
(iii) Now effluent is passed to settling tank where flocs sediment. This sediment is called activated sludge.
(iv) Activated sludge sediment is sent back to aeration tank as inoculum. Rest part is into anaerobic sludge digester.
(v) In digesters anaerobic bacteria digest the other bacteria, fungi and gases like methane, Hydrogen sulphide and `CO_2` are evolved.
(vi) Gases released are used as biogas and treated effluent is released into natural water bodies.