Define sewage .How are Microbes used in sewage treatment plant.

Sewage treatment in Sewage plants

Sewage is municipal waste containing large amounts of organic matter, Human excreta, and Pathogenic bacteria

Generally sewage is drained into water bodies like rivers, ponds etc. Before discharging into water bodies, sewage is made less polluting so sewage is treated in sewage treatment plants.

Sewage treatment is carried out in two steps. Physical treatment and Biological treatment. 

Physical treatment in sewage plants

Physical treatment includes removal of large and small particles by filtration by passing through a wire mesh.

The grit, soil, pebbles are removed by process of sedimentation in a tank. After physical treatment, the supernatant is known as effluent.

This effluent is undergone the Biological treatment.

Biological treatment in sewage plants

In biological treatment, the effluent is passed into an aeration tank where aerobic microbes are grown rapidly and form a group.

This group of aerobic microbes is called flocs. Flocs are masses of bacteria and look like a mesh -like structure.

These aerobic bacteria consumes organic matter present in sewage and reduces Biochemical  oxygen demand (BOD). BOD refers to the amount of oxygen required for consumption  of all organic matter in one litre of water oxidised by microbes.

BOD measures the amount of organic matter in water measuring the rate of oxygen uptake by microbes. Once  Biochemical  oxygen demand is reduced then effluent is called as activated sludge.

This sludge is passed out in anaerobic sludge digesters. In these digesters, heterotrophic microbes are grown anaerobically to digest the pathogenic bacteria and fungi.

In the digesters, during this process a mixture of gases such as methane hydrogen sulphide and carbon dioxide are released that are stored as biogas.

In 1985, Ministry of Finance and Environment started Ganga and yamuna action plan. Under this plan, sewage treatment plants have been set up near the major river to protect from water pollution, sewage treatment plants make seewage water from more polluted to less polluted before discharging into water.