Microbes in Human Welfare - Sewage Treatment

Sewage is the municipal waste water containing large amounts of organic matter and pathogenic microbes, requiring treatment before discharge into natural water bodies.

Primary Treatment: A physical process involving sequential filtration and sedimentation to remove floating debris and grit (soil and small pebbles). The settled solids form the 'primary sludge' and the supernatant forms the 'effluent'.

Secondary Treatment (Biological Treatment): The primary effluent is passed into large aeration tanks where it is constantly agitated. This allows the growth of 'flocs'—masses of bacteria associated with fungal filaments to form mesh-like structures.

Biochemical Oxygen Demand ($BOD$): It refers to the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria. The $BOD$ test measures the rate of uptake of oxygen by micro-organisms; hence, $BOD$ is a measure of the organic matter present in the water.

The relationship between pollution and $BOD$ is direct: Higher $BOD$ $\implies$ Higher polluting potential of the waste water.

Anaerobic Sludge Digestion: Once $BOD$ is significantly reduced, the effluent is passed into a settling tank where bacterial flocs are allowed to sediment, called 'activated sludge'. A small part is reused as inoculum, while the rest is pumped into anaerobic sludge digesters.

During anaerobic digestion, bacteria produce a mixture of gases such as methane ($CH_4$), hydrogen sulphide ($H_2S$), and carbon dioxide ($CO_2$), which form 'biogas' and can be used as an energy source.

The Ministry of Environment and Forests has initiated the Ganga Action Plan and Yamuna Action Plan to build sewage treatment plants ($STPs$) and save major rivers from pollution.