Explain the role of microbes in the treatment of sewage.

Explanation

Sewage is the municipal waste water having human excreta and ofter organic wastes and various disease causing microbes.
It is made less polluting by passing it through STP (Sewage Treatment Plants).
There are 2 stages of treatment as follows-
1. Primary Treatment.
`implies` It removes floating and suspendid solids from sewage through two process-filtration and sedimentation.
`implies`First floating matter (like polythene bags etc.), is removed through sequential filtration.
`implies` The filtrate is kept in large open settling tanks where grit (soil and small pebbles) settles down. The sediment is called primary sludge while the supernatant is called effluent.
`implies` The effluent from the primary settling is taken for secondary treatment.
2. Secondary Treatment. There are several methods of secondary treatment, e.g., oxidation tanks, trickling filter and activated sludge system.
`implies` In activated sludge system, the primary effluent is taken to aeration tanks where it is constantly agitated mechanically, Air is also pumped into it.
`implies` A large number of aerobic heterotrophic microbes grow in the aeration tank to form floes masses of bacteria, held together by sline and fungal filaments to form mesh like structures.
`implies` The microbes digest a lot of organic matter, converting it into microbial biomass and releasing a lot of minerals. As the
BOD (biochemical oxygen demand) of the waste matter is reduced to 10-15% of raw sewage, it is passed into settling tank where the bactrial floes undergo sedimentation.
`implies` The effluent is generally passed into natural waters like rivers and streams. BOD refers to the amount of oxygen that would be consumed, if all the organic matter in one litre of water were oxidised by bacteria.
`implies` The sediments of settling tank is called activated sludge. A part of it is used as inoculum in aeration tank. The remaining is passed into a large tank called anaerobic sludge digester.
`implies` Anaerobic microbes digest the organic mass as well as aerobic microbes of the sludge.
`implies` Methanbgenic bacteria produce a mixture of gases containing methane `H_2S` and `CO_2`. The mixture called biogas is inflammable and is a source of energy. The spent sludge can be used as manure or part of compost.