How do microbes help in the secondary treatment of sewage?

The correct option is A The aerobic microbes breakdown the organic matter in the sewage
Sewage is the municipal waste water generated in cities and towns. Apart from containing harmful inorganic and organic wastes, sewage is also rich in microbes, most of which are pathogenic (having the ability to cause a disease). Thus, sewage needs to be treated in sewage treatment plants (STPs) before being dumped into water bodies such as streams, rivers, etc.

Sewage treatment is done in 2 major stages
• primary treatment
• secondary treatment
Primary treatment of sewage involves physical removal of the particles in the sewage. This involves filtering out the floating debris in sewage by passing it through several filters of varying pore sizes. The filtrate is then allowed to sit undisturbed in large sedimentation tanks where the heavier stones and pebbles, etc settle down or sediment as primary sludge under the influence of gravity. The clear fluid collected above is called the primary effluent.

The primary effluent is subjected to secondary treatment. During the secondary treatment, the primary effluent is passed into huge aeration tanks where it is continuously agitated (disturbed) by stirring and pumping it with air. This leads to the rapid growth of heterotrophic aerobic microbes into flocs (masses of bacteria and filamentous fungi) that digest the organic wastes present in the sewage and degrade it. The microbial flocs use the pumped air to oxidise all the organic matter present in the sewage, thus the BOD (biochemical oxygen demand) is significantly reduced. BOD 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. More the organic matter in water, higher is the BOD, indicating high level of pollution. Secondary treatment is continued until the BOD of the waste water is significantly reduced.

After the secondary treatment, the effluent is passed through a large settling tank where the microbial flocs settle down. This sediment is called activated sludge. The effluent from the secondary treatment is released into water bodies.

The activated sludge is fed into an anaerobic sludge digester tank where anaerobic microbes digest the bacteria and fungi present in it in the absence of oxygen. During this process, gases like methane, carbon dioxide and hydrogen sulphide are released. These form the biogas which can be used for cooking.