Microbes in Sewage Treatment & in Production of Biogas | Biology Class 12 - NEET PDF Download

Microbes In Sewage Treatment

Municipal waste-water which contains large amounts of organic matter is called sewage.
Before disposal, hence, sewage is treated in sewage treatment plants (STPs) by the heterotrophic microbes to make it less polluting.
Sewage treatment is carried out in two stages.

Primary Treatment

• These treatment steps basically involve physical removal of large and small particles.
• Initially, floating debris is removed by sequential filtration and then the grit are removed by sedimentation.
• All solids that settle form the primary sludge, and the supernatant forms the effluent.
• The effluent from the primary settling tank is taken for secondary treatment.

Secondary Treatment or Biological Treatment

• The primary effluent is passed into large aeration tanks where it is constantly agitated which allows vigorous growth of useful aerobic microbes into flocs.
• Flocs are the masses of bacteria associated with fungal filaments to form mesh like structures.
• While growing, the microbes significantly reduces the BOD (biochemical oxygen demand) which is the amount of oxygen required to oxidize total organic matter in the effluent.
• The BOD test measures the rate of uptake of oxygen by micro-organisms, the greater the BOD of waste water, more is its polluting potential.
• The effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment and the sediment is called activated sludge .
• A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.
• The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters where other kinds of bacteria grow anaerobically which digest the bacteria and the fungi in the sludge.
• During digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide which form biogas .
• The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

Sewage Plant

Microbes In The Production of Biogas

Biogas is the mixture of gases produced by the microorganisms. It is a renewable source of energy. Methane is the predominant gas present in the biogas mixture.
Certain bacteria grow under anaerobic conditions and produce a large amount of methane along with carbon dioxide and hydrogen. The bacteria which produce the gaseous mixture are collectively known as methanogens. Methanobacterium is one such methanogen.
Methanobacterium is present inside the rumen of the cattle and the sludge produced during sewage treatment. The Methanobacterium present in the food of the cattle digests the cellulose present. The dung then produced by the cattle contains these methanogens which can be used for the production of biogas also known as the gobar gas.
Cattle dung is available in the rural area in very large quantities. Therefore, we can find biogas plants more often in rural areas. The biogas produced can be used for lighting and cooking purposes.
The conversion of waste into energy takes place in a biogas plant, by the activity of certain microbes.

Biogas Plant

• The biogas plant consists of a source to supply the feedstock, a digestion tank for biogas production, a biogas recovery unit to isolate the produced biogas, and heat exchanger to maintain the temperature of the digester.
• The biowaste and the slurry of dung are fed into an anaerobic digester.
• The slurry is covered with a floating cover. The gas produced due to microbial activity makes the cover rise upwards.
• The produced biogas is supplied to the respective places through connected pipes and can be used for cooking and lighting.
• The used slurry is removed through an outlet and can be used as fertilizer later.

Biogas Plant

Biogas Production in Landfill
Apart from the biogas plant, the biogas is also produced in the landfills.

• The organic matter naturally decomposes inside the landfill, i.e. inside a pit in the land, and biogas is produced by the activity of the microbes.
• The Methanobacteria present in the organic waste decompose the waste and produce the mixture of gases known as the biogas.
• There is a network of interconnected pipes in the landfill to collect the gas produced.
• The composition of the gas varies after a certain time interval. After a year, the composition of methane and carbon dioxide is 60% and 40% respectively.
• This method is gaining acceptance due to the fact that it prevents the explosion caused by the collection of methane inside the landfill, and also prevents the loss of methane in the atmosphere.
• The biogas thus produced is used to create electricity.

Substrates Required in Biogas Production

Advantages of Biogas

1. Biogas is a safe, cheap, renewable source of energy.
2. Biogas can be burnt in stoves to provide heat.
3. It is used for domestic and street lighting, and cooking.
4. It is eco-friendly and does not cause any pollution.
5. It is also used for driving engines.
6. It is easy to generate, transport and store.
7. It improves the sanitation of the surroundings.

The use of biogas is environment-friendly. It implies the conversion of animal and plant waste into useful energy, thereby, reducing the production of methane. This is because of the biogas combustion which results in a net decrease in the emission of greenhouse gases.