Secondary treatment or Biological treatment : The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into Flocs (masses of bacteria associated with fungal filaments to form mesh like structures).

While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (Biochemical Oxygen Demand) of the effluent. BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria.

The sewage water is treated till the BOD is reduced. The BOD test measures the rate of uptake of oxygen by microorganisms in a sample of water and thus, indirectly, BOD is a measure of organic matter present in the water. The greater the BOD of waste water, more is its polluting potential.

Biogas is a mixture of gases such as carbon dioxide and methane and H2. It has predominantly gas is methane about 60%. The technology of biogas production was developed in India mainly due to the efforts of Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC).

Bacteria are collectively called methanogens, that act on cattle dung. and one such common bacterium is Methanobacterium. These bacteria are commonly found in the anaerobic sludge during sewage treatment. These bacteria are also present in the rumen (a part of stomach) of cattle.

Cattle dung is available in large quantities in rural areas where cattle are used for a variety of purposes. So biogas plants are more often built in rural areas.

Alexander Fleming

Antibiotics produced by microbes are regarded as one of the most significant discoveries of the twentieth century and have greatly contributed towards the welfare of the human society.

Alexander Fleming while working on Staphylococci bacteria, once observed a mould growing in one of his unwashed culture plates around which Staphylococci could not grow. He found out that it was due to a chemical produced by the mould and he named it Penicillin after the mould Penicillium notatum.

However, its full potential as an effective antibiotic was established much later by Ernest Chain and Howard Florey. After Penicillin, other antibiotics were also purified from other microbes. Antibiotics have greatly improved our capacity to treat deadly diseases such as plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog), which used to kill millions all over the globe.

We have now realised that there are problems associated with the overuse of chemical fertilisers and there is a large pressure to switch to organic farming – the use of biofertilisers. Biofertilisers are organisms that enrich the nutrient quality of the soil.

The main sources of biofertilisers are bacteria, fungi and cyanobacteria. the nodules on the roots of leguminous plants formed by the symbiotic association of Rhizobium. These bacteria fix atmospheric nitrogen into organic forms, which is used by the plant as nutrient.

Other bacteria that can fix atmospheric nitrogen while free-living in the soil are Azospirillum and Azotobacter, thus enriching the nitrogen content of the soil. Mycorrhiza also play significant role for the plants. Anabaena, Nostoc and Oscillatoria also makes soil nitrogen rich.

People of a village in Gujarat are setting up a biogas plant to equip their village with renewable energy. As per the diagram, a mixture of cow dung and water is put in a digestion tank. In the anaerobic condition of a digestion tank, microorganisms decompose the organic matter. This process mainly produces gases like methane (CH4) and carbon dioxide (CO2). The biogas plant has an outlet, which is connected to a pipe to supply biogas to nearby houses. The semi-liquid material remaining after digestion comes out as sludge, which is an excellent natural fertilizer for agriculture.

A progressive group of farmers from Gujarat decided to adopt eco-friendly farming. The continuous use of chemical pesticides was reducing the fertility of the soil and destroying the beneficial flora. So they adopted biocontrol methods. To control harmful insects in the field, they used Bacillus thuringiensis (Bt), which destroys the larvae of certain types of insects.

To protect crop against fungal diseases. Trichoderma fungus was used, which suppresses the growth of pathogenic fungi, for pest control, NPV (Nucleo Polyhedro Virus) was used, which is a virus of the Baculovirus group and affects only specific target pests.

In addition, natural control of harmful insects like aphids was achieved by using ladybird insects in cotton and vegetable fields. All these biocontrol methods increased crop production and had no harmful effects on the environment and other beneficial organisms.