Sewage treatment is carried out in two stages:

Primary treatment: This step involves physical removal of particles from sewage through filtration and sedimentation. Floating debris is removed by sequential filtration. Grit is removed by sedimentation. All solids that settle form the primary sludge. The supernatant forms the effluent. The effluent is sent for secondary treatment.

Secondary treatment or Biological Treatment: The primary effluent is sent to large aeration tanks. Effluent is then constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful microbes into flocs. (Flocs are the masses of bacteria associated with fungal filaments to form mesh like structures). These microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD of the effluent.

Biological Oxygen Demand (BOD): The amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria; is called biological oxygen demand. Higher BOD means higher level of pollution in water. Higher BOD shows higher level of organic matter in water.

When the BOD of wastewater is significantly reduced, the effluent is sent to a settling tank. In this tank, the bacterial ‘flocs’ are allowed to settle at the bottom. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as inoculums. Remaining part of the sludge is sent to large tanks called anaerobic sludge digesters. In this tank, anaerobic bacteria digest the bacteria and fungi in the sludge. During this process, a mixture of methane, hydrogen sulphide and carbon dioxide is produced. These gases form the biogas. Biogas is used as a source of energy. The effluent from the secondary treatment plant is usually released into natural water bodies.


Different types of gaseous end products are produced by microbes during their growth and metabolism. Some microbes produce carbon dioxide, while some others produce methane (along with carbon dioxide and hydrogen). Bacteria which produce methane are called methanogens. These bacteria grow anaerobically on cellulosic material, e.g. methanobacterium.

Biogas Plant: It consists of a concrete tank about 10 to 15 feet deep. Bio-wastes and a slurry of dung is fed in this tank. A floating cover is placed over the slurry. The cover keeps on rising with incremental production of gas. An outlet takes the biogas to the house where the gas can be utilized for various energy needs. The spent slurry is removed through another outlet and can be used as fertilizer. The technology of biogas production was developed in India by Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC).


Biocontrol: The use of biological methods for controlling plant diseases and pests is called biocontrol. Use of synthetic insecticides, weedicides and pesticides causes much harm; like soil pollution, water pollution, bioaccumulation, etc.

Ladybird and dragonflies help in getting rid of aphids and mosquitoes.

Bacillus thuringiensis (a bacterium) is available as dried spores in sachets. The dried spores are mixed with water and sprayed onto vulnerable plants; like brassicas and fruit trees. The insect larvae feed on these bacteria. A toxin is then released in the gut of the larvae; which results in the death of the larvae. It only kills the caterpillar; leaving other insects unharmed.

BT Crops: Scientists have succeeded in incorporating the gene from Bacillus thuringiensis (BT) into some crops. It helps such crops in fighting against the insect pests. BT cotton and BT brinjal are examples of such genetically modified (GM) crops.

Trichoderma species are free-living fungi which are very common in the root ecosystem. They are effective against several plant pathogens.

Baculoviruses attack insects and other arthropods. Majority of baculoviruses used as biocontrol agents are in the genus Nucleopolyhedrovirus. These viruses are excellent for species-specific narrow spectrum insecticidal applications. They have been shown to have no negative impacts on plants, mammals, birds, fish or even on non-target insects.


Overuse of chemical fertilizers causes much harm; like soil pollution, water pollution, bioaccumulation, etc. Organisms that enrich the nutrient quality of the soil are called biofertilisers. Bacteria, fungi and cyanobacteria are the main sources of biofertilisers.

Rhizobium bacteria live in symbiotic association with leguminous plants. These bacteria help in nitrogen fixation in soil. Azospirillum and Azotobacter are the free-living soil bacteria which help in nitrogen fixation.

Many members of the genus Glomus (fungi) form mycorrhiza, i.e. symbiotic association with plants. These fungi help the plant in absorption of phosphorus from soil.

Cyanobacteria serve as an important biofertiliser in paddy fields. Blue green algae also add organic matter to the soil and thus increase its fertility. Many biofertilisers are commercially available.

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