Millions of tonnes of waste water, called sewage is released from houses and industries. Under the Ganga Action Plan and Yamuna Action Plan, sewage should be treated in sewage treatment plants before letting into rivers and streams to avoid pollution. In India, microbes are used to treat gallons of waste water in the sewage plants. Microbial treatment of sewage usually takes place in two stages: primary and secondary treatment.
In primary treatment, the larger and smaller objects found in sewage are removed. In this stage, the floating debris is removed by sequential filtration, whereas the grit, such as coarse soil or pebbles is removed via sedimentation in a primary settling tank. The settled organic and inorganic solids are called primary sludge while the floating grease and plastic materials comprise primary effluents, which are transferred for secondary treatment.
In secondary treatment, organic matter in the effluents is reduced. In this stage, effluents are agitated mechanically with rich supply of oxygen in aeration tanks. So, the aerobic microbes in the tank utilise oxygen and breakdown the organic matter present in the effluents. BOD or biochemical oxygen demand refers to the amount of oxygen needed by the microbes to oxidise all the organic matter in one litre of water. Therefore, the BOD is an indicator of the amount of organic matter present in water. The BOD of drinking water should be less than 0.5. However, the BOD of raw sewage can be as high as six hundred milligrams per litre.
Once the BOD of water decreases, the effluents are transferred to another aeration tank, where bacteria is gradually allowed to sediment. This sedimented ‘activated sludge’ is pumped into anaerobic sludge digesters. Here, anaerobic bacteria digest the organic matter present in the sludge and produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. This mixture of gases is also called biogas and is used as fuel, while the sludge is used as manure. The effluents from the secondary treatment plant are released into the natural water bodies.