Microbes and sewage

Every day, towns and cities produce huge amounts of waste water, sewage. It contains human excreta, a great deal of organic matter, and many pathogens, so it cannot be released into rivers untreated. It is treated in sewage treatment plants (STPs), in two stages.

Primary treatment is physical: removing solid particles.

• Filtration: floating debris is removed by sequential filtration.
• Sedimentation: grit (soil, small pebbles) settles out.

The solids that settle form the primary sludge; the liquid above, the effluent, goes on to the next stage.

Secondary treatment is biological: microbes do the work.

• The effluent is passed into large aeration tanks, where it is constantly stirred and air is pumped in. Useful aerobic microbes grow vigorously and form flocs: masses of bacteria held together with fungal threads in a net-like structure.
• As they grow, the microbes consume most of the organic matter, and so greatly reduce the effluent's BOD.

BOD (biochemical oxygen demand) is the amount of oxygen that would be used up if all the organic matter in one litre of water were oxidised by bacteria. It measures how much organic matter is in the water, so the higher the BOD, the more polluting the water.

When the BOD has fallen enough, the effluent passes to a settling tank, where the bacterial flocs settle: this is the activated sludge.

• A small part of it is pumped back into the aeration tank to serve as the inoculum (a starter of useful microbes).
• The rest goes into large tanks called anaerobic sludge digesters. There, anaerobic bacteria digest the bacteria and fungi in the sludge, producing a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. This is biogas, which can be used as fuel.

The treated water is finally released into rivers and streams.

Untreated sewage still pollutes many of India's rivers. The Ministry of Environment and Forests launched the Ganga Action Plan and Yamuna Action Plan to build more sewage treatment plants, so that only treated sewage reaches the rivers.