Secondary treatment and BOD

Secondary treatment is biological. The primary effluent is led into large aeration tanks, stirred mechanically all the time, with air pumped in.

This lets useful aerobic microbes grow vigorously into flocs: masses of bacteria joined with fungal filaments into mesh-like structures.

As they grow, the microbes use up most of the organic matter in the effluent, which greatly reduces its BOD.

BOD (biochemical oxygen demand) is the amount of oxygen that would be used up if bacteria oxidised all the organic matter in one litre of water.

The BOD test measures how fast micro-organisms in a water sample take up oxygen, so it is an indirect measure of organic matter. The higher the BOD of waste water, the more it can pollute.

Once BOD has fallen enough, the effluent goes to a settling tank where the flocs sediment; this sediment is activated sludge. A small part is pumped back into the aeration tank as inoculum.

Most of the sludge goes to large anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi of the sludge, giving off methane, hydrogen sulphide and CO₂. These gases form biogas, which burns and can be used as fuel.

The effluent of secondary treatment is usually released into rivers and streams. This microbial method has been used for more than a century, and no man-made technology has yet matched it.

Treatment plants have not kept pace with the growth of sewage, so untreated sewage often reaches rivers, polluting them and spreading water-borne diseases. The Ministry of Environment and Forests began the Ganga Action Plan and Yamuna Action Plan, which propose many new STPs so that only treated sewage enters these rivers.