Simulation · Biology · Class 12
The whole sewage plant
From the lesson Secondary treatment and BOD in Microbes in Human Welfare. Change the values and watch what happens.
The whole sewage plantBiology · Class 12
The idea behind it
NCERT §8.3
- 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.