Microbes in Sewage Treatment
Sewage (municipal wastewater) contains large amounts of organic matter and microbes. Sewage is treated in Sewage Treatment Plants (STPs) to make it less polluting. It includes two stages.
Primary Treatment
It is the physical removal of particles. It includes:
- Removal of floating debris by sequential filtration.
- Removal of the grit (soil and pebbles) by sedimentation.
The settled solids form the primary sludge, and the supernatant forms the primary effluent.
Secondary Treatment (Biological Treatment)
- Primary effluent is passed into large aeration tanks and constantly agitated. This allows vigorous growth of useful aerobic microbes into flocs (bacteria associated with fungal filaments to form mesh-like structures). These microbes consume the organic matter in the effluent, reducing the Biochemical Oxygen Demand (BOD).
- The effluent is then passed into a settling tank where the bacterial flocs are sedimented. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining sludge is pumped into large tanks called anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge, producing gases like CH4, H2S, and CO2. These gases form biogas.
- The effluent is released into natural water bodies like rivers and streams.
- The Ministry of Environment and Forests initiated the Ganga Action Plan and Yamuna Action Plan to save rivers from water pollution.
BOD: Amount of O2 consumed by bacteria to oxidize all organic matter in one litre of water. It is a measure of organic matter present in the water. The greater the BOD, the higher its polluting potential.
Microbes in Biogas Production
- Biogas is a mixture of gases (mainly CH4) produced by microbial activity. It is used for cooking and lighting.
- Methanogens grow anaerobically on cellulosic material and produce CH4. E.g., Methanobacterium.
- Methanobacterium is found in anaerobic sludge and the rumen of cattle (for cellulose digestion).
- The cattle dung (gobar) is rich in these bacteria. Dung can be used for generation of biogas (gobar gas).
- The Biogas plant consists of:
- A concrete tank (10–15 feet deep) to collect bio-wastes and slurry of dung. A floating cover is placed over the slurry, which rises as biogas is produced.
- An outlet connected to a pipe to supply biogas.
- An outlet to remove spent slurry (used as fertilizer).
Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC) developed the technology of biogas production in India.