Sewage treatment and biogas
Role of microbes in treating sewage and producing biogas.
The essentials
Sewage is municipal wastewater containing large amounts of organic matter and microbes, many of them pathogenic. It cannot be released directly into rivers, so it is treated in sewage treatment plants (STPs), mostly by the action of heterotrophic microbes naturally present in sewage.
Primary treatment is physical. Floating debris is removed by sequential filtration, and grit (soil and small pebbles) is removed by sedimentation. Solids that settle form the primary sludge; the liquid above it is the effluent.
Secondary (biological) treatment: The primary effluent goes to large aeration tanks where it is mechanically agitated and air is pumped in. Aerobic microbes grow vigorously as flocs (masses of bacteria associated with fungal filaments in a mesh-like structure). They consume organic matter and reduce the BOD (biochemical oxygen demand) of the effluent. BOD is the amount of oxygen consumed if all the organic matter in one litre of water were oxidised by bacteria; higher BOD means greater polluting potential. When BOD falls enough, the effluent goes to a settling tank where flocs settle as activated sludge. A small part is pumped back to the aeration tank as inoculum; the rest goes to anaerobic sludge digesters, where anaerobic bacteria digest it and release gases such as methane, hydrogen sulphide and CO₂ (biogas). The treated effluent is then released into rivers and streams.
Biogas is a mixture of gases, mainly methane, made by microbial activity. Methanogens such as Methanobacterium grow anaerobically on cellulosic material. They live in the rumen of cattle and in cattle dung (gobar). A biogas plant has a concrete tank in which slurry of dung is fed; a floating cover rises as gas is produced, and an outlet lets out spent slurry that can be used as fertiliser. The technology was developed in India largely by IARI and KVIC.
Key points
- Primary treatment: physical; secondary treatment: biological.
- Lower BOD after treatment means less organic matter remains.
- Biogas: mainly methane, with CO₂ and some H₂S.
- The Ganga Action Plan and Yamuna Action Plan aimed to build STPs to reduce river pollution.