Microbes in sewage treatment
Sewage is cleaned in two stages: primary treatment removes solids physically; secondary treatment uses aerobic microbial flocs to cut BOD, then anaerobic bacteria digest the sludge.
Primary treatment (physical)
Raw sewage cannot go straight into rivers: it is full of organic matter and pathogens. It is treated in **sewage treatment plants (STPs)** by the **heterotrophic microbes** naturally present in it.
- 1Floating debris is removed by sequential filtration.
- 2Grit (soil and small pebbles) is removed by sedimentation.
- 3★All the solids that settle form the primary sludge; the liquid above (supernatant) is the effluent.
- 4The primary effluent goes for secondary treatment.
★ key step
Secondary (biological) treatment
- 1The primary effluent flows into large aeration tanks, where it is stirred mechanically and air is pumped in.
- 2Useful aerobic microbes grow vigorously into flocs.
- 3★The flocs consume most of the organic matter, so the BOD falls sharply.
- 4When the BOD is low enough, the water goes to a settling tank, where the flocs settle as activated sludge.
- 5A small part of the activated sludge is returned to the aeration tank to serve as inoculum.
- 6The major part goes to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in it, giving off methane, hydrogen sulphide and CO₂ (biogas).
- 7The treated effluent is released into rivers and streams.
★ key step
Draw it: flow of a sewage treatment plant
From the settling tank, the clear effluent goes to a river; from the digester, biogas is collected.
What BOD tells us
BOD measures the rate of oxygen uptake by microbes in a water sample. Microbes use more oxygen when there is more organic matter to break down, so BOD is an indirect measure of organic matter. The higher the BOD, the more polluting the water. Sewage is treated until its BOD is reduced; clean water has a low BOD.
Saving our rivers
- Untreated sewage in rivers raises BOD, uses up dissolved oxygen, and spreads disease.
- The Government of India (then the Ministry of Environment and Forests) started the Ganga Action Plan and the Yamuna Action Plan.
- Under these plans, many sewage treatment plants are built so that only treated sewage reaches the rivers.
Large Indian cities run STPs that use exactly these flocs and digesters, and some even run their pumps on the biogas from the sludge. Not flushing plastics, oil or medicines down the drain helps these microbes do their job.
2 marks: “What is BOD? What does a high BOD of a water sample indicate?” The amount of oxygen consumed if all organic matter in one litre of water were oxidised by bacteria. High BOD means much organic matter, so high polluting potential.
3 marks: “Differentiate between primary and secondary treatment of sewage.” Primary: physical removal of debris (filtration) and grit (sedimentation); primary sludge and effluent. Secondary: biological; aerobic flocs in aeration tanks reduce BOD; activated sludge; anaerobic digestion gives biogas.
5 marks: “Describe how sewage is treated in a sewage treatment plant. What is the role of microbes at each stage?” Primary treatment; aeration tank with flocs consuming organic matter and lowering BOD; settling tank with activated sludge, part as inoculum; anaerobic sludge digesters producing biogas; effluent to rivers. Ganga and Yamuna Action Plans.
F-S-A-S-D: Filter, Settle, Aerate, Settle again, Digest. High BOD = Bad Organic Dirt.