Sewage Treatment Biogas

10.7 Microbes in Sewage Treatment

Every Indian city produces enormous quantities of waste water — called sewage. It is loaded with organic matter and pathogenic microbes. If discharged untreated, sewage devastates rivers, lakes and oceans — kills fish, contaminates drinking water, spreads cholera and typhoid. Microbes themselves help treat the very sewage they live in — a beautifully ironic solution.

Modern Sewage Treatment Plants (STPs) use two principal stages — primary treatment (physical) and secondary treatment (biological, involving microbes). Many advanced STPs add a tertiary (chemical/UV) polishing stage too.

Primary Treatment — Physical Removal

Primary treatment involves the physical removal of large and small particles from sewage:

1. Bar screens / sequential filtration: Remove floating debris — paper, plastic, twigs.
2. Grit chamber: Allows grit, sand, pebbles to settle.
3. Primary sedimentation tank (PST): Remaining particles settle as primary sludge; oils/fats float as scum. Both are removed.
4. The supernatant (clarified water with dissolved organics) is now called primary effluent and is passed to secondary treatment.

Secondary Treatment — Biological (the Microbe Stage)

This is the microbe-driven stage. Primary effluent is passed into large aeration tanks, where:

• Vigorous mechanical agitation and air pumping introduce oxygen.
• Naturally occurring aerobic microbes form masses called flocs — bacterial+fungal masses.
• Flocs feed on dissolved organic matter, breaking it down into CO₂, H₂O and biomass. BOD (biological oxygen demand) drops dramatically.

Once BOD is reduced, the effluent is passed to a settling tank, where the flocs (now called activated sludge) settle. A part of this sludge is pumped back to the aeration tank as inoculum. The rest is pumped into large anaerobic sludge digesters.

Anaerobic Sludge Digestion

In the digesters, anaerobic bacteria (including methanogens) digest the bacterial flocs and remaining organic matter, producing a mixture of gases — methane (CH₄), hydrogen sulphide (H₂S), and CO₂ — collectively called biogas. Thus, even the waste sludge yields valuable fuel.

The treated effluent (clean water) is generally released into rivers/streams. The Ganga Action Plan and Yamuna Action Plan were launched to install adequate STPs along Indian rivers; large progress has been made under the Namami Gange Mission.

10.8 Microbes in Biogas Production

Biogas is a mixture of gases (predominantly methane CH₄) produced by microbial activity in the absence of oxygen. Biogas can be used as fuel.

Methanogens — the Gas Producers

The bacteria that produce methane gas are collectively called methanogens. The most notable is Methanobacterium. They are also found in:

• Rumen of cattle — where they help digest cellulose. This is why cattle dung is rich in methanogens and produces biogas easily.
• Anaerobic sewage-sludge digesters — as seen in section 10.7.
• Paddy fields, marshes, swamps — sources of "marsh gas".

Fixed-Dome Biogas Plant (KVIC / Deenbandhu Models)

India's biogas plants are based on the constant availability of cattle dung in villages. The technology was developed by the Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC). The fixed-dome (Deenbandhu) design is most popular.

Components of a biogas plant:

1. Inlet pipe / mixing tank: Cattle dung mixed with water in 1:1 ratio is poured in.
2. Digester / dome: An underground brick chamber where anaerobic digestion occurs. Methanogens produce gas; the gas collects in the dome.
3. Gas outlet: Pipe at the top of the dome leads gas to nearby kitchens.
4. Outlet chamber (slurry tank): Spent slurry is pushed out as the dome fills with gas. Slurry is a rich manure (NPK).

Composition and Use of Biogas

Uses: Cooking fuel (replaces firewood, kerosene, LPG); electricity generation (rural micro-grids); vehicle fuel after purification (Bio-CNG); industrial heating.

10.9 Worked Examples