How Microbes Clean Sewage and Feed Crops

Towns and cities produce huge volumes of waste water every day, and farms need ways to control pests and replace nutrients without harming the soil. Microbes quietly do much of this work — cleaning sewage, producing fuel gas, killing pests and fixing nitrogen.

This part covers sewage treatment, biogas production, biocontrol agents and biofertilisers.

Primary treatment physically removes floating debris, grit and settleable solids, while secondary, biological treatment lets aerobic microbes grow as flocs that consume organic matter; biochemical oxygen demand, or BOD, measures how much oxygen microbes need to oxidise the organic matter in water, so a higher BOD means more pollution.

Primary treatment:

- Sequential filtration removes floating debris
- Sedimentation removes grit such as soil and small pebbles
- Settled solids form primary sludge; the liquid above, the effluent, goes on to secondary treatment

Secondary treatment:

- The effluent enters large aeration tanks, where it is constantly stirred and air is pumped in
- Useful aerobic microbes grow rapidly into flocs — masses of bacteria bound with fungal filaments in mesh-like structures
- The microbes consume most of the organic matter, sharply reducing BOD
- In a settling tank, the flocs settle as activated sludge; a small part returns to the aeration tank as inoculum, and the rest goes to anaerobic sludge digesters

BOD. The amount of oxygen that would be used if bacteria oxidised all the organic matter in one litre of water.

An everyday example. Sewage treatment plants built along rivers such as the Ganga and Yamuna aim to clean waste water before it reaches the river.

The substance. A high BOD harms rivers by using up dissolved oxygen, suffocating fish and other aquatic life.

In anaerobic sludge digesters, anaerobic bacteria break down the microbes in activated sludge and release biogas, a mixture of methane, hydrogen sulphide and carbon dioxide; methanogens such as Methanobacterium, also found in the rumen of cattle, produce the methane, and a biogas plant uses cattle dung to make fuel gas and manure.

Anaerobic sludge digestion:

- Activated sludge is pumped into large anaerobic sludge digesters
- Anaerobic bacteria digest the bacteria and fungi in the sludge
- The gases produced form biogas, a useful source of energy

Methanogens:

- Bacteria that grow anaerobically on cellulose-rich material and produce large amounts of methane with carbon dioxide and hydrogen
- Example: Methanobacterium
- Found in anaerobic sludge and in the rumen of cattle, where they help break down cellulose, so cattle dung is rich in them

Biogas plant:

- A concrete tank about $10$ to $15$ feet deep is fed with bio-wastes and a slurry of dung
- A floating cover over the slurry rises as microbes produce gas
- An outlet pipe carries biogas to nearby houses for cooking and lighting
- Spent slurry is removed through another outlet and used as fertiliser

An everyday example. Gobar gas plants in Indian villages give families clean cooking fuel and rich manure from the same cattle dung.

The substance. Biogas forms only without oxygen — methanogens are anaerobic, so a tank that lets air in stops making methane.

Biocontrol uses living organisms to control plant diseases and pests: Bacillus thuringiensis releases a toxin that kills caterpillars, the fungus Trichoderma controls soil-borne plant pathogens, and baculoviruses attack particular insects, making all three well suited to organic farming and integrated pest management.

Why biocontrol:

- Chemical pesticides are toxic to people and animals and pollute soil and water
- Organic farming aims to keep pests at manageable levels through natural checks and balances rather than wiping them out
- Integrated pest management (IPM) combines biological control with limited chemical use

***Bacillus thuringiensis* (Bt):

- Spores sprayed on crops such as brassicas and fruit trees are eaten by insect larvae
- In the larval gut a toxin is released that kills the caterpillars but leaves other insects unharmed
- Bt toxin genes have been introduced into crops such as cotton, producing insect-resistant Bt cotton

Trichoderma:

- Free-living fungi common in the root ecosystem
- Effective against several soil-borne plant pathogens

Baculoviruses:

- Viruses that attack insects and other arthropods, mostly of the genus Nucleopolyhedrovirus
- Species-specific and narrow-spectrum, with no harmful effect on plants, mammals, birds, fish or non-target insects

An everyday example. Bt cotton grown by farmers across India carries the Bt toxin gene to resist bollworms.

The substance. Biocontrol aims to manage pests, not eliminate them** — leaving some pests keeps their natural predators alive.

Biofertilisers are organisms that enrich soil nutrients: Rhizobium fixes nitrogen in legume root nodules, free-living Azospirillum and Azotobacter fix nitrogen in the soil, fungi of the genus Glomus form mycorrhizae that absorb phosphorus for plants, and cyanobacteria such as Anabaena and Nostoc fix nitrogen in paddy fields.

Why biofertilisers:

- Heavy use of chemical fertilisers has polluted soil and water
- Biofertilisers enrich soil naturally and support organic farming
- The main sources are bacteria, fungi and cyanobacteria

Bacteria:

- Rhizobium — lives symbiotically in the root nodules of legumes, fixing atmospheric nitrogen into forms the plant can use
- Azospirillum and Azotobacter — free-living in soil, also fixing atmospheric nitrogen

Fungi — mycorrhiza:

- Many species of Glomus form mycorrhizae with plant roots
- The fungus absorbs phosphorus from the soil and passes it to the plant

Cyanobacteria:

- Autotrophic microbes, many of which, such as ***Anabaena, Nostoc and Oscillatoria*, fix atmospheric nitrogen
- In paddy fields they are important biofertilisers and also add organic matter to the soil

An everyday example. Farmers who grow moong or chana between cereal crops** enrich their soil with nitrogen fixed by Rhizobium in the legume roots.

The substance. Mycorrhizae do not fix nitrogen — their main contribution is phosphorus uptake.