BOD value - 30mg/L - Secondary effluent from STP
BOD value - 8mg/L – River Water
BOD value - 500mg/L – Sewage Water
(Fact – The greater the BOD of water, the more is its pollution potential.)

Vinegar is produced by using fruit juices(apple), starchy vegetables, malted cereals such as barley, rice, wheat and corn. Vinegar production is carried out by Saccharomyces cerevisiae and Acetobacter species.

The small amout of curd added to the milk contains millions of lactic acid bacteria they start multiplying at the suitable temperatures.
LAB produce acides that coagulate and partially digest the milk proteins; thus, milk is coverted into curd.
Vitamin B₁₂ content is increased

The treatment of sewage Involves two steps:
(i) Primary treatment and
(ii) Secondary treatment.
(i) Primary treatment
1. Primary treatment is a physical process of removal of small and large particles through filtration and sedimentation.
2. The first step is to remove the floating objects (like polythene bags) by letting the sewage to pass through wire mesh screens of sequentially smaller pore sizes.
3. Then the sewage is passed into the grit chamber, where grit is sedimented.
4. The sewage is allowed to go into the primary settling tank, where the suspended materials settle down to form primary sludge.
The effluent is taken for secondary treatment.
(ii) Secondary Treatment
1. It is a biological process, that employs the heterotrophic bacteria naturally present in the sewage.
2. The effluent from the primary treatment is passed into large aeration tanks, where it is constantly agitated and air is pumped into it.
3. This allows the rapid growth of aerobic bacteria into flocs, which consume the organic matter of the sewage and reduce the BOD.
4. The effluent is passed into a settling tank, where the flocs are allowed to sediment forming the activated sludge.
5. A small part of the activated sludge is pumped back into aeration tank as inoculum.
6. The remaining major part of the sludge is pumped into anaerobic sludge digesters, where the anaerobic bacteria digest the organic matter and produce methane, hydrogen sulphide and carbon dioxide.
7. The effluent from this is passed into the waterbody.

(i) It would be helpful to humans as microorganisms are fast growing, the food is rich in nutrients and it reduces pollution.
(ii) Initially, there can be very low response, but it is expected to have high response (as of mushroom culture in the recent years).
(iii) Recycling nutrients and minimising pollution for the betterment of the environment.

Biofertilisers are those organisms which enrich the soil fertility.
Categories of organisms:
(i) Bacteria, e.g. Rhizobium, Azotobacter
(ii) Cyanobacteria, e.g. Nostoc, Aulosira
(iii) Mycorrhizae/fungi, e.g. Glomus
Functions:
1. Bacteria and Cyanobacteria fix atmospheric nitrogen and increase the nitrogen content of the soil.
2. Cyanobacteria also add organic matter to the soil through their photosynthesis.
3. The fungi decompose the organic matter of the soil and make the nutrients available to the roots.
4. Mycorrhizae help in absorption and storage of some mineral nutrients like phosphorus for the plants.