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BOD measures the milligrams of oxygen consumed during the biological decomposition of organic matter occurring in a litre of waste water during a specified period of time.BOD value is an indication of pollution , so if the BOD value is high the water is highly polluted and if it is low i indicates less pollution.
Untreated sewage water is most polluted as it contains high amount of human excreta, wash waters and industrial and agricultural wastes, so it has highest BOD value. River water is clean and contains less organic material, thus it has least BOD value among all. So they can be labelled as :

Sample	BOD Value	Type of Sample
A → B → C →	20 mg / It  → 8 mg / It  → 400 mg / It  →	Secondary effluent River water Untreated sewage water

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Biofertilizers play a vital role to solve the problems of soil fertility and soil productivity.

1. Anabaena, a cyanobacterium increases the fertility, enriches soil with organic material and fixes nitrogen. 

2. Rhizobium also fix atmospheric nitrogen as nitrates and nitrites and lives as a symbiont in the roots of leguminous plants.

3. Mycorrhizae formed by association of bacteria and roots of higher plants increase soil fertility.

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The order according to the importance will be
Penicillin - Biogas - Citric acid - Curd.


1.Penicillin. It is an antibiotic used to control number of bacterial diseases such as malaria , diphtheria, lung infection etc.

2. Biogas. It is an ecofriendly gas which is used as a fuel. The slurry produced by its production is used as a manure.

3. Citric acid. It is used as preservative in many food items.

4. Curd. It is a dairy product which is a rich source of vitamin B_^12.

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1. Cyclosporin A is produced by Trichoderma polysporum.

2. Statin is produced by Aspergillus.

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(a) Single cell protein (SCP) are basically produced from the microbes, for example Candida utilis, Chlorella, Spirulina, Saccharomyces. The microbes are grown on an industrail scale for this purpose and they provide a huge amount of protein in a short span of time.  The single cell proteins are used as food supplements for humans.

(b) Soil- Microbes in the soil play an important role in increasing the fertility of the soil. Microbes breakdown the oraganic materials and enrich the soil with nutrients. Rhizobium is a symbiotic bacteria which helps to fix nitrogen in leguminous plants.