(i) Bio-fertilizers are formulation of living microorganisms that enrich the nutrient quality of the soil. They increase physico - chemical properties of soils such as soil structure, texture, water holding capacity, cation exchange capacity and pH by providing several nutrients and sufficient organic matter.
(ii) The main sources of bio-fertilizers are bacteria, fungi and cyanobacteria. Rhizobium is a example for symbiotic nitrogen fixing bacteria. This bacterium infects the root nodules of leguminous plants and fixes atmospheric nitrogen into organic forms. Azospirillum and Azotobacter are free living bacteria that fix atmospheric nitrogen and enrich the nitrogen content of soil.
(iii) A symbiotic association between a fungus and the roots of the plants is called mycorrhiza. The fungal symbiont in these associations absorbs the phosphorus from soil and transfers to the plant. Plants having such association show other benefits such as resistance to root-borne pathogens, tolerance to salinity, drought, enhances plant growth and developments. For example, genus Glomus form mycorrhiza.
(iv) Cyanobacteria (or) blue green algae (BGA) are prokaryotic free-living organisms which can fix nitrogen. Oscillatoria, Nostoc, Anabaena, Tolypothrix are well known nitrogen fixing cyanobacteria. In water logged paddy fields Cyanobacteria multiply and fix molecular nitrogen. Cyanobacteria secrete growth promoting substances like indole-3-acetic acid, indole-3-butyric acid, naphthalene acetic acid, amino acids, proteins, vitamins which promotes plant growth and production.
(v) Biofertilizers are commonly used in organic farming methods.
(vi) This process involves the use of biological materials, avoiding synthetic substances to maintain soil fertility and ecological balance thereby minimizing pollution and wastage.

(i) Protecting soil quality using organic materials and encouraging biological activity.
(ii) Indirect provision of crop nutrients using soil microorganisms.
(iii) Nitrogen fixation in soils using legumes.
(iv) Weed and pest control based on methods like crop rotation, biological diversity, natural predators, organic manures and suitable chemical, thermal and biological interventions.

(i) Bacillus thuringiensis is a soil dwelling bacterium which is commonly used as a biopesticide and contains a toxin called cry toxin. Scientists have introduced this toxin producing genes into plants and have raised genetically engineered insect resistant plants. Eg. Bt-cotton.
(ii) When the insects ingest the toxin crystals their alkaline digestive tract denatures the insoluble crystals making them soluble. The cry toxin then gets inserted into the gut cell membrane and paralyzes the digestive tract. The insect then stops eating and starves to death.

1. Milk is converted into curd with the help of bacteria like Lactobacillus acidophilus Lactobacillus lactis and Streptococcus lactis are commonly called Lactic Acid Bacteria (LAB).
2. The LAB bacteria grow in milk and convert it into curd, thereby digesting the milk protein casein.
3. A small amount of curd added to fresh milk as a starter or inoculum contains millions of Lactobacilli, which under suitable temperature \(\left(\leq 40^{\circ} \mathrm{C}\right)\) multiply and convert milk into curd by fermentation.
4. Curd is more nutritious than milk as it contains a number of organic acids and vitamins.

(i) Biogas is a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen.
(ii) The slurry is drained through another outlet and is used as fertilizer.
(iii) Biogas is used for cooking and lighting.

BOD stands for Biological Oxygen Demand. BOD refers to the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria. The greater the BOD of the waste water, the more is its polluting potential.

Enzymes - Produced from Yeast (Saccharomyces cerevisiae) and bacteria
(i) Eg: pectinase, protease and cellulase - Bottled juices are clarified.
(ii) Rennet - separate milk into solid curds for cheese making.
(iii) Streptokinase (from Streptococcus) - Clot busier - (removing clots from the blood vessels of patients who have undergone myocardial infarction patients).
Cycloporin 'A' - Immuno suppressant used in organ transplantation (produced from the fungus Trichoderma Polysporum).

S. No	Primary Treatment	Secondary Treatment
i.	It involves the physical removal of solid, and particulate organic and inorganic materials fro the sewage through filtration and sedimentation. The supernatant is the effluent which is taken for secondary treatment.	The process takes place in large aeration tanks with vigorous growth of microbes. The organisms break down the organic matter and reduces the Biological oxygen demand it is further passed into a setting tank where are bacterial 'flocs' sediment to form the activated sludge.
ii.	It is a total physical process.	It is a biological process.
iii.	The solid waste is mainly removed.	The dissolved organic waste is removed by this process by aerobic and anaerobic means.
iv	It is a similar process.	It is more time consuming and a major part of sewage treatment.
v.	No by products are produced	Biogas production occurs.