(a) Secondary treatment of sewage be explained in detail.
(b) Purification of water before dumping it into the source in order to avoid water from being more polluted.
(c) Values-Not to disturb nature's balance, self sufficiency, develop confidence, concern about the environment.

(i) Aerobic microbes
The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent. The sewage water is treated till the BOD is reduced. Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial 'flocs' are allowed to sediment.
ii). This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as source of energy as it is inflammable. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

(i) SCP refers to the large-scale production of protein obtained from unicellular or filamentous microorganisms.
(ii) Spirulina, Chlorella.
(iii) He was concerned about the welfare of the society.

(i) Biofertilisers are the microorganisms which bring about soil nutrient enrichment, maximise ecological benefits and minimise environmental hazards.
(ii) Nostoc, Anabaena.
(iii) The scientist was concerned about the clean environment and, therefore, he suggested Rahul to use the ecofriendly technique.

1. The very familiar beetle with red and black markings- the Ladybird, and Dragonflies are useful to get rid of aphids and mosquitoes, respectively.
2. Role of Bacillus thuringinesis.
(i) But coming to microbial biocontrol agents, that can be introduced in order to control butterfly caterpillars is the bacteria Bacillus thuringiensis (often 'Written as Bt.)
(ii) These re available in sachets of dried spores which are mixed with, water and sprayed onto vulnerable plants such as brassicas and fruit trees, where these are eaten by the insect larvae.
(iii) In the gut of the larvae, the toxin is released and the larvae get killed.
(iv) The bacterial disease will kill the caterpillars, but leave other insects unharmed.
(v) Because of the development of the methods of genetic engineering in the last decade or so, the scientists have introduced B. thuringiensis toxin genes into plants.
(vi) Such plants are resistant to attack by insect pests. Bt.-cotton is one such example which is being cultivated in some states of our country.
3. Biological control of Plant pathogens. A biological control developed for use in the treatment of plant disease is the fungus Trichoderma. Trichoderma spp. are free living fungi that are very common in soil and root ecosystems. They are effective biocontrol agents of several plant pathogens.
4. Baculoviruses are pathogens that attack insects and other arthropods. The majority of baculoviruses used as biological control agents are in the genus Nucleopolyhedrovirus. These viruses are excellent candidates for species-specific, narrow spectrum insecticidal applications. They have been shown to have no negative impacts on plants, mammals, birds, fish, or even on non-target insects. This is especially desirable when beneficial insects are being conserved to aid in an overall IPM (Integrated Pest Management) programme, or when an ecologically sensitive area is being treated.

1. Aeration to prevent alcohol formation.
2. Optimum pH = 5.
3. Ammonium phosphate that stimulates then growth of yeast.

MOEF of Government of India has initiated the Yamuna Action Plan and Ganga Action plan for saving Yamuna and Ganga rivers respectively.
Ganga Action Plan:
1.Due to increased urbanisation and industrialisation there is a huge quantity of sewage produced; but there are not enough sewage treatments plants and hence the untreated sewage is let into major rivers.
2. The government has planned to build a number of sewage treatment plants, because the sewage and effluents from the areas nearby Ganga rivers can be treated before they are let into the river.
3. The objective of Ganga Action Plan is to save the river from pollution.
Yamuna Action plan :
In order to protect the major rivers of India from sewage pollution, the Ministry of Environment and Forests has initiated development of sewage treatment plants under the National River Conservation Authority, e.g., Ganga Action Plan and Yamuna Action Plan. Industries contribute chemical effluents to the river's pollution load and the toxicity kills aquatic animals in large sections of the river.

During aerobic condition, the decomposing bacteria multiply very rapidly and associate with fungal filaments to form flocs. The aerobic bacteria decompose large amount of organic wastes of wastewater in the presence of oxygen produced by algae. But during anaerobic degradation, some toxic gases are produced which kill the bacteria and fungi.

(i) Curd is a common food item that supplies nutrients like calcium,vitamin A,B phosphours, etc
(ii) Biogas is used as energy source
(iii) Penicillin is an antibiotic, used to control several diseases.
(iv) Citric acid is used as a preservative in fruit juices, squashes, etc.