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 Once the BOD of sewage reduced significantly, the effluent is then passed into the settling tank where the
bacterial ’flocs’ are allowed to sediment. This sediment is called activated sludge.
 Small part of activated sludge is pumped back to aeration tank to serve as the inoculums.
 The remaining sludge is pumped into anaerobic sludge digester.

 In the anaerobic sludge digester there is other kinds of bacteria which grow anaerobically, digest the
bacteria and fungi in the sludge.
 During this digestion bacteria produce biogas, (mixture of methane, hydrogen sulphide and carbon dioxide)
 The effluent from the secondary treatment plant is released into natural water body like rivers and streams.
 Ganga Action Plan and Yamuna Action Plan initiated by Ministry of Environment and Forest to save these
major rivers of our country.
 It is proposed to build a large number of sewage treatment plants so that only treated sewage may be
discharged into the rivers.
MICROBES IN PRODUCTION OF BIOGAS:
 Biogas is a mixture of gases (predominantly methane) produced by the microbial activity and is used as
fuel.
 Certain bacteria grow anaerobically on cellulosic material, produce large amount of methane along with CO2
and H2S. These bacteria are collectively called methanogens. One common bacterium
is Methanobacterium.
 These bacteria present in the rumen of cattle, plays essential role in nutrition of cattle by digesting cellulose.
Hence the excreta (dung) used for the production of biogas.
MICROBES AS BIOCONTROL AGENT:
 Biocontrol refers to the use of biological methods for controlling plant diseases and pests.
 Effect of use of chemical, insecticide and pesticide to control disease and pests:
o These chemicals are toxic and extremely harmful to human beings and animals
o Polluting our environment (soil, ground water), fruits, and vegetables.
o Soil is polluted through use of weedicides to remove weeds.
Biological control of pest and disease:
 Use of biocontrol measures will greatly reduce our dependence on toxic chemical and pesticides.
 The Ladybird and Dragonflies are used to get rid of aphids and mosquitoes.
 Bacillus thuringiensis (Bt) used to control butterfly caterpillars.

 Dried spores are mixed with water and sprayed onto vulnerable plants, where these are eaten by the insect
larvae.
 In the gut of the larvae, the toxin is released and the larvae get killed.
 Trichodermaa free living fungus used to control several plant pathogens.
 Baculoviruses are pathogen 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 application.
 They have no negative impacts on plants, mammals, birds, fish, etc.

 This is very use full in integrated pest managements (IPM).