MICROBES IN HUMAN WELFARE III

MICROBES IN SEWAGE TREATMENT

Municipal waste-water-  large amounts of organic matter -- sewage.

Before disposal, sewage -- treatment in sewage treatment plants (STPs) by the heterotrophic microbes -- less polluting.

Sewage treatment is carried out in two stages.

1. Primary treatment –

• physical removal of large and small particles.
• Initially, floating debris is removed by sequential filtration and then the grit are removed by sedimentation.
• Settled solid -- primary sludge
•  supernatant -- effluent.

• The effluent from the primary settling tank is taken for secondary treatment.

1. Secondary treatment or biological treatment-
2.  primary effluent --large aeration tanks where it is constantly agitated -- vigorous growth of useful aerobic microbes into flocs.

• Flocs = masses of bacteria + fungal filaments = mesh like structures.

• While growing, the microbes significantly reduces the BOD (biochemical oxygen demand) which is the amount of oxygen required to oxidize total organic matter in the effluent.
• The BOD test measures the rate of uptake of oxygen by micro-organisms, the greater the BOD of waste water, more is its polluting potential.

• The effluent -- settling tank where the bacterial ‘flocs’ are allowed to sediment -- 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 --large tanks called anaerobic sludge digesters where other kinds of bacteria grow anaerobically which digest the bacteria and the fungi in the sludge.

• During digestion, bacteria --mixture of gases such as methane, hydrogen sulphide and carbon dioxide --biogas .

• The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

MICROBES IN THE PRODUCTION OF BIOGAS

Biogas is a mixture of gases (mainly methane) produced by the microbial activity.

Certain bacteria grow anaerobically on cellulosic material and produce large amount of methane along with CO2 and H2 which are collectively called as methanogens.

One common methanogen is Methanobacterium

These bacteria are commonly found in the anaerobic sludge during sewage treatment and in the rumen of cattle to digest cellulose in the food of the cattle , thus the excreta of cattle, commonly called gobar can be used for generation of biogas, commonly called gobar gas.

Biogas plant

• The biogas plant consists of a concrete tank (10-15 feet deep) -- bio-wastes are collected and a slurry of dung is fed.
• A floating cover is placed over the slurry, which keeps on rising as the gas is produced in the tank due to the microbial activity.
• The biogas plant has an outlet, which is connected to a pipe to supply biogas to nearby houses.
• The spent slurry is removed through another outlet and may be used as fertilizer.
• The biogas thus produced can be used for cooking and lighting.