microbes in sewage treatment
Sewage is waste water having food residues, animal and human excreta, detergents, discharges from commercial and industrial
establishments and pathogenic microbes etc.
Sewage poses many harmful effects such as:- It results in dissemination of water-borne diseases caused by microorganisms.
- It may cause depletion of dissolved oxygen in water. Reduction in oxygen availability may kill aerobic aquatic organisms.
- Untreated sewage produces offensive odour.
Thus sewage treatment is essential for reducing harms done by it. Whenever untreated sewage is disposed into natural waters such as
streams, ponds, lakes, etc., the normal amount of dissolved oxygen present in water, gets quickly utilised by microorganisms as a
result of oxidation of organic matter present in sewage. Biochemical oxygen demand (BOD) is increased. BOD represents the amount
of dissolved oxygen that would be consumed if all the organic matter in one litre of water were oxidised by microorganism (at 20°C).
It is an indicator of water quality. Thus, high value of Biochemical Oxygen Demand (BOD) means the water is highly polluted by
organic matter. Low value of BOD means the water is either normal or less polluted by organic matter.
Sewage water can be purified by passing it through sewage treatment plants with the action of heterotrophic microorganisms. A
sewage treatment plant separates solids from liquids by physical processes and purifies the liquid by biological processes.
There are two stages of this treatment; primary or physical treatment and secondary or biological treatment.
primary treatment
It removes floating and suspended solids from sewage through two processes: filtration and sedimentation. First, floating matter
is removed through sequential filtration with small pore filter progressively. The filtrate is kept in large open settling tanks where
grit (sand, silt, small pebbles) settles down.
The sediment is called primary sludge while the supernatant is called effluent. The primary sludge traps a lot of microbes and
debris. It is subjected to composting or land fill where anaerobic digestion removes the organic matter.
secondary treatment
There are several methods of secondary treatment, e.g., oxidation
tanks, trickling filter, and activated sludge system.
In activated sludge system, the primary effluent is taken to aeration
tanks. A large number of aerobic heterotrophic microbes grow in the
aeration tank. They form flocs. Flocs are masses of bacteria held
together by slime and fungal filaments to form mesh like structures.
The microbes digest a lot of organic matter, converting it into microbial
biomass and releasing a lot of minerals.
As the BOD of the waste matter is reduced to 10-15% of raw sewage, it
is passed into settling tank. Thus secondary treatment is more or less
biological. The sediment of settling tank is called activated sludge. A
part of it is used as inoculum in aeration tank. The remaining is passed
into a large tank called anaerobic sludge digester. Anaerobic
microbes digest the organic mass as well as aerobic microbes of the
sludge. They are of two types, nonmethanogenic and methanogenic.
Methanogenic bacteria produce a mixture of gases containing methane, H 2 S and CO 2. The mixture called biogas is inflammable
and is a source of energy. The spent sludge can be used as manure or part of compost.
Finally the treated sewage effluent is subjected to chemical treatment for disinfection before releasing it into natural water bodies
like rivers and streams.
river action plan
Before 1985 very few cities and towns had sewage treatment plants. The municipal waste water was discharged directly into rivers
resulting in their pollution and high incidence of water borne diseases. 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 (GAP), Yamuna Action Plan, Sutlej Action Plan, Gomti Action Plan etc.