Primary treatment: These treatment steps involve physical removal of particles from the sewage through filtration and sedimentation. These are removed in stages; initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge,and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.
During secondary treatment, the primary effluent is passed into large aeration tanks. Air is also passed into these tanks and agitated. This enables the growth of aerobic microbes into flocs. While growing, the bacteria consume majority of the organic matter present in the effluent. This causes a significant reduction in the biochemical oxygen demand (BOD) of the effluent.
Essential to control pollution in natural water bodies, to check water borne diseases/pathogenic organism.
Detailed Answer:
Sewage water can be purified by passing it through sewage treatment plants with the help of the action of microorganisms. In this plant the solids are separated from liquid by physical processes and the liquids are purified by biological processes. There are three stages of this treatment l.e. primary, secondary and tertiary. The primary treatment is physical, secondary treatment is biological while the tertiary treatment is chemical. Sewage Treatment (In STP's Binary Treatment Plants)
(i) Primary Treatment: Physical particles like debris and soil, sand, silt etc. removed by :
(i) sequential filtration.
(ii) Sedimentation.
The substances that settle down forms primary sludge and the effluent is primary effluent.
(ii) Secondary Treatment :
Biological treatment in which primary effluent is passed to large aeration tanks and is constantly agitated and supplied with air/O2.
This causes creation of flocs or association of useful aerobic bacteria and fungal filaments into a meshlike structure.
In this process, the microbes use up organic matter in the polluted water and hence reduce the BOD Biological Oxygen Demand (amount of oxygen required to oxidize all the organic matter in 1 litre polluted water by microbes).
Once the BOD is sufficiently reduced, it is allowed to pass to a settling tank where flocs are allowed to settle down - activated sludge.
A little sludge is pumped back to the aeration tank to serve as inoculums but most is passed to the anaerobic sludge digester where it is degraded by useful anaerobic bacteria (e.g., methanogens) releasing a mixture of gases like CH4,CO2,H2,H2S etc.(biogas).
The water coming out after this treatment is hence purified and can be discharged into water bodies.
This treatment is essential to :
(i) Avoid pollution of the natural water bodies by excessive accumulation of harmful chemicals, organic matter and nutrients which may lead to eutrophication.
(ii) Prevent the spread of infectious water borne diseases caused due to the deadly pathogens in polluted water.
