Waste water (or) Sewage Treatment process

Objectives of waste water management :

The main objectives of waste water treatment are

i) To convert harmful compounds into harmless compounds.

ii) To eliminate offensive smell.

iii) To remove the solid content of the sewage.

iv) To destroy the disease producing micro-organisms.

Treatment Process :

The sewage (or) waste water treatment process involves the following steps.

i). Preliminary Treatment :

In this treatment, coarse solids and suspended impurities are removed by passing the waste water through bar and mesh screens.

ii). Primary Treatment (or) Settling Process :

In this treatment, greater proportion of the suspended inorganic and organic solids are removed from the liquid sewage by settling. In order to facilitate quick settling coagulants like alum, ferrous sulphate are added. These produce large gelatinous precipitates, which entrap finely divided organic matter and settle rapidly.

Al2(So4)3 + 6H2O --------------> 2 Al (OH)3 + 3H2SO4

iii) Secondary (or) Biological Treatment :

In this treatment , biodegradable organic impurities are removed by aerobic bacteria. It removes upto 90% of the oxygen demanding wastes. This is done by trickling filter or activated sludge process.

1. Trickling filter process

2. Activated Sludge process

iv) Tertiary treatment

After the secondary treatment, the sewage effluent has a lower BOD (25 PPM), which can be removed by the tertiary treatment process.

In the tertiary treatment, the effluent is introduced into a flocculation tank, where lime is added to remove phosphates. From the flocculation tank the effluent is led to ammonia stripping tower, where pH is maintained to 11 and the NH4 is converted to gaseous NH3. Then the effluent is allowed to pass through activated charcoal column, where minute organic wastes are organic wastes are absorbed by charcoal. Finally the effluent water is treated with disinfectant ( Chlorine ).

v) Disposal Of Sludge :

This is the last stage in the sewage treatment. Sludge formed from different steps can be disposed by

i) Dumping into low-lying areas.

ii) Burning of Sludge (incineration).

iii) Dumping into the sea.

iv) Using it as low grade fertilizers.

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