Question

Describe the steps involved in getting clarified water from wastewater.

Answer 1

Hint: The municipal waste-water including human excreta is also called sewage. Such waste contains large amounts of organic matter and microbes many of which are pathogenic. And cannot be discharged into natural water bodies like rivers and streams directly. Before disposal, it is treated in sewage treatment plants (STPs) to make it less polluting.

Complete Answer:
Treatment of waste water is done naturally by microbes which are heterotrophic in nature,present in sewage. This treatment is of following stages: (a) Primary treatment, (b) Secondary treatment. (C) Tertiary treatment.
1. Primary treatment: It is a physical process which involves removal of large and small particles from sewage through filtration and sedimentation .
a. Sequential filtration: Floating debris are removed by this filtration
b. Sedimentation: The soil and little pebbles are removed by sedimentation All solids that settle form the primary sludge, and the supernatant forms the primary effluent. This effluent from the primary settling tank is taken for secondary treatment.

2. Secondary treatment or biological treatment : In this step, the primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it.
- In Aeration tanks , useful aerobic heterotrophic microbes vigorously grow into flocs.
When they grow, these microbes consume the major part of the organic matter present in the effluent. This significantly reduces the BOD (Biochemical oxygen demand) of the effluent a. BOD: The amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The Pollution potential of waste water is more, if its BOD is greater.
- The wastewater is treated till its BOD is reduced. Once the BOD of sewage or wastewater is reduced significantly, the effluent of aeration tank is then passed into a settling tank where bacterial flocs are allowed to sediment. This sediment is called activated sludge.
- A small amount of this sludge is pumped back into the aeration tank to serve as an inoculum. The residual major part of the sludge is pumped into large tanks called anaerobic sludge digesters.

b. Anaerobic sludge digesters : Here, the bacteria and fungi present in sludge are digested anaerobically by methanogenic bacteria. During this digestion, bacteria produce a mixture of methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can also be utilized as a source of energy as it is inflammable.
The effluent from the settling tank of the secondary treatment plant is generally released into natural water bodies like rivers and streams..

3. Tertiary treatment: If the waste water is to be used for domestic purpose, then tertiary treatment is done. It is a physico-chemical process in which chlorine gas, ozone gas, zirconium, and perchlorate salts. UV rays or reverse osmosis etc., are used to remove DDT, pesticides, pathogens and turbidity in wastewater.

Note: River Action Plans
(1) Due to the increase of urbanisation, sewage is being produced in much larger quantities than ever before. However, the number of sewage treatment plants has not increased enough to treat such large quantities So untreated sewage is often discharged directly into rivers leading to their pollution and increase in water borne diseases.
(2) The Ministry of Environment and Forests has initiated Ganga Action Plan (1985) and Yamuna Action Plan to save these major rivers of our country from pollution. Due to these plans, it is proposed to build a large number of sewage treatment plants, so that only treated sewage may be discharged in the rivers.