Question

How many of the following processes are involved in the primary treatment of sewage?
1. Filtration
2. Microbial proliferation
3. Sedimentation

• A. 1, 2 and 3
• B. 1 and 3
• C. 1 and 2
• D. 2 and 3

Answer 1

The correct option is B 1 and 3

The municipal waste water consisting of organic wastes and microbes generated from the cities and towns every day is called sewage. The sewage cannot be released into the water bodies directly as it can cause the pollution of water making it unfit for drinking. The pathogenic microbes of the sewage cause harmful diseases. Hence, the sewage is treated in the sewage treatment plants (STPs) before it is released into the water bodies.

Sewage treatment is carried out in two stages: primary treatment and secondary treatment.

Primary treatment: it involves physical removal of the particles from the sewage through filtration and sedimentation. Floating debris is removed by sequential filtration. This is carried out in the filtration tanks. After filtration, the waste water is allowed to stand undisturbed for a certain time. The heavier particles such as soil and small pebbles settle down under the effect of gravity and form the sediment which is referred to as primary sludge. The supernatant formed is called the effluent. The effluent is taken for biological treatment/secondary treatment.

Secondary treatment: the effluent is passed into the aeration tank and is mechanically agitated with continuous air supply. This results in the growth of a large number of aerobic heterotrophic microbes. These microbes form flocs (mesh-like structures containing masses of bacteria associated with the fungal filaments). They break down the organic matter present in the effluent. After the digestion of the organic matter, the effluent is now passed to the final settling tank. Sedimentation of the flocs takes place in this tank. The sediment obtained is called activated sludge. The effluent after secondary treatment is finally released into the water bodies.

Hence, the primary treatment of sewage involves filtration and sedimentation. The secondary treatment involves the breakdown of the organic matter with the help of microbes. Therefore, microbial proliferation is required during the secondary treatment of the sewage.