Question

Study the given differences between primary sludge and Activated sludge and select the incorrect one

	Primary sludge	Activated sludge
(i)	It is sludge formed during primary sewage treatment	It is sludge formed during secondary sewage treatment
(ii)	It possesses flocs of Decomposer microbes	It does not possess flocs of Decomposer microbes
(iii)	It does not require aeration	Formation of activated sludge requires aeration
(iv)	A lot of decomposition occurs during the formation of primary sludge	Very little decomposition occurs during the formation of activated sludge

(a) (i) and (ii)
(b) (ii) and (iv)
(c) only (iv)
(d) (ii) and (iii)

Answer 1

Hint: Large quantities of water are generated every day in cities and towns. The municipal waste- water is also called sewage. It contains large amounts of organic matter and microbes. Before disposal sewage is treated in sewage treatment plants to make it less polluting. The treatment is carried out in two stages; Primary treatment and secondary treatment.

Complete step by step answer:
Primary treatment involves the physical removal of particles in large and small from the sewage through filtration and sedimentation. All the solids that settle down form the primary sludge and the supernatant forms the effluent. The primary effluent is passed into large aeration tanks. Where it is constantly agitated mechanically and the air is pumped into it. This allows the vigorous growth of useful aerobic microbes into flocs. While growing the microbes consume a major part of the organic matter in the effluent. This significantly reduces the BOD of effluent. The sewage water is treated until the BOD is reduced. Once the BOD of sewage is reduced significantly, the effluent is then passed into a settling tank where bacterial flocs are allowed to sediment. This sediment is called activated sludge. A small part of activated sludge is pumped back to the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into larger tanks called anaerobic sludge digesters.
So, the correct answer is '(c) only (iv)’.

Additional Information:
- BOD refers to the amount of oxygen that would be consumed if all organic matter in one liter of the water were oxidized by bacteria.
- The effluent from the secondary treatment is released into natural water bodies like rivers and streams.
- During digestion in anaerobic sludge digester, bacteria produce a mixture of gases like methane, hydrogen sulphide, and carbon dioxide. These form biogas and can be used as a source of energy as it is inflammable.

Note:
- Sewage treatment is the process of removing contaminants from municipal wastewater, containing mainly household sewage plus some industrial wastewater.
- Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater (or treated effluent) that is safe enough for release into the environment.
- The major aim of wastewater treatment is to remove as much of the suspended solids as possible before the remaining water, called effluent, is discharged back to the environment.