Question

During sewage treatment, the gases that are produced include:
A. Methane, Hydrogen sulfide, Carbon dioxide, and Hydrogen
B. Methane, Oxygen, and Hydrogen sulfide
C. Hydrogen sulfide, Methane and Sulphur dioxide
D. Hydrogen sulfide, Nitrogen and Methane

Answer 1

Sewage treatment is a process that removes the majority of the contaminants from wastewater or sewage and produces both a liquid effluent suitable for disposal in the natural environment and sludge. Wastewater containing faeces, urine, and laundry waste is referred to as sewage. Untreated sewage water in such areas can contaminate the environment and cause diseases such as diarrhoea. Sewage is treated in water treatment plants and is mainly biodegradable and most of it is broken down in the environment.

Complete answer:
Sewage contains a lot of germs and pathogenic bacteria and hence it is important to treat it before disposing it off into river streams. The treatment is done using the heterotrophic microbes already present in the sewage. The process involves two steps- primary and secondary treatment.
Primary Treatment: It is a physical treatment that involves the removal of large particles mainly by filtration and sedimentation. Firstly, the floating debris is removed from sewage by the process of sequential filtration. This results in the removal of insoluble large and small particles. Then the sewage is allowed to sediment and the soil particles are removed with the help of sedimentation. This results in the formation of ‘primary sludge’, that settles down during sedimentation and the effluent (supernatant) which is taken for further processing.
Secondary or Biological Treatment: The primary effluent is now passed in large aeration tanks. Air is continually pumped in those tanks and it is mechanically, constantly agitated. Due to constant agitation, the aerobic microbes present in effluent start growing vigorously and lead to the formation of flocs. Flocs are the masses of bacteria associated with fungal filaments to form mesh-like structures. When these microbes grow, they consume the maximum amount of organic matter present in the effluent which leads to reduction of the organic matter thereby reducing the Biological Oxygen Demand (BOD) (For BOD-see Note section). The sewage is treated unless the BOD is decreased in a significant amount; less is the BOD, less will be the pollution in water. After that the effluent is allowed to settle in the settling tank and the ‘flocs’ are allowed to sediment and this sediment is now called as ‘activated sludge’. To serve as the inoculum, a small portion of the activated sludge is fed back into the aeration tank. Generally, the effluent from the secondary treatment facility is discharged into natural water bodies such as rivers and streams.
Formation of gas: The remaining part of activated sludge apart from the ‘inoculum’ is transferred into large tanks called as “anaerobic sludge digesters”. In these tanks the bacteria that grows anaerobically starts growing and they digest the flocs- bacteria and fungi in the sludge. While digesting, the bacteria produce a mixture of gases such as methane, hydrogen sulfide, carbon dioxide and hydrogen. These gases are collectively called “biogas”. Biogas is non-flammable and can be utilized to generate electricity.
Methane, Hydrogen sulfide, Carbon dioxide, and Hydrogen: These gases are all part of the biogas produced during the sewage treatment.
So, option (A) is correct.
Methane, Oxygen, and Hydrogen sulfide: Oxygen is not produced during the formation of biogas in sewage treatment plants which makes Option (B) incorrect.
Hydrogen sulfide, Methane and Sulphur dioxide: In the above explanation, we can see that sulphur dioxide is also not formed during the formation of gases.
So, option (C) is incorrect.
Hydrogen sulfide, Nitrogen and Methane: It is also seen that the nitrogen gas is not formed during the formation of gases during sewage treatment.
Option (D) is incorrect.

The correct answer is option(A).

Note:
Biological Oxygen Demand: It refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria. The BOD test measures the rate of uptake of oxygen by microorganisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, more is its polluting potential.