Question

Secondary sewage treatment is mainly
A. Chemical process
B. Mechanical process
C. Biological process
D. Physical process

Answer 1

Hint: Sewage treatment plant is polluted water treatment plant which treats the polluted water which contains a large amount of solid waste like human feces, commercial liquid waste, heavy metal, high BOD and COD, high salt content which can be removed by several different kinds of scientific processes to make that water fit for human consumption for various purposes.

Complete answer:
The sewage treatment plant is mainly divided into three phases such as primary treatment, secondary treatment, and tertiary treatment which further consist of a mechanical process, chemical process, biological process, and physical process, in which polluted water convert into potable water.
Primary treatment which is also called a physical treatment that consists of shredding, churning, screening, and sedimentation, such process remove the large floating debris, suspended solids heavy articles, settle down, the sediments and the residue of sequential filtration constitute primary sludge, primary sludge is used for biogas generation, composting and combustion.
Secondary treatment is also called biological treatment which requires microbial treatment, in which microbial decomposition of sine organic matter is present in the effluent. The decomposing of microbial flora is called sewage fungus, the two common methods that are named as trickling filters and activated sludge treatment. The BOD decreases ten to fifteen percent of the wastewater taken to large settling tanks where flocs of sewage fungus settle down. The supernatant passed to the water body. It can be treated further. The organic sediment is passed into an anaerobic sludge digester.
Hence, the correct option is C.

Note: Tertiary treatment consists of various methods like UV filtration, chlorination, reverse osmosis, ultrafiltration, ozonation process, and many more. Mainly a chlorination process is performed, in which hypochlorite salt is used for the disinfectant to remove microbes in water.