Answer: Option C
:
C
1.The first step or primary treatment, the sewage is split into solid and liquid components. This is achieved by sedimentation and by filtration.
2.The liquid effluent is passed to the next phase, which is called secondary treatment or biological treatment. In this phase the effluent is pumped into a large aeration tank. Here, the liquid is constantly agitated to promote the growth of flocs. Flocs are masses of bacteria that are associated with fungal filaments to form mesh-like structures. In these aeration tanks, the introduced microbes consume the available nutrients.
3.Once BOD levels drop to a sufficiently low level, a second sedimentation process is carried out in a settling tank. In the secondary sedimentation the solid floc is separated from the liquid. The separated solid floc is called a secondary or activated sludge.After the activated sludge is separated from the effluent, the nutritionally and microbially poor liquid portion of the sewage is suitable for release into the environment.
4.The sludge is passed into anaerobic sludge digester tanks. These digesters have a set of anaerobic microbes. These anaerobic microbes consume the bacteria and fungi that form the sludge and in the process produce methane and hydrogen sulphide gas. These biogases are collected and used as fuel.