Name the two different categories of microbes naturally occurring in sewage water. Explain their role in cleaning sewage water into usable water. 

Study the graph given below. Explain how oxygen concentration is affected in the river when sewage is discharged into it.

The diagram above is that of a typical biogas plant. Explain the seQ uence of events occurring in a biogas plant. Identify a, b and c.

Explain the different steps involved during primary treatment phase of sewage.

What are methanogens ? How do they help to generate biogas?

108.

Mention the role of cyanobacteria as a biofertiliser. 

Name a free-living and a symbiotic bacterium that serve as bio-fertilizer. Why are they called so?