Explain the change that can be observed in the characterstics of river water when sewage is discharged into it and a few weeks after the discharge with respect to
(i) level of dissolved oxygen
(ii) Population of fresh water organisms.

Why does the oxygen concentration in the water decrease when sewae is discharged into a river?

Explain the process of sewage water treatment before it can be discharged into natural bodies.

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

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

Explain the process of sewage water treacment before it can be discharged into natural water bodies . Why is this treatment essential ?

Explain the process of sewage water treatment before it can be discharged into natural water bodies . Why is this treatment essential ?

Explain the process of sewage water treacment before it can be discharged into natural water bodies . Why is this treatment essential ?

Explain the process of sewage water treatment before it can be discharged into natural bodies. Why this treatment is essential?

A student puts his pencil into an empty trough and observes the pencil from the position as indicated in Figure. (i) What change will be observed in the appearance of the pencil when water is poured into the trough? (ii) Name the phenomenon which accounts for the above stated observation. (iii) Complete the diagram showing how the student's eye sees the pencil through water.