Why sometimes foaming is seen in river water near the place, where sewage water is poured after treatment?

When domestic sewage mixes with river water

Why does the oxygen concentration in the water decrease when sewage is dicharged into a river?

Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A,B and C, but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20 mg/L and 400 mg/L, respectively. Which sample of the water is most polluted ? Can you assign the correct label to each assuming the river water is relatively clean ?

Why do soaps not produce foam with hard water ?

When domestic sewage mixes with river water (a) small animals like rats will die after drinking river water (b) the increased microbial activity releases micronutrients such as iron (c) the increased microbial activity uses up dissolved oxygen (d) the river water is still suitable for drinking as impurities are only about 0.1 %

Why do we prefer to call secondary waste water treatment as biological treatment ?

A delta is formed at the meeting point of sea water and river water. Why?

Write notes on any three of the following each write two to three important points. (a) Net primary productivity (b) Morphological adaptations in xerophytes (c ) Layering (d) Pollen-pistil interaction (e ) Tertiary treatment of sewage water ,br> (f) Transcription unit