What does the image below represent?

The key substance between primary and secondary treatment in Sewage Treatment Plants $(STP)$ is?

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 $20\, mg/L, 8\, mg/L$ 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?

What does primary treatment of sewage involve?

Why is aerobic degradation more important than anaerobic degradation for the treatment of large volumes of waste waters rich in organic matter? Discuss.

$A$: More sewage treatment plants are required in proportion to the human population.
$R$: Biogas,which is a fuel energy,is produced in $STPs$.