Identify the incorrect statement.

In sewage treatment,secondary treatment is considered highly significant,because

In the treatment of waste water discharge,which treatment stage involves biological treatment?

Measuring of $BOD$ in sewage water gives an estimation of . . . . . .

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?

Select the appropriate option regarding the anaerobic sludge digester.