In anaerobic sludge digesters,what do anaerobic bacteria digest?

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 correct statement from the following.

During primary treatment,all solids that settle form $A$ and the supernatant forms $B$.

Which substances present in water are consumed by microorganisms,including bacteria?

When does the $BOD$ level in effluent decrease?