Consider the following statements about, secondary sewage treatment.

I. In secondary treatment, useful aerobic microbes grow rapidly and form flocs. Flocs are masses of bacteria associated with fungal filaments to form mesh-like structures.

II. The growing microbes consume organic matter and thus, reduce the biochemical oxygen demand.

WhenBODof sewage has reduced, the effluent is passed into settling tank.

III. In settling tank, the bacterial flocs settle and the sediment is called activated sludge.

IV. Asmall part of the sludge is used as an inoculum in the aeration tank and the remaining part is passed into large tanks called anaerobic sludge digesters.

V. In the digesters, heterotrophic microbes anaerobically digest bacteria and fungi in sludge producing mixture of gases such as, carbon dioxide, nitrogen and carbon monoxide which form the biogas.

Which of the statements given above are correct?