Wastewater treatm A. anaerobic digesters

B. floc

C. chemicals

D. oxidation pond.

Anaerobic digesters are used in waste water treatment plants for the treatment of large quantities of sludge. They reduce the organic sludge as these digesters have microbes which digests the sludge and produces a mixture of gases like methane, carbon dioxide and hydrogen sulphide which are constituents of biogas.

Chemicals and oxidation ponds are not used in waste water treatment for large quantities of sludge. Flocs are microbial biomasses which causes flocculation. Therefore, option a. anaerobic digesters are mainly used for treating a large quantity of sludge and it is the correct answer.