Answer :

(a) Primary Sludge: All the solids that settle down, during the primary treatment of
sewage water.
(b) Activated Sludge: Produced during the secondary treatment or biological
treatment of sewage, primary effluent + aerobic microbes flocs (bacteria and
fungus) – get converted to a sediment whose BOD has reduced significantly.
(c) Anaerobic sludge digesters: Large tanks where activated sludge is treated with anaerobic bacteria which digest the bacteria and fungi, and produce a mixture of CH4, H2S and CO2/ Biogas