Question

the sludge is decomposed by

Answer 1

sludge is scraped with a scraper
Sludge digestion is a biological process in which organic solids aredecomposed into stable substances. ... The sludge then flows into a second tank, where the dissolved matter is converted by other bacteria into biogas, a mixture of carbon dioxide and methane.

Answer 2

Sludge is mostly water with lesser amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge separated in secondary clarifiers includes treated sewage sludge from secondary treatment bioreactors.