Operator terms · Wastewater
What is sludge in wastewater treatment?
Sludge is the settled solids removed during wastewater treatment. Primary sludge settles out early; secondary sludge is the excess biological growth. Sludge is thickened, digested to stabilize it, and handled as biosolids. Managing sludge is a large part of running a wastewater plant.
Last reviewed 2026-08-03
Where it comes from
Primary clarifiers settle out raw solids as primary sludge, and the biological process produces excess biomass that settles as secondary sludge.
What happens to it
Sludge is thickened to cut its water, digested to stabilize it and reduce its volume, and then handled and disposed of as biosolids under the rules.
Common questions
- What is primary sludge?
- The raw solids that settle out in the primary clarifier.
- What is secondary sludge?
- The excess biological growth from the treatment process.
- What happens to sludge?
- It is thickened, digested, and handled as biosolids.