Once the flocs act on the nutrients in the effluent, the left over substance after sedimentation is called as
Sludge refers to the semi-solid residual material which is left behind from the treatment of wastewater. The two main types of sludge produced from the wastewater treatment process are primary and secondary sludge.
Primary sludge is a result of the capture of suspended solids and organics in the primary treatment process through gravitational sedimentation, typically by a primary clarifier.
The secondary treatment uses microorganisms (flocs) to consume the organic matter in the wastewater. It is done by the interaction of different types of bacteria and microorganisms, which require oxygen to live, grow and multiply in order to consume the organic matter. The resulting sludge from this process is called activated sludge.