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 process uses microorganisms to consume the organic matter in the wastewater. The microorganisms feed on the biodegradable material in the wastewater in the aeration tank then flow into a secondary clarifier where the biomass settles out and removed as secondary sludge.


When fed to an anaerobic digester the two sludge types have different biogas production potential due to the way in which they were treated and removed. Primary sludge contains higher biogas production potential because it was captured via gravity and therefor its energy content has not yet been consumed. Secondary sludge has lower biogas potential because the microorganisms in the secondary treatment process have consumed most of their energy content leaving behind mainly inert biomass.

ClearCove’s EPT technology captures the majority of organics in the primary treatment stage, preserving their energy content and increasing biogas generation by up to 3 times.