1.Primary sludge comes from the settling process. It is therefore made of easily decantable suspended particles: large and/or dense particles. It has a low level of Volatile Solids content (VS around 55% to 60%) and its dewatering ability is excellent. It is also very easy to concentrate this type of sludge with a static thickening step just before dewatering. The drawback is that this sludge ferments very easily.
2.Biological sludge comes from the biological treatment of the wastewater. It is made of a mixture of microorganisms. These microorganisms, mainly bacteria, amalgamate in bacterial flocs through the synthesis of exo-polymers. A simple decantation in the clarifier will easily separate the bacterial flocs from the treated water. Only part of this settled sludge is sent to dewatering: the excess biological sludge; part of it is recirculated to maintain the bacterial population in the reactor.
3.Mixed sludge is a blend of primary and biological sludges. The blending ratio is often as follows: ● 35% to 45% of primary sludge. ● 65% to 55% of biological sludge.