Characterising the clarification of the supernatant of activated sludges

The solid/liquid separation in activated sludge plants is traditionally achieved by gravity sedimentation in secondary settling tanks (SST). Thickening data, used for the design of SSTʹs is usually recorded by observing the descent of the sludge/supernatant interface vs batch settling time, revealing the sludge sedimentation curve. To get information about the clarification process an experimental device to record the evolution of supernatant transparency during batch settling was set up. The curves gotten, revealed that different activated sludges, showing the same kind of sedimentation curve, can produce supernatant very different in suspended solids concentration. Two distinct types of separation behaviour exist, which are different in the way the supernatant clarification process is effected. For the first type the clarification of the supernatant is only due to a dilution of the supernatant by the expelled water which is filtered by flowing through the thickening biomass during batch settling. This behaviour is called “cross-linked separation” because it only happens when the flocs are linked to each other in an immobile structure. The other type of supernatant clarification is due to sedimentation of supernatant suspended solids. This behaviour is called “disperse” separation because the flocs are suspended as mobile individuals, thus they are disperse. Furthermore a more transparent layer, located directly above the solid/liquid interface was found and is explained.