Flotation of activated sludge results in the formation of scum. The scum formation is abetted in particular by sludge floc with a low specific weight and a large specific surface area. Flocs with these properties are formed, for example, in the presence of filamentous bacteria.

Other prerequisites are the presence of free dispersed gas bubbles that provide buoyancy, as well as water repellents or surfactants that favor the separation between the sludge floc and the surrounding medium.

The floating sludge formation takes place by the consolidation of individual activated sludge flocs to larger units which are distended by gas bubbles. Gas bubbles are produced for instance during denitrification, but are also generated by the aeration in the activated sludge plant.

Floating sludge (scum) can lead to considerable operational problems because fore once the effluent quality can be severely impaired and, furthermore, the activated sludge is not available to the biological process. The S::Select^® process granulates the activated sludge and drastically reduces the risk of scum formation.


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