The surplus sludge is withdrawn from the final clarification with a low concentration (3 - 10 kg/m3, i.e. 0.3 - 1.0%). In order to be able to economically build and operate the subsequent and very cost-intensive treatment stages (digestion, dewatering), the sludge must first be thickened and the volume must be reduced significantly. Various static and mechanical methods (e.g., belt thickener, disk thickener) are used. Good results, i.e. high TS concentrations can be achieved with the addition of polymer.

In addition to the mechanical and process design of the thickening stages, the sludge characteristics are especially important for the thickening result. If a compact, ideally an aerobic granular activated sludge (AGS) can be produced in the biological stage, then considerably better thickening results can be achieved with a significantly reduced polymer consumption.

For example, in a plant (100'000 PE) a thickening result of 4.5% was achieved with conventional activated sludge plant operation and addition of 4 kg/t polymer (active substance). After upgrading the biological stage to the S::Select® system, which establishes and maintains an aerobic granular sludge, it was possible to consistently achieve a result of 7% TS with the same specific polymer consumption. Therefore, 35% less sludge volume was produced with a comparable specific polymer consumption.

This is of considerable importance with regard to the subsequent stages. Hence, 35% less volume need be heated up (mesophilic digestion) and the retention time in the digestion is correspondingly longer.

The longer retention time in the digestion means a higher degree of sludge stabilization and more gas produced. Thus, resulting in better dewaterability and in total significantly less sludge, which nowadays in many places needs to be disposed of by means of very cost intensive incineration.


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