This paper reports on lab-scale investigations in relation to pre-selection of flocculants for sludge dewatering with decanter centrifuges. Results obtained were compared with CST-measurements and discussed in relation to findings under field conditions. Experiments were carried out with sewage sludges of different origin and characteristics and a number of commercial flocculants. Kinetics of sedimentation and clarification were measured as well as the compression behaviour and shear sensitivity of sludge sediments. To measure flocculant performance stability against intensive shearing, total solids in the sludge cake obtained and dewaterability of the sludge cake during the first 20-50 s of centrifugation were compared. A screening test procedure was developed. Efficient flocculants should produce high residual total solids and good initial compressibility. Lab-scale investigations deliver more reliable results if the dynamic behaviour of the sludge under centrifugal acceleration is also investigated. The separation analyser LUMiFuge 114 can provide results about the compression behaviour of sludges in the range between 10 and 100 s. So far no other method or device is known which can deliver such results.

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