Simulation of liquid phase accumulation for the centrifugal dewatering of activated sludge

Abstract

Centrifugation of activated sludge is a frequently used dewatering process but, in spite of this, there is a need for a fitting function for moderate and high sludge concentrations. Liquid phase accumulation kinetics during centrifugation may be used as a source of information about the sedimentahon properties and the governing mechanism during cake formation. For this purpose, activated sludge at different concentrations was investigated on a laboratory centrifuge with a centrifugation factor of 2667. The following sludges were used in the experiments: 1. Activated sludge from a thickener (with and without flocculant treatment) 2. Sludge taken after an industrial centrifuge decanter, treated with Flottweg Z62-4 flocculant and collected from a dumping pound 3. Sludge (2) after anaerobic treatment 4. Sludge (2) with a dispersed solid additive. Using the experimental data for an asymptotical analysis, a dimensionless fitting function was obtained that adequately describes the dependence of liquid volume vs. time for the centrifugation of activated sludge. For the highly concentrated sludge it was found that the dewatering occurs through cake compression. Analysis of deformation models leads to the conclusion that for the compressible cake it is necessary to introduce a parameter characterising the cake plasticity dependence on the centrifugation time. The developed model was validated by fitting numerous experimental data. The main advantage of the proposed model is the possibility to fit the liquid phase accumulation kinetics during centrifugation over a wide range of activated sludge concentration values, from suspensions up to structured and paste-like cake consistency.