Conditioning for Municipal Sludge Dewatering. From Filtration Compression Cell Tests to Belt Press

Belt presses are commonly used to achieve sewage sludge dewatering. As this process essentially works by squeezing, it was interesting to better understand how the sludge conditioning affects cake expression. Experiments were carried out with conditioned municipal activated sludge on both a filtration compression cell (FCC) and a laboratory-scale belt press. Two cationic polyelectrolytes with a high charge density (80%) but different molecular weights were used. A significant impact of the conditioning was observed for FCC tests. The higher molecular weight polyelectrolyte led to much shorter filtration phases than with the medium molecular weight one. However, it produced wetter filter cakes. It also led to more elastic filter cakes but less susceptibility to creep. Higher final dryness was observed for sludge conditioned by the medium molecular weight polymer. Experiments carried out on the belt press device did not exhibit such impact. Consequently, the FCC test is not suitable to analyze the sludge dewatering by belt press, although it enables characterization of the expression phase. Indeed, some additional features involved in belt press, such as shear forces or progressive pressurization of the cake, are not characterized by classical FCC tests.