Assessment of the rheological methods for a correlation to sludge filterability

Biological sludge contains a large quantity of water and the decrease of its content constitutes one of the main issues for wastewater treatment plant operation. Laboratory tests to estimate the sludge dewaterability, i.e., its capability of being dehydrated, have been developed in the past, but there is no evidence of the tests’ direct correlation with the behavior of the real-scale dewatering devices. Different methods (specific resistance to filtration at different pressure, capillary suction time, expression tests, and drying test) have been applied on civil sludge conditioned with ferric chloride/lime or polymer/ferric chloride and results have been compared with operational data obtained from a full scale plant. The results presented in this paper highlight the role of the expression phase in the sludge dewatering using frame filter presses. To predict the sludge behavior and then to optimize the conditioning step, the laboratory procedure used for selecting the chemicals and doses has also to consider the effect of the expression of the cake.