Assessment of rheological methods for a correlation to sludge filterability

To achieve good dewaterability, the conditioning process should create sludge flocs providing a porous cake during compression but also a high resistance to erosional stresses. This suggests that sludge's rheological characteristics, some of which (like the yield stress or the storage modulus) represent its network strength, should be related to the material's dewatering properties. Previous efforts to verify such a correlation--which would allow prediction of full scale dewaterability--have not provided strong correlations. In this work, commonly accepted rheological and dewatering tests were applied to conditioned sludges. Correlations were seen between different rheological parameters, particularly yield stress and specific energy, but not between rheological and filterability properties. Even using high pressure filtration tests, which should reflect the sludge's responses to normal and shear stresses, results were related to the measured rheological properties only in an indirect manner: a threshold shear strength is required beyond which further strength confers no improvement in filterability. Thus, common rheological tests are unlikely to provide useful information regarding full scale filterability.