Dewatering of urban residual sludges: Filterability and hydro-textural characteristics of conditioned sludge

The hydro-textural properties of different urban residual sludges, characterized by their organic matter content, were estimated and correlated with their limit water content, corresponding to their minimum water content obtained after constant high-pressure filtration. A conventional compression-permeability (C-P) cell was used to determine the limit water content (wlim)(wlim) whereas standardized soil mechanics tests were run to determine the liquid limit (wL)(wL) and plastic limit (wP)(wP) content, characterizing the hydro-textural properties. The results demonstrated that wlimwlim, wLwL and wPwP parameters depend on sludge composition: high organic matter content ratio is unfavorable to a low wlimwlim but favorable to a high plastic range and so a high deformation of the sludge. It appeared that the wlimwlim value was close to the wPwP value, but lower than that obtained after mechanical dewatering in the wastewater treatment plants (WWTPs). It appears thus that the totality of the mechanically extractable water is not extracted by the WWTPs mechanical dewatering devices.