SEWAGE SLUDGE RHEOLOGYCAL PROPERTIES EVALUATION BY EXTRUSION TEST

Rheological properties have a great importance in sludge management, as they have been successfully used for predicting performance of several sludge treatment processes. Sewage sludge is a non-Newtonian fluid, because shear stress is not linearly proportional to shear rate or velocity gradient. Among different equations describing non-Newtonian fluids, the Bingham equation seems to better account physical behaviour, i.e. the presence of initial yield stress until deformation occurs. Rotating viscometers are generally used for measuring rheological properties, but they are costly and difficult to operate in the field. Therefore, research has been carried out to try and measure such properties by adopting simple and applicable field procedures. Several sewage sludges at different solids concentrations were tested using an extrusion apparatus (Kasumeter). Values of rheological parameters were estimated and compared to those measured by viscometer. Results showed that yield stresses and apparent viscosity cal be easily evaluated by Kasumeter and their values are well correlated with those by viscometer. For sludge samples of the same type and origin yield stress appeared strictly correlated to solids concentration, being relationships dependent on sludge type, thus evidencing that other physical, chemical and biological characteristics influence sewage sludge rheology.