Rheological characterisation of sludge coming from a wastewater treatment plant

The aim of this study was that of studying the rheology of sewage sludge using two different rheological test protocols taken from literature and comparing them in order to evaluate which useful information are given from every protocol. Approach: Two different protocols have been used taking particularly into account the problems connected to sludge heterogeneous composition and to the interaction between solid-solid and solid-water particles in order to completely understand the rheological behavior of this suspension; moreover, the consequences of particular effects connected to test geometry and conditions have been considered. Two fundamental parameters have been modified in the samples: The total solids content and the polyelectrolyte addition. Sludge with 3 and 5% of total solids have been investigated, with or without polyelectrolyte using also microscope analysis to understand the effect of polyelectrolyte on the sludge. Results: As expected, it was noticed that sludge viscosity grows up increasing the total solids content and with the presence of polyelectrolyte. The effect of polyelectrolyte is that of separating the liquid-phase from the solid-phase of the sludge giving a more space-heterogeneous suspension with higher viscosity and higher non-Newtonian behavior. Conclusion: This study proved that combining two different protocols of analysis can be useful to furnish important and complementary information on sludge rheology especially when some parameters change from sample to sample. Moreover, in order to have good and consistent results, it is necessary to use particular attention on samples pretreatment