Experimental difficulties often encountered with sludge rheological properties determination and advices to perform reliable measurements

Rheological parameters being of great importance for sludge process management, they are increasingly studied. However, experimental procedures may strongly impact their determination. Sample volume, measuring device depth and roughness but also mechanical history, have to be well-controlled to ensure reproducible results. Indeed, even if shear history can be erased with a sufficient preshear for diluted sludge, this paper clearly established that no steady state can be achieved for concentrated sludge. The longer the shear history, the lower the rheological characteristics: Reproducible results are hardly obtained. More importantly, slippage appeared to occur even with surfaces of moderate roughness and the phenomenon is all the more important that the dry matter is high. From all these observations, an experimental procedure based on the control of preshear and rest periods has been defined. Advices and precautions to observe are given in order to ensure reproducible and obtain unaltered results.