Changes in waste activated sludge viscosity during thermal treatment

Effective treatment of sewage sludge is an ongoing challenge in wastewater treatment processes. Many advanced technologies are emerging to further reduce the volumes of bio-solids produced. Thermal treatment methods are increasingly applied to help achieve this. They are used for sludge conditioning, as in the case of thermal hydrolysis pre-treatment for anaerobic sludge digestion, or even used as a complete sludge treatment method, as in the case of wet oxidation sludge treatment. These processes generally subject sewage sludge to elevated temperature (>100 degreesC) and pressure (>1 bar) conditions. In the case of thermal hydrolysis, it has been established the final viscosity of the treated sludge is significantly reduced. However, this change in sludge viscosity as the treatment progresses has not been fully examined. A deeper understanding on the rheological behaviour of sludge under thermal treatment conditions is valuable in the optimization and design of these processes. In this study, the apparent viscosity of thickened waste activated sludge (10 wt% solids) was measured at shear rate of 100 s-1, in real-time, while the sludge was subjected to high temperature (80 - 145 degreesC) and pressure (5 bar) conditions over 1 hour. At lower temperatures, the apparent viscosity of sludge was found to decrease rapidly in the first 15 minutes before becoming more gradual in the subsequent treatment period. At higher temperatures, the sludge apparent viscosity decreased only gradually with time. At any point in time during the 1-hour period, the apparent viscosity decreased linearly with increasing temperature.