Rheological measurements as a tool for monitoring the performance of high pressure and high temperature treatment of sewage sludge

Hydrothermal processing plays a significant role in sewage sludge treatment. However, the rheological behaviour of sludge during these processes is not fully understood. A better understanding of the sludge rheology under hydrothermal processing conditions can help improve process efficiency. Moreover, sludge rheology is easier to measure than chemical analyses. If a relationship could be established, it provides a possibility of using rheological measurement as a basis for monitoring the performance of hydrothermal processing. The rheological changes in thickened waste activated sludge (7 wt%) was investigated using a pressure cell-equipped rheometer during 60-min thermal hydrolysis (TH) at various temperatures (80-145 degrees C) and constant pressure (5 bar). Changes in the soluble chemical oxygen demand (COD) were measured using a separate reactor with a similar operating condition. The sludge behaved as a shear-thinning fluid and could be described by the Herschel-Bulkley model. At constant temperature, the yield stress and high-shear (600 s(-1)) viscosity of sludge decreased logarithmically over 60 min. At constant time, the yield stress and the high-shear viscosity decreased linearly with increasing TH temperature and these values was much less than corresponding properties after treatment and cooling down to 25 degrees C. The soluble COD of sludge also increased logarithmically over 60 min at constant temperature, and increased linearly with increasing temperature at constant time. Furthermore, the yield stress and high-shear viscosity reduction showed a linear correlation with the increase in soluble COD. (C) 2017 Elsevier Ltd. All rights reserved.