An investigation 39 on rheological behaviour of thermally treated digested sludge: dependency on solid concentration

This study investigated the effect of temperature and duration of heat treatment on rheological properties of different concentration of digested sludge. The flow curve and the yield stress of 2%, 3% and 3.5% digested sludge have been measured before and during heat treatment at different temperatures between 20 and 80°C for different heating duration of 1, 15 and 30min. The effect of thermal history on rheology of sludge was then studied after cooling back sludge to 20°C (thermal history at 50, 60, 70 and 80°C). In addition, the effect of temperature and thermal history on solubilized chemical oxygen demand (COD) for sludge with different solid concentration was also investigated to prove the composition change of sludge during the heat treatment. The result showed that the irreversible effect of temperature was more evident at higher temperatures and concentrations. At higher concentrations (3% and 3.5%), by increasing the temperature, the yield stress decreased after an initial increase while for 2% sludge yield stress consistently decreased with increasing temperature and showed a linear relationship with decreasing particulate COD. The yield stress followed a great agreement with an exponential decreasing function of temperature and the temperature of thermal history.