Effect of high-temperature thermal hydrolysis on rheological properties and dewaterability of sludge

The disposal processes like dewatering and anaerobic digestion (AD) are commonly utilized to reduce the volume of sludge and recover energy. Thermal hydrolysis process is widely used as a pretreatment for sludge AD, which can change rheological properties and dehydration of sludge irreversibly. The experiment was designed to evaluate the effect of high-temperature thermal hydrolysis (120, 130, 145, 160 and 170°C) for 60 min on sludge rheological properties, as well as its dewaterability. Rheological tests were performed at (20 ± 0.1)°C with a HAAKE Viscotester 550 Rotary Viscometer. Both raw and thermal hydrolysis sludge has a considerable reduction on apparent viscosity and yield stress, but a somewhat increase in thixotropy. With the increase of temperature, the sludge flow behaviour index n increases linearly, while the consistency coefficient k follows the law of linear decline, showing that thermal hydrolysis can weaken the non-Newtonian fluid properties and then improve sludge fluidity. The dewaterability increases linearly with the temperature. Besides, the dewaterability of high-temperature thermal hydrolysis processes sludge was always significantly better than raw sludge as its mud cake has a much larger solid content. Notably, the increase in flow performance index n and the decrease of thixotropy kinetic coefficient K caused by thermal hydrolysis are all linear with the enhancement of dewaterability, which demonstrates that rheological indicators can be a new tool to evaluate the dewaterability of sludge.