Evaluation of Thermal Hydrolysis Efficiency of Sewage Sludge via Rheological Measurement

Thermal hydrolysis processes (THP) play an important role in sewage sludge treatment. In this study, both low-temperature (60°C–90°C) and high-temperature (120°C–170°C) thermal hydrolysis experiments were performed on the initial sludge (at a total solid of a 7.03% and 6.45% by weight). The results have shown that the release of organic matter is logarithmic with the duration during low-temperature thermal hydrolysis (LTHP). For high-temperature thermal hydrolysis (HTHP), the release of organic matter is linear with a treated temperature. Good linear relationships were obtained between the release of a soluble chemical oxygen demand (rSCOD) and soluble proteins/polysaccharides. The pH decreased with the increase of the treatment temperature. The apparent viscosity of the sludge decreased after the thermal hydrolysis, especially at a high temperature. The yield stress of the sludge disappeared, and the thixotropy increased. Moreover, the flow behavior index increased, and the consistency coefficient K decreased with the increase of the thermal hydrolyzed temperature. Meanwhile, we found a linear relationship between the rSCOD and increasing infinite viscosity. It demonstrated that the rheological measurement could be used as an indicator to evaluate the extent of organic matter solubilization in THP.