In China, over 17 million tons dewatered sewage sludge cake (DSSC), with about 80% water content, was generated from wastewater treatment plants in 2010. High water content is the bottleneck of sludge treatment and disposal. In this study, the combination of hydrothermal and mechanical treatments has been chosen in order to improve sludge dewaterability. Sludge thermogravimetry analysis was conducted to determine 180 degrees C as the upper-limit hydrothermal temperature. Five temperatures (60, 80, 120, 150, 180 degrees C) were chosen to study the effects of hydrothermal treatment temperature and the holding time on dewaterability. The higher the hydrothermal temperature, the better was the dewaterability character. The water contents of solid products were positively correlated with the hydrothermal holding time at predetermined temperatures in this study. Degradation of macromolecules into acidic compounds could be the reason of pH decrease of separated liquid. Destruction of zoogloe and decomposition of organic matters improved the sludge dewaterability. Sludge dewaterability experiencing hydrothermal processes in this study was negatively correlated with extracellular polymeric substance (EPS) content. With the rising temperature, sludge flocculate disaggregated to small particles generally, this could also be one of the important reasons for sludge dewaterability.
Implications: High water content is the bottleneck of sludge treatment and disposal. Up to now, only a small amount of research has been conducted to determine whether the dewaterability of dewatered sewage sludge cake can be improved by hydrothermal pretreatment. The mechanism of sludge dewaterability by hydrothermal pretreatment is uncertain. In this study, a new sludge disposal method and corresponding parameters were given. The mechanism of sludge dewaterability was analyzed extensively by extracellular polymeric substances, scanning electron microscope images, element contents, and caloric values, etc. This study will be helpful for knowing about sludge hydrothermal pretreatment technology and its mechanism.