Study On Thermal Hydrolysis Characteristics Of Sewage Sludge And High Performance Dewatering Technology

Municipal sewage sludge generation is continuously escalating. It is extremely urgent to dispose the sludge safely and economically. However, mechanically dewatered sludge still has a moisture content of almost80wt%, which cannot meet the requirements of the sludge disposal. Traditionally, thermal drying is often required to further reduce the moisture content, consuming large amounts of energy. It is necessary to research and develop one kind of sludge dewatering technology with high performance and low energy consumption. Thermal hydrolysis is an effective sludge pretreatment technology. It can improve the dewaterability of waste activated sludge (moisture content99%) or thickened sludge (moisture content96%). But due to the high moisture content, the energy consumption is still high. In current dissertation thermal hydrolysis of mechanically dewatered sludge is studied. Compared to treating waste activated sludge or thickened sludge, the volume of mechanically dewatered sludge was reduced to5-20%of the original one. The energy consumption is significantly reduced. By modifying the liquid-solid bonding within sludge, it meets the requirements of mechanical dewatering again. The water in sludge can be removed in liquid form. This study is aimed to find a technical route different from thermal drying, which can reduce energy consumption and significantly reduce the moisture content of sludge, to provide theoretical basis for the development of high performance dewatering technology. The main contents and conclusions in current dissertation are as follows:1) The thermal hydrolysis characteristics of mechanically dewatered sludge are studied and the effects of thermal hydrolysis temperature, reaction time and sludge initial moisture on the sludge dewatering performance are discussed. The results indicate that thermal hydrolysis of sludge at temperature above150℃improves sludge dewaterability. The optimum hydrolysis temperature is about180℃. The moisture content of sludge cake is reduced to about30%after thermal hydrolysis and dehydration. The sludge dewatering performance is improved at a lower temperature when adding the acid or alkali to assist thermal hydrolysis. For instance, after adding Ca(OH)2to sludge the moisture content is decreased to50%at120℃.The moisture content of hydrolyzed sludge fully meets the disposal requirements of sludge landfill or land utilization.2) After thermal hydrolysis treatment part of organic components in the sludge is dissolved and hydrolyzed. The ratio of organic components in hydrolyzed sludge to that in original sludge decreases with increasing temperature and the COD concentration of the filtrate increases significantly. Adding acid or alkali to sludge further promotes the release of sludge organic components from solid phase to water phase. The heating value of dry basis of hydrolyzed sludge decreases by15%at180℃and the heating value of as-received basis of sludge cake increases as the moisture content of dewatered sludge decreases.3) The the migration of heavy metals in sewage sludge by thermal hydrolysis treatment is studied, including the total amount, the species distribution and leaching toxicity of heavy metals in sludge. Most of the heavy metals remain in solid phase and exist in a more stable form. The leaching toxicity of heavy metals is reduced by35-70%. The results on tannery sludge shows that the migration of Cr from solid phase to liquid phase increases as pH values decreases. The total amount and leaching toxicity of Cr are reduced by more than90%by combination of H2SO4extraction and subsequently thermal hydrolysis.4) A sludge thermal hydrolysis and incineration system is designed and simulated using Aspen Plus. Simulation results indicate that the heating value of sludge after thermal hydrolysis and dehydration is sufficient for auto-thermal combustion when hydrolysis temperature is above180℃. The amount of steam generated meets the demand for heating sludge and there is still about20%steam left. When sludge is preheated by flash steam, the amount of steam required for heating sludge is reduced by30%. Thermal hydrolysis of sludge has a net energy savings of about65%compared with sludge drying. Thermal hydrolysis and incineration process provides a new route for sludge treatment with high efficiency and low consumption.