Transformation Characteristics During Thermal Hydrolysis Pretreatment And Anaerobic Digestion Of High-solid Sludge

The increasing amount of municipal sludge has already exceeded the treatment ability, due to high operating cost and low efficiency of conventional sludge anaerobic digestion(AD). To enhance its hydrolysis rate, as the limit step of AD, many researches of pretreatment and AD were carried out. Thermal hydrolysis pretreatment(THP) could hydrolysis the organics in sludge sufficiently and accelerate the reaction process of AD. The solid content of sludge was also increased to 10% to reduce the reactor volume and energy cost, integrated as the high-solid sludge THP-AD process. The researches of high-solid sludge THP-AD process were focused on the operation parameters optimization and system stabilization, but the research of transformation characteristics was rare.The transformation characteristics of carbon, nitrogen, phosphorus and sulfur of THP(165℃,50 min), AD(mesophilic and thermophilic, HRT=20d) and acidogenesis process in dewatering sludge from municipal wastewater treatment plant(WWTP) was investigated by using thermal hydrolysis reactor, completely mixed AD reactor and acidogenesis phase reactor. The analysis results of organic compounds transformation are valuable for the application THP and AD of high-solid sludge and the other treatment schemes of pretreated high-solid sludge. Main conclusions are as follows:(1) THP could release and hydrolysis the organics as the cell was crashed. The main content of dissolved organic matter in hydrolysate was dissolved protein and carbohydrate after THP at 165℃ for 50 min. There are different transformation characteristics of nitrogen, phosphorus and sulfur in thermal hydrolysis pretreatment. The main components of nitrogen, phosphorus and sulfur inhydrolysate were dissolved organic nitrogen, phosphate and dissolved organic sulfur.(2) The digestion efficiency of pretreated high-solid sludge mesophilic AD was over 49%. There was no detected of acid accumulation during digestion, and the retention time of mesophilic AD could be shorten. The digestion efficiency of unpretreated high-solid sludge mesophilic and thermophilic AD was under 25%.(3) The dissolved nitrogen and phosphorus in sludge were ammonia and phosphate after AD. The inhibition of ammonia should be considered during high-solid AD. The H2 S concentration in biogas during high-solid digestion was under 200 ppm.(4) There is no need to supply energy in the process of THP and AD of high-solid sludge. The volume of the digester could be reduced 2/3 compared with the conventional AD process. The heat recovery system in the pretreatment unit and the mixing system in digestion unit should be improved to reduce the energy consumption.(5) The pretreated high-solid sludge was easy to acidification. The main reaction in acidogenesis phase during HRT≤2d and 4~5d was acidogenesis and acetogenesis, respectively. There was no significant detected of acetic acid in thermophilic acidogenesis phase.