Studies On Energy Recovery And Nutrient Enrichment Of Organic Waste

In this study, the energy recovery and nutrient enrichment from two typical municipal organic waste –sludge and human feces was investigated. According to the differences of organic matter and nutrient content in sludge and human feces, anaerobic digestion and aerobic composting was considered as the optimal treatment technology for energy recovery from sludge and nutrient enrichment from feces, respectively. Further research was carried out to solve the propionic acid accumulation in anaerobic digestion and the nitrogen loss in aerobic composting. Then the optimization methods were proposed to improve the efficiency of energy recovery and nutrient enrichment.It was found that volatile fatty acids(VFAs)(especially propionic acid) started to accumulate when organic loading rate(OLR) beyond 8.17 g-COD/L·d in thermophilic anaeorobic co-digeation of sludge. When the OLR increased to 14.6 g-COD/L·d, the concentration of propionic acid climbed to 2g-COD/L and this thermophilic system tend to fail. Then, the kinetic characteristic of anaerobic digestion of substrates and the reason of propionate accumulation was found out by batch experiment. In order to overcome the propionic acid accumulation, sulfate as an electron accepter was introduced into reaction system at low concentration(500 mg/L), then, the accumulated propionic acid was degraded sharply after 20 days acclimation. The maximum OLR for stable thermophilic co-digestion of sludge was promoted from 4.06 g-COD/L·d to 15.2 g-COD/L·d. Therefore, lower level of sulfate was beneficial for overcoming the propionate accumulation and promoting the OLR and stability of thermophilic anaerobic co-digestion of sludge and coffee ground. The more effective and stable energy recovery could also be achieved.Batch experiments were conducted for human feces composting using sawdust as bulky matrix under different temperature conditions. Compared the effects of temperature on initial and whole composting process, it was found that under mesophilic condition, organic nitrogen was converted into NH4+-N fast by the action of ammonification bacteria which was mesophilic bacteria. Then NH4+-N lost as the form of gasous NH3 with p H increase and lead to the nitrogen loss reached 20%. However, under thermophilic could not only enhanced organic removal rate, but also suppress the ammonification process which result in the less organic nitrogen loss of 5.51%. It was clear that organic nitrogen could be retained in compost by continuously thermophilic condition effectively and higher nitrogen-containing fertilizer could be obtained.In this study, according to the differences of organic waste, thermophilic anaerobic digestion and thermophilic anaerobic composting was demonstrated as an effective way for energy recovery and nutrient enrichment from sludge and feces, respectively. High efficiency of energy recovery and nutrient enrichment in these two process could be achieved by optimizing the reaction condition.