Development and microbial community analysis of high efficiency sludge digestion process using thermophilic aerobic digester

DoctorBiological wastewater treatment process has long been used to purify municipal and industrial wastewater. Although biological treatment is considered to be the most effective and ultimate method for wastewater treatment, waste sludge which is generated as byproduct of biological treatment has become a serious problem recent days. Until 2012, waste sludge was discharged to the ocean in many countries. However, disposal of waste sludge can cause marine environmental problem because it contains high amount of heavy metals and pathogens. Thus, discharge of waste sludge to the sea was prevented by the activation of London convention. There are several waste sludge treatment methods except of marine disposal such as landfill, incineration and biological digestion. Among them, anaerobic or aerobic biological digestion has been considered as the most ecofriendly method because it didn’t generate secondary pollutants while other methods have secondary pollution problem such as air pollution. Anaerobic digestion is widely used biological sludge digestion method as its energy production ability. However, anaerobic digestion has several disadvantages including sensitive operating conditions and requirement of large occupation areas due to slow digestion rate. On the other hands, thermophilic aerobic digestion method shows very high solid removal rate but it requires some energy for aeration and does not produce any energy or useful materials. In our research, we combined mesophilic anaerobic sludge digester and thermophilic aerobic digester to complement disadvantages of each reactor. We also improved combined digestion process by applying several pre-treatment to influent waste and internal recirculation to increase sludge digestion rate and methane production of combined process. In chapter II, A lab-scale combined sludge digestion process consists of mesophilic anaerobic digester (MAD) and thermophilic aerobic digester (TAD) was developed. Various pre-treatment methods of influent sludge including thermal, thermal-alkaline and long-term alkaline treatment were applied to examine the effects to sludge removal and methane production efficiency. After sludge pre-treatment, TCOD removal was maintained over 75%. Methane production rate was also dramatically increased from 100mL/L/day to 250mL/L/day. PCR-DGGE and quantitative PCR were used to investigate microbial community of combined process. Clostridium straminisolvens was the major bacterial species while Methanosaeta concilli was major archeal species in MAD. In TAD, Ureibacillus species which found in thermophilic aerobic reactors was the most abundant species. In this experiment, we investigated that sludge pre-treatment could improve COD removal and methane production of combined sludge digestion process by the induction of hydrolysis step. In chapter III, combined sludge digestion process was improved with recirculation of solid sludge in the effluent. The recirculation effect to the combined sludge digestion process was investigated. The strength of pre-treatment was decreased by each phase to find economically effective pre-treatment method. The VSS removal rate was increased from 76% to 97% by applying of internal recirculation and maintained stable until experiment ends. The concentrations of VSS in each digester also showed steady profiles. On the other hands, concentration of TSS was slightly increased due to accumulation of inert materials in the reactors. Methane production of combined process was decreased from 260mL/L/day to 170mL/L/day as the strength of pre-treatment is decreased. At phase five, the influent and recirculated sludge was supplied without any pre-treatment. In this phase, mesophilic anaerobic digester was seriously damaged by invasion of thermophilic microorganisms. Methane production rate was dropped from 170mL/L/day to 40mL/L/day and rapid accumulation of TSS and VSS was occurred. However, thermophilic aerobic digester was operated stable and accumulation of VSS was not observed. This result means that sludge removal ability of combined process could be maintained stably. In this experiment, we investigated that sudden introduction of thermophilic microbial community can break mesophilic sludge digestion activity. On the other hands, thermophilic aerobic digester has enough capacity for sludge digestion by its high activity. Thus, pre-treatment of influent and recycled sludge is necessary for energy production in the improved combined process but not essential for sludge removal