Pilot-scale 3단계 발효시스템을 이용한 유기성 폐기물로부터 고효율 메탄 생산 기술 개발

Abstract

A novel pilot-scale three stage fermentation system was developed for high-efficiency methane production from food wastes. This system consisted of three reactors and total reactor volume was 2.5 m3. The first stage was a semi-anaerobic hydrolysis/acidogenic process which converted food wastes into variable sugars, amino acids, and volatile fatty acids (VFAs). The operation temperature and pH were 45℃, and 3.8 ~ 4.0, respectively. The hydraulic retention time (HRT) was 2 days. Eleven microbial strains (K1 ~ K11) were isolated and characterized through this stage. Total chemical oxygen demand (tCOD) was 71,234 mg/L. The second stage was an anaerobic acidogenic process which can produce large amounts (14,560 mg/L) of VFAs including acetate, propionate, butyrate, valerate, and lactate. The operation temperature and pH were 35℃, and 3.5 ~ 3.8, respectively. The HRT of this stage was also 2 days. Five strains (KA1 ~ KA5) were isolated and characterized in this stage. The third stage was a strictly anaerobic methane fermentation process producing methane and carbon dioxide from VFAs. The operation temperature and pH were 50℃, and 7.1 ~ 7.5, respectively. The HRT was 12 days. Seventy two percent of methane from total biogas was generated and the yield was 360 L/Kg COD. Through out these stages, 91% of tCOD was removed. The effects of temperature and HRT on the methanogenesis were examined. Operation temperature was adjusted from 30℃ to 55℃, and the HRTs ranged from 8 to 12 days. The rate of soluble chemical oxygen demand (sCOD) removal correlated with digestion time according to the first-order kinetic model developed by Grau et al[Water Res., 9, 637-642 (1975)]. With liquor food waste, thermophilic digesters showed a higher rate of sCOD removal than mesophilic digesters. The rates of biogas and methane production by thermophilic digesters were higher than those by mesophilic digesters regardless of HRT. Although maximum biogas production occurred when the HRT of 10 days was used, the methane yield was the highest in the reactor when an HRT of 12 days was used (223 L CH4/kg sCODdegraded). However, digestion stability decreased when an HRT of 8 days was used. The concentration of NH3-N generated in this experiment did not inhibit anaerobic digestion. The process characteristics of anaerobic methane fermentation using several organic wastes were investigated. The substrates used were food wastes, vegetable wastes, cow manures, pig manures and wastewater sludge. The substrates were mixed with inocula (methanogenic fluid) at a ratio of 1:1, and several parameters including total solid (TS), volatile solid (VS), sCOD, and biogas production were monitored. Anaerobic digestion of food wastes and pig manures were occurred rapidly in the initial stage of fermentation, whereas that of vegetable wastes were occurred slowly in the late stage. Cow manures were degraded in the both initial and late stages of digestion. Total 10 m3 pilot-scale methane fermentation system was operated under the same condition as total 2.5 m3 methane fermentation system. The operation temperature and pH of the first stage were 45℃, and 3.8 ~ 4.0, respectively. The HRT was 2 days. tCOD was 56,734 mg/L. The second stage was an anaerobic acidogenic process which can produce VFAs including acetate, propionate, butyrate, valerate, and lactate. The operation temperature and pH were 35℃, and 5.0 ~ 5.5, respectively. The HRT was 2 days. The third stage was a strictly anaerobic methane fermentation process producing methane and carbon dioxide from VFAs. The operation temperature and pH were 50℃, and 7.3 ~ 7.8, respectively. The HRT was 12 days. Sixty eight percent of methane was contained in the biogas. Through out these stages, total 93.5% of tCOD was removed.