Anaerobic digestion using various organic wastes could be promising environmental technology because it can provide not only waste treatment also bioenergy production. Due to low efficiency and effluent quality of conventional anaerobic digestion, several new or modified systems for improving performance have been developed, and post treatment is inevitably applied for water reuse. Good performance using phase-separation technology has been reported because of providing optimal environments for each major group of microorganisms and their associated biological reactions. In a two-phase process, overall process efficiency is determined by acidogenic phase since it includes hydrolysis, referred to the overall rate-limiting step, so optimization of acidogenic phase is necessary. Having all merits of anaerobic digestion, anaerobic membrane bioreactors (AnMBRs) can offer high quality effluent, but the major obstacle for wide application of AnMBRs is fouling problem which is directly related to the operational cost. In contrast to aerobic MBRs, fouling characteristics of AnMBRs has not been studied a lot. Therefore, this study was performed to improve the methane production rate and effluent quality by modified two-phase anaerobic digestion system consisted of optimized acidogenic fermentation and AnMBRs. Additionally, characteristics of membrane fouling in submerged AnMBRs were evaluated though ultrafiltration of sludge in AnMBR at various HRTs.
Firstly, anaerobic batch tests were conducted to find out optimum co-substrate condition for maximum acidification. Three kinds of substrates, livestock wastewater, sewage sludge and food waste leachate, were used and an optimized substrate condition was determined by maximum VFA increase and SCOD increase simultaneously using statistical analysis. Suggested optimum mixing ratio of livestock wastewater, sewage sludge and food waste leachate was 0.4 : 1.0 : 1.1, respectively on COD basis.
Secondly, response surface methodology (RSM) was applied to find out optimum operating parameters, hydraulic retention time (HRT) and substrate concentration, for continuous acidogenic fermentation of mixed organic wastes. Continuous experiment was performed according to three steps of RSM. Three standards - total volatile fatty acids (TVFA) increase, acidification efficiency and acidification degree - were considered simultaneously in optimization process. The optimum condition was 2 days of HRT and 29,237 mg COD/l. Predicted TVFA increase, acidification efficiency and acidification degree was 73%, 20% and 0.471 respectively. Empirical equations were obtained through regression analysis. To confirm the validity of the statistical experimental strategies, a confirmation experiment was conducted under the obtained optimum condition (14.5 g COD/l/day as organic loading rate (OLR)). Experimental values of acidification standards were 76%, 18% and 0.414, respectively. Relative errors between theoretical and experimental values were 4%, 10%, and 12%, respectively, and this could be negligible because the physical and chemical characteristics of each organic waste were subject to wide fluctuation. Therefore, the RSM approach is appropriate for optimizing the acidogenesis of organic wastes.
Lastly, AnMBRs were operated as a methanogenic reactor in two-phase anaerobic digestion system for improving effluent quality and methane production rate (MPR). Three submerged AnMBRs with HRTs of 14, 16 and 20 days were operated. Feed was synthesized final effluent from optimized acidogenic reactor. AnMBR with HRT 14 days showed best performances in terms of MPR $(0.28 m^3/m^3/d)$ and COD removal (99.6%) including soluble COD removed by cake/gel layer (7.2%) on the membrane surface. To investigate fouling characteristics of AnMBRs at various HRTs, metabolic substances (EPS and SMP) production was evaluated. As HRT decrease, total EPS concentration increased: 29.4, 27.3 and 24.2 mg/gVSS at HRT of 14, 16 and 20 days, respectively. The EPS carbohydrate increased at low HRT because excess carbon substrates could be converted to polymers that accumulated as EPS at such high OLR. The supernatant DOC representing SMP also increased from 105.2 to 357.1 mg/l as HRT decreased from 20 to 14 days due to stress under high OLR. The increased EPS and SMP production at low HRT made cake/gel layer on the membrane surface, which increased total resistance of membrane (2.56, 2.96, and $3.39 \times10^{13}m^{-1}$ at HRT of 20, 16 and 14 days). As HRT decreased, therefore, EPS and SMP production increased, and consequently membrane resistance increased due to cake/gel layer formation on the membrane surface. Although increased fouling potential, cake/gel layer enhanced the effluent quality because it acted as a dynamic membrane to reject small particles or macromolecules passing through. Therefore, AnMBRs operation at low HRT made cake/gel formation on the membrane surface which caused severe fouling, but high effluent quality could be maintained due to the dynamic membrane. Relative contributions of supernatant containing colloids and solutes to the total membrane fouling were 63, 59, and 52% at HRT of 14, 16 and 20 days, respectively. On the contrary, the summation of $R_c$ and $R_f$ form microbial floc had similar values: 1.26, 1.22, and $1.24\times10^{13} m^{-1}$ at HRT of 14, 16 and 20 days, respectively. From the results, solutes and colloids, mainly resulting from the lysis of bacteria, were most likely to have higher fouling potential on UF membrane than microbial flocs, and the contribution of solutes and colloids to membrane fouling appeared severer at lower HRT. Distinctively, supernatant was more responsible for a membrane fouling in case of UF filtration when a reactor was operated at a lower HRT.
대표적인 유기성 폐기물인 음식쓰레기, 하수슬러지, 축산폐수를 혐기성 소화를 통해서 처리하는 기술은 유기물의 안정화가 가능할 뿐만 아니라 부산물로서 메탄과 같은 바이오 가스를 얻을 수 있는 장점이 있다. 유기성 폐기물의 혐기성 소화에는 크게 산생성 단계와 메탄생성 단계로 분리하여 운전하는 이상 소화가 도입되고 있다. 각 단계에서는 서로 다른 특성을 가지는 미생물이 관여하기 때문에 최적의 미생물 생장환경을 제공해 전체적인 공정 효율 증대를 추구한다. 이 중 산생성 단계는 율속단계인 가수분해를 포함하는 단계로 후속 단계인 메탄 생성조에서 주요 기질로 사용되는 유기산을 생성하는 중요한 단계이며, 수리학적 체류시간(HRT)과 기질농도는 소화조의 유기물 부하를 결정하는 핵심 운전인자로써 최적화가 필요하다. 혐기성 막결합 생물반응조 (AnMBRs)은 기존 혐기성 소화조의 단점인 낮은 유출수질 문제를 해결할 수 있지만 호기성 MBR과 달리 막오염(fouling)에 관한 연구가 부족하여 실플랜트의 확대 적용이 어려운 실정이다. 따라서, 본 연구에서는 유기성 혼합 폐기물을 처리하기 위해 기존의 이상소화에 산발효 단계에는 최적화 기법을 메탄 발효 단계에서는 AnMBRs을 적용하여 새로운 개념의 이상소화 시스템을 구축?제안하고자 하였다. 또한, AnMBRs 운전에서 HRT 변화에 따른 막오염의 경향을 파악하기 위한 연구도 함께 수행되었다.
먼저, 세가지 유기성 폐기물의 최적 혼합비를 도출하기 위하여 회분식 실험을 진행하였다. 통계학적 분석 결과, VFA와 SCOD 증가를 최대로 하는 최적혼합비는 COD 기준으로 축산폐수, 하수슬러지, 음식쓰레기 순으로 0.4 : 1.0: 1.1로 나타났다. 도출된 최적의 혼합 기질의 효율적인 산발효조 연속운전을 위하여 실험계획법의 일종인 반응표면법(RSM)을 적용하여 최적의 HRT와 기질농도를 구하였다. 최적의 조건은 유기산 증가량(TVFA increase), 산발효 효율(acidification efficiency), 산발효 정도(acidification degree)의 3가지 조건을 기준으로 결정하였고, 각 기준에 대한 경험적 모델식을 얻을 수 있었다. 3가지 기준을 동시에 만족하는 최적의 조건은 HRT 2일, 기질농도 29,237mg COD/l인 것으로 나타났으며, 각각의 모델식에 따른 예측된 결과는 76%, 18%, 0.414 인 것으로 예측되었다. 모델식의 정확도 테스트를 위한 검증실험 결과, 이론치와 실험치의 오차는 4%, 10%, 12% 로 성상의 변화가 큰 실폐기물의 대상으로 하였음에도 불구하고 높은 정확도를 나타내었다. 따라서, 반응 표면법을 이용한 최적화 방안은 물리화학적 성상변화가 큰 유기성 폐기물에서도 적용이 가능한 것으로 판단되었다.
메탄 반응조에서는 침지형 AnMBRs을 HRT 14, 16, 20일 조건으로 운전하였고, 기질은 최적화된 산발효조의 유출수를 모사한 합성기질을 이용하였다. HRT 14일 조건에서 가장 높은 메탄생성률$(0.28 m^3/m^3/d)$과 COD 제거율(99.6%)을 나타내었고, 멤브레인 표면에 케익층에 의해 제거된 양(7.2%) 역시 가장 높은 것으로 나타났다. HRT가 짧아질수록 EPS와 SMP는 증가하여 멤브레인 표면에 케익층을 형성하여 결과적으로 전체 여과 저항을 증가시켰다. 케익층은 여과 저항을 증가시켜 파울링이 증가되는 단점이 있었지만, 2차 멤브레인 역할을 하여 유출수질을 향상 시키는 장점이 있었다. 각각의 HRT 조건에서 운전된 AnMBRs의 슬러지를 미생물 플록과 상등액으로 구분하여 여과실험한 결과, HRT가 짧을수록 전체 막저항과 케익층에 의한 막저항이 증가하였으며, 이는 대부분 상등액의 용존물질이나 콜로이드 입자에 의한 여과 저항(63%)인 것으로 나타났다. 따라서, 낮은 HRT에서 AnMBRs의 운전은 높은 메탄 생성률과 유기물 제거율을 얻을 수 있고, 특히 케익층에 의한 2차적인 유기물 제거가 가능하지만 여과저항이 증가한다는 특성을 가지고 있는 것으로 나타났다.
