Study On The Optimization Of The “Integrated Ethanol-methane Fermentation Circulation” Process

The goal of waste water zero emission in fuel ethanol production was basically achieved by the “integrated ethanol-methane fermentation” process. Currently, the mesophilic anaerobic digestion effluent(ADE) has been successfully resued. However the mesophilic ADE has high alkalinity and pH, therefore a lot of sulfuric acid was required to reduce the slurry pH to about 6.0. To resolve this problem, portion of distillery waste was used to reduce pH of the mesophilic ADE. Currently, this new integrated process has been carried out in full-scale. However, due to enlargement of the process on time and space, higher residual total sugar concentrations, acidification and other issues of ethanol fermentation were exposed. Reasons that caused problems of higher total sugar residual concentrations and acidification were studied and corresponding solutions were explored in this study.As for the problem of higher residual total sugar concentrations in ethanol fermentation, we found that inhibition of the distillery waste to the liquefaction process led to this problem. The insoluble solids and some metal ions in distillery waste were the main inhibitors, in which the metal ions were the most important inhibitors. Results showed that α-amylase was mainly inhibited by Fe3+, Al3+ and Li+, Wherein Fe3+ is a major inhibitor and 1.40 mmol·L-1 Fe3+ decreased the α-helical content by 24.40% and the enzyme activity by 19.18%.Meanwhile, it was found that Mg2+ could stimulate the α-amylase activity. Addition of 16 mmol·L-1 Mg2+ to the distillery waste could promote liquefaction of starch and decrease the total sugar residual concentrations by 10.29 g·L-1 in the ethanol fermentation, and its application feasibility was the best. In addition, 60% increase in the usage of amylase could increase reducing sugar production by 8.03 g·L-1, However production cost per ton of ethanol was increased by 15.66 yuan. The liquefaction time extended to 130 min could increase reducing sugar production by 7.13 g·L-1 in the liquefaction process, However long time of liquefaction may cause the production of caramel and result the waste of carbon source.The study on acidification problem of ethanol fermentation found that acidogens from the mesophilic ADE could utilize the nutrients contained in the distillery waste to growth and produce a large number of small molecular organic acids(mainly including butyric acid and acetic acid) when the mesophilic ADE was mixed with the distillery waste, Synthesis rates of volatile organic acids reached 116.50 mg·L-1·h-1 within 28 h. The content of butyric acid was more than 80% and the concentrations reached 2648 mg·L-1 Results showed that number of S.cerevisiae in ethanol fermentation was decreased by 48.20% with 3000 mg·L-1 of butyric acid. Thus S.cerevisiae lost their dominant position and the probability of contamination increased significantly.It was found that mesophilic ADE was ultrafiltrated by the tubular organic membrane, synthesis rates of volatile organic acids was controlled to 8.35 mg·L-1·h-1 within 28 h and this can solve the problem of contamination and acidification in the ethanol fermentation. At the same time, none new substances were introduced to the circulation process. Long-term use of penicillin, kejunling and other chemical drugs could cause resistant of microorganisms. In addition, Thermal treatment the mixed water(115℃, 10-15 min) could also avoid acidification phenomenon, Synthesis rates of volatile organic acids was controlled to 7.25 mg·L-1·h-1 within 28 h, However it may lead the fouling phenomenon of mixed water.