Process On High-Solids Anaerobic Fermentation For Converting Food Waste And Excess Sludge To Biogas

With the rapid development of economy and great improvement of people's life, more and more organic solid waste, such as food waste and excess sludge, were released into ecosystem, which would be harmful to human beings and environment. Carbohydrate, protein, lipid and cellulose were the primary organic composition of solid waste, which could be converted into biogas according to anaerobic fermentation process.In a normal start-up of an anaerobic fermentation reactor, a certain amount of inoculum should be added together with the substrate to provide the required microorganisms to start reactions. Effect of various feed to inoculum ratios on high-solids anaerobic fermentation of food waste was conducted in laboratory scale, which was operated at mesophilic (35℃±1℃) and high-solids (12% to 18% total solids) conditions. The biogas yield, properties of fermentation broth mixture and removal efficiency of TS and VS were investigated. Results showed that inoculation over 50% had a higher biogas production rate, cumulative biogas yield and methane content than those by inoculation ratio less than 25%. Inoculum addition could not only enhance buffer capacity of fermentation system with stable and appropriate pH values, which lied between 6.8 to 7.2, but also reduce the concentration of total nitrogen, phosphorus and COD_cr. As the feed to inoculum ratio of 1.0, methane content as well as reduction rate of TS and VS reached the maximums, 64%, 43.6% and 52.8%, respectively, for which the cumulative biogas yield was 228.6 mL/gVS.Considering the disadvantages of the acids accumulated and the pH decreased while food waste as a single substrate, co-fermentation of food waste and excess sludge was operated at mesophilic (35℃±1℃) and high-solids (12% total solids) conditions, which could also enhance the utilization of reactor volum. Results showed that compared with other mixing ratios, a mixture ratio of 30:30 could obviously enhance the anaerobic efficiency of biogas production, and the cumulative biogas yield, cumulative methane yield, biodegradability as well as VS removal rate increased to 612 mL/gVS, 327 mL/gVS, 76.9% and 63.6%, respectively. Moreover, it was found that acidification and methanogenesis were the rate-limiting steps in early and later periods of multi-food waste case, respectively. The biogas production rate exhibited a rapid increase in earlier 5 days of co-fermentation, but the content of methane was still below 60% throughout the high-solids anaerobic co-fermentation, volatile fatty acids were mainly inhibition factor. As far as the case of multi-sewage sludge, hydrolysis was considered as the rate-limiting step in early period of anaerobic co-fermentation, while both hydrolysis and acidification were limit factors for the conversion of the mixed organic waste in later period. The peaks of biogas production rate were obtained from the 10th to the 25th day. It was also observed that methane content rapidly increased to 50% in earlier five days and then stabilized at approximately 70%.The proportion of organic substance, such as carbohydrate, protein and lipid, had a significant effect on methane convertsion efficiency. This paper had made up various mixing ratios of carbohydrate, protein and lipid, via food waste and excess sludge, to evaluate the effects of mixing ratios on methane conversion and degradation efficiency of organic substance, which operated at mesophilic (35℃±1℃) and high-solids (12% total solids) conditions. Results showed that the maximum values of specific methane production potential (P_s), specific methane production rate (R_s) and methane yield were all achieved at mixing ratio of 55:36:9, and they increased to 404.1 mL/gVS, 11.2 mL/gVS·d, and 326.7 mL/gVS, respectively. Compared with other mixing ratios, the degradation process of oragnic substance was also more effective and more stable at the ratio of 55:36:9. Adding appropriate carbohydrate could not only enchance the degradation itself, but also improve the degradation efficiency of protein and lipid. Moreover, it was found that when carbohydrate in the substrate accounted for more than 65%, the high concentration of total volatile fatty acid (VFA), some single VFA and total undissociated acid became the primary factors resulting in methane production inhibition. Increasing the proportion of protein could lead to the prolonged star-up period as well as fermentation time. When protein increased up to 48%, a switch to the inhibition of methane production by total ammonia (TNH₃-N) and free ammonia (NH₃-N) occurred.