Study On Characteristics Of Anaerobic Fermentation With Food Wastes

In China, with the improvement of people's living standard, the output of food wastes is increasing day by day. The ecological environment and human health would be badly affected if the massive food wastes couldn't be treated timely and effectively. Using anaerobic fermentation technology to treat food waste, can not only obtain biogas but also alleviate environmental pollution, so, it is effective to solve the problem of energy shortage and environmental pollution. Currently, lots of studies on anaerobic fermentation technology of food wastes have been completed. The present researches indicate that anaerobic digestion of food wastes will soon be prone to acidification because of its easy degradability and higher content of organic matter. The inhibition of acidification on fermentation may be reduced to some extent by the method of adding chemical reagent to adjust pH to be neutral. However, the cost and ion concentration of biogas slurry would be enhanced, and it is not conducive to the further utilization of digestion residue. In addition, this way is limited to improve contribution of reactor's organic loading, the capacity utilization is still very low, and it is unable to achieve sTablele and efficient production of methane with food wastes.In order to obtain the process of improving efficiency of food wastes anaerobic digestion, a large number of experiments have been conducted in this research. First, single fermentation characteristics and methanogenesis potential of starch, protein and fat were investigated, and then the influence of organic ingredients on gas yield property was determined using mixture design experiments, thus the suiTablele ratio of organic compounds was ascertained. Second, on the basis of single factor results, the interaction of inoculation proportion, concentration and temperature on initiating process of food wastes anaerobic fermentation was explored with response surface method, the process parameters was optimized and initiating condition was confirmed in the meantime, this experiment was to solve the problems that food waste was easily going to acidification in the process of anaerobic digestion and the reactor's initiating was difficult. Third, for the sake of realizing continuous and sTablele methane yield, and also with the purpose of boosting the utilization ratio of anaerobic fermentation reactor, the effect rule of organic loading on production of acid and methane was explored in the process of anaerobic digestion with food waste using semi-continuous single-phase anaerobic fermentation. On this basis, two-phase sludge return process was adopted to treat food waste, this made butyrate type directional acidification of acidification phase come to be true, meanwhile, alkalinity and the number of microorganism of methanogenesis phase raised significantly, and the efficiency of anaerobic fermentation for treating food waste was greatly improved. The main research contents and achievements are as follow:(1) The study result indicated that while food waste was used as anaerobic digestion material, fermentation process would be affected by the mixing ratio of fat, starch and protein. It was inhibited in different degree by long-chain fatty acid, volatile fatty acid and ammonia nitrogen when the ratio was unsuiTablele. While the ratio was appropriate, the inhibition of intermediate meTableolites could be avoided effectively on account of the synergistic effect, and then the organic loading was boosted appropriately. The optimum mixing ration was as follow:fat:starch:protein was36:30:33, the corresponding maximum biochemical methane potential and VS degradation ration were451.36and79.62%, respectively. The protein content of actual food waste was a little lower, but the starch content was higher, therefore, in order to enhanced the organic loading, mixing fermentation of food waste and high nitrogen content material should be put to use. Comparing with the optimum mixing ratio, the protein content of actual food waste was lower and the starch content was high.(2) Lactic acid was easily generated in the process of anaerobic digestion with food waste, the lactic acid bacteria was mainly carried by food waste. The degradation ration of lactic acid was highest, lactic acid mainly converted into acetic acid and propionic acid, and both ratio was close to1:3. Under the condition that the lactic acid loading was10mmol·L-1·d-1, the phenomenon of insufficient buffer ability appeared in the anaerobic system after30days of fermentation, the cause was that propionic acid could not be conversed timely and caused accumulation which was yield after the degradation of lactic acid, accordingly, propionic acid and lactic acid should be prevented to go into anaerobic system.(3) The result of initiating experiment of anaerobic fermentation with food waste manifested that anaerobic system would turn into acidification while inoculation ratio was less than50:50and pH was lower than5.5. Conversely, when inoculation ratio was more than50:50and pH was higher than7.0, gas yield situation was in good condition and methane content could reach60%. Multi-objective optimization of level combination of material concentration, temperature and inoculation ratio was conducted using Box-Behnken design method and on the basis of regression model, and the optimum process parameters were determined as follow:material concentration, temperature and inoculation ratio were5%,35?and65%, respectively. Under optimum condition, the result of verification test which was consistent with predicted value was obtained, that was the average values of VS methane yield ration and VS degradation ratio were469.87mL/g and81.95%, respectively.(4) On the basis of batch initiating experiment, the effect of organic loading of single-phase anaerobic fermentation on the balance of each stage of anaerobic digestion was explored. Result showed that while organic loading was less than4kg·m-3·d-1, methanogenesis fermentation was the main characteristic for anaerobic system, on the contrary, acidification fermentation was the main characteristic. When pH was less than5.5, the fermentation characteristic of anaerobic system was acetic acid type, When pH was5.0, it was butyrate acid type, and When pH was lower than5.0, it was butyrate acid type lactic acid type.(5) The sludge retention time of two-phase sludge return fermentation system was extended, the organic loading that system could sustain was7.36kgVS·m-3·d-1, under this organic loading the average VS methane yield ration was399.95mL·g-1. There was exponential function relationship between VS concentration and organic loading that system could sustain, and the expression was y=1.61e023x (correlation coefficient R2=0.99187, P<0.0001). Where x represented organic loading and y represented VS concentration. So, it could be concluded that the maximum organic loading that wet fermentation could sustain was8kgVS·m-3·d-1.