Effect Of Temperatures On Hydrolysis Acidification Of Food Waste And Methane Conversion Of The Acidified Products

Showing high stability and resistance to the impact of feeding load, two-phase anaerobic digestion is an important technology in harmless and resourceful treatment of food waste. Volatile fatty acids, after hydrolysis acidification, are the substrates for methanogens. The composition and concentration of VFAs have great impacts on methane phase. Temperature shows a significant effect on the metabolite in hydrolysis acidification phase.In this study, after nine generations’ enrichment, composite acidification strains which could fastly and stablely promote hydrolysis acidification of food waste were achieved at mesophilic (35℃), thermophilic (55℃) and hyperthermophilic (70℃) conditions, respectively. Furthermore, the composite acidification strains obtained after enrichment were used as inoculums to investigate the difference of hydrolysis acidification at three temperatures. Meanwhile, the effects of factors (including initial pH, feeding load, reaction time and inoculum volumes) on hydrolysis acidification of food waste at three temperatures were discussed. The acidification performances of food waste at three temperatures were optimized by response surface methodology (RSM). Finally, the methane conversion effects of acidified products at three temperatures were discussed by methanation at corresponding temperatures and35℃, respectively.It was indicated that the strains after enrichment enhanced the concentration of total VFAs all at three temperatures. The amounts of total VFAs generated at three temperatures were11.56,5.35and5.87times higher than that in the control groups, respectively. Temperature has a significant effect on the acidified products of food waste. The concentration of total VFAs at35℃(20383.29mg·-L-1) was the highest among three temperatures, which was45.02%and76.08%higher than that at thermophilic(55℃) and hyperthermophilic (70℃). Besides, highest ethanol concentration was obtained at mesophilic (35℃) while highest butyric acid concentration was achieved at thermophilic (55℃). Isobutyric acid concentration was high at both thermophilic (55℃) and hyperthermophilic (70℃). Factors led to the change of the concentration of total VFAs at hyperthermophilic (70℃). Second order polynomial models of the concentration of total VFAs at three temperatures were fitted through RSM in which the concentration of total VFAs was the dependent variable (response value) while initial pH, feeding load, reaction time and inoculum volumes were the independent variables. The models of35℃and70℃showed persuasive descriptions between total VFAs concentration and those independent variables with the adjusted multiple correlation coefficients were0.9701and0.9786, respectively. Although the model of55℃was significant, the adjusted multiple correlation coefficients was only0.8935. The effects of four factors to the total VFAs were different at three temperatures. The impact level orders of factors from big to small were feeding load, inoculum volume, reaction time and initial pH at35℃and55℃while the sequence changed to volume, feeding load, reaction time and initial pH at70℃. Hydrolysis acidification of food waste was greatly affected by microorganism at hyperthermophilic condition. Gas production per unit VS at55℃was689.30mL·g-1, which was18.68%and102.14%than that at35℃and70℃, respectively. Gas production per unit VS in the methane-producing phase in mesophilic condition by using the discharging after55℃acidification was593.79mL·g-1, which was13.37%and16.73%higher than that at35℃and70℃, respectively. It was indicated that higher gas production per unit VS could be achieved by using the discharging of acidification at thermophilic (55℃) together with55℃methanation with a shorter digestion time and quickly rising of methane content to70%. Results of this study provided base data supports and references for the technical selection and parameters setting of food waste anaerobic digestion.