Study On Optimization Technologies Of Anaerobic Digestion Of Food Waste And Yak Dung

The amount of food waste and yak dung produced in China is huge and has been increasing year by year,and resource utilization can be realized through anaerobic digestion.However,food waste is easy to acidify and yak dung is difficult to hydrolyze,which often lead to failure of the digestion system and low efficiency.In order to improve the stability of the anaerobic digestion process and the efficiency of biomass conversion,this thesis takes Tibetan organic wastes as the research objects.The temperature mode and substrate to inoculum ratio（S/I）of anaerobic digestion of food waste were optimized,the pretreatments and nano-additives for anaerobic digestion of yak dung were selected,and the mechanism of anaerobic co-digestion of food waste and yak dung was discussed on this basis.Finally,the kinetic analysis and evaluation were carried out.The main conclusions obtained are as follows:（1）The batch anaerobic digestion of food waste had the fastest methane production rate(R_m=46.86 m L g^-1 VS d^-1,S/I=1:4)at 35℃,while became rancid at 15℃.The stepwise temperature changes modes reduced the rate of methane production,but did not significantly reduce cumulative methane production.Temperature had a greater impact on microbial diversity and community structure than substrate/inoculum ratio（S/I）.The decrease in temperature caused the digestion pathway to shift from acetoclastic to the hydrogenotrophic pathway gradually.As for a small-scale batch AD system without temperature control,the net energy was maximized at 25-15℃ with S/I=1:2,and the highest waste disposal was achieved at 35℃ with S/I=1:1(99 kg VS m^-3 y^-1).（2）The methane production of yak dung increased significantly after alkali pretreatment and thermal alkali pretreatment,improved 17.55%and 22.34%,respectively.The methane production of thermal pretreatment group increased slightly after adding nano zero valent iron（NZVI）or nano Fe₃O₄.Alkali pretreatment and thermal alkali pretreatment accelerated the rate of hydrolysis and increased the maximum content of volatile fatty acids from 0.185 g·L^-1 to 1.897 g·L^-1 and 2.386 g·L^-1,respectively.After alkali pretreatment and thermal alkali pretreatment,Methanosarcina（59%relative abundance）replaced Methanosaeta（57%relative abundance）as the dominant bacteria.Cost analysis showed that alkali/thermal alkali pretreatment reduced the net profit（-0.055$/Kg VS）of anaerobic digestion of yak dung,and n Fe₃O₄ slightly increased the net profit（0.107$/Kg VS）of anaerobic digestion of yak dung after thermal pretreatment.（3）Anaerobic co-digestion of food waste and yak dung was carried out at 35℃ and S/I ratio of 1:1,and n Fe₃O₄ was added at a mixing ratio of 25:75.The results showed that the cumulative methane production,methane content,maximum VFAs concentration,and volatile solids（VS）removal rate of anaerobic co-digestion of food waste and yak dung increased with the increase in the proportion of food waste.Due to the rapid accumulation of VFAs,the mono-digestion of food waste was inhibited in the early stage.The addition of yak dung improved the buffering capacity of the system.The cumulative methane production of co-digestion with 75%（VS basis）of food waste showed the highest synergistic effect.The microbial community in anaerobic co-digestion was more stable than mono-digestion.Adding n Fe₃O₄ changed the microbial structure and slightly increased the methane production.（4）The modified Gompertz model fitted well with the experimental values of each optimization technique.Reducing the S/I ratio shortened the lag phase of anaerobic digestion of food waste.Thermal alkali pretreatment extend the lag phase of yak dung.The lag phase of co-digestion with 25%of food waste was the shortest.A higher maximum methane production rate could be obtained for food waste at high temperatures,and the Rm of yak dung was highest(13.29 m L g^-1 VS d^-1)after thermal alkali pretreatment.When the mixing ratio of food waste and yak dung was 75:25（VS basis）,the Rm of anaerobic co-digestion was the highest(34.63 m L g^-1 VS d^-1).