Synergic Effect Of Alkali Pretreatment And Bentonite Addition On Kitchen Waste Anaerobic Digestion And Microbial Community Structure

Kitchen waste has high moisture content and is easy to be degraded due to its high organic content.Serious pollution to air and water environment will be caused if kitchen waste is not properly treated.High solid anaerobic digestion can make up for the defects of traditional anaerobic digestion such as too large reactor volume,too high energy consumption and too much biogas slurry and slag.But there are still some problems such as unstable performance of reaction system and low methane production rate.Therefore,it has become a key problem that how to improve the methane production rate of kitchen waste high solid anaerobic digestion and improve the performance of the reaction system.Some studies have found that the methane production rate of anaerobic digestion can be improved by alkali pretreatment.But the conclusion of optimal concentration and optimal time of alkali pretreatment has not been unified.Our previous study found that 0.6 g/g-VS bentonite addition can improve the methane production rate of kitchen waste anaerobic digestion.At present,the mechanism of how alkali pretreatment combined with bentonite enhance the performance of kitchen waste anaerobic digestion has not been studied,and the effects of alkali pretreatment combined with bentonite on kitchen waste anaerobic digestion under mesophilic and thermophilic intermediate zone and high temperature conditions have not been studied.Based on this,this study selected calcium oxide as the material of alkali pretreatment.Research is carried out in the following three aspects.(1)Under the medium temperature condition(37 ± 1℃),the effects of alkali pretreatment concentration with / without 0.6 g/g-VS bentonite on the kitchen waste high solid anaerobic digestion were investigated.The results showed that the cumulative methane production rate could be significantly increased by alkali pretreatment with / without 0.6 g/g-VS bentonite.Bentonite can promote the gas production rate at the initial stage of anaerobic digestion and shorten the stagnation period effectively.Considering the cumulative methane production rate and amount of Ca O,“3% Ca O pretreatment” and “3%Ca O pretreatment with bentonite” are recommended.Based on this,two continuous experimental reactors were started.(2)Based on the optimal alkali pretreatment concentration(3%Ca O(w/w))obtained by batch tests at moderate temperature,the effects of alkali pretreatment time on kitchen waste high solid anaerobic digestion were investigated at mesophilic and thermophilic intermediate zone(45 ± 1℃)and high temperature(55 ± 1℃).It was found that the reaction systems were acidified under the condition of high temperature while the methane production rate of anaerobic digestion could significantly be improved by alkali pretreatment with bentonite addition under the condition of mesophilic and thermophilic intermediate zone.(3)After the medium-temperature batch test,the optimal conditions “3%Ca O pretreatment with bentonite”(denoted as AB3)and “3%Ca O pretreatment”(denoted as A3)and the control group were selected for microbial community analysis.It was found that both A3 and AB3 increased the abundance and diversity of bacteria and the relative abundance of dominant archaea.A3 and AB3 increased the relative abundance of SAO bacteria Syntrophomonas.It is noteworthy that Syntrophomonas may establish DIET with methanogens.A3 and AB3 increased the relative abundance of Methanosarcina,Methanoculleus and Methanomassiliicoccus,and changed the main methanogenic pathway from acetic acid methanotrophic to hydrogen methanotrophic.In conclusion,alkali pretreatment and alkali pretreatment combined with bentonite can effectively improve the microbial community structure of kitchen waste anaerobic digestion system,which increased the methane production rate.According to the above three experimental studies,alkali pretreatment and bentonite can significantly improve the performance of anaerobic digestion reaction system,optimize the microbial community structure of anaerobic digestion,and achieve harmlessness,reduction and resource recovery in the process of food waste treatment,which is in line with China’s dual carbon policy.