| Aiming at the technical problems of excessive accumulation of volatile fatty acids（VFAs）during the co-digestion process of food waste and waste activated sludge to limit the popularization and application,there search analyzes the accumulation characteristics of VFAs in anaerobic systems under different organic loading rate（OLR）,reveals the causes of typical VFAs from the perspective of microbial growth kinetics and thermodynamics.On this basis,for the purpose of strengthening the methane production of VFAs on anaerobic digestion system,the persistent effect of biochar as external electroactive material on the co-digestion process of food waste and waste activated sludge was studied,combining the kinetics,thermodynamics and microbial analysis,clarified the promotion mechanism of biochar on VFAs methanogenesis.The following conclusions are mainly obtained:（1）By examining the accumulation characteristics of VFAs during the thermophilic co-digestion of food waste and waste activated sludge under different OLR（in F/S）,it was found that the concentration of total volatile fatty acid（TVFA）gradually increased to 4685.86mg COD/L,main accumulation type changed from acetate to butyrate,and the methanogenesis was suppressed.The comparison of syntrophic methanogenesis properties at different concentrations（500 mg COD/L-8000 mg COD/L）of four typical VFAs showed that microbes degraded acetate at the fastest specific growth rate(0.33 d-1),followed by butyrate(0.15 d-1);and propionate produces microbial inhibition at high concentrations（greater than 5000 mg COD/L）,which enables severe thermodynamic inhibition of valerate in the degradation process with propionate as intermediate（Gibbs free energy=1.45～2.11 KJ/mol）.The disadvantages of microbial growth kinetics and thermodynamics make it difficult for VFAs produced in the hydrolysis and acidogenesis stage to convert to methane in a short time,leading to the accumulation of VFAs such as propionate and butyrate in the system.（2）In order to strengthen the methanogenesis process of VFAs to avoid excessive accumulation,this study added 10g/L biochar to the co-digestion experiment of food waste and waste activated sludge,by comparing the promotion effect of biochar on the hydrolysis,acidogenesis,acetogenesis and methanogenesis of the continuous three-period experimental,it is found that the persistent promotion effect of biochar on the syntrophic methanogenesis（acetogenesis and methanogenesis stage）is the crucial cause for significantly improving the maximum methane production rate of co-digestion system in each period.Compared with the control group,although the VFAs types accumulated in the co-digestion system are still mainly acetate and butyrate,the syntrophic methanogenesis rate of VFAs was significantly accelerated and the TVFA accumulation was significantly reduced,including in the reaction to day 7.3,the TVFA in the biochar group was only 34.8%of the control group,confirming that the addition of biochar can effectively alleviate the VFAs accumulation problem.（3）On the basis of clarifying the effect of VFAs co-digestion of food waste and waste activated sludge,combined with the thermodynamics,electronic transfer path and microbial community analysis,further reveals the promotion mechanism of biochar for VFAs syntrophic methanogenesis,the results show that biochar can reduce the limitation of VFAs syntrophic methanogenesis by reducing high hydrogen partial pressure,expanding the thermodynamics window to promote the electronic transfer process of interspecies hydrogen transfer（IHT）;The biochar surface is rich in redox groups,which may trigger a more efficient direct interspecies electron transfer（DIET）pathway through the electron gain and loss process;In addition,the functional microbial Methanosarina is efficiently enriched on the biochar surface（relative abundance increases to 49.6%,21.7 times the control group）,significant increases expression of the related genes,further promote the syntrophic methanogenesis process. |
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