Effects And Mechanisms Of Salinity,fat,oil And Grease(FOG) On Short Chain Fatty Acids Production From Food Waste And Waste Activated Sludge Anaerobic Fermentation

With the acceleration of economy and urbanization,lots of food waste and waste activated sludge are produced annually.Food waste has the characteristics of high organic matter content,high salt and high oil,and easy to spoilage and breed viruses.Waste activated sludge,the by-product of sewage treatment process,also contains high levels of organic matter,pathogens and other toxic and harmful substances.If food waste and waste activated sludge are not effectively treated and disposed,they will result in waste of resources and secondary pollution.Anaerobic fermentation has become the first strategy for the treatment and resource utilization of food waste and waste activated sludge by the unique advantages of pollution control and energy recovery.Short chain fatty acid（SCFA）is an intermediate product of anaerobic fermentation of organic matter,it can be used as high-quality carbon source for biological nitrogen and phosphorus removal in wastewater treatment plant,electricity generation from microbial fuel cell,and the production of bio-plastics.Anaerobic fermentation of mono food waste often results in unsatisfactory energy recovery and pollutant removal due to unbalanced C/N and low impact load capacity.Anaerobic co-fermentation of food waste and waste activated sludge can balance C/N,dilute toxic and harmful substances,and cooperate with microbial communities to improve the stability and efficacy of anaerobic fermentation system.The salinity and oil contained in the food waste will inevitably affect the subsequent resources utilization.Although previous studies have reported the effect of salinity and/or fat oil and grease（FOG）on anaerobic digestion of food waste,previous studies focused on the production of gas phase.The effect of salinity and/or FOG on the production of SCFA from food waste mono-fermentation and food waste and waste activated sludge co-fermentation are rarely explored,and the corresponding mechanisms remained unknown.Therefore,in this study,food waste and waste activated sludge were studied.In the sequential batch anaerobic reactors,the variations of SCFA,soluble organic,key enzyme activities in hydrolysis,acidification and methanogenesis,and microbial communities under different salinity and FOG concentrations were analyzed to establish the effect of salinity and FOG on the production of SCFA and reveal the relevant mechanisms.Firstly,using food waste as a single digestive substrate,under mesophilic temperature condition,different salinity gradients were set up to investigate the effect of salinity on the production of methane and SCFA,organic compounds disintegration,model compounds degradation in hydrolysis,acidification,and methanogenesis,and the activities of key enzymes.The results showed that the presence of NaCl in food waste not only affected the maximum accumulation of methane,but also affected the optimal digestion time.When the mass concentration of NaCl increased from 0 to15.0 g/L,the maximum accumulation of methane decreased from 253.3 mL/g to 22.7mL/g,and the optimum digestion time extended from 23 d to 27 d.NaCl helped to improve the solubility of carbohydrate and protein in food waste,when the mass concentration of NaCl increased from 2.0 g/L to 15.0 g/L,the maximum content of soluble carbohydrate and protein increased from 8596 mg/L and 2156 mg/L to 12054mg/L and 3124 mg/L,respectively.NaCl promoted the hydrolysis process of food waste,but lower than 5.0 g/L NaCl helped the acidification process,while higher than5.0 g/L NaCl inhibited the acidification process.The presence of NaCl can inhibit methanogenesis.With the same NaCl mass concentration,digestion of food waste and waste activated sludge can improve methane production compared to the mono digestion of food waste.In addition,the study found that anaerobic co-fermentation of food waste and waste activated sludge can slow down the inhibition of NaCl.Secondly,food waste and waste activated sludge co-fermentation can improve the system stability and efficiency,the researcher took the food waste and waste activated sludge as the research object,in mesophilic environment,the effect of salinity on food waste and waste activated sludge anaerobic co-fermentation was investigated and the related mechanism was also revealed.The results showed that the effect of NaCl on the production of SCFA was dose-dependent.As the mass concentration of NaCl increased from 0 to 8.0 g/L,the production of SCFA increased from 367.6 to 638.5 mg/g.However,the further increase of NaCl inhibited the production of SCFA.Mechanism studies showed that the existence of NaCl not only accelerated the release of soluble substances in food waste,but also destroyed the extracellular polymeric substances（EPS）and cell wall/membrane,and promoted the bio-transformation of proteins released from fermentation system.Low levels of NaCl enhanced the hydrolysis and acidification processes,but inhibited methane production,and the processes of acidification and methane production were severely inhibited by high NaCl content.Moreover,FOG is the substance in food waste.The researcher used food waste as the mono fermentation substrate,and set up different levels of FOG in mesophilic environment.The influence of FOG on the production of SCFA from anaerobic mono fermentation of food waste was investigated,and the related mechanism was also revealed.The results showed that when the FOG content increased from 0 to 4.0 g/L,the maximum yield of SCFA increased from 32.7 g/L to 38.5 g/L.When the FOG content continued to increase to 8.0 g/L,16.0 g/L and 32.0 g/L,the maximum yield of SCFA decreased to 26.5 g/L,21.6 g/L and 19.8 g/L,respectively.In addition,the optimal fermentation time of SCFA was prolonged by high concentration of FOG.FOG self-degradation would produce SCFA,but its contribution was not great.High concentration of FOG had inhibitory effect on SCFA production from anaerobic digestion of food waste,and the higher the FOG content,the more obvious the inhibition of SCFA was.The effect of FOG concentration on the maximum value of SCOD was not obvious.The maximum value of SCOD was in 89.4⁹1.5 g/L in the application range of this study（0³2.0 g/L）.When the FOG content was 32.0 g/L,the presence of FOG prolonged the time required to reach the maximum value of SCOD.Low concentration of FOG promoted the hydrolysis and acidification of food waste,but inhibited methanogenesis,while high concentration of FOG inhibited hydrolysis,acidification and methanogenesis.Afterwards,the researchers used food waste and waste activated sludge as research objects to explore the influence of FOG content on the production of SCFA and related mechanisms.When the FOG content was 4.0 g/L,the FOG can promote the accumulation of SCFA,and shorten the fermentation time,however,when the FOG content was 8.0,16.0 and 32.0 g/L,the yield of SCFA was lower than the control group,and the optimum fermentation time had different degrees of delay.FOG content had little effect on the amount of co-fermented SCOD,but high concentration of FOG could prolong the time required to reach the maximum value of SCOD.4.0g/L FOG benefited the hydrolysis and acidification processes.When the FOG content further increased to 32.0 g/L,hydrolysis and acidification processes were severely inhibited.No matter how much the FOG content was,it had a serious inhibitory effect on methane generation.In addition,palmitic acid produced during anaerobic degradation of FOG had the strongest inhibitory effect on hydrolysis,acidification and methanogenesis.The low-concentration FOG promoted the activities of key enzymes responsible for hydrolysis and acidification,and increased the relative percentages of Bacteroidetes and Firmicutes to 35.9%and 39.2%.The microbial communities at the genera level showed low concentrations of FOG increased the relative abundance of Bacteroides,Clostridium and Bacillus and Proteiniborus,and they were 19.5%,21.3%,10.3%and 9.4%,respectively.Finally,the researcher explored the effect of the combination of salinity and FOG on the production of SCFA from food waste and waste activated sludge co-fermentation.In the low salt and low FOG reactor,the maximum value of SCFA was 651.2 mg/g,which was significantly higher than that of low salt group 519.6mg/g and low FOG group 421.3 mg/g,indicating that low salt and low FOG was helpful for anaerobic co-fermentation to accumulate SCFA.The effect of the combination of salinity and FOG on the solubility of food waste and waste activated sludge was mainly dependent on the salinity.When the salinity was high,the amount of soluble organic matter in the mixed fermentation system was large,while when the salinity was little,the organic matter release in the mixed fermentation system was relatively little.The low salt and low FOG group promoted the release of PO₄^3--P and NH₄⁺-N,while the high salt and high FOG suppressed the release of PO₄^3--P and NH₄⁺-N.Moreover,the low-salt and low FOG promoted the hydrolysis and acidification enzymes,and the high salt high FOG inhibited the hydrolysis and acidification enzymes.