Effects Of Citric Acid And Citric Acid-Fe²⁺ Chelate On VFA Production From Anaerobic Digetion Of Food Waste

The high organic content,high moisture content,and high biodegradability of food waste make it an ideal substrate for anaerobic digestion to produce volatile fatty acids（VFA）,a potentially renewable carbon source.However,the poor stability of anaerobic digestion caused by the lack of trace elements is one of the main factors limiting its popularization and application.The content of Fein food waste is low.Feis an essential element for the formation of microbial cells and various enzymes in anaerobic fermentation systems.Supplementing Fecan improve the stability of anaerobic digestion systems.The addition of trace elements in the form of trace element-chelating agents can be cost-effective while enhancing the microbial activity associated with anaerobic digestion.Citric Acid（CA）has good chelating properties with Fe,and can participate in the tricarboxylic acid cycle,which is closely related to microbial activity,and promote microbial energy metabolism to accelerate cell growth.Therefore,supplementing Feinto the anaerobic digestion system of food waste in the form of CA-Fe²⁺chelate is a win-win strategy for trace element supplementation.This study comprehensively considered the multi-stage nature of anaerobic digestion and used the method of sequencing batch experiment to investigate the effect of CA and CA-Fe²⁺chelate on the acid production performance of anaerobic digestion of food waste,and to find a suitable Fesupplementation strategy.At the same time,the influence mechanism of CA-Fe²⁺chelate on the hydrolysis,acidification,and methanation of food waste during anaerobic digestion was determined.（1）CA can promote the production of VFA by anaerobic digestion of food waste.When the concentration of CA was 75μmol/L,the maximum VFA accumulation amount was11352.8 mg/L,an increase of 44.81%compared with the control group.The yield and conversion of VFA were the maximum at this time,0.14 g/g VS（fed）and 0.55 g/g VS（removed）,respectively.In addition,the addition of CA can promote the release of ammonia nitrogen,so if the food waste digested liquid is used as an additional carbon source in a sewage treatment plant,certain measures should be taken to remove ammonia nitrogen before its application.Considering the anaerobic digestion efficiency（VFA concentration,VFA yield and conversion rate,VS removal rate）and the cost of CA addition,the optimal concentration of CA was selected as 75μmol/L.（2）FeCl₂ has a positive effect on the acid production system of anaerobic digestion of food waste.When the Fe²⁺concentration was 800 mg/L,the VFA concentration in the reactor was the highest,which was 10950.7 mg/L,which was 22.2%higher than that of the control group.When the Fe²⁺concentration was greater than 400 mg/L,the yield and conversion of VFA were the highest,which were 0.13 g/g VS（fed）and 0.44 g/g VS（removed）,respectively.Appropriate addition of trace metal elements can improve the removal effect of organic matter.The VS removal rate of substrates in each reactor is 29%-36%.When the Fe²⁺concentration is 25 mg/L and 50 mg/L,the VS removal rate increases slightly,and then with the increase of Fe²⁺concentration.The addition of Fe²⁺generally showed a decreasing trend.Taking into account the anaerobic digestion efficiency（VFA concentration,VFA yield and conversion rate,VS removal rate）and the cost of FeCl₂ dosing,the optimal FeCl₂ dosing concentration is 800mg/L.（3）The addition of CA,Fe²⁺and CA-Fe²⁺chelate all promotes the hydrolysis and acidification stages of carbohydrate components and protein components in the anaerobic digestion process.The promoting effect:CA-Fe²⁺chelate>Fe²⁺>CA.In addition,the addition of CA,Fe²⁺and CA-Fe²⁺chelate can inhibit the activity of methanogens,and the inhibition strength:CA-Fe²⁺chelate>Fe²⁺>CA.The above results indicated that supplementing Fein the form of CA-Fe²⁺chelate into the food waste anaerobic digestion system is a suitable trace element supplementation strategy.The addition of CA-Fe²⁺chelate allows more soluble polysaccharides and proteins to be released into the digestive juice,acid-producing bacteria use glucose to accumulate more VFAs,and amino acids are used to obtain greater ammonia nitrogen dissolution,while the activity of methanogens Inhibition is produced,which is conducive to the accumulation of VFA.The research results provide a theoretical reference for the efficient resource utilization of food waste.