Enhanced Mesophilic Anaerobic Codigestion Of Waste Activated Sludge And Food Waste With Compound Iron Oxide As Additive

A large amount of waste activated sludge（WAS）and food waste（FW）are produced every year in China.These two substances gradually become the main urban biomass organic waste,and improper treatment will cause serious harm to the environment.Due to the low content of organic matter in WAS in China,the resource recovery rate of direct anaerobic digestion is low.FW is rich in organic matter and nutrition,but anaerobic digestion alone is easy to produce inhibition of acidification.In view of their different properties and the performance of single digestion,co-digestion is an important means to improve their methane yield and system stability.Sludge hydrolysis can also buffer the excess organic acids formed by FW acidification,and then maintain the stability of the system;the addition of FW can also adjust the C/N ratio to a reasonable range.Fe₃O₄has attracted more and more attention because of its excellent electrical conductivity and its ability to promote anaerobic digestion.Bentonite is a natural mineral material with good adsorption and large specific surface area,which can alleviate the accumulation of organic acids.Therefore,this experiment attempts to add Fe₃O₄,bentonite,and Fe₃O₄modified bentonite into the co-digestion system of WAS and FW,to clarify the advantages of composite iron oxide compared with single carrier material and single iron oxide in reducing inhibitor concentration,buffering p H and promoting methane generation in anaerobic digestion,and to provide the basis for the subsequent mechanism analysis.WAS and FW were digested in different VS ratios of 1:0,1:0.5 and 1:1,and different doses of Fe₃O₄ particles were added as exogenous additives.In order to understand the mechanism of Fe₃O₄ particles promoting ad performance under pressure.The results showed that the addition of Fe₃O₄ had different performance under different digestion and acidification stress caused by different was/FW ratios.For group A with VS ratio of 1/0,low concentration of Fe₃O₄（20 mg/g VS based on iron）could promote methane production,which was 27%higher than that of the control group,while high concentration of Fe₃O₄ could inhibit microbial activity.The promotion of methane formation by Fe₃O₄ is due to the promotion of sludge hydrolysis.The concentration of VFAs in the reactor was low,and Methanosaeta was the main methanogen.For group B with VS ratio of 1/0.5,the direct interspecific electron transfer（DIET）between bacteria and methanogens was triggered by 60 mg/g VS of Fe₃O₄,which was 18.07%higher than that of the control group.The concentration of VFAs in the reactor is lower than the inhibition threshold,and there may be mixed pathways（hydrogen trophic and acetic acid cleavage）for methanogenesis.Methanosarcina and Methanosaeta are the main methanogens.For group C with VS ratio of 1/1,the optimal dosage was 200 mg/g VS（based on iron）,which was slightly higher than that of the control group.The reactor was under acid stress,and Methanosarcina was the main one with stronger acid tolerance.Bacteria secrete more non-conductive polysaccharides in EPS to resist the adverse environment,thus preventing them from contacting with Fe₃O₄ particles.Therefore,in this case,Fe₃O₄ is difficult to trigger DIET and promote digestive performance.The mixture of WAS and FW with VS ratio of 1:2 was used as co-matrix.Glass powder,Fe₃O₄,bentonite and three kinds of Fe₃O₄ with different Fe/bentonite were added to modify bentonite.In order to understand the mechanism of Fe₃O₄ modified bentonite in promoting ad performance under different digestion conditions,three reactors with no conductivity and no adsorption（S1）,only conductivity（S2）,only adsorption（S3）,and both conductivity and adsorption（A,B,C）were simulated.The results showed that the cumulative methane production of B2 and C2 in the reactor with Fe₃O₄ modified bentonite was higher than that in the control reactor S1 by 142.56%and152.03%respectively.The cumulative methane yields of S2（bentonite）and S3（Fe₃O₄）were 2343.28 m L and 2053.07 m L respectively,which were 135.88%and 106.67%higher than that of S1.By comparing the cumulative methane production of reactor B2 with that of reactors S2 and S3,it is found that the combination of bentonite and Fe₃O₄ can further improve methane production.For p H,the p H of the reactor with more bentonite and Fe₃O₄modified bentonite can be maintained above 5,which avoids the inhibition of acid producing bacteria and is conducive to the stability of anaerobic digestion system.The microbial community diversity of reactors S2 and S3 was more abundant than that of reactors S1 with glass slag,and the microbial community diversity increased with the increase of the dosage of Fe₃O₄ modified bentonite.However,when the dosage of Fe₃O₄is too much,the microbial diversity will decrease.Fe₃O₄,bentonite and Fe₃O₄ modified bentonite with different Fe/bentonite can effectively increase the abundance of Methanosarcina in the reactor,while the abundance of Methanobacterium in the reactor S1 with glass powder is higher.The addition of Fe₃O₄ modified bentonite also promoted the DIET between co cultured bacteria and methanogens.Therefore,the conversion or consumption of fatty acids,especially butyrate and propionate,to acetate is accelerated.