A Research Of Biochemical Methane Potential And Mechanism Of Oil,Fat And Grease Deposits By Anaerobic Digestion

The formation of fat,oil,and grease deposits（FDs）in sewage networks is a global challenge to maintain the smooth operation of sewage collection systems.With the continuous increase of consumption of edible oil in China,a large amount of waste grease is produced.Although a large part of the waste grease in the sewage pipe network system stays in the grease trap,a large amount of waste grease is still discharged into the sewage pipeline FDs are formed,which leads to blockage of the sewage pipe network and even overflow of sewage.A large amount of FDs are removed from the sewage pipe network every year.Because the saponified FDs reduce the inhibition of long chain fatty acids（LCFAs）under the action of Ca²⁺,the biochemical methanogenic potential of FDs may be higher than that of fat,oil,and grease（FOG）,which provides an opportunity to increase methane production through anaerobic co-digestion with waste activated sludge（WAS）.Based on the correlation analysis of the substrate/inoculum（Substrate/Inoculum,S/I;VS）ratio with cumulative methane production and average methane production percentage,it is estimated that the ideal S/I ratio range of FDs as a single substrate is between 0.25 and 0.75,The optimal S/I ratio is 0.5.Referring to the single anaerobic digestion S/I ratio of FDs,and setting the anaerobic co-digestion WAS and FDs experiment,when the ratio of substrate to inoculum（S/I）is 0.25 to 1.2,it is found that methane production is greatly improved.The maximum cumulative methane production（685.7±24.1 m L/gVSadded,after 42 days of incubation,under the condition of S/I=0.5）is 4 times higher than the control group（WAS only）.Under the same organic load（S/I=0.5）,when the COD load of FOG is 2 times higher than the COD load of FDs,the cumulative methane production by FDs co-digestion is 10.2%higher than that of FOG co-digestion（p<0.05）.Gompertz model fitting,if longer digestion time is allowed,methane production will increase to 17.4%.When the organic load in FDs（S/I=0.25）is reduced to half of the organic load in FOG（S/I=0.5）（the COD load of FDs is almost a quarter of FOG）,the cumulative methane production between the two groups There was no significant difference（p=0.32）.Microbial community analysis showed that Geobacter was highly enriched in FDW-0.25,followed by FDW-0.5 and FOGW-0.5.The relative abundance of Methanosaeta in the analysis of archaea communities is significantly higher than that of other methanogens.The relative abundance of Geobacter with direct intermediate electron transfer（DIET）was positively correlated with Methanosaeta in the FDW-0.25,FDW-0.5,FOGW-0.5 group（Pearson′s coefficient=0.97）,indicating that DIET may promote co-digestion of FDs.Microorganisms may form biofilms on FDs and enhance the electron transfer through the conductive fimbriae of Geobacter in biofilms,thus accelerating the production of methane.Calcium soaps produced by pretreatment of FOG with different calcium sources and different calcium ion concentrations of the same calcium source have different effects on cumulative methane production,methane production rate,and surface microbial aggregation.Due to the effect of calcium-oxygen bond,the calcium soap substrate is not as fast as the control group（FOG）,but the maximum cumulative methane production of the Ca（OH）₂-0.25 group is higher than that of the control group（FOG）,The cumulative methane production of the Ca（OH）₂-0.5 and CaSO₄-0.25 groups was close to that of the control group.The CaSO₄-0.25,CaSO₄-0.5 and Ca（OH）₂-0.25,Ca（OH）₂-0.5 groups corresponded to the calcium soap surface.The degree of microbial aggregation was also strong,showing a strip-shaped network structure.The accumulation of anaerobic microorganisms on the surface of calcium soap caused by Methanosaeta is likely to accelerate the biodegradation of LCFAs and increase methane production.This research aims at maximizing the recovery of renewable energy under anaerobic digestion conditions for FDs in the sewage pipe network system.Adding calcium to pre-treat waste grease to form calcium soap improves its methanogenic potential,which is of great significance to promote the recovery of waste grease in sewage pipe network system,reduce pipeline blockage caused by FDs,and promote the application of waste grease energy in sewage pipe network system.