Study On Inhibition Of Methanogenesis By Hydrochar Prepared From Anaerobic Digestion Sludge Residue

With the increase in the scale of wastewater treatment in China,the production of waste activited sludge（WAS）,a by-product of biological treatment of wastewater,is gradually increasing.Organic matter is abundant in WAS,which has a large potential for resource utilization.Anaerobic digestion technology converts organics in WAS into volatile fatty acids（VFAs）,methane and other energy sources through anaerobic microorganisms.Anaerobic digestion is a widely used technology for resource utilization of WAS,and the intermediate VFAs have higher applicability than methane.At the same time,anaerobic digestion of sludge also generates problems of continued treatment and disposal of sludge digestion residues.In this paper,hydrochar and ammonia were used to inhibit methane production in anaerobic digestion of sludge to achieve the purpose of VFAs enrichment in the system.Hydrochar was made from the residue produced by the anaerobic digestion of sludge rich in humic acid,and ammonia would be produced during the preparation of hydrochar and anaerobic digestion of sludge.Firstly,hydrochar was made from the residue of sludge anaerobic digestion,and the suitable preparation temperature of hydrochar and the inhibition effect of hydrochar in methanogenesis of sludge anaerobic digestion system were studied.Secondly,the inhibition of ammonia on methanogenesis,an endogenous inhibitor of sludge anaerobic digestion,was explored.Finally,the synergistic inhibition of ammonia and hydrochar on methane production in sludge anaerobic digestion system was discussed.The main research results are as follows:（1）Study on the appropriate preparation temperature of hydrochar prepared from digested sludge residues for inhibiting methane production from sludge.The suitable preparation temperature of hydrochar was obtained by comparing the inhibition effect of hydrochar made under temperature conditions of 145℃,170℃,195℃and 220℃on the methanogenic process of sludge.The results showed that the hydrochar H195 prepared at 195℃temperature condition had the highest humic acid content of 46.58%,and the XPS and FTIR analysis showed that the surface of H195 was rich in oxygen-containing functional groups.The results of the reactor operation showed that the maximum VFAs concentration in the R195reactor with the addition of H195 increased by 24.93%compared to the blank group,the cumulative methane production decreased by 9.64%,and the activity of coenzyme F420decreased by 53.19%.The results of microbial community structure analysis showed that H195promoted the enrichment of Firmicutes flora and increased the relative abundance of acid-producing bacteria such as Enterococcus,Trichococcus,Macellibacteroides,Proteiniclasticum and Lactococcus.Methanosarcina,a methanogen-producing bacteria in the reactor,decreased by 21.15%.H195 is rich in oxygen-containing functional groups that bind to key enzymes such as acetyl coenzyme A during anaerobic digestion,inhibiting methanogenic activity while promoting the enrichment of hydrolytic and acid-producing microorganisms.（2）To investigate the effect of ammonia concentration on the methanogenesis of anaerobic digestion of sludge.Since ammonia nitrogen is an endogenous inhibitor of methanogenesis in anaerobic digestion of sludge,the effect of ammonia nitrogen on the inhibition effect needs to be considered in the application study of hydrochar for the inhibition of methanogenesis in anaerobic digestion.Ammonia at concentrations of 1g/L,2g/L and 3g/L was injected into the sludge anaerobic digestion reactor.The experimental results showed that1g/L concentration of ammonia inhibited the hydrolysis and acid production processes in the reactor,and theα-glucosidase activity in the system was reduced by 20.17%compared with the blank group,and the maximum VFAs concentration was 6.43%lower than that of the blank group.2g/L concentration of ammonia inhibited the methane production in the sludge anaerobic digestion system more significantly,and the cumulative methane production was reduced by22.84%compared with the blank group,and the coenzyme F₄₂₀ activity decreased by 11.40%compared with the blank group,and had almost no effect on the maximum yield of VFAs;3g/L concentration of ammonia increased the maximum concentration of VFAs in the system,but the methanogenic inhibition effect was weakened in the later stage of the reactor operation.The results of microbial community structure analysis showed that the relative abundance of Firmicutes phylum in the N3 group with 3g/L ammonia injection was high,while the content of Trichococcus in the N3 group was 44.24%higher than that in the blank group on the 6th day of reactor operation,which was the highest among the four reactor groups.VFAs accumulated in the reaction,resulting in a decrease in system p H from 7.55 to 6.93 and a decrease in the concentration of free ammonia in the system,with the resulting weakening of methanogenesis inhibition.Therefore,2g/L ammonia concentration has the best effect on methanogenesis inhibition and has almost no effect on acid production.The accumulation of VFAs in the system may cause a decrease in p H,which affects the inhibitory effect of free ammonia on methanogenesis.（3）To investigate the synergistic effect of hydrochar and ammonia in anaerobic sludge digestion system.The combination of ammonia and hydrochar at different concentrations was injected into the anaerobic digestion reactor.The experimental results showed that the combination of 3g/L ammonia plus 10g/L hydrochar and 3g/L ammonia plus20g/L hydrochar inhibited the methanogenic process in the sludge anaerobic digestion system more significantly and had almost no effect on the acid production process in the system.The N3+C10 group with 3g/L ammonia plus 10g/L hydrochar combination significantly inhibited methanogenesis,and the cumulative methanogenesis was 59.24%lower compared to the blank group,and the activity of coenzyme F₄₂₀ was the lowest,42.50%lower than that of the blank group.According to the results of microbial community structure analysis,the combination of3g/L ammonia and 10g/L hydrochar inhibited the activity of acetoclastic methanogens in the sludge anaerobic digestion system,and the content of Methanosarcina and Methanosaeta in the N3+C10 group decreased by 36.72%compared to the blank group on the 12th day of reactor operation.The combination of 3g/L ammonia plus 10g/L hydrochar showed the best inhibition of the methanogenic process with less hydrochar dosing.Digested sludge residue-based hydrochar,ammonia and their synergistic effects can effectively inhibit methanogenic and achieve accumulation of VFAs in residual sludge systems,which is an effective way to achieve sludge resource utilization.