Enhanced Methane Production From Anaerobic Digestion System Of Sewage Sludge With The Additve Of Exogenous Iron

Anaerobic digestion?AD?is an important way for sludge stabilization,but the process is vulnerable to the adverse factors such as acid inhibition and sulfide poison,which result in the low treatment efficiency and biogas quality.The introduction of iron material into AD system can effectively reduce the adverse influence of these factors,while the effects are different due to chemical forms of iron.In this research,we selected zero-valent iron?ZVI?with different particle sizes,different ferric salts and chelate iron,and then established ZVI-AD system,Fe?III?-TAD system and chelate Fe-TAD system.The main physicochemical parameters,organic matters transformation and the changes of microbial community structure were discussed to set forth the mechanism of iron material on sludge AD process.The main results are as follows:?1?The results of ZVI-AD system indicated that ZVI could improve the AD process effectively and the dosage of 10 g/L corresponded to the optimum methane production efficiency under the mesophilic AD condition.The cumulative methance production could reach 264.9 mL/g·VS,which was 113.9%higher than the control group.Meanwhile,the effects of ZVI with different particle sizes on hydrolysis-acidification and methanogenesis processes were examined,the results showed that ZVI had the positive effects on hydrolysis-acidification and intensified the acetic acid formation simultaneously.Among them,NZVI was the most effective.The cumulative methane production in the assays with ZVI-100,ZVI-1000 and NZVI were171.6 mL/g·VS,142.7 mL/g·VS and 108.2 mL/g·VS,which were 131.6%,92.6%and46.1%higher than the control group.Further analysis showed that micrometer-scale ZVI not only could create a suitable substrate environment for microbial growth metabolism and improve the system stability,but also could enhance the methane production by enriching Methanosaeta.NZVI could contribute to the buffer capactity by enriching Methanosarcina,and intensify the methanation of H₂/CO₂ by enriching hydrogenotrophic methanogens.?2?ZVI could help to the rapid establishment of thermophilic anaerobic digestion?TAD?during one-step startup from mesophilic anaerobic digestion?MAD?.ZVI could assist the smooth transition of hydrolytic bacteria from MAD to TAD,and meanwhile create the suitable substrate environment for methanogens by optimizing the composition of organic acids.Further analysis of methanation from the perspective of dynamics indicated that the accumulation of volatile organic acids?VFAs?would result in the lag phase during the tradiitional one-step of TAD process.ZVI could effectively promote the degradation and transformation of organic acids and shorten the lag phase.Meanwhile,ZVI could optimize the microbial community structure and enrich the Methanosarcina to realize the rapid methanation of organic matters with the stable metabolism balance system.?3?The results of Fe?III?-TAD system indicated that exogenous supplementation of FeCl₃ could enhance biogas production and sludge stabilization with the cumulative biogas production of 357.0 mL/g·VS,which was 79.6%higher than the control group.Exogenous supplementation of Fe₂?SO₄?₃ could not enhance the biogas production obiviously,while the supplementation of Fe?NO₃?₃ even resulted in the failure of AD process.Exogenous FeCl₃ could promote the hydrolysis-acidification and convert the released organic matters efficiently,which created a suitable substrate environment for microbial metabolism.Conversely,Fe?NO₃?₃ would induce ORP to increase rapidly and result in the substrate environment deterioration with the accumulation of organic acids.It was found that a clear dynamic response between cumulative biogas production and dosing time of FeCl₃ and the assay with the dosing time of 72^nd h achieved the best performance with the cumulative biogas production of 293.1 mL/g·VS.The appropriate dosing time of FeCl₃ could disinhibit the excessive VFAs,while dosing too early was adverse to methanogens growth and reproduction and dosing too late could not control the acid inhibition.Under the experimental conditions,9.92 mgFe/gDS dosages of FeCl₃ had the best performance.The cumulative methane production was 236.8mL/g·VS,which was 2.2-fold higher than the control group.When the dosage was less than 9.92 mgFe/gDS,the cumulative methane production increased with the increased dosage;An irreversible damage to TAD system would be observed when the dosage was more than 19.84 mgFe/gDS.The fitted results of the cumulative methane production increment and dosage showed that the theoretical dosage of FeCl₃ was 8.95mgFe/gDS.?4?The dosage of 50 mg/L citric acid-Fe corresponded to the best performance with the cumulative methane production of 204.3 mL/g·VS,which was 26.3%higher than the control group.Appropriate dosage of citric acid-Fe could improve specific acetotrophic methanogenic activity?SAMA?with reduced specific acetotrophic methanogenic activity?SHMA?,and the enhanced methane production resulted from the synthetic effects.The bioavailability analysis of Fe indicated that the system with appropriate dosage of citric acid-Fe had the higher biological potency,which contributed to the enhanced methane production.Additionally,Clostridia and Methanosarcina were enriched in the assay with citric acid-Fe.?5?EDTA-Fe could enhance the methane production and the best performace was obtained in the assay with 50 mg/L,which was 33.5%higher than the control group?198.8 mL/g·VS?.The higher biological potency of Fe per unit was obtained in the system with appropriate dosage of EDTA-Fe and the enhanced methane production resulted from the synthetic effects of biochemical process.In the initial stage of AD,the excessive EDTA had a toxic effect,but EDTA could help to improve the bioavailability of endogenous Fe and enrich Methanothermobacter.The exogenous supplementation of EDTA exhibited the positive effect on the whole methanogenesis process.