| Sustainable wastewater treatment is critical for low-carbon operation of wastewater treatment plants.As the main by-product of municipal wastewater treatment,waste activated sludge with amounts of organic components is a potential biomass resource.The organic components can be recovered in the form of methane by anaerobic digestion,which is beneficial to the utilization of sludge.However,traditional anaerobic digestion has the disadvantages of long sludge retention time but low methane production.Currently,zero valent iron(ZVI)has been widely applied to anaerobic digestion of waste activated sludge for improving methane production.The corrosion of ZVI accomplished with hydrogen contributes to the establishment of interspecies hydrogen transfer(IHT),which is beneficial for hydrogenotrophic methanogenesis.However,the particle agglomeration and oxidation film during anaerobic digestion will inhibit the corrosion of ZVI,which reduced the iron atom in the surface of ZVI.Therefore,it is important to find a regulation method for promoting ZVI corrosion process to improve the promotion effect of ZVI on anaerobic digestion.Firstly,for improving the methane production performance of anaerobic digestion with ZVI addition,this project proposed a strategy combined citric acid with ZVI to improve the performance of anaerobic digestion.The dissolved organic matters(DOMs),short chain fatty acid(SCFAs),and methane production under untreated sludge treatment or sludge with thermal treatment were investigated.The treatment combined citric acid with ZVI contributed to the release of DOMs.More importantly,the enhancement for DOMs release increased with the increased dosage of citric acid.Besides,the production of SCFAs and the proportion of acetic acid also increased with increased dosage of citric acid.The methane production reached 93 mL/g volatile suspended solids(VSS)with 6mM citric acid,which increased by 78.8% and 63.2% respectively compared with control and solo ZVI.The methane production was further improved after thermal treatment with low temperature.When the dosage of citric acid was 6 mM,the methane production was173 mL/g VSS,which increased by 86% compared with untreated sludge.These results showed that the combined strategy of citric acid and ZVI contributed to solubilization,acidification,and methanogenesis for improving methane production performance from anaerobic digestion of waste activated sludge.Secondly,the influence of citric acid on solubilization,hydrolysis,acidification,and methanogenesis have been investigated to explore the mechanism for improving methane production of anaerobic digestion with the strategy combined with citric acid and ZVI.It was found that citric acid promoted solubilization potential of waste activated sludge,the enhancement also increased with increased dosage of citric acid.Citric acid resulted in the decline of pH,which led to the cell lysis.However,citric acid played a few parts in the improvement of hydrolase activity.Adding citric acid alone inhibited the hydrolysis stage,and the hydrolysis efficiency of dextran decreased from 99.2 % to 51.9 %.Similarly,citric acid inhibited the processes of acidification and methanogenesis.The influence of citric acid on acidification and methanogenesis were strengthened with increased dosage of citric acid.When the concentration of citric acid reached 12 mM,the methane production decreased by 91.9 % compare with control.Thus,solo citric acid treatment could significantly inhibit methanogenesis during anaerobic digestion.The inhibition of citric acid on methanogenesis could be alleviated by ZVI,which promoted citric acid participation in the process of ZVI corrosion.Then,the acidity induced by citric acid could be neutralized by ZVI corrosion for maintaining system stability.Finally,the corrosion performance of ZVI and microbial community structure have been studied for understanding the mechanism of the strategy combined with citric acid and ZVI on anaerobic digestion.The hydrogen production of citric acid with ZVI treatment increased by 2.42 times compared to that with solo ZVI.This result suggested citric acid contributed to the corrosion of ZVI.The predominant phylum of microbial community structure changed from Chloroflexi to Actinobacteriota and Firmicustes after citric acid addition.The later were reported to participate in the process of hydrogen production.The abundance of Clostridium＿sensu＿stricto＿10 in control was 1.25%,while it increased to 6.38% in the treatment combined citric acid with ZVI,which indicated that citric acid promoted the hydrogen evolution efficiency of ZVI during anaerobic digestion.Euryarchaeota was still the main dominant phylum,and the relative abundance of Euryarchaeota reached 67.36%.Besides,Methanobacterium was the main dominant genus with the combined treatment of ZVI and citric acid,which was hydrogenotrophic methanogen.The relative abundance of Methanobacterium reached 51.03%,indicating that citric acid played an important role in the establishment of IHT.The strategy of using citric acid combined with ZVI to promote the corrosion of ZVI have been proposed,contributing to hydrogenotrophic methanogenesis enhancement,and the methane production performance from anaerobic digestion of waste activated sludge can be improved.The findings of this project can provide theoretical basis and technical support for methane production enhancement of anaerobic digestion. |
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