Effect Of Nano-zerovalent Iron (NZVI) On Anaerobic Digestion Of Sludge

Anaerobic digestion is a key step in achieving"carbon neutrality"in wastewater treatment plants.Sludge anaerobic digestion can not only solve the ecological and environmental problems caused by a large amount of sludge,but also use the organic matter in the sludge to generate methane and other energy substances to ease the current tension between energy supply and demand.However,the traditional sludge anaerobic digestion process is limited by the slow hydrolysis rate of organic matter,insufficient hydrolysis,and low methane production efficiency,making energy recovery and utilization efficiency low.Therefore,how to solve the problem of sludge anaerobic digestion,maintain the stability of anaerobic digestion,increase the rate of anaerobic digestion,and develop a new and efficient sludge anaerobic digestion process are the top priorities for the future development of sludge anaerobic digestion treatment technology.Based on the above considerations,this study carried out a series of studies on the effect and mechanism of NZVI on sludge mesophilic anaerobic digestion by introducing nano-zero-valent iron(NZVI)in conventional anaerobic digestor.The main findings are as follows:(1)Through the simulation experiments,the enhancement of different concentrations of NZVI and ZVI on anaerobic digestion of sludge was investigated.The addition of different concentrations of NZVI(10,20,30 mM)all promoted the generation of methane.The cumulative methane production of the 30 mM NZVI group was the highest,which was 37.5%higher than that of the blank group.NZVI also promoted the degradation and solubilization of sludge,and the degradation rate of sludge COD reached 73%,which was significantly higher than that of the blank group.The dynamic changes of hydrogen content and VFAs indicated that the faster hydrogen evolution rate of NZVI compared to ZVI makes the hydrogen partial pressure in the later system lower,which is beneficial to the conversion of propionic acid.Microscopic analysis found that NZVI adsorbed on anaerobic microbial extracellular polymeric substance(EPS),which destroyed the decayed microorganisms with less EPS and released intracellular substance,while the surrounding EPS protects the active microbial cell membrane from NZVI damage.(2)In response to the problems of NZVI's easy flocculation and short life,activated carbon supported NZVI material(AC/NZVI)was prepared to explore its influence and mechanism during anaerobic digestion.In the start-up phase,the cumulative methane production of the AC,NZVI and AC/NZVI groups increased by 14.3%,26.3%and 34.1%respectively compared with the blank group.The addition of AC/NZVI also promoted the hydrolysis and acidification of sludge,and exerted the synergistic effect of its different components on the anaerobic digestion of sludge.After the additive is removed,the direct interspecies electron transport(DIET)path formed during the start-up phase was disrupted,affecting the process of anaerobic digestion.The cumulative methane production of the AC group and AC/NZVI group was 21.7%and 13.5%lower than that of the blank group,respectively.The modified Gompertz model fitting found that the addition of AC,NZVI and AC/NZVI increased the methanogenic potential(P_m)and the maximum daily methane production(R_m).In addition,the change of microbial community structure showed that AC/NZVI significantly increased the abundance of hydrogenotrophic methanogens.(3)Through the two-phase anaerobic digestion test,the different mechanisms of NZVI on the hydrolysis acidification process and the methanogenesis process were explored.In the hydrolysis and acidification stage,the SCOD increased with the increase of NZVI concentration(1?30 mM).When the concentration of NZVI increased to 50mM,SCOD decreased instead,which means that excessive concentration of NZVI may have an inhibitory effect on the metabolism of microorganisms.In the methane production stage,the optimal dosage of NZVI was 10 mM,which was an increase of35.62%compared to the cumulative methane production of the blank group,while the cumulative biogas production of the 30 and 50 m M NZVI groups was reduced by 9.35%and 16.21%.In the hydrolytic acidification and methanogenesis stage,the NZVI dosage showed a positive correlation with pH,that is,the higher the NZVI dosage,the higher the pH.Similarly,the more the amount of NZVI added in the system,the faster the rate of oxidant consumption,so that the ORP value of the anaerobic digestion system can be reduced to a lower level.