MEC And Iron Materials Enhancing Anaerobic Digestion Of Waste Activated Sludge And Its Mechanisms

Waste activated sludge(WAS),as the byproducts of municipal wastewater treatment process,needs to be properly disposed due to its huge production and environemntal risks,with the aim for realizing its stabilization,reduction and reclamation.Anaerobic digestion(AD)has been considered as an efficient and sustainable strategy for the disposal of WAS due to its abilities to transform organic matter into methane.However,a major factor limiting the efficiencies of AD of WAS that results in the declined performances of sludge reduction and methanogenesis is the relatively slow hydrolysis including the disintegration of sludge flocs and cell walls.Recently,it was reported that microbial electrolysis cells(MECs)could promote AD of WAS especially for accelerating the sludge hydrolysis and acidification.Considering the role of zero-valent iron(ZVI)and Fe3O4 in the promotion of AD of WAS,the current study was to investigate whether the hydrolysis/acidification of WAS could be enhanced in the MECs combined with iron materials,which finally improved the efficiency of AD of WAS.The main results are as follows:(1)In order to investigate the effects of ZVI and C electrode on AD of WAS,Fe-C MEC and C-C MEC were established(Ag/AgCl served as the reference electrode).The results showed that methane production and COD removal rate in C-C MEC were 15.1%and 10.9%higher than that in the control reactor(without electric field),which however in Fe-C MEC were 7.3%and 0.9%lower than that in the control reactor.It shoud be pionted out that,no CO2 was detected in Fe-C MEC,but the formate(672.3 mg/L)was largely detected in the sludge-digestion liquid,accompanied by a significant release of H2(45.8 mL).The electrochemical experiments(CO2 could be reduced in Fe-C MEC under the same conditions)confirmed that the production of formate was closely associated with the cathode reduction of CO2.The production of H2 should be ascribed to cathode reduction of H+,which was in accordance with the increment of liquid pH(9.3)after AD,since the weakly alkaline pHs could inhibit the methane production.However,the performances of Fe-C MEC without electric field were nearly the same as that of C-C MEC,suggesting that the mechanisms involved in the enhancement of AD of WAS with Fe-C and C-C electrode were different.(2)In order to further investigate the effects of C-C MEC on AD of WAS,C-C MEC and C-C reactor(open circuit)were established.The results showed that both C-C MEC and C-C reactor could improve the efficiencies of AD of WAS.Compared with the control reactor(without electrode),the methane production in C-C MEC and C-C reactor increased by 30,2%and 12.9%,respectively,and the COD removal efficiency in C-C MEC and C-C reactor increased by 36.7%and 27.6%,respectively.The calculation of the current indicated that methane production from the cathodic reduction of CO2 did not exceed 86.8 mL,only accounting for 27.6%of the increased amount of methane.The high-throughput 16S rRNA gene pyrosequencing revealed that Methanosaeta species were enriched in C-C MEC and C-C reactor.It was well known that Methanosaeta species were capable of accepting electrons for the reduction of CO2 into methane via direct interspecies electron transfer(DIET).Geobacter species capable of participating in DIET were also enriched in C-C MEC and C-C reactor.Besides,both the content of c-type cytochrome Omcs and sludge conductance were increased in C-C MEC and C-C reactor.Therefore,it could be reasonably inferred that DIET was established and played an important role in C-C MEC and C-C reactor.As a result,the methane produced from DIET in C-C MEC accounted for about 70%of the increased amount of methane.(3)In order to investigate the effects of iron on AD of WAS,ZVI and Fe3O4 were added into the digesters.The results showed that both ZVI and Fe3O4 could increase methane production and organic removal,and the activities of enzymes relative to hydrolysis were also increased.However,it was found that the mechanisms involved in the enhancement of AD of WAS with ZVI and Fe3O4 were distinct.Specially,ZVI had almost no direct effect on the solubilization,hydrolysis,and acidification,which however significantly promoted the methanogenesis.The rapid utilization of the fermentative intermediates(acetate,H2 and CO2)of WAS hydrolysis in the methanogenic stage drove the hydrolysis and acidification in thermodynamics,enhanced the activities of enzymes and finally promoted the whole AD of WAS.Interestingly,although Fe3O4 as the electron accepter could compete the substrates and inhibit methanogenesis,it could significantly enhance the solubilazition,hydrolysis and acidification.Due to the promotion of hydrolysis-acidification process that produced acetate or CO2/H2 for methanogenesis,the whole AD of WAS was promoted.(4)In this study,Fe3O4 was used to promote the anaerobic co-digestion(AcoD)of WAS and wheat straw(WS).The results showed that,when Fe3O4 powder was added into the co-digester,the acidic pHs were effectively relieved and the efficiencies of AcoD were significantly improved.Besides,it was demonstrated that Fe3O4 was capable of optimizing the composition of VFAs by reducing the proportion of propionate and increasing the proportion of acetate.Microbial community analysis indicated that the abundance of Ethanoligenens species known as the ethanol-type fermentation genus in the Fe3O4-added reactor was 2.9 times higher than that in the reactor without Fe3O4,suggesting that Fe3O4 could improve the efficiencies of AcoD by changing the fermentation type.