Effects Of Multiwalled Carbon Nanotubes On Wastewater Treatment And Microbial Community Structure In Anaerobic Sludge

Nanomaterials are not only used in military,aerospace and other high-tech fields,but also are applied to people's daily life gradually by nano-products.They are bringing us convenient while also trigger a series of worries.With the commercialized production and application of nanomaterials,the nano-waste can be abandoned to the environment,which will increase load and risk in the environment.Currently,researches have showed that many nanomaterials can pose a threat to human health,when these nanoparticles reached into water environment.It will enter into the sewage treatment system through various channels and may make a risk to the sewage treatment system as new type of pollutants.This has aroused great attention around the scholars at home and abroad.Facing this problem,we chose multiwalled carbon nanotubes(MWCNTs)which were widely applied,and focused on the impacts of MWCNTs to the upflow anaerobic sludge blanket(UASB)on the wastewater treatment and microbial community structure in short-and long-term.The response of anaerobic sludge exposed to MWCNTs with different concentrations(1,20 and 100 mg·L-1)in short-term anaerobic digestion process(7 d)was investigated.Compared with the control,the changes of COD,pH and VFA in the effluent of the reactors with MWCNTs at 1,20 and 100 mg·L-1 are consistent;MWCNTs at 100 mg-L-1 induced much faster substrate utilization.There was no significant difference between reactors with MWCNTs and the control in gas production and methane content(p>0.05).Using the modified Gompertz equation to fit the curve of methane yield and calculating maximum specific methane production rate(Umax?CH4)and maximum ratio of COD removal rate(Umax,COD),we found there was no adverse effect on characteristics about gas production of methanogenes.Scanning electron microscopy(SEM)showed that a large number of anaerobic microorganisms were adsorbed on the surface of MWCNTs,and the effect on sludge containing 100 mg·L-1 of MWCNTs was obvious.It was further observed that a large number of extracellular polymeric substances(EPS)were produced on the surface of anaerobic microorganisms.The long-term experiment(180 d)adding MWCNTs(20 mg·L-1)to the wastewater was further performed.The results showed that the removal of COD from UASB after long-term effect of MWCNTs was consistent with the short-term results,while the removal of NH4+-N was increased firstly and decreased lastly,and the removal rate of TP increased 29.34%.In addition,the effluent VFA concentration decreased significantly,and pH maintained between 6.5-7.6.In the aspect of biogas producted by UASB,biogas production and methane content did not change significantly after long-term effect of MWCNTs,and the content of CO2 in biogas was reduced slightly and no H2 was detected.Regarding to sludge characteristics,MWCNTs improved the conductivity of anaerobic sludge,meanwhile the total amount of EPS in the sludge increased after long-term effect.Miseq sequencing results showed that the microbial diversity did not decrease after long-term effect of MWCNTs,but the microbial community structure changed significantly.Saccharibacteria replaced Proteobacteria as the highest abundance of phylum in bacterial community.In archaea,the relative abundance of Euryarchaeota accounted for more than 99.7%.Methanobacterium and Methanosaeta were the main methanogen genera.The long-term effect of MWCNTs on archaea community structure was significant.Its conductivity promoted the differentiation of methanogens structure,and relative abundance of related archaea increased significantly.