Impacts Of Silver Nanoparticles On Water Treatment Efficiencies Of SBR System And Microbial Communities

In rencent years,with the wide use of silver nanoparticles(AgNPs), itsecological effect on environment has caused great attention.After sewage withAgNPs entering into wastewater treatment plant(WWTP),the interaction of AgNPsand activated sludge may affect the treatment efficienc ies of WWTP.Thus,focusing on AgNPs, the inhibition effect of AgNPs on water treatment system atdifferent concnetrations and its influence on the microbial community structure ofactivated sludge were studied to predict the possible harm to WWTP in this thesis.The influence of AgNPs on bio logical treatment effic iencies wereinvestigated at short-term and long-term (50d) models by exposing the simulatedsewage treatment system under differernt AgNPs concentrations(0,0.1,1.0and10mg/L). No obvious influence was observed at the short-term exposure experiments,ind icating that the temporary shocks of AgNPs on water treatment system did notreduced the system performance. When the system was exposed to AgNPs inlong term, low concentrations (<0.1mg/L) AgNPs almost had no influence on thefunction of activated sludge system, while the effect at higher concentrations (>1mg/L) appeared gradually with the exposure time. The remova l effic iencies ofCOD, ammonia nitrogen and solub le orthophosphate (SOP) were all inhib ited athigher AgNPs levels, and the inhibition effect increased with AgNPs concentration.Moreover, nitrite concentration in effluent increased with AgNPs levels, whilenitrate decreased. When the influent concentration of AgNPs was10mg/L, theremova l rate of COD, ammonia nitrogen and SOP dropped to70%,44%and55%,respectively. Nitrite in effluent increased to2.2±0.3mg/L, while almost no nitratewere detected. Both the bio mass and bio logical activity of activated sludge werefound to be inhib ited by AgNPs through measuring the changes of extracellularpolymers (EPS), MLVSS/MLSS and specific oxygen uptake rate (SOUR). It wasalso found that most of AgNPs accumulated in activated sludge by measuring thesilver concentration in sludge and water phase.At the stabilization stage of AgNPs long term exposure, it was found thatsludge surface gradually turned to rough, microbial community became to less,bacteria proliferation was restrained, cell morphological became abnorma lities,membrane integrity was broken, and death cells were appeared with the increase ofAgNPs concentration by observing the microorganisms morphology and sludgesurface. The impacts of AgNPs on microbia l community structure at long-termexposure were characterized by using high-throughput sequencing.Theexperimental results showed that with the increase of AgNPs concentrations, the biological community bacame more diverse to resist the impact of AgNPs. Theresults of microbial community d iversity showed that the communities in systemwith AgNPs was different with the control one, while that in system with10mg/LAgNPs existed obvious difference with other three systems. Microbialcommunities showed differences at the phylum, class and genus levels. It was alsofound that bacteria with remova l abilities of nitrogen and phosphorus wereinhibited by AgNPs,which lead to the poor removal of nitrogen and phosphorus.