The Effects Of Zinc Oxide Nanoparticles On Wastewater Biological Niteogen And Phosphorus Removal

With the dynamic development of nanotechnology, ZnO nanoparticles are beingused within a great variety of applications. However, there are now significant concernsabout the adverse effects of nanoparticles (NPs) on human health and the environment.The increasing use of nanoparticles introduces them intentionally or unintentionally intowastewater treatment plants, which will pose a significant operational security challengeto the wastewater biotreatment system. The aim of this study was to investigate theeffects of different dosages of ZnO NPs on biological nitrogen and phosphorus removalafter short and long-term exposure, and to offer references for establishing andimproving the emission limits for metal oxide nanoparticles in wastewater.Acute (24h) and chronic (30d) exposures of nitrification-denitrification processesand phosphorus adsorption-release process were carried out with three dosages (1mg/L,5mg/L and25mg/L) of ZnO NPs to assess the adverse impacts on biological nitrogenand phosphorus removal. In long-term respects, the effects of different concentrations ofZnO NPs on the composition and contents of EPS from denitrifying activated sludgeand dephosphorizing activated sludge were investigated. Furthermore, changes inmorphology of bacterial flora and biomass, as well as the adsorbed amount of ZnO NPson two kinds of activated sludge and Zn ion concentrations in the in the supernatantwere analyzed. Finally, the possible inhibition mechanism of ZnO NPs onphosphate-accumulating organisms and nitrifying bacteria was studied based on theabove results. In short-term respects, the influences of ZnO NPs not only on thetreatment performance of both systems, but also on the transformations of intracellularpolyhydroxyalkanoates (PHA) and glycogen were explored.The current study showed that ZnO NPs could inhibit the ammonia oxidation andnitrate denitrification in various degrees, and higher concentrations of ZnO NPs havemore remarkable influence on nitrate denitrification than ammonia oxidation in theshort term exposure experiments. When activated sludge was exposed to5mg/L and25mg/L ZnO NPs, the average NH4+-N removal efficiencies significantly decreased to74.5%and32.2%, respectively, with increasing exposure time to the first several days.However, it soon recovered to almost the same level as what observed in the control test, suggesting that the microbes in biological nitrogen removal system possessed certainself-recovery function. After24h of exposure to25mg/L ZnO NPs, the phosphorusremoval process displayed only9.3%removal of SOP, which indicated that ZnO NPshad significantly acute effect on phosphorus adsorption. Also, compared with nitrifyingbacteria, it was found that the PAOs were more sensitive to the toxicity of ZnO NPsafter long-term exposure, as the removal of SOP could not recover to the extent ofcontrol test, and the effluent SOP concentrations increased gradually.Overall, the EPS of nitrification-denitrification process produced slightly higherprotein and polysaccharide content than the phosphorus adsorption-release processunder all ZnO NPs concentrations. This was further evidence that nitrogen-cyclingbacterium functional group had a higher resistance to toxicity of ZnO NPs, which wasconsistent with the experimental results that the performance of nitrification-nitrification process is superior to phosphorus adsorption-release process. The acutephosphorus removal test with high ZnO NPs concentration (25mg/L) demonstrated thatthere was more impact for PHA degradation than the PHA synthesis, and the significantincrease of glycogen in an operation cycle was observed.It was found that long-term exposure to1mg/L ZnO NPs induced different degreesof increase to the biomass in both biological nitrogen and phosphorus removal system.However when the concentration was increased to5mg/L and25mg/L, the biomasshappened to take a slightly and a significant decrease, respectively. Scanning electronmicroscope (SEM) analysis showed that most of ZnO NPs were adsorbed onto activatedsludge, and that lead to the gully increase on the surface of the microbial communityafter long-term exposure to25mg/L ZnO NPs. The microbial cells presented consider-able damage with an affected morphology and a relatively disorganized cell wall underthe observation of Transmission electron microscope (TEM).