The widespread use of silver nanoparticles (AgNPs) may lead to the release of AgNPs into urban wastewater treatment plants (WWTPs), and endanger the operation of WWTPs due to its potential toxicity. In order to assess the risks of the long-term AgNP exposure, we firstly constructed a bench scale membrane bioreactor (MBR), which mainly consisted of anaerobic zone, anoxic zone, oxic zone, and a membrane module in the oxic zone. PVA-coated AgNPs of 0.1, 1, and 5 mg/L was then successively spiked into the anaerobic zone. The reactor was operated for 284 d in all. The results showed that 0.1 and 1 mg/L AgNPs did not affect removal of NH4+�N and total nitrogen (TN), and also did not increase the concentration of NO2��N and NO3��N in the effluent. However, 1 and 5 mg/L AgNPs decreased the removal efficiency of PO43��P from 55.5 � 13.0% to 37.9 � 19.2% and 39.2 � 9.7%, respectively. AgNPs of 5 mg/L could distinctly reduce the removal of organic compounds. The particle size analysis indicated that the addition of AgNPs obviously decreased the particle size of activated sludge. Principle component analysis (PCA) results from terminal restriction fragment length polymorphism (T-RFLP) profiles revealed bacteria community structure in the activated sludge could be evidently affected by 0.1, 1 and 5 mg/L AgNPs and definitely self-recovered with the persistent exposure, but could not regain the initial state totally. Interestingly, the scanning electron microscopy (SEM) observation on MBR membranes clearly displayed that 0.1 and 1 mg/L AgNPs caused the disintegration and detachment of fouling layer on the membranes surfaces, which meant the AgNPs could reduce the membrane fouling. On the other hand, metallic silver was not found in the interior of membrane material via examining the cross-section of membrane by SEM and energy dispersive x-ray (EDX) analysis. The concentration of dissolved silver in effluent was sharply increased in the first 20 d of the exposure of 0.1 mg/L AgNPs, and then rapidly decreased to the AgNP-unexposed concentration. The addition of 5 mg/L AgNPs increase the dissolved silver in effluent from 3.4 � 1.3 �g/L to 7.8 � 0.7 �g/L averagely. Moreover, metallic silver was distinctly accumulated to 28.2 � 1.6 mg/g-MLSS in the sludge mixed liquor along with the successively addition of 1 and 5 mg/L AgNPs. Our work suggests the long-term exposure of AgNPs can negatively affect the biological wastewater treatment system, but profitably can reduce the membrane fouling.

Tolerance of biological wastewater treatment system to silver nanoparticles

247th National Meeting (2014)

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