Long-term impacts of silver nanoparticles in an anaerobic–anoxic–oxic membrane bioreactor system

• The removal efficiency of phosphorus in A2O-MBR was decreased by 1 mg/L AgNPs.
• AgNPs could reduce the particle size of activated sludge, especially for larger flocs.
• Bacteria community structure could be affected by the exposure of AgNPs.
• The exposure of AgNPs could affect the membrane fouling in A2O-MBR.

The widespread use of silver nanoparticles (AgNPs) may lead to the release of AgNPs into urban wastewater treatment plants (WWTPs) and influence the operation of WWTPs due to its potential toxicity. We constructed a bench scale anaerobic–anoxic–oxic membrane bioreactor (A2O-MBR) to assess the impact of the long-term AgNPs exposure. Stepwise increases of polyvinyl alcohol-coated AgNPs (0.1, 1, and 5 mg/L) were spiked into the anaerobic zone, and the reactor was operated for 285 d. The results showed that AgNPs of 1 and 5 mg/L decreased the removal efficiency of PO43−–P from 56 ± 13% to 38 ± 19% and 39 ± 10%, respectively. Principal component analysis of terminal restriction fragment length polymorphism profiles revealed that bacteria community structure in the activated sludge was affected with the addition of AgNPs and was stable with the persistent exposure. Metallic silver was found to accumulate in the sludge mixed liquor with the successive addition of 1 and 5 mg/L AgNPs. Our work has demonstrated that the long-term exposure of AgNPs could cause minor effects on the A2O-MBR system, and did not cause reactor failure even under the highest AgNPs dose of 5 mg/L.

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