Assessment of the fate of silver nanoparticles in the A2O-MBR system

- AgNPs would aggregate and form Ag-sulfur complexes in the activated sludge.
- A small amount of dissolved silver released from AgNPs was detected in the A2O-MBR.
- More than 99.5% of AgNPs could be removed by A2O-MBR.
- Most of the silver contents were accumulated in the A2O-MBR and wasted excess sludge.
- The adsorbed AgNPs in activated sludge still could release dissolved silver.

In this study, we employed a bench scale A2O-MBR (anaerobic-anoxic-oxic membrane bioreactor) system to systematically investigate the behavior and distribution of silver nanoparticles (AgNPs) in the activated sludge. The results showed that AgNPs would aggregate and form Ag-sulfur complexes in the activated sludge, and the dissolved silver only reached 13.6 μg/L when AgNPs of 5 mg/L was spiked into the A2O-MBR. The long-term mass balance analysis showed that most of the silver contents were accumulated in the bioreactor and wasted excess sludge. Only a small fraction (less than 0.5%) of silver could get across the hollow fiber membranes with 0.1 μm nominal pore size in the effluent. In addition, the comparison between total AgNP concentration in aerobic sludge supernatant and effluent suggested that the membrane modules played a role in controlling the discharge of AgNPs into the effluent, especially under a higher influent concentration of AgNPs. Our results also showed that the adsorbed AgNPs or silver complexes in activated sludge still could release dissolved silver at the ambient pH. Thus, since activated sludge could be a sink for AgNPs, the risks of AgNPs in wasted excess sludge during utilization and disposal should be further studied.