Recirculation of hollow fiber nanofiltration concentrate to biological treatment to increase removal of micropollutants from wastewater

Reverse osmosis and nanofiltration membranes are known to remove organic micropollutants (OMPs) from water. They could be used to increase OMP removal from wastewater and improve surface water quality. However, the membrane concentrate contains elevated concentrations of OMPs and requires further treatment. In this study, recirculation of the concentrate to the preceding biological treatment is investigated to increase concentration and OMP residence time in the biological treatment, resulting in an increase in removal. A continuously operated pilot with an influent flow of 1 m3 h−1 is used, which includes a biological treatment and a commercially available hollow fiber nanofiltration (HFNF) membrane. The system operated in a stable manner for 45 days with concentrate recirculation. Compared with the performance of the base case (no recirculation), it is found that the main functionality of the biological treatment to remove organic matter and nutrients is comparable, while the sludge properties are similar. During recirculation, a significantly increased sulfate concentration was found of up to 1200 mg L−1 in the concentrate (base case of around 300 mg L−1). The removal of 11 OMPs is monitored closely. The removal of amisulpride and diclofenac is significantly improved by including the HFNF with recirculation, from 30 % to 70–80 %, due to high retention by the NF. Some other OMPs hardly show an increase in removal, due to a combination of low bioremoval and low retention. A broader range of larger OMPs shows that total removals beyond 80 % can be obtained, even with low bioremoval.