Abstract

Long-term operation of a forward osmosis - reverse osmosis (FO-RO) closed loop pilot plant is presented. The system was fed with a single batch of clean tap water into which organic micropollutants were spiked; RO permeate was recirculated to the FO feed. The system was operated in two different runs, with NaCl and MgCl2 as draw solutes. Over the course of the pilot plant trials, the organic micropollutants migrated into the draw solution closed-loop, reaching an equilibrium with the FO feed. This implies that there was strong accumulation of the organic micropollutants in the draw solution, up to a factor of 50 relative to the FO feed. FO rejection was variable between 55 and 91% for the different micropollutants, while RO rejection was high in all cases: in excess of 99% for all but one organic micropollutant. The extent of micropollutant accumulation in the draw solution was related to high RO selectivity and internal concentration polarization within the FO membrane's support layer. In turn, FO permeability and the extent of accumulation were found to correlate negatively: well-rejected compounds accumulated slower in the draw solution loop, but to a higher extent. The FO membrane's structural parameter, as obtained from clean water flux tests using mineral salt draw solutes, was a poor predictor of organic micropollutant internal concentration polarization.