Enhancing pressure retarded osmosis performance with low-pressure nanofiltration pretreatment: Membrane fouling analysis and mitigation

The advancement of pressure retarded osmosis (PRO) has drawn much attention recently to its feasibility of practical operation. Membrane fouling is one of the main challenges, especially when real wastewater is used as feed solution in the PRO system. Low pressure nanofiltration (NF) pretreatment was thus adopted in current study to pretreat the wastewater reverse osmosis (RO) brine collected from a wastewater treatment plant prior to feeding to the PRO process. Three NF membranes were compared in terms of the membrane properties, NF permeate (i.e., PRO feed) quality and the PRO membrane performance. Results showed that the PRO water flux could increase to 30.5 L/m²/h (LMH) at 16 bar applied pressure by adopting the pretreated solution using an in-house made low-pressure NF hollow fiber membrane module, in contrast to the water flux of 9 LMH for untreated wastewater RO brine. A systematic analysis of water chemistry and membrane characterization (electron dispersed X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) depth profiling) revealed that calcium salts, organic compounds, and silica could be the main contributors to the membrane fouling in PRO. Low-pressure NF was able to mitigate the fouling potential from multivalent ions and organic matters, but silica scaling in PRO needs to be further addressed.