Differential Behavior of Salt and Organic Matter Passage in 2-Pass RO Systems for Ultrapure Water Production

This study investigates how membrane transport characteristics affect permeate quality in a 2-pass reverse osmosis (RO) system for ultrapure water (UPW) production. Unlike conventional RO, UPW-RO operates in an ultra-low concentration range. Seven commercial 4-inch RO membrane modules spanning a wide range of water (A) and salt (B) permeability coefficients were evaluated under various second-pass RO feed concentrations (specifically, total dissolved solids (TDS) and total organic carbon (TOC)). Second-pass RO permeate TDS remained almost constant regardless of membrane specifications, whereas the permeate TOC was strongly membrane-dependent. RO permeates from high-permeability membranes showed significantly higher TOC than those from high-selectivity membranes. The experiments also revealed that a high-permeability membrane configuration for both RO passes resulted in excessive TOC leakage, while a high-selectivity membrane configuration mitigated TOC passage at the cost of a high operating pressure requirement. A combination of a high-permeability membrane (the first pass) and a high-selectivity membrane (the second pass) could achieve an acceptable TOC passage with a moderate operating pressure requirement in UPW-RO systems.