Effect of Membrane Aging and Operating Conditions on the Rejection of Trace Organics for Potable Reuse

The aim of this study is to provide a better understanding of the effect that membrane degradation and operating conditions have on the removal of selected organic compounds via reverse osmosis (RO). Originally, the study included laboratory experiments in which ESPA2-LD membranes from Hydranautics were to be chemically damaged and the passage of the suite of trace organics through the membranes was to be evaluated. Unfortunately, the degradation experiments were cut short due to lab closures associated with the COVID-19 pandemic. The study then changed direction to provide insight into the role operating conditions have on the removal of trace organics from water via RO treatment for potable reuse application. Equations that describe mass transfer of water and solutes through RO membranes were used to investigate the relationship between solute rejection and operating conditions, specifically net-driving pressure (NDP) and recovery. The effect of NDP on solute rejection was compared to experimental data collected from a lab-scale RO treatment system. Increased NDP increased rejection, while increased recovery decreases rejection although rejection is not uniform for all organics and depends on the mass transfer coefficient. A model was developed that allowed the modeling of multi-stage, multi-element full-scale RO systems. The process model demonstrated that operating conditions have a more significant effect on the rejection of trace organics that the rejection of salts. These findings can aid in the design of potable reuse plants to achieve better rejection of trace organics.