Elucidating Foulant Diversity during Full-Scale Potable Reuse: Forensic Analysis of Lead and Lag Elements of a Three-Stage Reverse Osmosis System

We examined how colloidal, biological, organic, and inorganic fouling varied in 4-year-old reverse osmosis (RO) membranes taken from lead and lag ends of pressure vessels of each of the three stages from the world's largest potable reuse facility. Colorimetry, microscopy, spectroscopy, and microbial analysis revealed varying combined fouling extents in all cases. Bioorganics dominated all but the last element. In the first and second stages, non-siliceous multivalent inorganics accumulated more on lead elements, whereas silicon-based foulants prevailed in lag elements. The third-stage lag element suffered the heaviest inorganic and the lowest bioorganic fouling. Cleaning small swatches cut from each element with a mixture of sodium tripolyphosphate and sodium dodecylbenzenesulfonic acid fully recovered the permeability of only the lag element of the first stage, which had endured the lowest inorganic fouling. Partial permeability recovery of all of the other elements suggested that cleaning was only effective against bioorganics, and harsher cleaners and/or interstage treatment were needed to combat silicon fouling. Cleaning agents did not appear to penetrate and remove entrapped foulants, resulting in faster performance deterioration when the membranes resumed service post-cleaning. Results emphasize the importance of systematically autopsying elements from each stage and evaluating their post-cleaning characteristics to optimize RO pretreatment and cleaning protocols.