Reverse osmosis (RO) membranes used to treat municipal wastewater for reuse become fouled with inorganic, organic and biological materials present in the feedwater, causing a decrease in RO efficiency. The feedwater matrix varies continuously throughout the RO feed channel; in a 3-stage RO system, the feedwater solute concentration increases over 6-fold. A test system was developed to assess RO membrane performance at each of four critical locations in the 3-stage RO train: the beginning of the first, second and third RO stages and the end of the third RO stage. The study objective was to correlate loss of membrane performance (normalized specific RO product flux) with accumulation of materials on the membrane surface. Measured membrane surface parameters included protein, carbohydrate, total bacteria, total viable aerobic heterotrophic bacteria, and elemental atom/carbon atom ratios (determined by SEM-EDX) of O, N, F, Na, Mg, Al, Si, P, S, Cl, Ca, K, Fe, and Cu. Linear regression analysis was used to relate accumulation of materials on the membrane and Vexar spacer surfaces at each of the stages with observed reduction in performance. At the beginning of stage 1, the loss of specific flux was not strongly related to biofouling, but was mostly associated with materials on the spacer; most prominently Si, followed by Cl, K, Na, N, Mg, Cu, F, Ca and O. Evidence of biofouling was also not observed in the SEM images, although bacteria were observed on the spacer. At the beginning of the second RO stage, protein and Si on the membrane surface were most closely linked to flux decline. SEM images showed evidence of nanoparticulate deposition at the membrane surface. The spacer at this stage appeared to have less bacterial contamination compared to the previous stage. At the transition of the second and third RO stages, accumulation of total aerobic heterotrophic bacteria on the membrane was the sole factor negatively related to specific flux decline. However, organisms were not predominately visible by SEM on the membrane surface or the spacer. At the end of the third RO stage, no strong relationships ( percent R-squared less than 90 percent) were observed between any material on the membrane or on the spacer and specific product flux. P, Fe, and Cu element ratios described 78 percent of the observed variation in the membrane specific flux in the tail end of the process but were not statistically significant at ?95 percent confidence level. SEM images of the end of third stage showed the membrane and spacer evenly covered with round nodules (20-200 nm) with the elemental make-up of P, Fe, and Cu, along with C, O, F, Na, Al, Si, S, Cl, and K. The study results suggest that factors related to membrane fouling change significantly along the RO feed channel from stage to stage.
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- Jana Safarik / Donald W. Phipps, Jr.
- Orange County Water District
- AMTA/AWWA Membrane Technology Conference, Long Beach, CA
- AMTA/AWWA Membrane Technology Conference
- Membrane, Research, Reverse Osmosis, Fouling