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Feed Temperature Effects on Organic Fouling of Reverse Osmosis Membranes: Competition of Interfacial and Transport Properties

Dr. Santiago Romero-Vargas Castrillon

We investigated the effect of feed temperature on organic fouling of reverse osmosis (RO) membranes. Experiments were conducted over the range 27 < T < 40 °C, relevant to feed temperatures in arid, near-equatorial latitudes. Fouling by alginate, a major component of extracellular polymeric substances, was investigated at the nanoscale by means of AFM-based temperature-controlled colloidal-probe force spectroscopy (CPFS). The CPFS results, complemented by interfacial property characterisation (contact angle, surface roughness and charge) conducted under temperature-controlled conditions, enabled us to rationalize the observed fouling kinetics in cross-flow fouling experiments. We observed less severe flux loss at 35 °C (J/Jo = 75%, t = 24 hr) compared to 27 °C (J/Jo = 65%), which is due to weaker adhesion forces with rising temperature. The observed variation in the magnitude of adhesion forces is consistent with the temperature dependence of hydrophobic interactions. At 40 °C, the observed flux loss (J/Jo = 68%) was similar to that at 27 °C, despite the fact that adhesion forces are relatively weak (and similar to those at 35 °C). Analysis using a series-resistance model shows that the foulant layer hydraulic resistance is equal at 35 and 40 °C, consistent with the CPFS results. More severe fouling was observed at 40 °C compared to 35 °C, however, due to the higher water permeability coefficient at 40°C, which resulted in a greater flux of foulant to the membrane. Our experiments further show that the fouling layer develops within ~2 hours, during which the flux sharply decreases by 26% at 27 °C, 19% at 35 °C, and 22% at 40 °C; thereafter, flux losses are small and temperature independent. CPFS experiments show that this behaviour is due to the foulant layer, which results in weak, often repulsive, and *T</em|-independent foulant-foulant interactions, which hinder further foulant deposition.