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A bench-scale study was performed to optimize backwash frequency and air sparging conditions during ultrafiltration (UF) of natural surface waters in order to maximize water production and minimize irreversible fouling as well as operating and maintenance costs. Surface shear stress representing different air sparging conditions (continuous coarse bubble, discontinuous coarse bubble, and large pulse bubble sparging) was applied in combination with various backwash frequencies (0.5, 2 and 6 h) and fouling was assessed. Results indicated that air sparging with discontinuous coarse bubbles or large pulse bubbles significantly reduced the irreversible fouling rate while providing cost savings when compared to the baseline condition, which assumed a 0.5 h-backwash frequency and no air sparging during filtration. Cost savings were more pronounced at lower backwash frequencies, due to value associated with extra water produced over longer filtration times and longer membrane life resulted from fewer recovery chemical cleans because of lower irreversible fouling.

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Air‐sparging and coagulation/flocculation were compared as fouling control strategies during ultrafiltration of surface waters. Fouling was assessed following coagulation (0.5 and 15 mg/L alum) and surface shear stress representative of different air‐sparging conditions: continuous coarse bubble, intermittent coarse bubble, and large pulse bubble. Results indicated that 0.5 mg/L of alum reduced membrane fouling, especially for waters with higher concentrations of organic matter (> 4 mg/L dissolved organic carbon). A 15‐mg/L alum dose did not significantly improve membrane performance relative to the low dose. Air‐sparging reduced fouling, but the benefits were not additive in combination with coagulation. Potential cost savings were calculated based on longer permeation times made possible by reduced fouling, and the value associated with water produced relative to energy costs (air‐sparging) and chemical costs (coagulant). For the water investigated, 0.5 mg/L of alum or large pulse bubble air‐sparging was optimal for membrane fouling control.

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