Coagulation–bubbling–ultrafiltration: Effect of floc properties on the performance of the hybrid process

• A novel hybrid process of coagulation–bubbling–ultrafiltration was proposed.
• Relationship of air bubble size and gas flow rate was studied.
• Bubbles and floc properties were measured to explain membrane performance.
• Concentration polarization resistance was reduced with air bubbles injected.
• Floc separation by air bubbles holds great promise in reducing membrane fouling potential.

A novel hybrid process of coagulation–bubbling–ultrafiltration was proposed to study membrane fouling phenomena by surface water. Relationship of bubbles, flocs and the hollow fibers was explored. When applying less than 20 mL/min gas flow rate, membrane fouling was accelerated with air bubbles introduced. When gas flow rate increased further to 40 mL/min and 60 mL/min, TMP showed a two-stage development trend, which was a fast development in the first few hours followed with a relatively slow development after about 4 h. Unified membrane fouling index (UMFI) increased from 0.00216 (without bubbles) to 0.00274 m2/L (40 mL/min gas flow rate) and 0.00219 m2/L (60 mL/min gas flow rate). As gas flow rate increased, bubble size became bigger, and its distribution range became wider, resulting in higher shear rate in the ultrafiltration column, which led to severe floc breakage. Flocs of small size and compact structure accelerated membrane fouling, resulting in highest UMFI value under 40 mL/min gas flow rate. However, under 60 mL/min gas flow rate, with largest bubbles and highest shear rate examined in this study, concentration polarization was effectively limited. As a result, TMP development slowed down when pore blockage reached equilibrium.