Effect of Fouling Conditions and Cake Layer Structure on the Ultrasonic Cleaning of Ceramic Membranes

Abstract Homogeneous alumina membranes fouled by polystyrene latex particles at different pH values and ionic strengths were subjected to ultrasonic cleaning. Cleaning was more effective at high and low pH than at neutral pH. At low pH values, less repulsive particle‐particle interactions resulted in the removal of millimeter‐scale aggregates and highly effective cleaning. At near‐neutral pH, stronger repulsive particle‐particle interactions caused detachment to occur as individual particles from the cake layer rather than as flocs, which was a slightly less effective cleaning mechanism. Ultrasonic cleaning of cake layers formed at high ionic strength (>0.3 M KCl) was less effective than cleaning at lower ionic strength (<0.3 M KCl). High ionic strength caused particles to coagulate in solution and deposit as flocs on the membrane surface forming a highly permeable fouling layer. This fouling layer was resistant to ultrasound at the sub‐optimal cleaning conditions used in this study, perhaps due to particle attachment occurring within a primary energy minimum. Membrane cleaning experiments performed with particles of varying size showed that particle size was less important than the surface potential of the particles. For a given mass, particles that possessed the largest surface potential formed the thickest fouling layer, irrespective of particle size, and showed the greatest improvement in flux with ultrasonic cleaning. These results demonstrate that solution conditions influence ultrasonic cleaning of membranes primarily by modifying particle‐particle and particle‐membrane interactions as well as cake layer structure, rather than by impacting the extent or magnitude of cavitation events.