Ultrasonic cleaning of latex particle fouled membranes

Membrane filtration has arguably been the most significant development in the area of water purification in the past 50 years. It has enabled many processes to become more reliable and to achieve purities that have not been achieved previously. One of the drawbacks of membrane use is fouling which is a decrease in the permeate flux with time. Fouling causes a decrease in production and requires some sort of treatment to remove or reverse its effects. In this research, the ultrasonic cleaning of particle-fouled membranes was investigated. Specifically, the mechanisms responsible for ultrasonic cleaning, the solution conditions and cake layer effects on ultrasonic cleaning and the implementation of ultrasound into an existing membrane cell were evaluated.; In initial studies, ceramic membranes were fouled by sulfate polystyrene latex particles in a dead-end filtration cell and subsequently subjected to ultrasound in a separate ultrasonic cleaning vessel at different frequencies, power intensities and durations of treatment. Results indicate that increased power intensity and lower frequency increase particle removal from a fouled membrane. These data along with SEM images suggest that cavitational mechanisms (i.e., microstreaming and microstreamers) are important in detaching particles from the membrane surface while turbulence associated with ultrasound (i.e., acoustic streaming) plays a role in the transport of particles away from the surface following detachment.; In further studies, ceramic membranes were fouled in a dead-end filtration cell at different pH values and ionic strengths using polystyrene latex particles of differing sizes and subsequently subjected to ultrasonic cleaning in a separate ultrasonic cleaning vessel. Ultrasonic cleaning at high and low pH values was more effective than at neutral pH because of charge interactions among the particles. The particle-particle interactions at the lowest pH were strong and brought about large floc removal while the highest pH had the greatest repulsion among particles leading to rapid detachment of individual particles. Ultrasonic cleaning of fouling layers formed at higher 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 settle as flocs on the membrane surface forming a highly permeable fouling layer. This fouling layer was resistant to ultrasound at sub-optimal cleaning conditions used in this study due to the strong particle-particle interactions in the fouling layer. (Abstract shortened by UMI.)...