Direct Observation and Dynamics of Gel-like Deposits on Nanofiltration Membranes

Ben Ukrainsky, Environmental Engineering, Technion, Haifa, Israel
Naama Segev, Environmental Engineering, Technion, Haifa, Israel

Membrane-based water production can be severely limited by organic and biological fouling, the prevention and management of which is a long standing theme of research. Such fouling materials are often characterized by gel structures comprised of long chain polymers such as polysaccharides. Here, we report a direct observation method that was used to monitor alginate fouling and cleaning with the aid of fluorescent marker beads. These markers are trapped in the alginate layer and their motion is hindered due the visco-elastic medium. Observation of these marking particles trapped at various distances from the membrane, enables to follow the micro-rheology and formation kinetics of the deposited layer.

Fouling and cleaning experiments were conducted using with and without the presence of calcium, enabling controlled manipulation, albeit qualitatively, of gel rigidity – the presence of calcium ions form a more rigid alginate layer, which is manifested through the near immobilization of the marker beads; this is contrasted by the more fluid-like layer formed in the absence of calcium. Interestingly, rigidity of the deposited ‘gel’ correlated with a faster permeate flux decline. Finally, direct observation experiments suggest that detachment occurs more rapidly and completely when the applied pressure is shut off, while significantly less detachment was observed when only the permeation flux was shut off during a cleaning stage. This insight into the dynamics of ‘soft’, deformable deposits reveals interesting mechanisms of fouling and important implications for the development of efficient cleaning protocols.