(399s) Silica Scaling of RO Membranes – Real Time Detection and Scaling Kinetics

Silica scaling of RO membranes is detrimental at high recovery desalination of various brackish water sources as well as in water reuse applications. Early detection of silica scaling is critical for developing effective scale mitigation strategies. Accordingly, an approach to detection of the onset of silica scaling on RO membranes was developed based on direct and real time membrane surface imaging. In addition a comprehensive investigation was conducted of the impact of silica scaling (for a range of silica supersaturation) on flux decline and the scaled membranes were characterized via both AFM and SEM imaging. Using the developed membrane surface monitoring approach silica scaling was detected significantly earlier (by a factor of 2–7) relative to the assertion of scaling via the detection of measurable flux decline (i.e., ≥5%). Membrane surface image analysis enabled quantification of silica scaling kinetics in terms of both the number density of silica particles and surface area coverage by scale. Silica particle nucleation on the membrane surface followed classical nucleation theory and particle growth was governed by a classical diffusion mechanism. Based on analysis of real time surface imaging, SEM surface imaging and AFM, it suggested that silica scaling occurs through the formation of primary silica particles and their agglomeration, in addition to silica gel-like film formation. The surface silica gel can be smooth or rough with embedded silica particles. At low levels of supersaturations, silica gel formation is favored and the film formed is smoother and less porous than that which forms at high silica supersaturation. Over the range of silica supersaturation studied (about 1.5-3 above saturation) the membrane surface roughness increased by a factor of 2-8 relative to the native membrane. As expected, the silica scale layer thickness was in the range of 0.1 – 3.5µm and increased with silica supersaturation. Given the ability for early detection of silica scaling, the present approach was successfully evaluated for antisclant selection and dose optimization and to optimize the mitigation of scale formation through novel process operational control.