Real-time direct detection of silica scaling on RO membranes

Abstract

Silica scaling of RO membranes was evaluated via real-time direct surface imaging demonstrating a capability for detecting the onset of silica scale formation and its evolution. Silica scaling was detected significantly earlier than by traditional flux decline measurements. The observed rate of silica particle nucleation followed classical nucleation theory while the growth of individual silica particles, at the early stages of silica scaling, was governed by diffusional growth. SEM and optical images of the membrane surface suggest that silica scaling occurs through the formation of both primary silica particles and their agglomerates (~1–30µm), as well as a gel-like silica film embedded with silica particles both of which contribute to permeate flux decline. At low silica saturation index at the membrane surface (SIm≤1.93) silica gel film formation resulted in a smoother and less porous film than at higher silica saturation (SIm≥2.72). At the higher silica saturation levels (SIm=2.72–3.50), silica scaling resulted in larger observed particles as well as rapid permeate flux decline. The silica scale layer thickness was in the range of ~0.1–3.5µm, with surface roughness being higher by a factor of 2.6–8.3 relative to the native membrane. Results of the present study suggest that there is merit in exploring the application of the present approach for early detection and monitoring of silica scaling in RO plants in support of strategies for silica scale mitigation.