Abstract
To evaluate the significance of reverse osmosis (RO) and nanofiltration (NF) surface morphology on membrane performance, productivity experiments were conducted using flat-sheet membranes and three different nanoparticles, which included SiO2, TiO2 and CeO2. In this study, the productivity rate was markedly influenced by membrane surface morphology. Atomic force microscopy (AFM) analysis of membrane surfaces revealed that the higher productivity decline rates associated with polyamide RO membranes as compared to that of a cellulose acetate NF membrane was due to the inherent ridge-and-valley morphology of the active layer. The unique polyamide active layer morphology was directly related to the surface roughness, and was found to contribute to particle accumulation in the valleys causing a higher flux decline than in smoother membranes. Extended RO productivity experiments using laboratory grade water and diluted pretreated seawater were conducted to compare the effect that different nanoparticles had on membrane active layers. Membrane flux decline was not affected by particle type when the feed water was laboratory grade water. On the other hand, membrane productivity was affected by particle type when pretreated diluted seawater served as feed water. It was found that CeO2 addition resulted in the least observable flux decline, followed by SiO2 and TiO2. A productivity simulation was conducted by fitting the monitored flux data into a cake growth rate model, where the model was modified using a finite difference method to incorporate surface thickness variation into the analysis. The ratio of cake growth term (k1) and particle back diffusion term (k2) was compared in between different RO and NF membranes. Results indicated that k2 was less significant for surfaces that exhibited a higher roughness. It was concluded that the valley areas of thin-film membrane surfaces have the ability to capture particles, limiting particle back diffusion.
Highlights
In reverse osmosis (RO) and NF membrane treatment processes, fouling is one of the major issues related to the deterioration in membrane performance
The effect of nanoparticle concentration on the productivity of RO and NF membranes at a constant feed ionic strength are shown in the following figures
The difference between the permeate flux with nanoparticles in the feed stream and the baseline indicates the net contribution of nanoparticles to membrane productivity
Summary
In RO and NF membrane treatment processes, fouling is one of the major issues related to the deterioration in membrane performance. Colloidal fouling of membranes is caused by different mechanisms. For RO, NF, and perhaps some tight UF membranes, colloidal fouling is caused by the particles accumulating on the membrane surface to develop a so-called cake layer. This cake layer provides an additional hydraulic resistance to water permeating through the membrane, reducing the water flux. For MF and UF membranes, pore plugging is another factor that causes membrane fouling besides the particle accumulation on the surface. The extent of pore plugging and cake formation depends on the relative size of the particles compared to the membrane pores sizes [4]
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