Overall porosity is a sign of voids and holes in the membrane matrix. How does it affect on membrane rejection percent?
Considering that the separation process is based on particle size, to have a high selectivity, pores on the membrane need to be smaller than the particles in the mixture/solution. Therefore, The degree of selectivity is depend on both membrane porosity and charge. As you know, membranes can be classified base on size to Microfiltration, Ultrafiltration, Nanofiltration, and Reverse Osmosis. Microfiltration for example with largest pore has therefore lowest selectivity as it allows a range of particles with different sizes to pass, so has low selectivity and so commonly used as a pre-treatment or clarification step prior to other membrane processes. As the pores gets smaller the membrane become more selective.
Sometimes porosity is confused with the pore size (which is wrong). With correct definition of porosity (as a volume fraction of void space) there may be an indirect dependence of rejection on it due to the dependence of extent of concentration polarization on the volume flux. At higher porosity the flux will be larger and rejection lower.
Dear Shavandi & Yaroshchuk,
Thanks for your time and information. We have two kinds of porosity: surface porosity and total porosity. My question was about the second one. Maybe the sentence which I saw was about surface porosity!
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