The Process and Water Quality Specialists

Desalination
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Classes of Pressure Driven Membranes

Pressure driven membranes have been classified into four categories based on the membrane rejection properties as follows:

  • Microfiltration (MF) membranes - have the largest pore size (0.1 - 3 micron), require low transmembrane pressure (1- 30 psi), and are used for turbidity reduction, removal of suspended solids, parasites (e.g. giardia and cryptosporidium), bacteria and some viruses
  • Ultrafiltration (UF) membranes - have a smaller range of pore sizes than MF membranes (0.01-0.1 micron) require low transmembrane pressure (1- 30 psi), and are capable of removing viruses as well as some color, odor, and organics removal, along with everything that the MF process can remove
  • Nanofiltration (NF) membranes - are relatively new porous membranes that have a pore size < 0.002 micron require moderate transmembrane pressure (75- 150 psi), and are primarily used for natural organic matter (NOM) removal for controlling disinfection byproduct precursor, water softening and sulfate removal
  • Reverse osmosis (RO) membranes - are effectively non-porous membranes that require high transmembrane pressure (150-500 psi) and are used for monovalent salt removal

MF/UF membranes are being increasingly employed in the desalination process to shield RO/NF/ED membranes from suspended solids and larger colloidal material that are detrimental to their performance. In other words, neither MF nor UF membranes are capable of salt rejection and, as a result, both are only relevant to desalination membrane systems as a pretreatment process. NF membranes can be visualized as tight UF membranes that not only reject materials (i.e. suspended solids, colloidal material, and bacteria) based upon a size exclusion, but also remove hardness (e.g. multivalent ions) based upon a charge repulsion mechanism. However, NF membranes poorly reject monovalent salts. Therefore, a majority of desalination is performed by non-porous RO membranes that provide physical barrier to a wide range of contaminants, including monovalents.

The EPA has designated RO as a best available technology (BAT) for removal of numerous inorganic contaminants, including antimony, arsenic, barium, fluoride, nitrate, nitrite, boron, selenium, radionuclides, and emerging contaminants, including endocrine disrupting compounds (synthetic and natural hormones), and several pharmaceutical compounds. Click here for summary various contaminants that would nominally be rejected by the various types of membranes.

The most common configuration of RO/NF element is the spiral-wound element, wherein a large number of flat sheet membranes are wrapped around a perforated PVC pipe. Because this configuration results in a densely packed module, a significantly fewer number of membrane elements are required, which reduces the overall footprint of the desalination facility. Generally, four to eight elements are arranged in series in a pressure vessel. During operation, when pressurized feed water enters the first element, a portion passes through the membrane material is collected as product water while the fraction remaining on feed side, now concentrated, becomes feed for the next element. Thus, the salt concentration on the feedwater side increases in each succeeding elements, with the last element receiving the most concentrated feed solution. The elevated feedwater concentration can cause increase in (1) concentration gradient across membrane that reduces the product water quality (e.g. higher TDS); (2) osmotic pressure that in turn reduces pure water flux or requires additional pressure to maintain this flux; and (3) inorganic scaling (precipitation) on the membrane surface. Click here to see the geometry and function of a spiral wound RO module and installation of modules into a typical RO pressure vessel.


Concentration of reject streams leaving each element is directly related to the system recovery, i.e. fraction of feed water, which is recovered as product water. High recovery implies that a small amount of concentrated waste will be generated (which has economic and other advantages), but results in poor water quality and flux decline. On the other hand, low recovery operation translates into better membrane performance, but results in a large waste stream that is less economic and difficult to dispose of. Clearly, there is a trade-off between system recovery, overall membrane performance and project costs.