The Process and Water Quality Specialists
Classes of Pressure Driven Membranes
Pressure driven membranes have been classified into four categories based on the membrane rejection properties as follows:
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.