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Membrane technologies

The pressure driven membrane separation technologies of microfiltration (MF), ultrafiltration (UF),

nanofiltration (NF) and reverse osmosis (RO), as a group, provide the greatest range of contaminant

removal. As a result, these technologies are often the lynchpins for potable water production systems.

In particular, membrane technologies possess certain properties that make them unique when compared to other solid or liquid separation operations. These include: * Continuous process, resulting in automatic and uninterrupted operation * Low energy use involving neither phase nor temperature changes * Modular design – no significant size limitations * Minimal moving parts with low maintenance requirements * No effect on form or chemistry of contaminants * Discreet membrane barrier to ensure physical separation of contaminants * No chemical addition requirements Simply put, these technologies are continuous filters. The type of contaminant removed is a function of membrane polymer selection and its pore size. Although they all provide separation of contaminants from the solvent (usually water), each performs a specific function and has advantages and disadvantages when compared to the others in a particular application.

The development in filtration technology known as “crossflow” or “tangential flow” filtration allows for continuous processing of liquid streams. In this process, the bulk solution flows over and parallel to the filter surface. Because the system is pressurised, water is forced through the filter medium and becomes “permeate”. The remaining water exits the membrane device as “concentrate” and carries out the contaminants rejected by the membrane. Turbulent flow of the bulk solution across the surface minimises the accumulation of particulate matter on the filter surface and facilitates continuous operation of the system.

Microfiltration

Generally, microfiltration involves the removal of particulate or suspended materials ranging in size from approximately 0.01 to 1 microns (100 to 10,000 angstroms).

Ultrafiltration

Ultrafiltration is used to separate dissolved, non-ionic materials typically smaller than 0.01 micron (100 angstroms). The removal characteristics of UF membranes can be described in terms of “molecular weight cutoff” (MWCO), the maximum molecular weight of compounds that will pass through the membrane pores. MWCO terminology is expressed in Daltons. Basically, ultrafiltration is used to remove dissolved organic contaminants, while suspended solids are removed by microfiltration.

Nanofiltration

This is an intermediate process between ultrafiltration and reverse osmosis. The molecular weight cut-off (MWCO) properties of nanofiltration membranes are in the range of 400 to 800 Daltons (<10 angstroms). Ionic rejections vary widely depending upon the valence of the salts; multivalent salts such as magnesium sulfate (MgSO4) are rejected as much as 99%, while monovalent salts such as sodium chloride (NaCl) may have rejections as low as 20%.

This process will remove almost all dissolved organic (non-ionic) solids with molecular weights above approximately 100 Daltons, as well as a high percentage of ionic materials.

**Reverse osmosis

**Membranes are not perfect; they will typically remove 95 to 99% of the ionic contaminants.