Membrane Filtration

Membrane filtration is widely used for water purification. All modern membrane filters are of the crossflow design, typically using spiral-wound or hollow-fiber membrane configurations. The pore sizes of these membranes cover a very wide range that reaches from less than 0.001 micrometers to more than 2.0 micrometers.

This range has been subdivided into four filter categories that with their decreasing pore sizes will retain ever smaller particles or molecules while always letting the water pass. Decreasing pore sizes require higher operating pressure (i.e. higher energy consumption) and a corresponding higher capital cost and higher operating cost. For a certain filtration task therefore, the category chosen is always the one that permits the largest pore size without letting any of the undesired components pass through the membrane.

The four categories, their retention capabilities, their pore size ranges and their operating pressure ranges are shown on the graph below.

For most water purification applications spiral-wound R.O. or hollow-fiber UF/MF membrane systems are being chosen, for two fundamentally different purposes.

R.O. is always required when salt has to be retained. For a typical sea water desalination application working with 35,000 ppm of TDS (salt) the pore size is 0.0008 ηm and the operating pressure is ±850 psig. Due to the higher osmotic pressure higher salt concentrations will require even higher operating pressure, while for brackish water or only slightly saline water the operating pressure can be as low as 200 psig.

UF or MF systems are widely used to filter sweet (non-saline) water where particles in the macromolecular and microparticulate ranges have to be removed. These ranges include all bacteria and most viruses.

NF systems are only used in rare cases where specific components in the molecular size retention range of these filters need to be removed.

Often however, MF and UF are used for prefiltration upstream of R.O. systems. For very large desalination systems NF can be found to prefilter the feed to R.O. systems.