Membrane Filtration Systems
Polymeric or inorganic membranes (usually man-made) allow sieving down to the molecular level. The openings in these membranes (pores) are so small that significant pressure is required to drive fluid through them. Unlike traditional filters, all the influent does not pass through the filter element. Instead, some of the influent carries concentrated contaminants away from the filter element. The pores are so fine that they would plug up and foul instantly unless systems are designed to carry the contaminants away.
Membrane Classification
There are now four commonly accepted classes of membranes, defined based on the size of the contaminants they will remove.
They are (from smallest to largest):
- Reverse Osmosis (RO) - output from RO membranes approaches that of Mixed Bed DI systems - ultrapure water.
- Nanofiltration (NF) - often called loose RO, these bridge the gap between RO and UF membranes. With pore size of approximately 1 nanometer, they will remove all but monovalent salt ions.
- Ultrafiltration (UF) - pore sizes of 100 to 100,000 nanometers.
- Microfiltration (MF) - pore sizes of 1 to 10 MWCO. These elements overlap traditional straight-through filters, but utilize crossflow to provide lower cost operation and longer media life.