Ultrafiltration (UF)

As with most conventional filtration methods, sand filters and media filtration require consistent raw water quality to deliver quality effluent, which is not always possible. They also don't provide an absolute barrier. Traditional media filters typically remove particles to down to about 5 microns.

Ultrafiltration (UF), however, does not suffer from those limitations. This technology uses an ultrafiltration membrane barrier to exclude particles 0.02 to 0.05 microns, including bacteria, viruses, and colloids, meeting increasingly stringent water-quality standards around the world, and providing a stable, reliable, and consistent water quality.

UF offers:

• High and consistent product quality measured by turbidity (NTU) or silt density index (SDI).
• Tolerance to feedwater quality upsets.
•  Limited to no use of pretreatment chemicals (polymer, coagulant, pH adjustment) and associated costs for sludge disposal.
• Small footprint and less weight than media filters.

When used as a pretreatment for reverse osmosis (RO), UF also helps reduce fouling of the RO membranes, which can lead to:

• Reduced chemical cleaning frequency and consequently, lower operating costs and downtime.
• The potential to operate RO at higher flux, with fewer membranes and vessels — leading to lower capital cost for the RO system.

Rejecting particles, colloids, and macromolecules

In ultrafiltration, water and low-molecular-weight substances are passed through an ultrafine membrane fiber by the application of pressure or vacuum to the system; flow through the membrane is achieved by applying a pressure gradient between the outer and inner walls of the membrane structure. For pressurized UF, feed pressure is from 4 to 100 psig (typically < 30) and transmembrane pressure is generally <14 psig.

Ultrafiltration membranes have pore sizes in the range of 0.02 – 0.05 microns, which contributes to the production of high-quality water; the pore-size range means that the purification process is characterized by a high removal capability of bacteria, viruses, colloids, and silt.

Ultrafiltration membranes have a service life of three to seven years (and potentially longer). They are commercially available in hollow fiber, tubular, plate, and frame, as well as spiral wound configurations.