Ultrafiltration (UF) is becoming more popular for pretreatment to reverse
osmosis systems for its ability to remove fine particulates and colloidal silica,
which can foul RO membranes. UF membranes typically have pore sizes under
0.1 micron and as small as 0.001 micron, so in addition to particulates, they are
often used for removing bacteria and viruses in high purity polishing or drinking
water applications as well. UF in industrial purified water systems is a crossflow
separation process, similar to reverse osmosis except the membrane is designed
to remove discrete particles and high molecular weight organics, but allows ionic
species to permeate the membrane.

UF membranes come in tubular, spiral wound and hollow fiber designs, and
are generally constructed from polypropylene, polyethersulfone (PES), and
poyvinylidenefluoride (PVDF). UF membranes are typically rated for a molecular
weight cutoff, rather than a micron removal rating. With the variety of UF
configurations and materials, it’s important to match the membrane to the
application, and in some cases more than one type of membrane may be
appropriate. It is also important when designing a system to select a membrane
material that is compatible with the chemicals that will be used for cleaning and
sanitizing the system.

For pretreatment to reverse osmosis, a hollow fiber element is most
commonly used, as these membranes exhibit good physical strength, are less
prone to breakage, and can be cleaned both chemically as well as scoured with
air. In pretreatment applications, conventional filtration is normally used
upstream to remove relatively large suspended particles, to permit the membrane
to do the job of removing finer particles and organic molecules. These modules
are typically mounted vertically, so the UF skids themselves are fairly compact;
however, the system also requires auxiliary equipment for backflushing and
cleaning, so the complete system can take up some floor space. It should be
noted that in many of these pretreatment applications, similar results can be
achieved using the proper coagulant aid at the proper dosing rate with
conventional filtration; so if capital is limited (or even if it isn’t), this approach
warrants serious evaluation.

For polishing applications, where there is not as much particulate loading, a
spiral configuration may be the most appropriate choice. In high purity water
systems, UF is increasingly used instead of the conventional 0.2 or 0.1 micron
polishing cartridge filters. When a UF membrane is used in lieu of a cartridgetype
final filter, particle removal efficiency is improved, plus the UF membranes
can be sanitized rather than having to be replaced if bacterial growth is
experienced.

Yet another opportunity to use this technology is in the growing demand for
water reuse and recovery. UF is effective for removing emulsions and many of
the organic compounds found in industrial wastewaters. In general, these
applications require a pilot study to determine the most appropriate membrane
for the application and to generate data for design of the full-scale system.

For further information or for assistance in evaluating a potential application,
contact your Process Solutions, Inc. sales representative!

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