Membranes used in VSEP systems are made
of various polymers such as polyethersulfone,
polyamides, and other thin film composites.
Over 200 different membranes types are routinely
employed in VSEP systems.
manufacturers are routinely coming on-line
all around the world, and a number of exciting
technological advances have been made in
recent years (California is considerd by many
to be the epicenter of the new membrane technology
wave). Many of these advancements are aimed
at increasing the capabilities of membrane
separations in harsh process water and wastewater
environments. For example, there are nanofiltration
membranes coming out that can withstand extreme
temperatures and pH ranges.
Membranes are available in a variety of sizes
and configurations. In VSEP systems, the membranes
are affixed to a steel tray and stacked in
an array, much like a traditional plate and
frame configuration. (See our VSEP
system components page for more information
on the VSEP filter pack configuration.)
Essentially, membranes allow
some things to pass through while rejecting
the rest. Thus, any feed stream sent to a membrane
system such as a VSEP will be split into two. These
two streams are called permeate and concentrate
(these are sometimes referred to as product
water and retentate, respectively).
of the stream that can pass through the membrane
is called the permeate. The permeate is the
"clean water". The part of the stream that
is rejected by the membrane is called the concentrate.
In a product recovery or concentration context,
the concentrate is the desirable material;
in a wastewater or product clarification application,
the concentrate represents a concentrated slurry
of undesirable contaminants.
Four basic categories of membranes exist,
and they are characterized as follows:
Microfiltration (0.1µ - 2.0µ)
The microfilters used in VSEP systems are almost
exclusively PTFE (Teflon®). Microfilters
or MF membranes are used to remove small suspended
solids, large colloidal material, some emulsions,
and most bacteria. MF membranes do not hold
back any dissolved solids.
MF membranes are especialy useful in dewatering
slurries such as titanium dioxide and calcium
microfiltration applications were the focus
of most VSEP applications.
Teflon MF membranes
are the most robust of all the membrane types;
they can withstand temperatures of 130°C (266°F) and
as they are chemically inert, can handle continous
pH levels of 0-14. MF membrane operating pressures
are usually between 30 and 100 psi (~2 to ~7
Ultrafiltration (0.008µ - 0.1µ)
or UF membranes
in a variety
of VSEP applications
goal is to
100% of the
and colloids. UF membranes can be used
in VSEPs to break emulsions without using
Ultrafiltration can also be used as primary treatment where a particularly
dirty wastewater is being recycled using a secondary reverse osmosis membrane
system. UF membranes commonly employed in VSEP systems include polyethersulfone,
PVDF (Kynar®), and regenerated cullulose.
UF membranes are robust performers and come in a wide variety of constructions.
Depending on the configuration and application, UF membranes operate between
30 and 250 psi (~2
to ~14 bar). UF membranes' upper temperature limits average around 90°C (194°F)
and their pH tolerances range from 1 - 14.
Nanofiltration (0.001µ - 0.01µ)
Nanofiltration or NF is the newest membrane type. NF membranes are essentially
"loose" reverse osmosis membranes. These semi-permeable membranes are
constructed of materials such as sulfonated sulfone, polyamides, and other
thin film composites.
NF membranes can be used to remove organics and many dissolved materials such
as hardness. NF membranes are often used in wastewater treatment
to remove BOD. NF can also be used as pretreatment to a RO VSEP or spiral RO
system; the permeate from a nanofiltration membrane is a "soft" water.
NF membranes operate at pressures from 200 to 600 psi (~14 to ~41 bar) and can
withstand a range of pH's from 1 to 14 (although many are limited to a range
Reverse Osmosis (30 daltons - 0.001µ)
Reverse Osmosis or RO is the "tightest" of all the membrane types.
This is the type of membrane used in seawater desalination. RO membranes are
designed to hold back sodium chloride (NaCl) and are rated by their ability
to do so. For example, seawater desalination membranes are typically rated to
reject 99.5% NaCl.
In VSEP systems, RO membranes are often used to remove organics, trace oil,
and trace metals in a single unit operation. RO membranes have been much maligned
in the industrial context due to their high fouling potential. VSEP's vibration
mitigates this risk, thus opening the door
to a huge variety of applications where removal of low molecular
weight contaminants from a wastewater stream is desired.
While early reverse osmosis membranes were made of cellulose acetate, today's
RO membranes are typically constructed of proprietary thin film composites.
These membranes operate at 300 to 1,000 psi (~21 to ~69 bar) and can tolerate
a pH range of 2-12.