| July Spotlight: Reverse Osmosis
An Overview on Advances
in POU Technology
By Victor Zaldivar, David Carlile
& David Powell
| Summary: There's been much talk about movement
toward a "tankless" RO system. And, while advances in technology
and design have meant less cramped space under homeowners' sinks,
limitations of high output membranes still make that a far flung notion.
The following article discusses the state of the RO industry today.
The point-of-use (POU) water treatment industry has changed
and grown immeasurably in the last 20 or so years. This is especially
true for reverse osmosis (RO) systems that today do more and are better
and faster than the RO systems commercially available 10 or even five
years ago. Advances in membrane and filtration technologies, coupled with
refinements in RO control systems have spawned an era of truly affordable,
high quality drinking water systems for homes and small businesses.
Residential RO systems
Since introduction of the first RO system for home use in the mid-1960s,
refinement and better affordability have been the mission. RO membranes
separate many contaminants from water by forcing the water molecules,
under pressure, through a semi-permeable membrane. The resulting "product
water" or permeate can contain as low as 5 percent of the original
dissolved solids content (depending on the feed water makeup and membrane
properties). This product water is usually stored in a pressurized storage
tank for use by the customer and delivered through a dedicated faucet
on the sink-top. Incorporation of high capacity prefilters and postfilters
and design refinements complete the product to ensure customer satisfaction.
The first home ROs were based on cellulosic membrane materials (cellulose
acetate or CA; followed later by cellulose tri-acetate or CTA). CA was
the first version of an RO membrane, but is virtually unused today. More
advanced CTA membranes became the mainstay of the home RO channel since
they were readily available and fairly inexpensive. They also exhibit
a fairly high tolerance toward oxidizing chemicals such as chlorine. Since
most U.S. municipal water supplies are chlorinated to control bacteria
levels, this chlorine tolerance was an important contributor to membrane
However, CTA as a material exhibits some
distinct disadvantages as well. It's a comparatively low flux membrane
material. Flux is defined as the amount of water in gallons that can be
passed through one square foot of membrane and is represented by gallons
per square foot per day (GFD). This results in more square inches of membrane
in a roll required to achieve the same flow rate as that of a comparable
thin film membrane element. Also, although some recent advances by membrane
manufacturers have improved CTA's pH tolerance, they're still limited
in application. If feed water pH is higher than 8.5, CTA membranes begin
to quickly degrade and lose total dissolved solids (TDS) rejection performance.
When this happens, the CTA membrane is said to hydrolyze, a condition
characterized by high output and poor rejection. Lastly, CTA is more sensitive
to high feed water temperatures (see Figure 1). A typical CTA-RO
system has an upper limit of 85°F for feed water temperature. And
feed water temperatures approaching 100°F (38°C) aren't uncommon
in many parts of the world.
Figure 1. Production rate vs.
feed water temperature
SOURCE: WQA RO Manual
In spite of these disadvantages, CTA dominated
the point of use RO industry for the first 15 years or so. Then in the
early '80s, the first thin film membrane--which is made of an ultrathin
active layer of polyamide polymer coated on a much thicker polysulfone
polymer support layer-appeared on the market for home use. CTA still had
a price advantage over thin film and, therefore, continued to enjoy a
good percentage of the market. The advantages of thin film-as well as
advances resulting in lower, more comparable pricing-however, have resulted
in what is, for all practical purposes, a full swing to thin film membranes
for home RO systems.
Thin film composite membranes
Thin film membranes enjoy multiple performance advantages over their cellulose-based
cousins. Thin film membranes actually "reject" or exclude a
higher percentage of many common water contaminants resulting in a more
"pure" finished product (see Figure 2 & 3). They're
more tolerant of higher feed water temperatures as standard thin film
RO products can operate up to 100°F. Thin film membranes also exhibit
a higher tolerance to extremes of pH, operating normally within a pH range
of 3 to 11. The main disadvantage of thin film membranes is that they
have almost no tolerance for chlorine or other oxidizers.
Figure 2. Percent rejection
vs. feed water TDS level
SOURCE: WQA RO Manual
Systems manufacturers have compensated for
this by inclusion of an activated carbon filter as a prefilter. However,
this is the single biggest weakness in RO system design because the RO
membrane will only remain effective as long as the prefilter is removing
a very high percentage of the feed water chlorine concentration. Make
sure the manufacturer includes a high quality, high capacity carbon filter
before installing any thin film RO system on chlorinated water.
Figure 3. Percent rejection
of salts vs. net pressure
SOURCE: WQA RO Manual
Thin film membranes have a higher flux rate,
allowing manufacturers to use less membrane material to achieve the desired
flow rate from a given sized element. Advances in thin film technologies
have resulted in large flux increases. Elements that were limited by their
size (2-inch x 12-inch) to a maximum of 20 gallons per day (gpd) a decade
ago can now be manufactured to provide up to 75 gpd. A 100-gpd residential
elements appear to be on the near horizon with at least one membrane manufacturer
test marketing the design.
Flux and flow rates
In general, membrane materials have a maximum flow rate, which results
in premature fouling of the membrane if exceeded. Typical under sink RO
designs are equipped with some form of reject water flow control. By controlling
the amount of flow to drain, manufacturers of residential RO systems can
walk a balance between premature fouling (by allowing some amount of water
to exit the system, constantly flushing the membrane surface of contaminants)
Higher flux thin film membrane materials
have been developed for industrial applications where much attention to
prefiltration and water treatment (i.e. softening, chemical feed and bed/depth
filtration) offers some amount of added protection not available in a
standard residential application. These higher flux membranes allow more
gallons to permeate per square foot, but this is compensated for by the
increased attention to prefiltration. The end result is new thin film
membrane materials that can almost double standard thin film GFD flow
rates. This allows the industrial user to operate at lower feed pressures
to produce comparable amounts of water. In addition, this concentration
on composite polyamide chemistry-thin film membranes-has helped in the
development of ranges of both RO and nanofiltration (NF) membranes, all
having their own unique rejection and flux characteristics. For example,
a standard 2-inch diameter by 12-inch long membrane designed for use in
a residential system could be either an RO or NF membrane. Containing
about 4.8 square feet of active membrane area, the high rejection polyamide
element (RO) will produce around 35 gallons a day, while the NF element-containing
the same amount of active membrane area-will produce about 80 gallons
a day under similar operating conditions.
The natural tendency would be to apply these materials for home use. However,
the general application of high flux elements must be considered very
carefully. Undersink RO designs can be susceptible to a phenomenon known
as "TDS creep." This is the tendency for the product water in
the storage tank to slowly increase in TDS over time. RO membranes take
a little bit of time to return to optimal rejection level after sitting
idle for some time. This is because, without forward pressure applied
by the feed water driving water molecules through the membrane (RO), the
natural phenomenon of diffusion will apply. That means, when the membrane
is idle, the concentration of salts on the feed and product sides will
move toward equilibrium (i.e., the same TDS on feed as on product side).
When the system restarts, this "slug" of high TDS water will
enter the tank. Secondly, thin film membranes increase in rejection over
the first five to 15 minutes of operating time at full driving pressure.
So, although they're not operating optimally, they still deliver this
first water to the tank. The faster the membrane can refill the tank (i.e.,
the higher output membrane with more surface area), the worse this TDS
creep can be. A well designed RO system will take this phenomenon into
account by balancing RO membrane output with shutoff valve characteristics.
A poorly designed RO or one with too much membrane for the application
will result in lower quality product water.
A properly installed RO system will make use of some form of an air gap
device. This is usually incorporated into the RO spigot. Since proper
system design dictates a reject water to product water ratio of anywhere
from 3:1 to 5:1 or more, the air gap device must be able to handle the
reject flow rate. However, since a common consumer complaint is "air
gap noise," one should be careful when considering very high output
membranes. Basically, this translates to the higher the membrane rating,
the more noise you can expect at the RO system air gap and/or drain connection.
Undersink RO designs currently all have a storage tank. The tank is typically
a captive air design (air charged with a bladder or diaphragm to hold
the treated water). Most undersink RO systems are controlled with some
form of water conserving automatic shutoff valve. These valves are intended
to maintain available water in the storage tank at some volume that maximizes
customer satisfaction. In other words, the shutoff valve must reopen the
feed supply before the tank runs out of water. The most efficient designs
will turn off at a pressure high enough to store a satisfactory amount
of water (but not so high that finished product quality suffers from TDS
creep) and turn back on before the tank reaches a critically low level
of storage. A well designed RO system optimizes this balancing act by
keeping water in the tank fresh and low in TDS while ensuring that, in
all but the highest use scenarios, the customer has water left in the
tank when needed.
Quicker servicing options
Certain RO system manufacturers have also developed disposable, encapsulated
elements or modules for easier servicing of the system. The RO element
is permanently sealed inside a plastic housing with easy-threaded or other
convenient end connection fittings so the user can easily replace the
spent module. A simple hand twist slides the fitting into the bracket.
Change-out of modules is quicker, cleaner and more efficient with an encapsulated
element. Since there's no actual contact with the spent membrane, servicing
of the RO system is also more sanitary.
So what does the future hold? We feel new efforts are to simplify membrane
systems in order to reduce size and cost. A chlorine resistant thin film
composite membrane would greatly reduce prefiltration while extending
membrane life. Membrane manufacturers continue to drive more water through
small membrane elements. Could this eventually result in "tankless"
RO for the home? Perhaps. So far, even a 100-gpd membrane can only deliver
0.07 gallons per minute (gpm) to the faucet with fairly warm water and
good pressure. This drops significantly as temperature and pressure drop.
This wouldn't be acceptable to the standard consumer, so a tank is still
required. When a small, low cost element can deliver 0.5 gpm of reduced
TDS water, we may see a new system on the market. In the interim, Figure
4 offers a good comparative chart to use when weighing your options between
Figure 4. Comparison of
POU RO Membrane Parameters
| Maximum pH
| Maximum operating
| Bacterial resistance
| Free Cl2
Good (12,000 ppm-hrs)
Poor (1,000 ppm-hrs)
| Max 2"x12"
| TDS rejection@60
| Nitrate rejection@60
1. There are many different formulations
of CTA membranes (generally a blend of cellulose di- and tri-acetate
polymers forming a continuous asymmetric structure), each with
its own individual pH tolerance characteristics. Consult with
the supplier for detailed test data on pH tolerance. When the
maximum pH is exceeded, rapid loss of TDS rejection occurs due
to hydrolysis deterioration of the membrane.
2. RO membranes will maintain structural integrity at temperatures
higher than those listed here. However, hydrolysis of the CTA
membrane may be accelerated, which will result in loss of TDS
rejection. In POU RO applications, the TFC membrane should be
limited to 100°F for practical reasons.
3. Due to its limited resistance to bacteria , the CTA membrane
should be used on water supplies that are regularly disinfected.
4. The chlorine tolerance shown is the number of hours of operation
on feed water containing 1 ppm of free chlorine before loss
of TDS rejection occurs. There are other factors such as pH,
which may increase or decrease this number. Consult with your
supplier for details of this parameter.
5. Membrane production rates are approximate for standard 1.5-to-2"
diameter (nominal) X 12" long membrane elements used for
POU RO applications. TFC membrane elements in this size may
be available with considerably more production, but should be
used only where the needs of the customer warrant it. Ideally,
POU RO membrane elements should be in operation as continuously
as possible to minimize fouling and bacterial growth.
6. The pressure referred to is the net pressure across the membrane.
When the membrane element is incorporated into a "system"
with an air/water storage tank the actual TDS and nitrate rejection
may be considerably less.
SOURCE: Slovak, Robert, "A Practical Application
Manual for Residential, Point-of-Use Reverse Osmosis Systems,"
Water Quality Association, Lisle, Ill., March 2000.
About the authors
Zaldivar, of Irvine, Calif., is a special accounts sales manager
with CUNO/Water Factor Systems, which is based in Meriden, Conn.
Previously, Zaldivar was with CUNO's Scientific Application Support
Services department. He can be reached at (949) 588-7385, (949)
588-7393 (fax) or email: email@example.com
David Carlile, of
Irvine, Calif., is West Coast regional sales manager for CUNO/Water
Factory Systems. He can be reached at (949) 588-7385, (949) 588-7393
(fax) or email: firstname.lastname@example.org
David Powell is
RO marketing specialist for CUNO Inc. in Meriden, Conn. He can
be reached at (203) 238-8776, (203) 238-8701 (fax) or email: email@example.com