Six Pieces of the
Home RO Puzzle
Summary: Providing a reverse osmosis (RO)
system is not always a straight-line procedure. Variances in requirements and
conditions, and the ever-present "problem water," create an ongoing challenge
for treating some water. A thorough knowledge of RO components can help you
purchase and maintain the most effective system possible.
The perfect home reverse
osmosis (RO) system does not exist. Some designs are better than others, but no
one design is best for all situations. Most dealers and manufacturers will use
and recommend different models or designs for different feedwater
or end-use requirements, but there are situations where this is not enough and
system customization will be required. Installing an RO system is as easy as
connecting plumbing. Sometimes this works, but not in all cases.
A good overall RO design is
typically conservative with respect to operating conditions. Generally
speaking, the more expensive an RO system, the more "what if"
scenarios have been considered (and compensated for) in the design.
However, compensating for every possible feedwater
problem would be prohibitively expensive. "Problem waters" will
always exist. Because they are also the ones most likely to need treatment,
dealers can expect to come across situations that require custom modifications.
By understanding the
importance of critical RO components, you can provide the best general RO
design. With this knowledge, you can also customize a system to perform in
problem water areas.
When feedwater
is not problematic, you can remove conservative features from the design to get
a better value. The end result is a lower overall cost, a technical advantage
over other systems.
The critical components
relative to system operation are:
- RO membrane
- Flow restrictor
- Sediment prefilter
- Pre- and post-carbon
filters
- Storage tank
- Pump
Even the simplest of
systems cannot operate properly without the first three components, while the
last three are employed to meet specific wants or needs. Proper selection and
use of each is vital to system operation.
RO MEMBRANE
The RO membrane is
obviously the heart and soul of an RO system. System design begins with
membrane selection, and all other components are designed around the membrane
requirements.
Following are the three
types of RO membrane available and the advantages/disadvantages of each type
(see Membrane Types chart).
· Cellulose acetate/triacetate blends
(CTA)
· Thin film/thin layer composites
(TLC®)
· Modified polysulfone
(SPSF)
CTA membranes provide the
lowest cost-per-gallon of water. Lower flow rates limit CTA membranes in cases
of high-water consumption, but their resistance to oxidation allows
self-sanitization from chlorinated feedwater. The
rejection and pH tolerance of today's more advanced formulations generally make
CTA a good choice for typical treatment needs. Also, by allowing chlorine to go
to storage tanks, you will have a lower heterotrophic
plate count (HPC) bacteria level.
TLC membranes combine high
flow rates, high rejection and high pH tolerance, making them ideal for many
needs. For situations with high-volume consumption, low feedwater
temperature/pressure, high-nitrate concentration or high-pH water (greater than
9.0), it is advisable to use a system with TLC membranes.
With TLC membranes, there
is a growing concern over system sanitization and the high bacteria levels
which may develop in storage tanks as a result of dechlorinating
the feedwater. TLC membranes are still the most
expensive on a dollar-per-gallon basis.
The SPSF membranes
available today are definitely better than those marketed in years past, but
they have not matched the performance or CTA costs. There is still some concern
regarding the ability of SPSF membranes to handle non-softened feedwater. SPSF membranes are the best value for softened
feed with a high pH level or where nitrate levels are a primary concern. CTA or
TLC membranes are probably best in most other cases.
FLOW RESTRICTORS
Next to the membrane, the
flow restrictor is the most critical component. It controls the recovery of the
system (i.e., the amount of water produced compared to the water put to drain).
The flow restrictor must be designed to allow enough feed flow (crossflow) across the membrane surface to keep the membrane
clean. If the flow restrictor is undersized, the crossflow
will be too low and the membrane will foul or plug due to chemical
precipitation, particle agglomeration or a combination of the two. Because pure
water is continuously pulled away from the feed stream, contaminant
concentration increases across the length of the membrane element. When the crossflow velocity is too low, the contaminants are not
flushed away from the membrane surface.
If you want to get more
volume from an RO system, simply installing a membrane with a higher flow rate
may not be the answer. You may get higher flow rates initially, but the
membrane will plug prematurely because too much of the feedwater
is being sent through the membrane, and not along the
length of the element to provide the crossflow
cleaning action.
However, because feedwater temperature, pressure and total dissolved solids
(TDS) all affect flow rates, the same system design may be able to accommodate
a different membrane's flow-rate requirements at different sites. Typical
system design assumes 77°F (25°C), 60 psig (4.2 bars) and 500 parts per million
(ppm) TDS, and the flow restrictor is sized according
to the anticipated permeate rate at these conditions.
If you have a lower
temperature and pressure or significantly higher TDS, you can use a higher-flow
membrane without altering the basic system design. Or, you could install a
lower volume restrictor and prevent money from going down the drain. Graphs 1
and 2 on page 38 show the relative effects of temperature and pressure on
membrane flux.
The most
important thing to do when changing the flow restrictor or using a membrane
with a different flow rate than specified is to calculate the recovery. Recovery = [permeate flow ¸ (concentrate + permeate flow)] x
100 percent Membrane manufacturers recommend that your recovery rate be 15
percent or less for maximum life Most home RO systems are designed at 25
percent or more. You are generally safe if you stay between these numbers and
have good pretreatment.
SEDIMENT PREFILTER
Regardless of your crossflow velocity and/or recovery rate, your membrane will
plug if particles are not sufficiently removed from the feedwater.
A 5-micron nominal cartridge will typically remove enough particulate, but it
may not be sufficient when there is a high-silt content. Most "silt"
is less than 5 microns in size, and it will clump together and plug the
membrane.
If the membrane plugs
prematurely the prefilter, not the membrane, may be
to blame. A filter with a lower micron rating is recommended in cases of
high-silt levels, because the additional cost is minimal and the increased
pressure drop is almost indistinguishable at typical home RO feed-flow rates.
Regardless of the silt
content, the added cost of a high-quality 5-micron filter may be justified by
the extra protection it would provide, especially compared to the greater
expense of supplying a new RO membrane.
CARBON FILTERS
If you are using TLC
membranes, you need to use a carbon prefilter to
remove oxidizing agents, even on nonchlorinated well
water. In addition to chlorine, other chemicals-- including bromine, iodine and
various iron complexes-- will cause salt passage through a TLC membrane to rise very rapidly (graph 3). Therefore, always use a carbon prefilter and use one that lasts. Again, the cost of a
carbon filter is much lower than a TLC membrane element, so it pays to include
a carbon filter.
Most systems also use a
carbon filter downstream right before the tap. These filters remove the bad
taste associated with chlorine (in CTA and SPSF systems) and absorb some of the
more harmful organic chemicals that may still be present. Larger organics tend
to foul the surface of the carbon media, allowing the smaller, often more
harmful organics to pass. The RO membrane will reject the larger organic
chemicals and other fouling contaminants, allowing the postcarbon
filter to effectively remove the remaining, smaller organic contaminants by
adsorption.
On feedwaters
with organics suspected of being carcinogenic or otherwise harmful, the
post-carbon filter is crucial to meeting the consumer's needs and should always
be included in the design.
STORAGE TANKS
The importance of the
storage tank is often overlooked, but it is vital to the proper design of the
RO system. The tank must be large enough to meet the "on-demand"
requirements, but if it is too large (and of a bladder tank design), the
backpressure created when it is nearly full significantly reduces the rejection
and efficiency of the system. Lower quality water is produced and a higher
percentage of water goes to drain. For these reasons, a larger storage tank for
greater on-demand capacity should not be added without careful consideration.
THE FINAL
RESULT--SATISFACTION
Home RO system design must
be varied according to the characteristics of the water being treated. A
thorough understanding of critical components is required to handle
"problem water" and to provide the best system to meet your
customers' needs. Although other components may be important to the creation of
a functioning, attractive system, they would not necessarily need to be
modified for specific installations.
Often, on-site component
modification or an application-specific system selection will allow a home RO
system to work effectively on any tap-water source. Knowing how to apply the
appropriate system design will allow you to increase the size of the RO market
by making RO work where it did not before. It will also increase your share of
the market by providing technologically superior service, and decrease the
costs of handling customer complaints and returns by solving problems quickly--
or by preventing them from happening in the first place.
MEMBRANE TYPES
|
CA/CTA |
Thin Film/TLC |
Polysulfone |
|
Advantages: |
Advantages: |
Advantages: |
|
Chlorine tolerant |
High rejection High flow |
Chlorine tolerant |
|
Low price |
pH tolerant |
pH tolerant |
|
Disadvantages: |
Disadvantages: |
Disadvantages: |
|
pH intolerant |
Chlorine intolerant |
Low rejection |
|
Medium rejection |
High price |
Hardness intolerant |
|
Low flow |
|
Low flow |
|
Applications: |
Applications: |
Applications: |
|
Municipal water |
Low temperature water |
Softened water with high pH or
nitrates |
|
Chlorinated wells |
High nitrate water |
|
There are three types of RO
membranes currently available. Due to superior flow and
rejection. TLC membranes are the most common. Because of their chlorine
tolerance, CTA membranes are also abundant. Polysulfone
membranes are beginning to gain some market share.