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Things
a consumer should know about reverse osmosis
Reverse Osmosis or RO: Is the passing of a greater concentrate under
pressure to a lesser concentrate through a semi permeable membrane. Tap water having the
higher concentration of impurities than that of cleaner water. The pores of a membrane are
very small at around .001 thousands of a micron. To compare the human hair is around 100
microns thick. The effectiveness of a membrane is determined by how the surface of the
membrane maintains its integrity against contact with your tap water. Membrane failure can
be caused by many different reasons but to keep it simple once the membrane develops a
hole there is no saving it.
Pre filter: These range in size from 30 microns down to 1 micron. This
filter works on the larger particles in the water. The finer the pre filter the faster it
will clog up with impurities. You will want to protect the pre-carbon in a thin film
composite or TFC system from fouling by sediment. A 5 to 10 micron pre filter is best
suited for residential systems. In a cellulose tri-acetate or CTA system the sediment
filter is all that stands between the incoming tap water and your expensive membrane. It
works as the first line of defense for both types of systems. If you live in an area that
has a lot of sediment than you should change this filter more often, typically it is
sufficient to change it every 6 months.
Pre carbon: Should only be used with a TFC system it will remove the
chlorine before it comes in contact with your membrane this will extend the life of a TFC
membrane. If used on a CTA system it will cause the membrane to fail. The reason is
because a CTA needs chlorine in the water to protect it from bio fouling. This filter
should be replaced every 6 months.
Membrane: Of coarse is the key to why all RO systems work. There are two
types of residential membranes first is the CTA or cellulose tri-acetate membrane, this
membrane works well in chlorinated water but has a lesser rejection percentage than the
TFC or thin film composite membrane. The CTA also has a limitation with the pH that it can
handle it will not survive in pH above 8.5. The TFC on the other hand can survive in feed
water with pH of up to 11. However the TFC membrane cannot handle more than 1000 hours of
contact with chlorine so this is why all residential TFC systems have a carbon filter
ahead of the membrane. Many 4-stage systems are TFC systems because of the pre-carbon
filter. If you see a 3 stage system chances are you are looking at a CTA system. Always
read the label most membranes are marked TFC or CTA. You should replace a CTA membrane
every year and a TFC membrane every 2 years this will ensure optimal performance. In some
parts of the country you can get a longer interval for replacement but you will need a
total dissolved solids meter to verify when it is time for replacement.
Check valve: A very important part in the RO system. The check valve
stops water from running backwards through the membrane after the shut off cuts off the
inlet pressure. A non-working check valve can ruin your membrane in less than 1 month if
it is left unchanged. It is inexpensive insurance for your expensive membrane.
Flow control: Also very important to the survival of your membrane. A
flow control regulates how much water passes across the surface of the membrane and then
to drain. You will want to know how much water is running to drain of your new system so
that you will know if it decreases you will have to replace it. You can do this with a
small cup and wristwatch. Measure how much water runs into the cup from the drain line in
15 seconds and then multiply it by 4 to get 1 minutes worth. A typical 15-gallon per day
residential RO system uses 100 to 150 milliliters of water per minute. Mark the cup and
save it. When you service your system use the cup to check the flow to drain if the amount
of water decreases or increases by any more than half the original amount then it is time
for a new flow control.
Post carbon: This is a polishing filter that removes taste and orders. It
usually contains smaller more fine carbon than the coarser pre carbon. Most post carbon
filters are encapsulated or are in the form of a cartridge. What's inside the filter is
simply coconut shells that are burned at 2500 degrees in a furnace. This gives the water a
very pleasant taste and should be replaced every 6 months.
Encapsulated filter/membrane: This is a filter that comes sealed inside
its own filter housing. The old filter is simply removed and thrown away. The new filter
or membrane will not be handled or exposed to outside contamination so it is the best way
to ensure a sanitary filter replacement.
Storage tank: This is where the purified water
that the RO system produces is stored before use. The
reason most RO systems require a storage tank is to utilize the time when no water is
needed to make water. One draw back to this
is that once the water passes through the purifier it has no more chlorine or chlorimine
left in it to keep bacteria from colonizing in the storage tank. This is why all manufacturers of RO systems
recommend disinfecting the storage tank with bleach and then rinsing it to drain on a
regular basis. Follow the manufacturer
recommendations. RO/CONN connector systems make this a simple operation.
I
recommend sanitizing the RO system at least once a year. The best time to do this is while
changing your filters. First, shut down the system in the same manner as with a Filter
Change. Drain the system, including the tank, and then disconnect the blue tubing from the
tank shutoff valve. Put ¼ to ½ teaspoon of household bleach into this tube, using an
eyedropper for best results. Reconnect the blue tube to the shutoff valve and follow
normal startup procedures, including draining the tank. If you have an icemaker hookup
installed, be sure the ball valve in the line to the refrigerator is in the closed
position during this procedure. The air pressure of the tank should be checked at this
time with the tank empty. The standard 3.2 gallon RO tank will use 8 psi to displace all
the water inside. Use a low pressure tire gauge to check pressure and if you need to add
air use a bicycle tire pump. Check the storage tank pressure every time you change the
membrane.
Ultra Violet: Called UV this is the best way to ensure that the pure
water stored in the storage tank stays that way. In this application water comes from the
storage tank and before it goes through your faucet it passes through a high intensity
ultra violet light. This will kill all forms of bacteria that may or may not exist in the
storage tank. Think of it as an insurance system. The UV lamp should be replaced once a
year.
Shut off: All new residential systems are now required to have a full
tank shut off. This shuts off the inlet pressure to the system when the storage tank
becomes full. Older RO systems without this feature should have a full tank shut off
installed so that the system does not continue to pass water to drain even though the
storage tank has reached its capacity. This device alone will save thousands of gallons of
water a year. They do not however last forever. Shut off's should be checked once a year
to see if they are functioning this can be bone by turning off the storage tank valve and
making sure that no concentrate water is running to drain. If you see water running to
drain after 10 minutes in a steady stream you should then check your check valve to make
sure that it is functioning. If your check valve is OK then you should replace your shut
off.
Twist off filters or Bayonet filters: These are filters that can be
removed more easily than conventional filters. By simply turning the filter with your hand
it will release from the system. These replacement filters are always encapsulated witch
make for more sanitary servicing and require no tools to replace the elements in a system.
Air gap faucet: This protects your entire system from potential
contamination from a drain backup. The water running to the drain is vented at the faucet
that comes with your system. A proper air gap is a minimum of one inch above the top of
your sink. With this gap if your sink backs up and your system is shut off water will not
run back up into the drain line of your system contaminating it.
Rejection of impurities: This is what your membrane is removing from your
tap water. A new reverse osmosis system will remove about 95% to 99% of all TDS or total
dissolved solids. As your membrane gets older its rejection of impurities will fall. Once
it falls below 85 % it is time to replace the membrane.
TDS meters: TDS stands for total dissolved solids. The best way to check
if your system is operating at its peak performance is to do a water quality check with a
TDS meter. Test the tap water and then the purified water. Take note of the readings for
example tap water containing 500 ppm of TDS. A typical new TFC RO system will get this
number down to around between 5 or 9 ppm. Most of these meters give you a reading in parts
per million or ppm of total dissolved solids witch is basically everything in the water
that cannot be boiled out of it. A TDS meter can range in price from $45.00 all the way to
$500.00. You should keep your TDS meter in good condition and check it against an absolute
solution of 100 parts per million.
pH Test: This is especially a good thing to know before your
installation. Knowing the feed pH will determine the type of membrane you should use. On
high pH conditions that is any thing over 8.5 pH a thin film composite membrane or TFC
should be used. If the water is heavily chlorinated and the pH is below 8.5 then a
cellulose triacetate membrane may be used. Keep in mind that if the feed water has over
1000 ppm of total dissolved solids than a TFC should be used regardless. Keep your pH
meter in good condition by checking it against an absolute solution of pH 7.
Hard water: The hardness of your tap water will greatly affect how long
your RO systems membrane will last. If you have hard water than you should also consider
using a water softener. However if your water exceeds 1500 ppm of TDS than this will cause
your water softener to fail. And you will have to factor the cost.
Reverse osmosis systems are a great way to deal with questionable tap water quality or bad
taste that cannot be handled with simple carbon filtration. One must remember that service
to these systems is imperative to how long they will last. Following the manufacturer
recommendations on filter replacements is the best way to ensure the system you are using
is doing what it is suppose to. If you live in an area that has problem water than trial
and error will be your guide to keeping your system in optimum condition. You may
experience better performance with different types of elements or membranes.
Considerations must be made when choosing an RO system to install. Install your system
with the idea that maybe someday you may have to get to it and service it. Keep up with
the maintenance of your system, it can give you many years of service. The use of reverse
osmosis systems around the world has steadily increased over the past 10 years. When this
technology was first introduced the systems were clumsy and large much like the first
computers were. Now the technology is so advanced some states are electing to process
seawater with reverse osmosis to satisfy ever-increasing demands on traditional water
sources. Tap water testing now is beginning to be tested into the range of parts per
trillion for impurities. As the testing methods improve so must the treatment
technologies. We all have a responsibility to keep pollution out of our constantly
shrinking fresh water sources.
About the author, David F. Walling has been in the water
treatment business for 12 years and has patented products in use with reverse osmosis,
water treatment systems. He can be reached through his web site at www.roconn.com or 1-800-617-1474, fax: 602-311-1122
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Copyright © 1998 R/O CONN. All Rights Reserved
U.S. Patent # 5,660,720
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