|
Once you have decided that you would like to purchase an
RO system, you will need to learn and understand some new terms and concepts in order to compare various systems
that you may be offered. You should also ask your sales person the following questions-
Will the unit remove the contaminants I want to remove?
RO's are not effective for removing-
- most volatile organic compounds(VOC) and other low weight
molecular compounds
- dissolved gases
- chloramines
The person selling you the RO should be able to provide you
with the percent rejection of his RO for the particular substance(s) you want removed. The ability of a RO to remove
a particular substance is measured in percent rejection.
Example-
If the water starts with 100 mg/l(milligrams/liter) of salt and the rated rejection for salt is 98%, then the treated
water will have 2% of the original amount or
100mg/l - .98 X 100mg/l = 2 mg/l of salt
If the salt content of the untreated water goes above 100mg/l,
then, the treated water salt content will go up. This is very important to understand when dealing with health
related contaminants. You should always allow for a possible increase in the contaminant.
Is the RO NSF rated for the substance you are trying to
remove?
The National Sanitation Foundation(NSF) International is
an internationally accredited, independent, third party certifier which validates product performance claims. It
is not only important, that the RO is NSF rated, but is it RATED FOR THE CONTAMINANTS YOU ARE REMOVING. There should
be an NSF label on the unit that states the tested contaminants.
What is the rated output of the unit and what can I expect
for the actual output in GPD?
You should have a unit that will produce 1 gallon per day(GPD)
for each person in the house. A family of four should have a unit that can produce at least 4 gallons per day.
Please see rated and actual output.
How much storage does the unit have?
RO units produce water very slowly and therefore need to
store water so that it is available when you need it. Try to decide what the greatest single draw you will want
to have and don't get a storage tank with a capacity any larger than this. The tank should be as small as possible
and provide you with your needs because this will help reduce TDS
creep. The tank should be rated in "draw down"
capacity not the actual volume of the tank.
Do I need a Booster pump?
If your pump is in the basement, you will definitely need
a booster pump because these types of pumps can not produce enough pressure to operate the RO efficiently. You
will also need a booster pump if you plan to have the water delivered to a tap on a floor above where the RO is
installed.
Do I need a smart faucet?
A smart faucet is a faucet that is connected to a TDS monitor.
TDS stands for total dissolved solids. These dissolved materials are what the RO is removing from the water (see
"What is.. Reverse Osmosis"). All of these smart faucets have a light indicating the quality
of the water the RO is producing. We insist on installing these on all systems installed to treat health related
parameters like arsenic, gross alpha and nitrates.
Will I know when to change the pre and post filters ?
All reverse osmosis systems require pre and post filter changes.
Many system leave this up to the homeowner to remember. These cartridges do have a life expectancy. NSF rated units
must have the cartridges changed based on gallons of water used to maintain their rated protection. Some units
provide an indicator that lets the customer know how much useful capacity is left and even shuts the water off
from the RO if the capacity is exceeded. This provides an added level of safety and security to the system. Kinetico
systems use both capacity indicators (purometer) and shutdown (macguard filters) features.
Rated and Actual Output-
The rated production stated in all manufacture's literature
is only true under very specific conditions. These conditions do not exist in the field and should only be used
for comparison of one unit to another not for the actual output of an installed unit. There are two things that
will affect the rated production; one is net pressure exerted on the membrane, and the other is the temperature
of the water.
The net pressure is the pressure exerted by your pump minus
the back pressure of a storage tank and the osmotic
pressure. Most of the under-the-counter units are
rated at 60 psi net pressure. Most storage tanks will range in pressure from 7 psi empty to 40 psi full. This means
the pressure on the membrane can be reduced up to 40 psi. because of the storage tank. If the total dissolved solids(TDS)
of the water is high the osmotic pressure will be high. Water with a TDS of 1000 mg/l will reduces the net pressure
by an additional 10 psi.
The water temperature at which the units are rated is 77 oF. The membranes lose about 1.5% of the rated output for every 1 oF
the temperature is below 77 oF. This means that in Maine where the water temperature is approximately
48 oF
the output of the unit is decreased by about 34%. Or, in other words, if a unit has a rated capacity of 14 gallons
per day (GPD), the actual output would be reduced to just over 9 gallons per day because of the colder water.
If one takes into account typical net pressures and the colder
temperatures found in Maine, the output for an RO installed in Maine is about 50% of its rated capacity. So a 14
GPD unit will produce approximately 7GPD.
Draw Down-
You have a choice of an air pressurized or water pressurized
storage tank. All air pressurized RO storage tanks contain a bladder that holds compressed air (usually at 7 psi
when no water is in the tank). This means that some of the capacity of the tank must be used to hold this air.
For example, a 3 gallon tank may only be able to hold 2 gallons of water because the rest of the tank is holding
the air. This means that once the tank is full it will only be able to deliver 2 gallons of water. The 2 gallons
would be considered the draw down capacity of the tank. Many resellers will tell you that you are getting a 3 gallon
storage tank and not mention that its capacity is only 2 gallons.
Water pressurized storage tanks (like the Kinetico Quik-flo
Tanks) uses the pressure of your water system to deliver water to your faucet. Water pressure is delivered to the
outside of a bladder. The pressure exerted by this water forces treated water from the tank and to your faucet.
The draw down on this type of tank is the total volume of the tank. The advantages of this type of tank is -
- The full volume of the tank can be used for storage ( a
three gallon tank provides 3 gallons of storage).
- There is no back pressure exerted on the system. This means
both higher quality water and more water.
- This type of system dramatically reduces TDS creep.
- The flow delivered to the faucet is constant. The pressure
does not depend on constantly changing air pressure as the tank empties.
TDS Creep
The quality of the water a RO produces is affected by the
net pressure exerted on the membrane. The higher the net pressure the better the quality of the water. The net
pressure is the pressure your pump exerts against the membrane minus the back pressure. At a given TDS a loss in
net pressure occurs under two conditions.
- When the pressurized storage tank is filling
- When the RO turns off (zero net pressure)
Under these conditions the TDS of the product water will
increase. This increase in TDS is called TDS creep and will slowly reduce the average quality of the water.
To reduce TDS creep, you should -
- Choose a unit that flushes the RO of untreated water each
time the unit shuts off- like Kinetico's
- Choose a unit with a booster pump or non-pressurized storage tank.
- Periodically drain the storage tank.
|
