pressure is used to force water molecules through a membrane that has extremely tiny
pores, leaving the larger contaminants behind. Purified water is collected from the "clean" side of the
membrane, and water containing the concentrated contaminants is flushed down the
drain from the "contaminated" side. The average RO system has several
components consisting of:
sediment/chlorine pre filter
- the reverse-osmosis membrane
- a water storage tank
- an activated-carbon post filter
They cost from about $150 to over $1,500
for point of use systems.
The advantages of Reverse Osmosis include:
Reverse osmosis significantly reduces salts, most other inorganic material
present in the water, and some organic compounds.
With a high quality activated carbon filter to remove any organic materials that gets through the
filter, the purity of the treated water approaches that produced by distillation.
||Microscopic parasites (including viruses)
are usually removed by properly functioning RO units, but
any defect in the membrane would allow these organisms to flow undetected into the
"filtered" water - they are not recommended for use on
biologically unsafe water.
slower than a carbon or sediment water filter, RO systems can typically purify more water per day
than distillers and are less expensive to operate and maintain.
||Reverse Osmosis systems also do not use
electricity. However, because they do require relatively high water pressure to
operate, they may not work well in some emergency situations.
The disadvantages of Reverse Osmosis include:
of Use RO units make only 12 - 24 gallons of treated water a day for drinking or cooking
- which is ok for most homes since the treated water is stored
in a tank for use.
waste water. Two to four gallons of "waste" water are flushed down the drain for
each gallon of filtered water produced.
||Some pesticides, solvents and other volatile organic chemicals
(VOCs) are not completely removed by RO.
A high quality activated carbon post filter is
recommended to reduce these contaminants.
||Many conditions affect the RO membrane's efficiency in reducing the amount of contaminant in the water.
These include the contaminant concentration, chemical properties of the contaminants, the membrane type and condition, and operating conditions (like pH, water
temperature, and water pressure).
||Although RO filters do not use electricity, they depend on a relatively high water pressure to force the
water molecules through the membrane. An electric booster pump can
be used to increase water pressure if needed. In an emergency situation where water pressure has been lost, these systems
will not function.
* However, if a high quality activated carbon
filter is used for the post filter, it could be disconnected and
used to siphon water through in an emergency to reduce
||RO systems require maintenance. The pre and post
filters and the reverse osmosis membranes must be
changed according to the manufacturer's recommendation,
and the storage tank must be cleaned periodically.
||Damaged membranes are not easily detected, so it is hard to tell if the system is functioning normally and safely.
You might want to check NSF
International to see if the Reverse Osmosis system you are
interested in purchasing is certified.
||A reverse-osmosis system is a good treatment
option for people who have unacceptably high levels of dissolved inorganic
contaminants in their drinking water which can not be removed effectively or
economically by other methods. Water from shallow wells in agricultural areas
that contains high nitrate levels is a good example of a situation where RO would make sense. Most people, however, who are using municipal water water that is subject to
EPA regulations usually have acceptably low levels of nitrates. Maximum nitrate
levels should be able to be determined by requesting a water quality report from
your local water provider.
I have received a number of questions asking if
distilled water is either bad for a person's health or
beneficial for health relative to purified water that still contains
ions (usually calcium and magnesium). This question pertains
equally well to water from a RO system, because most ions have been
removed by the treatment. Click here
to view my
response to that question.