Best for: salts of sodium,
ferrous iron, fluoride, nitrate, lead
May help with: organic contaminants
Not for: high levels of hardness minerals
A reverse-osmosis or RO system resembles a sieve.† The water isn't strained in the usual
sense.† Instead, ions (charged
particles) and large molecules are excluded as water and small organic
molecules pass through.† Pressure in the
water line does the work, pushing the water against a cellophane-like plastic
sheet known as a semipermeable membrane.†
Most units specify a minimum pressure of 40 psi.
Reverse osmosis removes salt and most other inorganic
material present in the water.† For this
reason, RO lends itself to use in places where the drinking water is brackish,
and/or loaded with heavy metals, nitrate or fluoride.† RO has been used for several years in plants on the Persian Gulf
and the Florida coast to remove salt from sea water.
Reverse osmosis systems available for home use make a few
gallons a day for drinking or cooking.†
Some RO units fit on a countertop, others are installed under a sink.
Typically, an RO package consists of:
2. the reverse-osmosis membrane
3. a storage tank
4. an activated carbon filter
Putting on the
Most RO units do a good job of removing salt from
water.† The best remove about 95%. Toxic
metals such as lead are removed impressively well - 95% reduction is typical.
Calcium can hinder the working of the reverse-osmosis
membrane by clogging its pores, but most RO units can handle a small amount of
calcium (hardness).† Several
manufacturers advise against using a reverse osmosis system if the calcium
level exceeds 10 grains of hardness per gallon (gpg).† Most systems also removed organic contaminants from the water -
thanks mostly to the activated carbon filter which is part of many systems.
Down the drain
Only 10 to 25 percent of the water passing through an RO
unit is forced through the membrane.†
The rest goes down the drain.†
Some under-sink models run all the time, even when their tank is full!† Such
models waste water every day even if you aren't using them
A watched pot
Reverse osmosis is a slow process.† Many under-sink units need 3 to 6 hours to process one gallon of
drinking water.† Countertop units were
even slower, requiring 4 to 21 hours per gallon.
Membranes donít last forever.† Producing two gallons of water a day, a new membrane will be
required each year.† Sediment and
activated carbon filters which cost about $25 are replaced in most systems when
the membrane is changed. The annual upkeep on an RO system ranges from 10 to 36
cents per gallon.
Reverse osmosis membranes are made from either thin-film
composite (TFC) or cellulose triacetate (CTA).†
TFC does a faster, more efficient job, but degrades in
the presence of chlorine. TFC can be used with chlorinated water as long as its preceded by an activated carbon
filter which removes chlorine. A TFC cartridge costs $100 to $250.
A reverse-osmosis system makes sense only for people who
have unacceptably high levels of dissolved solids, lead, or other inorganic
contaminants in their drinking water.†
Can one justify wasting lots of water for the sake of a few gallons of
clean drinking water?