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:

1. a sediment filter
2. the reverse-osmosis membrane
3. a storage tank
4. an activated carbon filter

Putting on the squeeze
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.

Which membrane?
Membranes don’t 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.

• Cellulose membranes produce water more slowly, but cost about half as much and hold up well in chlorinated water.

Recommendations
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?