REVERSE OSMOSIS is a unique water treatment technique that uses a water permeable membrane to remove ions from drinking water. It is called reverse osmosis because the pressure applied to the untreated water counteracts the osmotic pressure and forces drinking water through the membrane, leaving behind the undesirable ions in the wastewater.

Reverse osmosis (RO) systems are effective in a wide range of water quality levels and can achieve certification under ANSI/NSF Standard 58 for a variety of contaminants, as shown in the table:

Contaminant	ANSI/NSF Challenge Level (mg/L)	Maximum Allowable Allowable Effluent (mg/L)
Arsenic (V)*	0.30	0.025
Barium	10.0	2.0
Cadmium	0.03	0.005
Chromium**	0.30	0.1
Copper	3.0	1.3
Fluoride	8.0	1.5
Lead	0.15	0.010
Mercury	0.006	0.002
Nitrate/nitrite	30.0 / 3.0	10.0 / 1.0
Selenium	0.10	0.05
Radium 226/228	25 pCi/L	5 pCi/L
Total dissolved solids (TDS)	750	187
Turbidity	11 NTU	0.5 NTU
Asbestos	107 fibers/L	99%
Cysts	5 x 104 cysts/mL	99%
*Only on chlorinated waters     **Hexavalent and trivalent

A typical point-of-use RO system like the one below has a carbon pre-filter to remove chlorine that can attack some types of membranes. The water then flows through the RO membrane and into a drinking water storage tank. Typically, the water goes through a post-filter on its way to the faucet.

This "polishing" filter for the RO unit can also be certified by NSF when it removes contaminants such as volatile organic compounds (VOCs). A shut-off valve is an integral part of the system that helps reduce the amount of water directed to waste. RO systems produce excellent drinking water, but quality comes at a cost. A typical under-counter RO system will send 8 gallons of water to waste for every gallon of treated water. This wasted water can be minimized by the shut-off valve that sends water to waste only while the RO system is actually producing treated water.

Two measurements in ANSI/ NSF Standard 58 compare the amount of water produced to the amount sent to waste: percent recovery and percent efficiency.

Percent recovery measures the highest efficiency that the RO system can achieve. This measurement is taken with no backpressure on the membrane and is not the best indicator of the system's efficiency.

Percent efficiency is calculated for RO systems with storage tanks and shut-off valves. Percent efficiency, a more realistic indicator, expresses the amount of clean water produced as a fraction of the total water used. Typical efficiencies for a point-of-use RO system range from 8 to15 percent.

Reverse osmosis systems are sensitive to the quality of the untreated water. Particulates, iron, and very hard water can foul the membrane and reduce water production rate and quality.

In many RO installations, the pre-filter can remove particulate, but a softener should be used if the hardness is above 10 grains per gallon. RO systems should never be used on microbiologically unsafe water.

Treated water production rate and quality are directly affected by the water pressure applied to the system. Systems certified for nitrate or nitrite reduction should not be used with less than 40 psi inlet pressure. If the RO system is installed in a house with an onsite well water system, the minimum pump turn on pressure should not be below 40 psi.

In some homes, an installed RO system may be large enough to serve all faucets. Because the water from these systems is typically very low in dissolved solids, corrosion issues can become a problem. Multiple faucet RO systems should use tubing and faucets that are designed to carry water containing low dissolved solids.

Reverse osmosis systems can solve many water quality problems, but they must be maintained and installed properly. NSF-Certified systems are evaluated for water contact material safety, structural integrity, claims verification, and literature content.

For more information about reverse osmosis or other DWTU systems, contact Shannon Murphy at 1-800-NSF-MARK.