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Contaminants removed from water by reverse osmosis

Reverse osmosis (RO) systems frequently are used to reduce the levels of total dissolved solids and suspended particles within water. These systems remove a variety of ions and metals as well as certain organic, inorganic and bacterial contaminants. Some contaminants treated effectively by RO are listed in Table I. This table is not an exhaustive list of contaminants that RO may remove, but rather lists those for which RO can be a practical treatment method for treating household drinking water. Most RO systems also include activated carbon (AC) filters and the carbon provides the treatment for some contaminants, as noted in the table. The RO membrane alone may not be an effective method for total removal of these contaminants, but a properly designed system may be effective in reducing these contaminants to safe levels. Contaminant removal by the system may vary depending on operating conditions and equipment. Refer to the equipment section of this guide for further explanation of activated carbon filters combined with RO.

Reverse osmosis can remove microorganisms. However, it is not recommended for that use (i.e., only coliform-free water should be fed to the system) because membrane deterioration can occur due to the bacteria, and contamination may occur through pinhole leaks. Typically, RO treatment systems have a few components:

1. Prefilter - Typically a particle filter to remove particles, rust, and other debris.
2. Possibly a water softener is the total hardness is elevated and a smaller diameter particle filter.
3. RO treatment unit with treated water storage tank and a flush to waste line.
4. Most treatment activated carbon filter.
5. Post-treatment submicron filter.

Table I. Contaminants removed by dialysis center reverse osmosis units.

Ions and Metals Arsenic, Aluminum, Barium, Cadmium, Calcium, Chloride, Chlorine1, Chromium, Copper, Fluoride, Iron, Lead, Magnesium, Manganese, Mercury, Nitrate, Potassium, Radium, Radon1, Selenium, Silver, Sodium, Sulfate, Zinc
Organic Chemicals Benzene1, Carbon tetrachloride1, Dichlorobenzene1, Toluene1, Trichloroethylene1, Total Trihalomethanes (THM's)
Particles Asbestos, Protozoan cysts, Cryptosporidium
Pesticides 1,2,4-trichlorobenzene1, 2,4-D1, Atrazine1, Endrin, Heptachlor, Lindane, Pentachlorophenol

Activated carbon filters, commonly included in RO systems, can provide treatment for these contaminants.

They do not effectively remove some organic compounds. Mechanical filters and activated carbon filters are almost always used with a RO unit. First, the mechanical filter (1) removes dirt, sediment and other impurities, that would otherwise clog the reverse osmosis membrane. The RO (2) unit is installed next. The water is held in a pressurized water storage container (3). An activated carbon filter (4) then removes organic compounds which pass though the RO unit. A separate water tap (5) is used for this water.

Reverse Osmosis uses large amounts of water. Typically, about 75% or more of the water is discarded with the contaminant.

Contaminants not removed from water by reverse osmosis

There are some contaminants not removed from water by RO systems. These include dissolved gases such as hydrogen sulfide, a common nuisance contaminant with characteristic rotten egg odor, which passes through the RO membrane. The RO membrane's efficiency in reducing the amount of contaminant in the water depends on the contaminant concentration, chemical properties of the contaminant, the membrane type and condition, and operating conditions.