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Reverse Osmosis Treatment of Drinking Water Supplies

Effective Against: 

  • inorganic contaminants such as: dissolved salts of sodium, dissolved (ferrous) iron, nitrate, lead, fluoride, sulfate, potassium, manganese, aluminum, silice, chloride, total dissolved solids, chromium, and orthophosphate
  • some detergents
  • tastes
  • color and odor-producing chemicals
  • certain organic contaminants
  • some pesticides

Not Effective Against: 

  • dissolved gases
  • most volatile and semi-volatile organic contaminants including some pesticides and solvents
  • Alone, reverse osmosis units are not recommended for treatment of bacteria and other microscopic organisms.

How Reverse Osmosis Works

A complete reverse osmosis (RO) system consists of an RO module, a storage tank, and a separate faucet. The module contains a semi-permeable membrane that allows water to pass through and collect in the storage tank. The contaminants being treated by the RO unit are rejected and then washed off the membrane into a waste stream.

It is not practical to treat all water entering a home with an RO system because about 75 percent of the water introduced is wasted. Thus, four gallons of raw water into the system produce about one gallon of treated water. This treated water comes out much slower than water from a regular tap, so a tank is used to store the treated water. Each manufacturer’s RO units differ, but the time needed to produce one gallon of water ranges from 2-7 hours. Therefore, the treated water is often used only for drinking and cooking.

The line pressure of the water system in the home usually supplies the pressure for RO. To reduce membrane fouling, a sediment pre-filter and an activated carbon pre-filter can be installed. A post-filter might be included to complete treatment. The pre-filter removes sand, silt, and sediments, while the activated carbon filter removes the organic materials and any dissolved gases not treated by the RO membrane.

Types of Membranes

Several kinds of reverse osmosis membranes are available. Each has certain advantages and disadvantages. Certain membranes are used to remove certain contaminants. It is important to know what contaminant(s) you are trying to remove before buying a treatment unit.

Check with the manufacturer to see exactly what the RO unit will remove, as effectiveness depends on the membrane in the treatment unit, the water pressure, and the contaminant present and its concentration. Most manufacturers specify the efficiency of an RO membrane by its salt rejection percentage. The higher the salt rejection percentage, the better the membrane. When comparing RO units, be wary of exaggerated claims. RO membranes cannot be certified to remove 100 percent of anything. In addition, it is important to verify that the treatment system you are purchasing has been tested and certified by a third party to ensure manufacturer’s claims. See the section on Product Certification at the end of this fact sheet.


Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless maintained in accordance with the manufacturer’s recommendations for maintenance, cleaning, and part replacement. Keep a logbook to record equipment maintenance and repairs.

The RO water treatment unit is often more costly than simpler treatment methods. For example, using an ion exchange unit to remove dissolved (ferrous) iron may cost less than an RO unit. RO has low water recovery and recycling the wastewater in home systems is not cost effective. The device might require continuous operation to maintain peak membrane performance. Once the storage tank is filled in some systems, surplus water is discarded and water loss from the unit is frequently excessive. Clogged or torn RO membranes require replacement. A reverse osmosis system that uses pre- and postdevice treatment units adds to your purchase and maintenance costs. High levels of particulate matter in the water (like iron particles) can foul or clog the membrane, decreasing water flow.

Other Considerations

Ensure the system you choose is installed and operated according to the manufacturer’s instructions. After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory to ensure it is working properly and removing the contaminants. You should continue to test the quality of both the untreated and treated water annually or more frequently (quarterly or semi-annually) if high levels of contaminants are present in the raw water. Frequent testing will also help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary.

The efficiency of a reverse osmosis treatment unit can be affected by the presence of turbidity, iron, manganese, hydrogen sulfide, and chlorine in water. Because RO devices are “point-of-use” units, it may be necessary for a “point-of-entry” device to be installed to treat these problems if they exist.

RO devices produce what is called “reject water.” This is water that is not treated by the membrane and stays on the raw-water side of the unit. Reject water is discharged to a sewer, dry well, or septic system, and can be a concern because it contains a higher level of contaminants then the original water entering the tank.

Questions to Ask Before You Buy

Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present. This will help you determine if reverse osmosis is an effective treatment method for your situation. See the fact sheet Questions to Ask When Purchasing Water Treatment Equipment for more information.

Consumers should inquire about the following before purchasing a reverse osmosis system:

  • Has the treatment system been tested and certified by a third party to ensure that it meets manufacturer’s claims?
  • How much treated water will the unit produce per day?
  • How much water does the unit reject?
  • What kind of maintenance is involved?
  • How often will the membranes need to be replaced? What will this cost? Can the homeowner change the membranes or does a factory certified person need to do this?
  • What pre- or post-treatment devices are needed in conjunction with the RO system?
  • Are there any special installation requirements that may add to the equipment cost, for instance changes to your household plumbing?

Product Certification

NSF International is a non-profit organization that sets performance standards for water treatment devices. Because companies can make unsubstantiated statements regarding product effectiveness, the consumer must evaluate test results of the device to determine if claims are realistic. Products that have been tested or evaluated by NSF and meet their minimum requirements are entitled to display the NSF listing mark on the products and in advertising literature for products. Manufacturers and models that meet NSF’s standard are included in a listing published twice a year. For more information contact NSF at: 800-NSF-MARK (800-673-6275) or


Center for Agriculture Food and the Environment

This fact sheet is one in a series on drinking water wells, testing, protection, common contaminants, and home water treatment methods available on-line
and Cape Cod Cooperative Extension: 508-375-6699

MA Dept. of Environmental Protection, Division of Environmental Analysis
Offers assistance, information on testing and state certified laboratories: 617-292-5770
For a listing of MassDEP certified private laboratories in Massachusetts

U.S. Environmental Protection Agency, New England Office
Information and Education on where drinking water comes from; drinking water testing and national laws; and how to prevent contamination

US Environmental Protection Agency

For a complete list of primary and secondary drinking water standards

MA Department of Conservation and Recreation, Division of Water Supply Protection
Maintains listing of registered well drillers, information on well location and construction: 617-626-1409, 

NSF International

The NSF International has tested and certified treatment systems since 1965. For information on water treatment systems: 800-NSF-MARK

Water Quality Association

The Water Quality Association is a not-for-profit international trade association representing the household, commercial, industrial, and small community water treatment industry.

Last Updated: 
June 2007