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WQ-14

3/91

Cooperative Extension Service,
Purdue University

Reverse osmosis (RO) is becoming a common home treatment method for contaminated drinking water. RO is probably best known for its use in desalination projects (turning seawater into drinking water). However, it is also effective for treating home water quality problems.

RO reduces the amounts of organics, inorganics, bacteria and particles found in contaminated drinking water. The efficiency of removal of various contaminants varies and should be evaluated when considering using RO for home treatment. Other home treatment methods may be better for a specific contaminant .

The first step toward solving a suspected water quality problem is to have your water analyzed by the local health department or a reputable laboratory. A water analysis not only verifies whether a water quality problem exists, but is also essential to determine the most appropriate solution to the problem. State or local health officials can interpret water analysis results. Some laboratories also provide this service.

Home water treatment should be considered only a temporary solution. The best solutions to a contaminated drinking water problem are to end the practices causing the contamination or change water sources.

Reverse Osmosis

RO is based on the process of osmosis. Osmosis involves the selective movement of water from one side of a membrane (a plastic film that looks similar to cellophane) to the other. Pressure forces contaminated water through the membrane. Since contaminants do not move with the water across the membrane, purer water collects on the other side of the membrane. The treated water collects on one side of the membrane and then can be used or stored.

A specific amount of pressure is necessary to separate the contaminants from the water. The pressure required depends on the type and level of contaminants in the water. For example, producing purified water from sea water requires more than 10 times the applied pressure than regular tap water requires. Supplying more pressure than required provides better separation and a higher production rate. Figure 1 shows how reverse osmosis works.

Figure 1. How the reverse Osmosis process works.

RO treatment reduces the levels of most dissolved compounds and suspended matter present in water. However, the process does not efficiently remove all compounds. The efficiency of the membrane to reject the contaminant molecules depends on the pollutant concentration and chemical properties of the pollutant. Membrane type and operating conditions also affect the degree of pollutant removal. Table 1 lists some potential water contaminants and their typical ranges of rejection with an RO system.

The term `rejection percentage' describes a membrane's efficiency of removal. Rejection percentage is the percent of a particular contaminant not crossing the membrane, i.e., rejected by the membrane. However, rejection percentages do not tell the whole story. For example, the rejection percentage for nitrate can be as high as 90 percent with some systems, indicating the membrane is highly efficient in rejecting nitrate. However, for an incoming nitrate concentration of 110 mg/l (milligrams per liter) - an unrealistically high level - 90 percent removal would still leave 10 percent of the nitrate in the treated water, or 11 mg/l. This is greater than the 10 mg/l maximum contaminant level for nitrate allowed in drinking water supplies. You should know the rejection percentage and incoming pollutant concentrations to effectively reduce contaminant concentrations in the drinking water to safe levels.

RO Systems

Basic parts of an RO system should include a prefilter to remove fouling agents such as rust and hardness; an RO module containing the membrane; an activated carbon postfilter to remove residual taste, odor and some organic compounds from the treated water; a storage tank; and various valves, including a shut-off valve to stop the water flow when the storage tank is full. The system must also provide for waste flow to drains. Prefilters containing activated carbon are commonly used to protect chlorine-sensitive membranes. All of these components can be purchased from the dealer.

Other useful features available include an automatic membrane flush which periodically cleans the membrane, pressure gauges to help determine how effectively the unit is operating and a sanitizing procedure to kill any bacteria present in the system.

A high conversion rate - the percentage of treated water obtained from incoming water - is another important consideration. Conversion rates can be quite variable and range from as low as 10 percent up to 50 percent.

RO systems are typically installed as point-of-use (POU) devices. This means they are generally placed at a tap, usually in the kitchen. RO systems come in countertop or under-the-sink models. A separate faucet is often installed to bypass the RO unit so treated water is used only for cooking and drinking. A bypass increases the life of prefilters, postfilters and the membrane, making treatment more economical. RO systems vary in capacity; however, a common range is 3 to 10 gallons per day.

Membrane selection, an important characteristic of RO systems affecting performance, is based on various water characteristics such as acidity, hardness, total dissolved solids and chlorine content. A water analysis provides information on your water's characteristics.

Water pressure influences an RO system's performance. The higher the water pressure, the better the rejection of pollutants and the more purified the water. Typical water pressure in most homes may be adequate for RO treatment. Booster pumps can be added if it is not. Doubling the net pressure across a membrane more than doubles the output flow rate of treated water.

Keep in mind home water pressures vary significantly, which changes the efficiency of the unit. In addition, water production and rejection rates in some units decrease as the storage tank fills. As the tank fills pressure increases on the treated water side of the membrane. Consequently, for maximum benefit, the tank should be emptied daily and accurately sized to fit the water use rates.

Water temperature will also influence the production rate of treated water. The higher the water temperature, the better the production rate. A drop in temperature from 75 degrees to 45 degrees cuts the production of treated water virtually in half.

Operation, Maintenance and Cost

To continually perform well, RO systems, like all other home water treatment devices, require regular maintenance and replacement of various parts. Prefilters and postfilters need replacement on a regular basis. The length of time between changing prefilters depends on the water quality, especially the concentration of solids. The contaminant concentration, membrane rejection percentages and efficiency of activated carbon removal determine when postfilters should be replaced. RO membranes should typically last for one to three years, depending on operating conditions, membrane type and prefilter performance.

Unfortunately, it is difficult to know when to replace the various parts of an RO system. This is a major disadvantage of any home treatment system. A system's performance can only be determined by chemical testing. However, a professional with access to your water analysis and your water use rate can usually determine when filters and membranes need replacement, if contaminant concentrations have not increased.

A major disadvantage of a RO system is the large amount of contaminated waste water generated. The amount can be as much as 50 to 90 percent of the incoming water. The amount depends largely on the pressure difference across the membrane. The larger the pressure difference, the smaller the wastage rate.

The cost of RO systems ranges from $300 to $900 - expensive compared to some other treatment systems. Membrane replacement costs range from $70 to $140. Prefilter and postfilter costs vary, with an average price of about $20. (NOTE: Dollar values are provided as a rough guide to compare costs of different systems. Current prices are likely to be higher than those quoted.)

Certification and Validation

Independent testing laboratories, such as the National Sanitation Foundation (NSF), provide certification of treatment products. NSF test results give a good measure of the effectiveness of devices designed to treat water for both esthetic and health reasons. The Water Quality Association (WQA), is a self-governing body of manufacturers and distributors. WQA offers voluntary validation programs to its members. Validation is less stringent than certification. Certification or validation will not ensure effective treatment; all systems must be designed for each particular situation and maintained properly.

Summary

RO is a home water treatment process which effectively treats various contaminants from all major classes of drinking water pollutants, such as organic chemicals, inorganics, bacteria and particulates. If a water quality problem from several different contaminants exists, RO may be the most cost-effective removal method.

To determine the appropriateness and optimal operation of an RO system, your water should be analyzed and water use rate measured. As with all home treatment systems, it takes regular maintenance to ensure the quality of the treated water.

For Further Information

For further information on water quality contact your county Cooperative Extension office or local health department. The following bulletins in the WQ series may also be helpful:

WQ 1 "Water Testing Laboratories"

WQ 2 "What Is Ground Water?"

WQ 3 "How to Take a Water Sample"

WQ 4 "Why Test Your Water?"

WQ 5 "Interpreting Water Test Results Part One: Inorganic Materials"

WQ 6 "Buying Home Water Equipment"

WQ 7 "Animal Agriculture's Effect on Water Quality Pastures and Feedlots"

WQ 8 "Animal Agriculture's Effect on Water Quality Waste Storage"

WQ 9 "Water Quality for Animals"

WQ 10 "Wetlands and Water Quality"

WQ 11 "Sulphur Water Control"

WQ 12 "Distillation For Home Water Treatment"

References

Kamrin, Michael, Nancy Hayden, Barry Christian, Dan Bennack and Frank D'Itri, WQ-24 ``Reverse Osmosis for Home Treatment Of Drinking Water,'' Cooperative Extension Service, Michigan State University, 1990.

*Reviewed and revised by Adel Pfeil, Extension Specialist, Department of Consumer Sciences and Retailing.

Editor: Cheri L. Janssen, Department of Agronomy

Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an affirmative action/equal opportunity institution.