Michael Kamrin, Nancy Hayden Center for Environmental Toxicology Barry Christian, Dan Bennack Community Assistance Program in Environmental Toxicology Frank D'Itri Institute of Water Research Reverse Osmosis (RO) is becoming a common 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 water quality problems in the home. RO can reduce the amounts of organics, inorganics, bacteria and particulates that can be found in contaminated drinking water. Because the efficiency of removal of various contaminants can vary, homeowners should evaluate this 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 your local health department or a reputable laboratory. A water analysis will not only verify whether a water quality problem exists, but it is also essential to determine the most appropriate solution to the problem. State or local health officials can interpret water analysis results. Some laboratories may also provide this service. Note that home water treatment is considered only a temporary solution. The best solutions to a contaminated drinking water problem are to either 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. To make the process work, pressure is applied to the contaminated water, forcing water through the membrane. Since contaminants do not move with the water as it moves across the membrane, purer water collects on the other side of the membrane. The purified water that accumulates on one side of the membrane can then be used or stored.

A specific amount of pressure is necessary to separate purified water and contaminants. This required pressure is based on the type and concentration 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 even more pressure to the contaminated water than is required provides better separation and a higher production rate. The levels of most dissolved compounds and suspended matter present in water can be reduced by RO treatment. However, not all compounds can be efficiently removed by this process. The efficiency with which membranes reject the contaminant molecules depends on the pollutant concentration and chemical properties of the pollutant. Membrane type and operating conditions will also affect the degree of pollutant removal. Table 1 lists some potential water contaminants and their typical ranges of rejection with an RO system. Efficiency of removal is often described using the term "rejection percentage," which is the percent of a particular contaminant that doesn't cross the membrane, i.e., is 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 milligrams per liter (mg/l) - an unrealistically high level - 90 percent removal would still leave 10 percent of the nitrate in the purified water, or 11 mg/l. This is greater than the 10 mg/l maximum contaminant level for nitrate allowed in drinking water supplies. It is important to know not only rejection percentages, but also incoming pollutant concentrations to effectively reduce contaminant concentrations in the drinking water to safe levels. RO SYSTEMS BASIC COMPONENTS of an RO system should include a prefilter to remove fouling agents such as rust and lime; an RO module containing the membrane; an activated carbon postfilter to remove residual taste, odor and some compounds from the purified water; a storage tank; and various valves, including a shut-off valve that stops the water