Reverse Osmosis for Home Treatment of Drinking Water
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REVERSE osmosis (RO) is REVERSE OSMOSIS becoming a common home treatment method for RO IS BASED ON THE process of osmosis. Osmosis contaminated drinking involves the selective movement of water from one side water. RO is probably best of a membrane (a plastic film that looks similar to known for its use in cellophane) to the other. To make the process work, desalination projects (turning pressure is applied to the contaminated water, forcing seawater into drinking water through the membrane. Since contaminants do not water). However, it is also move with the water as it moves across the membrane, effective for treating water purer water collects on the other side of the membrane. quality problems in the The purified water that accumulates on one side of the home. membrane can then be used or stored. A specific amount of pressure is necessary to separate RO can reduce the amounts of organics, inorganics, purified water and contaminants. This required pressure bacteria and particulates that can be found in is based on the type and concentration of contaminants contaminated drinking water. Because the efficiency of in the water. For example, producing purified water from removal of various contaminants can vary, homeowners sea water requires more than 10 times the applied should evaluate this when considering using RO for pressure than regular tap water requires. Supplying even home treatment. Other home treatment methods may be more pressure to the contaminated water than is better for a specific contaminant. required provides better separation and a higher production rate. Fig. 1 shows how reverse osmosis The first step toward solving a suspected water quality works. problem is to have your water analyzed by your local health department or a reputable laboratory. A water The levels of most dissolved compounds and suspended analysis will not only verify whether a water quality matter present in water can be reduced by RO problem exists, but it is also essential to determine the treatment. However, not all compounds can be most appropriate solution to the problem. State or local efficiently removed by this process. The efficiency with health officials can interpret water analysis results. Some which membranes reject the contaminant molecules laboratories may also provide this service. depends on the pollutant concentration and chemical properties of the pollutant. Membrane type 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. and operating conditions will also affect the how effectively the unit is operating; and a degree of pollutant removal. Table 1 lists sanitizing some potential water contaminants and their procedure to kill any bacteria that have typical ranges of rejection with an RO accumulated the system. system. Efficiency of removal is often described A high conversion rate - the percentage of using the term "rejection percentage," which treated water obtained from incoming is the percent of a particular contaminant that water - is another important consideration. doesn't cross the membrane, i.e., is rejected Conversion rates can be quite variable and by the membrane. However, rejection range from as low as 10 percent up to 50 percentages do not tell the whole story. For percent. example, the rejection percentage for nitrate can be as high as 90 percent with some RO systems are typically installed as systems, indicating the membrane is highly point-of-use (POU) devices. This means efficient in rejecting nitrate. However, for an they are generally placed at a tap, usually incoming nitrate concentration of 110 in the kitchen. RO systems come in milligrams per liter (mg/1) - an urealistically countertop or under-the-sink models. A high level - 90 percent removal would still separate faucet is often installed to bypass leave 10 percent of the nitrate in the purified the RO unit so that the treated water is water, or 11 mg/l. This is greater than the 10 used for cooking and drinking purposes mg/l maximum contaminant level for nitrate only. This increases the life of prefilters, allowed in drinking water supplies. It is postfilters and the membrane, thereby important to know not only rejection making treatment more economical. RO percentages, but also incoming pollutant systems vary in capacity; however, 3 to concentrations to effectively reduce 10 gallons per day is a common range. contaminant concentrations in the drinking water to safe levels. Membrane selection, an important characteristic of RO systems that affects RO SYSTEMS performance, is based on various water characteristics such as acidity, hardness, BASIC COMPONENTS of an RO system should total dissolved solids and chlorine content. include a prefilter to remove fouling agents such as rust You can get information about water and lime; an RO module containing the membrane; an characteristics from a water analysis of a particular activated carbon postfilter to remove residual taste, odor water source. and some compounds from the purified water; a storage tank; and various valves, including a shut-off valve that Another important characteristic influencing an RO stops the water flow when the storage tank is full. The system's performance is water pressure. The higher the system must also provide for waste flow to drains. water pressure, the better the rejection of pollutants and Prefilters containing activated carbon are commonly the more purified the water. Typical water pressure in used to protect chlorine-sensitive membranes. All of most homes may be adequate for RO treatment, these components can be purchased from the dealer. although booster pumps can be added if it is not. Doubling the net pressure across a membrane more than OTHER USEFUL FEATURES that are available doubles the output flow rate of purified water. Keep in include an automatic membrane flush that periodically mind that home water pressures vary cleans the membrane; pressure gauges to help determine significantly, thus changing the replaced, efficiency of the unit. In addition, water presuming that contaminant concentrations production and rejection rates in some do not units decrease as the storage tank fills, increase over time. since this increases pressure on the purified water side of the membrane. A particularly major disadvantage of RO is Consequently, for maximum benefit, the the large tank should be emptied daily and sized amount of contaminated wastewater accurately to fit the water use rates. generated. This can be as much as 50 to 90 percent of the Water temperature will also influence incoming water. the production rate of purified water. This amount depends largely on the The higher the water temperature, the pressure difference better the production rate. A drop in across the membrane. The larger the temperature from 75 degrees to 45 pressure difference, the smaller the degrees cuts the production of purified wastage rate. water virtually in half. The cost of RO systems ranges from $300 OPERATION, MAINTENANCE to $900 -expensive compared to some AND COST other treatment systems. Membrane replacement costs range from $70 to $140. TO CONTINUALLY PERFORM WELL, RO systems, Prefilter and postfilter costs vary, with an like all other home water treatment devices, require average price of about $20. regular maintenance and replacement of various components. Prefilters and postfilters need to be (NOTE: Dollar values are provided as a rough replaced on a regular basis. The length of time between guide to compare costs of different systems. Current changing prefilters will depend on the water quality, prices are likely to be higher than those quoted.) especially the concentration of solids. The contaminant concentration, membrane rejection percentages, and CERTIFICATION AND VALIDATION efficiency of activated carbon removal determine when CERTIFICATION OF TREATMENT postfilters should be replaced. RO membranes should PRODUCTS is available from independent typically last for one to three years, depending on testing laboratories, such as the National operating conditions, membrane type and prefilter Sanitation Foundation (NSF). Results from NSF tests performance. provide good measures of the effectiveness of devices designed to treat water for both esthetic and health Unfortunately, it is difficult to know when to replace the reasons. The Water Quality Association (WQA), a self- various components of an RO system. This is a major governing body of manufacturers and distributors, offers disadvantage of any home treatment system. voluntary validation programs to its member. Validation Verification of a system's performance can only be is less stringent than certification. Note that certification determined by chemical testing. However, a professional or validation will not ensure effective treatment; all who has access to your water analysis and who knows systems must be designed for each particular situation your water use rate can usually accurately and maintained properly. determine when filters and membranes need to be SUMMARY For questions about water testing, test RO IS A HOME WATER TREATMENT PROCESS interpretation and that can effectively treat various contaminants from all treatment systems, contact: major classes of drinking water pollutants, such as organic chemicals, inorganics, bacteria and particulates. Your local health department (listed under city or If a water quality problem exists because several country in the phone book). different contaminants are present, RO may be the most Michigan Department of Public Health cost-effective method for their removal. Division of Water Supply 3 423 North Logan Street To determine the appropriateness and optimal operation P.O. Box 30195 of an RO system, it is necessary to have your water Lansing Michigan 48909 analyzed and your water use rate measured. As with all (517) 335-9216 home treatment systems, it takes regular maintenance to ensure the quality of the purified water. For further information on water quality and home water treatment, consult the following This material is based upon work supported by the U.S.