Excerpted from NDSU publication
Contributors: Fred Bergsrud, Bruce Seelig & Russell Derickson
What impurities will reverse osmosis remove?
Reverse osmosis (RO) has become a common method for the treatment of household drinking water supplies. Effectiveness of RO units depends on initial levels of contamination and water pressure. RO treatment may be used to reduce the levels of:
- Naturally occurring substances that cause water supplies to be unhealthy or unappealing (foul tastes, smells or colors).
- Substances that have contaminated the water supply resulting in possible adverse health effects or decreased desirability.
RO systems are typically used to reduce the levels of total dissolved solids and suspended matter. The principal use of reverse osmosis in mid-western states is for the reduction of high levels of nitrate, sulfate, sodium and total dissolved solids.
RO units with carbon filters may also reduce the level of some SOCs (soluble organic compounds) like pesticides, dioxins and VOCs (volatile organic compounds like chloroform and petrochemicals). An RO unit alone may not be the best solution for these types of contaminants, but installing a custom- designed RO unit to reduce the levels of other contaminants may provide a reduction in SOCs and VOCs.
How to test your water
Before installing any water treatment system be sure to have the water tested. The test will identify the bacteria and level of minerals that are present. Interpretation of the test results will help determine whether treatment is needed and what type of system or systems to consider. The intended use of the water (drinking only, drinking and cooking, laundry, or all household uses) will also help determine the extent of treatment needed and the type of system to select.
Note: RO systems are normally used to treat only drinking and cooking water supplies so may not be preferred where larger supplies are being treated. RO systems are not appropriate for treating water supplies that are contaminated by coliform bacteria. ( Check your city’s water here )
The water test analysis and interpretation will provide information about both naturally-occurring substances and those resulting from human activity. Treatment of contaminated water supplies should be considered only as a temporary solution. The best solution is to remove the source(s) of contamination and/or obtain a new water supply.
How reverse osmosis works
Reverse osmosis is sometimes referred to as ultrafiltration because it involves the movement of water through a membrane which has microscopic openings that allow water molecules, but not larger compounds, to pass through. Some RO membranes also have an electrical charge that helps in rejecting some chemicals at the membrane surface. Proper maintenance is essential to retain effectiveness over time. Some units are equipped with automatic membrane flushing systems to clean the membrane.
What types of equipment make up an RO System?
A typical home reverse osmosis treatment system is normally located beneath the kitchen sink since it is used to treat water for drinking and cooking purposes, and each RO system consists of a pre-filter, RO membrane unit, a pressurized storage tank for the treated water, a post-filter and a separate delivery tap for the treated water supply.
1. Prefilter: The prefilter is sometimes referred to as a sediment filter. It removes small suspended particles to extend the life of the membrane. Some membrane units are damaged by chlorine and others by bacterial growth. Where chlorine is present, a carbon prefilter may also be recommended.
2. RO Membrane: Several kinds of reverse osmosis membranes are available. The most common materials are cellulose acetate or polyamide resins. Mixtures or variations of these materials are also used. Each product has certain advantages and limitations and these need to be considered carefully.
Some of the factors that should be investigated are:
- The contaminant(s) involved and their initial concentration(s).
- The water supply rate, or whether the system will deliver enough water to meet normal daily drinking and cooking requirements.
- The rejection rate, or the percentage of contaminants to be removed by the membrane.
- The water pressure required to meet the supply and rejection rates. That is, can this unit be operated on the normal operating pressure of a home water system or will a booster pump be required?
- How can the system’s performance be monitored? That is, how can leaks or other problems be detected or how is the time for servicing or replacement determined? Some systems have built-in monitors, but many do not. Conductivity meters, pressure gauges and other devices can be used to detect problems where monitors are not included. Where coliform bacteria or other special contaminants are a known or suspected problem, periodic testing is recommended.
3. Storage Tank: Most RO units supply treated water at very low rates so a storage tank of 2 to 5 gallons is used to provide a suitable supply. These units are pressurized to produce an adequate flow when the tap is open. Under sink storage requires minimum pressure to deliver water. Other locations may require increased delivery pressure which may reduce membrane performance.
4. Post-Filter: The main reason for postfiltration is to remove any undesirable taste and any residual organics from the treated water. Usually a carbon filter is used for this purpose. Where a carbon filter is used as a part of the prefiltration step, postfiltration is normally eliminated.
5. Delivery Tap: A separate delivery tap for the treated water is used so that both treated and untreated water are available.
6. Other: No special controls are required on most systems since they operate by the use of pressure-sensitive switches, check valves, or flexible bladders. Shut-off valves are important to conserve water during low use periods. Monitoring gauges or servicing lights are becoming increasingly common and assist greatly in knowing whether the system is or isn’t working.
Summary
Reverse osmosis is a proven technology that has been used successfully on a commercial basis. One of the better known uses of RO is the removal of salt from seawater. Household RO units typically deliver small amounts (2 to 10 gallons per day) of treated water and waste 3 to 20 times the amount of water treated. Reverse osmosis units remove many inorganic contaminants from household drinking water supplies. The removal effectiveness depends on the contaminant and its concentration, the membrane selected, the water pressure and proper installation. RO units require regular maintenance and monitoring to perform satisfactorily over an extended period of time. Before purchasing an RO unit or any other water treatment equipment, purchasers should test their water to be certain that treatment is needed and that the equipment being selected is appropriate to the problem requiring treatment. All costs need to be considered when comparing competitive systems and when making purchase or rental decisions.
Article sponsored by www.soundwatersolutions.com
September 9, 2008 
systems. These are typically the best-performing RO units in that they offer more stages of treatment than smaller countertop models. Also, many under-sink ROs employ booster pumps that maintain a higher water pressure, which allows the RO membrane to perform at its peak. When booster pumps are used, some run on electricity and some are powered by water pressure. All under-sink ROs require installation, and almost all come with a separate faucet that is installed on the countertop. Some rapid flow ROs do not require storage tanks.
