Drinking Water Treatment - Reverse Osmosis

Drinking Water and Human Health December 06, 2010 Print Friendly and PDF

EFFECTIVE AGAINST: Inorganic contaminants such as: dissolved salts of sodium, dissolved (ferrous) iron, nitrate, lead, fluoride, sulfate, potassium, manganese, aluminum, silica, chloride, total dissolved solids, chromium, and orthophosphate. Also effective in removing some detergents, some taste, color and odor-producing chemicals, certain organic contaminants, and 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.

Contents

How Reverse Osmosis Works

Reverse osmosis (RO) is a point-of-use treatment system. The simplest reverse osmosis (RO) system consists of a housing for a semi-permeable membrane, a storage tank, and a separate faucet. The membrane allows water to pass through and collect in the storage tank.

Untreated water is forced through the membrane. The contaminants being treated by the unit do not pass through the membrane and are removed as waste.

It is not practical to treat all water entering a home with an RO system because four gallons of raw water into the RO treatmen system produce about one gallon of treated water. Therefore, the treated water is often used only for drinking and cooking.

Graphics of membrane will go here.

Types of Membranes

Several kinds of reverse osmosis membranes and membrane structures exist for RO devices. The two types of RO membrane structures are spiral-wound and hollow-fiber. In each case, the structure is designed to increase the surface area that comes into contact with untreated water. The larger the surface area, the greater the flow rate. Spiral-wound membranes are designed to treat water with high levels of dissolved solids. Hollow-fiber membranes are easily clogged by hard water, but they require less space and are somewhat easier to maintain than spiral-wound membranes.

Each type of membrane 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.

Capacity

RO units have a limited flow rate capacity and therefore usually treat only drinking and cooking water. Units that operate at household water pressure yield about 3 to 5 gallons per day.

Maintenance

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 log book to record water test results, equipment maintenance and repairs.

RO membranes have a lifetime of three to five years. Eventually the RO membrane tears, allowing contaminants to pass through. The membrane must then be replaced. An RO system must be well-maintained to ensure reliable performance. Clogged membranes, filters, or flow controls will decrease water flow and the system’s performance. If fouling is detected in the early stages, the membrane can often be cleaned and regenerated. The cleaning procedure varies depending on the type of membrane and fouling. Completely clogged or torn RO membranes must be replaced. Pre- or post-filters must be replaced once a year or more often, depending on the volume of water fed through the system and the quality of the feed water. Damage to RO membranes cannot be seen easily. The treated water must be analyzed periodically to determine whether the membrane is intact and doing its job. Many systems now have a built-in continuous monitor that indicates whether the system is operating properly.

Bacteria and germs, dead or alive, can clog RO membranes. To prevent this clogging, the units need to be disinfected periodically with chlorine or other products provided by the manufacturer.

Special 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.

Drawbacks to RO systems are that they waste water, can be expensive, generally produce water only for cooking and drinking, and often may require pre- and post-treatment. Any pre- or post-treatment devices require additional maintenance. High concentrations of iron, manganese, and hydrogen sulfide; excessive hardness; and high turbidity can damage the membrane and clog the device. Generally, RO systems also include a cartridge sediment pre-filter to remove suspended solids, such as sand, silt, and sediments while an activated carbon post-filter removes undesirable tastes or odors and some organic chemicals. Some systems require an activated carbon pre-filter to remove chlorine that could damage certain membranes.

The efficiency of a RO 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.

Water temperature also has a significant impact on performance –- the higher the temperature, the better the performance. Many devices are tested at an incoming water temperature of 77 degrees Fahrenheit. Groundwater temperature is about 55 degrees Fahrenheit. The amount of treated water from a unit decreases by 30 to 40 percent if the water temperature is 55 degrees instead of 77 degrees.

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.

Check with the manufacturer to see exactly what the RO unit will remove, as the 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.

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 Questions to Ask Before You Buy A Water Treatment System for more information.


Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.

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USDA / NIFA

This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.