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Reverse Osmosis

 

Introduction
Principles of Reverse Osmosis
Types of Reverse Osmosis Membranes
Reverse Osmosis System Selection
Operation and Maintenance
Cost

Introduction
Reverse osmosis (RO) is becoming a common home treatment method for contaminated drinking water. RO, probably best known for its use in water desalinization projects, can also reduce chemicals associated with unwanted color and taste. It also may reduce pollutants like arsenic, lead, and many other types of organic chemicals. RO can treat moderately saline to saline water, reducing the amounts of common minerals, including hardness, by 80-95%.

RO treatment is not effective for the removal of dissolved gases such as radon, or for some pesticides and volatile organic chemicals such as solvents. For example, RO will not effectively remove disinfection by-products like chloroform. Consumers should check with the manufacturer to determine what contaminants their unit will remove. The removal effectiveness (percent of removal) depends on membrane type, water pressure, and the amount and properties of each contaminant.

RO is recommended only for water that is free of sediments (particles) and pathogens. Pretreatments such as particle filtration (to remove sediments), carbon filtration (to remove volatile organic chemicals), chlorination (to disinfect and prevent microbial growth), pH adjustment or even water softening (to prevent excessive fouling produced by water with excessive hardness) may be necessary for optimum RO functioning.

Reverse osmosis (RO) process

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Principles of Reverse Osmosis
The simplest home RO system consists of a semi-porous membrane, a storage container for the treated water, and a flow regulator and valve to back-flush the membrane when it becomes clogged (or fouled). Tap water is passed through a membrane that filters out most of the contaminants. Eventually, the pores of the membrane become clogged with minerals and the flow-through of water slows down. To remove these residues, the membrane is back-flushed using tap water, which creates reject water high in salts. This brackish water is automatically discharged into the home drain system. When the membrane flow is restored, tap water can be treated again. The pressure for RO is usually supplied by the line pressure of the water system in the home. RO units installed in private water sources should have sediment and activated carbon pre- or post-filters.

Home RO units are often small, cylindrical devices, approximately 5 inches in diameter and 25 inches long, excluding any pre- or post-filtration devices. It is not practical to treat all water entering a residence with RO since small devices do not produce enough water to meet all household needs. Also, RO water can be very aggressive and should not be circulated through or stored in metal pipes or containers. Often, the unit is placed beneath the kitchen sink to treat water used for cooking and drinking.

Types of Reverse Osmosis Membranes
These membranes are made from organic chemicals like cellulose acetate, cellulose triacetate, aromatic polyamide resins, a mixture of these materials, and a variety of other materials. Membrane selection depends primarily on the quality of the water source. Some membranes are intended for use only with chlorinated water, others must have water with no chlorine, and still others may be used with either. Note that residual chlorine will quickly damage membranes not rated for chlorinated water.

All membranes used in home-size RO units are enclosed in a cartridge and are usually either hollow fiber or spiral wound. Spiral wound membranes, more common in home systems, are designed to treat water with high levels of suspended solids. Hollow fiber membranes are easily clogged by hard water, but they require less space and are somewhat easier to maintain than the spiral wound configuration.

Reverse Osmosis System Selection
Key questions to ask and issues to consider before purchasing an RO system include:

-- How well does the RO unit remove contaminants found in the water supply?
Note that removals are given in percent removed of the total present in the source water and that this varies for each contaminant. For example: if tap water has 100 mg/L sodium, a membrane with an 85% reject value should produce water with no more than 15 mg/L of sodium. Again, remember that this removal percentage for sodium may not be the same for other contaminants.

-- How much treated water can be produced per day?
Since some RO units operate continuously, an oversized system will result in excessive waste of treated water.

-- How much back-flushing water is needed per gallon of treated water?
This often is an overlooked cost that may be difficult to determine in home systems that drain the back-flush water directly into the sewer. Home RO systems may spend as much as 10 gallons of water to back-flush the membrane for every gallon of clean water they produce. In contrast, industrial RO units may need only 3 gallons of back-flush water for every 7 gallons of treated water. For home RO systems, the range of water treatment efficiency may vary from around 10 to 50 percent, depending on the TDS and hardness of your water source, membrane type, removal efficiency, and system pressure.

Operation and Maintenance
RO units increase home water use since tap water must be used to regularly flush the membranes. Some devices might require continuous operation to maintain peak membrane performance. This may lead to frequent and excessive losses of treated water. Clogged or torn RO membranes require replacement; however, well-maintained membranes should last two to three years. In addition, an RO system that uses pre-and post-treatment devices has added purchase and maintenance costs.

Membrane inspection is not practical, so regular analysis of the treated water (using a TDS meter or more specific and expensive contaminant testing methods) is necessary. Also, some beneficial minerals – such as calcium and magnesium – are reduced significantly in RO water. As drinking water is not the primary source of these nutrients in our diet, this can be of minimal importance.

Cost
RO devices available for the home range in price from $200 to $500, not including installation. Maintenance costs can range from $50–$120 a year.

Since extra tap water is needed to regenerate membranes, large home RO systems may result in significant increases in water use and fees. For example, a 10 gallon-a-day RO system with a 20% efficiency rating will require 1,500 gallons of extra water use each month. Additionally, if water softening is needed prior to RO treatment, those costs also should be considered. However, these extra monetary and environmental costs are often overlooked.

(Portions of this text have been adapted from Plowman, F.T., "Reverse osmosis," Durham: University of New Hampshire Cooperative Extension, Water Quality Fact Sheet 24, 1989.)

Go to other treatment methods:
Particle and Microfiltration Filters
Activated Carbon Filters
Distillation
Ion Exchange Water Softening
Disinfection of Drinking Water
Other Treatment Methods


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