Other fact sheets in this series identify reverse osmosis, commonly called RO, as a treatment process for removing many different contaminants from drinking water. This document provides greater detail concerning RO treatment.

WHAT IS REVERSE OSMOSIS TREATMENT?

Many decades ago, scientists noted that water molecules would spontaneously migrate through certain membranes that were separating a dilute solution from a concentrated solution. This phenomenon is called osmosis. They also noted that if pressure was added to contaminant solution, this natural flow could be "reversed". This reversal allows the contaminant solution to be concentrated further and allows purified water to be produced.

AMOUNT OF WATER TO BE TREATED

Where only a small volume of treated water is needed (say 0-10 gallons per day (gpd)), RO is typically the most flexible and cost efficient treatment process available for private home use.

Devices treating small volumes of water are often called "under-the-sink" or "point-of-use" sized treatment devices. Treatment devices which purify all water used in a home are commonly called "whole house" or "point-of-entry" sized water treatment devices.

CONTAMINANTS APPROPRIATE FOR RO TREATMENT

Certain contaminants in drinking water do not need to be fully removed from all of the water used in the home, but only from that water which will be consumed internally or used in food preparation. Some of these contaminants include:
 

Other contaminants needing treatment for all water used within a home include:
 

The above breakdown is generic and assumes only modest levels of contaminants in the water supply. Where contaminants are very high, whole house treatment is likely required for all contaminants.

THE RO TREATMENT DEVICE

An RO device typically consists of the membrane housing and "pre" and "post" filters. The stainless steel housing containing the membrane, would be at a minimum, approximately 3" in diameter x 10" long and the pre and post filters would be of similar size. The membrane would likely be of the spiral wound type. The housing would have three connection ports to accommodate the three flows regimes of a RO device:

1. A raw water "feed-in" port;

2. A treated water "outlet" port; and

3. A reject water "outlet" port.

EFFICIENCY OF THE RO PROCESS.

The efficiency of the RO process can be measured in two ways; using both purity and volume.

1. Efficiency based on the purity of the treated water. RO can typically remove 80-99 percent of most mineral contaminants. For example: if the concentration of contaminants in the raw water was 1.0 milligram per liter (mg/L) and the efficiency of the treatment device was 90 percent for that contaminant; the treated water contaminant concentration would be approximately 0.1 mg/L. Treated water quality is the most important measure of the efficiency of an RO device.

2. Efficiency based on a volume of treated water. In New Hampshire treatment efficiency based on the volume of water produced is poor; typically in the range of 20-30 percent. This is due to the typically cool temperatures of the states's groundwater. Example: Assuming the volumetric efficiency of an RO treatment device was 25 percent; if 10 gallons of raw water is fed into the device daily, only 2.5 gallons of water will migrate through the membrane to become treated pure water. The contaminants and the remaining 7.5 gallons of water will become reject water and will be discharged to a sewer, leachfield, or drywell. Typical New Hampshire groundwater temperature is 40-45 degrees Fahrenheit. This temperature is fairly constant year round. RO devices are typically rated at an operational temperature of 77 F at 60 pounds per square inch (psi) applied pressure. The problem causing this low rate of production is the viscosity (stickiness) of the water when cold.

WATER QUALITY CONSTRAINTS FOR RO DEVICES

RO treatment can be affected by some water quality factors including:

Turbidity (cloudiness)

To accomplish separation of water from contaminants, the passageways in the membrane must be very small. Thus these membranes are very sensitive to clogging by small solid particles. Where there are any particles in the raw water, a sediment prefilter must be installed. Sediment cartridge filters are so inexpensive that it is normally good practice to installed a prefilter before all RO units, regardless of the solids content of the raw water.

Where waters are very clear, the pre-filter size chosen is typically 5 microns. Where the raw water has more solids, the 5 micron filter could be preceded by a filter of larger pore size, approximately 25 or 50 microns.

Iron & Manganese

These naturally occurring contaminants often create particles which will clog RO membranes. A particle filter is critical where iron or manganese concentrations are high.

Hydrogen Sulfide

This material can create a gelatinous coating on the membrane. This may limit the life of an RO cartridge. Where hydrogen sulfide is high, other pretreatments may also be necessary such as oxidation.

Chlorine

Some RO membranes, called thin film composite (TFC), will be damaged by the presence of chlorine or other strong oxidizing compounds. Either a chlorine resistance membrane, such as cellose acetate (CA) must be used, or the chemical injection point for the chlorine should be placed after the RO unit.

Conservation of Reject Water

To prevent the constant wasting of water from the reject port, an automatic valve is often added to the waste discharge line. This valve shuts off the raw water feed as the treated water pressure tank approaches a near full capacity. This valve is available on more sophisticated RO designs.

Differential Pressure Across The Membrane

The greater the pressure differential pressure across the RO membrane, the higher the purity of the treated water. When the small accumulation pressure tank is nearly full, the differential pressure across the RO membrane drops to near zero. This creates the potential for excess contaminants to diffuse across the membrane and recontaminate the purified water. To prevent this recontamination, more sophisticated designs add two additional functions that activate when the device is not producing water. One turns off the feed water when the accumulation tank is near filled and the second purges the raw water side of the membrane, with treated water. These designs features are valuable but they also raise the cost of the RO device.

Pre and Post Activated CarbonFilters

We have already mentioned the importance of a sediment pre-filter before the RO unit. After the RO unit, the water conditioning industry normally advocates the installation of an activated carbon filter. The activated carbon targets taste and odor constituents which have escape treatment.

BACKUP AND REDUNDANCY

A very conservative design would provide a further treatment device down stream of the RO element to address the small concentration of contaminants that escaped through the main treatment device, particularly in point-of-use sized treatment. Such a device would address the possibility of rupture of the membrane or leakage of the internal membrane cartridge. Typically a media filter is used for this backup. The media type and associated contaminants are shown below.
 

PURCHASE AND INSTALLATION

If purchasing water treatment equipment, see the DES fact sheet WD-WSEB-2-5 entitled, "Considerations When Purchasing Water Treatment Equipment".

The purified water accumulated in the pressure tank. This water is typically dispensed by a special dedicated faucet mounted on the kitchen sink. This treated water can also be plumbed directly to the refrigerator ice maker.

The RO device can be installed in the basement or "under-the-sink" location. The basement location typically provides the greater ease of maintenance. Be sure to check the adequacy of the plumbing at both the water supply connection point and waste discharge connection point.

RO MAINTENANCE

In order to ensure proper operation, the home owner must establish and carry out a maintenance program for any treatment device. For RO system this program would consist of:

1. Periodic replacement of the particle pre-filter and or post filter;

2. Periodic water quality testing of the treated water;

3. Possible raw water testing ; and

3. Evaluation of RO membrane integrity.

FOR MORE INFORMATION

For more information, please call the DES Water Supply Engineering Bureau at call 271-3139. We would appreciate your comments of this fact sheet. For a full list of fact sheets please request WD-WSEB-15-2.  Drinking water fact sheets are available through the DES web site at: http://www.des.state.nh.us then select fact sheets.