Water Filtration

A typical home reverse osmosis treatment system is shown in the figure below. A reverse osmosis system is normally located beneath the kitchen sink since it is used to treat water for drinking and cooking purposes. RO systems consist of the 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.

A Typical Home RO System Includes: (1) particle filter, (2) reverse osmosis membrane unit, (3) pressurized treated-water storage container, (4) carbon adsorption post-filter, and (5) separate treated-water tap.

The water supply entering the RO unit should be bacteriologically safe. RO units will remove virtually all microorganisms but they are not recommended for that use because of the possibility of contamination through pinhole leaks or deterioration due to bacterial growth. Water softeners are commonly used in Minnesota and the Dakotas in advance of the RO system.

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.

You must be logged in to post a comment.