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Output Limitations of Pure Water by RO Systems - WCP Online

Water Conditioning & Purification Magazine

Output Limitations of Pure Water by RO Systems

By Bill Harty

Summary: Following are a few reminders regarding effective RO maintenance for dealers and installers.


Producing pure water from your reverse osmosis (RO) system may sound like a good idea, but many factors go into calculating such efficiency. Outlined here are 10 instances that can affect the outcome of desired water quality.

A few points to bear in mind:

  • Water pressure: Membrane ratings are reached at 60 pounds per square inch (psi); therefore, if water pressure is less than this, the membrane will produce less water proportionately. The choice of a higher gallon per day (gpd)-rated membrane can help offset this situation.
  • Water temperature: Membrane ratings are reached using water that’s 77°F. Water colder than this will result in less water production from the membrane. Again, choosing a higher gpd-rated membrane can help offset this situation. 
  • Water content: Very high total dissolved solids (TDS) levels—in excess of 2,000 parts per million (ppm)—can lower production due to osmotic pressure.

How POU RO systems work

1. To use and care for an RO system, it’s best to know what it does and how it works.

2. One approach will be to follow the water from the water line to the house to its final resting place in a storage tank.

3. To begin with, water leaves the house water line through whatever device has been used to tap into the house supply line. Some installers supply a stop valve adapter, kitchen valve adapter, saddle valve adapter or hose bib connection for this purpose. The water line supplies the RO system, and water enters the first canister through tubing.

4. The first canister may contain a carbon block cartridge or, more commonly, a sediment cartridge of pleated polyester or polypropylene fiber. The primary purpose of this cartridge is to remove sediment in generous quantities from your household water prior to further pre-filtering.

(Remember, the filter does this mechanically in the following manner—the cartridge is porous and water passes through these pores. The size of the pore hole determines the size of the particles that can pass through the cartridge. Just think of pores as holes in a window screen and sediment as dirt, which has particles of varying sizes. The sediment cannot get through if it’s bigger than the pore hole size. The word “micron” indicates the pore size. The human eye cannot see particles smaller than 40 micron (µm). The first canister cartridge usually has pore hole sizes of 5 µm, thereby stopping particles larger than 5 µm from traveling further through the central system.)

If this first canister contains a 5-µm carbon block cartridge, then it also reduces or removes chlorine, chloramines and other organic chemicals through adsorption technology as well sediment through micron pore size mechanical technology. The cartridge in the first canister is there primarily to remove particles in the water. If it does more than that, then such activity is a bonus. The replacement of this cartridge should be done annually or when sediment clogs the cartridge to the point that water exiting the first canister has lost considerable water pressure. A pressure gauge in your central system is a good way to monitor water pressure loss and determine frequency of cartridge replacement. Absent such a gauge, annual cartridge replacement is the minimal option for replacement.

5. The water tubing continues on to a second canister. In a bare-bones RO system, the first stage does the work of both canisters 1 and 2 before the water flows into the membrane housing. In the second canister, there’s either a carbon block cartridge or a granular activated carbon (GAC) cartridge. This cartridge’s purpose is to remove organic chemicals and chlorine that will damage a thin film composite RO membrane. The cartridge also removes taste and odors from household tap water. This cartridge should be changed annually absent any clogging or pressure loss, whichever one occurs first. Heavy use may require an earlier change-out. One should expect the standard cartridge to pre-treat 5,000 gallons of tap water or approximately 1,500 gallons of RO end product.

6. When water exits the second canister, it enters the membrane housing where the RO process takes place. What happens is the pre-filtered water passes over the surface of a semi-permeable membrane, which is similar, for example, to the skin found on a boiled egg between the shell and the egg white. Inside the membrane housing, the entrance is at one end with two exits at the other end. As the water passes over the membrane surface, there’s house water pressure behind this water flow. Some of the water passes through the membrane and becomes permeate water—pure water—which passes out of the membrane housing through a tube from one exit. The other water is reject water, which doesn’t pass through the membrane and finds its way out of the membrane housing through a tube from the other exit (wastewater). What happened is all the water was pressed against the membrane surface, but only “pure” water got through. The remaining water, which retains the large majority of TDS, goes down the drain.

7. What is TDS? In many water supplies, a large amount of TDS is dissolved calcium carbonate or related ions. Calcium will become a solid if boiled or frozen. That’s why a lot of ice cubes are cloudy, or boiled water develops a foamy substance floating on the top. The calcium comes out of solution and precipitates to a solid. In an RO system, the membrane separates the TDS from pure water and sends both in different directions.

8. The drain line exiting the membrane housing is where a flow restrictor is located. The purpose of the flow restrictor is to create a backpressure so water is forced under pressure through the membrane and separated from the TDS. If there’s no flow restrictor, the water would pass too quickly through the housing and very little pure water would be produced.

9. Flushing the membrane removes restrictions. After passage of time, suspended and precipitated solids may build up on the surface of the membrane and slow its ability to process pure water. It’s time to find the flow restrictor and remove it. Once the flow restrictor is removed, the system will allow a fast flow of water over the membrane surface and flush excess TDS build up off the membrane surface. This procedure should be followed every three months absent a pressure gauge, which would indicate when to do this maneuver. Replace the flow restrictor after about 10 minutes of flushing. The flow restrictor will either be a tube on the waste exit line, which will have to have a union installed in its place to flush the system, or a capillary type, which is installed inside the waste exit line tube at the exit from the membrane housing. If it’s a capillary style, just pluck it out of the waste exit line tube and reinstall the waste exit line tube without the restrictor for flushing purposes. When flushing is done, reinstall the capillary style flow restrictor inside the waste exit line tube to return to normal operation. Another flow restrictor used has a ball valve on the waste exit tube or a flow restrictor bypass ball valve. Merely turn the handle to open the ball valve to full flow for 10 minutes, and turn back again to return to normal.

10. The water coming out of the pure water exit tube is the finished product water.

Conclusion
To judge the effectiveness of your membrane, TDS meters are available to read the TDS content of finished product water. The life expectancy of your membrane is in excess of three years. If it’s flushed regularly and the carbon cartridge, which removes chlorine is changed annually, the system’s membrane should provide three or more years of service for which it’s designed. To read the system and its needs, use of a flow meter, a pressure gauge and a TDS meter can pinpoint all service requirements on any RO system. Such packages are available and easily added. Also, a ball valve flow restrictor or bypass is a good inexpensive addition to any RO system for effective operation and maintenance.

About the author
Bill Harty is president of The Foundry DM, of Holly Hill, Fla., a component part designer and manufacturer that distributes Dmfit® Quick fittings, faucets, RO membranes, housings, tubing, cartridges, RO clips and saddles, diverter valves, spouts, adapters, etc. Harty can be reached at (386) 257-5020, (386) 257-7090 (fax), email: fdmsales@cfl.rr.com or website: www.thefoundry.net

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