By Peter Cartwright, P.E., CWS-VI, and Tom Cartwright, CWS-I
Summary: This article looks at a few of the factors that affect efficiency of POU units and the resulting RO water quality. Initially, it’s important to understand how permeate, recovery and removal volume play in the equation. Once those are considered, it’s easier for water treatment professionals to pass along helpful information to their customers.
There seems to be significant confusion on the part of dealers, installers, salespeople and others trying to explain the relationship between reverse osmosis (RO) permeate (product water) quantity, quality and the volume of feed water required, or just plain trying to understand it themselves.
One of the fundamental engineering design parameters in an RO system is recovery, defined as that percentage of the feed water flow rate that passes through the membrane and becomes permeate. Recovery may be calculated by:
Percent Recovery = 100 × Permeate Rate ÷ [Permeate Rate + Concentrate (Reject) Rate]
The permeate rate of a given RO membrane element is fixed by the manufacturer and is dependent upon only these feed water characteristics—applied pressure, temperature and osmotic pressure, which is the pressure and potential energy difference that exists between two solutions on either side of a semipermeable membrane because of the tendency for water to flow from a region of higher purity to one of lower purity. Every 100 parts per million (ppm), or milligrams per liter (mg/L), of total dissolved solids (TDS) generates roughly one pound per square inch (psi) of osmotic pressure.
For residential drinking water applications, none of these characteristics can be easily altered—the water pressure depends upon the municipal supply (or the well pump pressure) less any pressure drop within piping up to the point-of-use (POU) unit. The permeate rate of cold water is significantly lower than that of warm water. Because the U.S. public equates cold water with “better tasting,” the goal is to have drinking water as cold as possible. Osmotic pressure is “back pressure” related to the kind of dissolved salts and their concentrations—usually less than 10 psi. The key is to understand how the existing POU/RO systems are designed and why they operate as they do.
The pressure necessary to force permeate through the membrane is typically created by a flow control device placed in the concentrate (reject) line of the RO system. This device is selected according to the permeate output of the membrane element (based upon 77°F water), and the resulting recovery of the system is usually between 10 percent and 30 percent. This means that if a membrane element producing 25 gallons per day (gpd)—based on the ambient water temperature—is installed with a 25 percent recovery setting, and the membrane element is used to its maximum capacity, 75 gpd of concentrate would go to the drain. Of course, almost nobody uses their RO system 24 hours a day and, to prevent the concentrate stream from continuously going down the drain, virtually all residential RO systems are now equipped with automatic shut-off valves.
Because POU/RO units typically don’t produce permeate fast enough to operate on a “demand” basis, they all utilize storage tanks, most of which are tanks equipped with a bladder (diaphragm) or a bag separating the incoming permeate from air (typically). As the permeate volume increases, the air on the other side of the bag is compressed, thereby increasing the tank pressure. This pressure forces the stored water out of the system, and is known as “net driving pressure.” As the tank fills, the tank pressure increases until the shut-off valve closes off the feed water supply to the unit shutting down the system. The shut-off valve is usually designed to close at about two-thirds of the line pressure. As an example of an actual system, if the line pressure is 50 psi, the shut-off valve settings and resulting water volumes in the storage tank might be as follows:
- Feed water pressure = 50 psi.
- Valve closing pressure = 33.5 psi; water volume = 2.5 gallons (g)
- Valve opening pressure = 16.5 psi; water volume = 0.8 g
The typical use pattern for residential POU/RO systems is that only a glassful, or perhaps a coffee-maker carafe-full, is taken from the storage tank at any one time. Since the typical storage tank holds 2-to-2-½ gallons of water when full, the normal incremental usage may be only in the range of 5-to-25 percent of the storage tank capacity. This removal volume might be enough to open the shut-off valve to start the RO system again, but the difference between the line pressure of the feed water and the pressure in the storage tank might only be 10-to-15 psi. It’s important to remember that although this back pressure in the permeate line will inhibit the flow through the membrane into the storage tank, the flow through the flow control device will be unchanged. If the line pressure is 50 psi, it will continue to be 50 psi up to this device, regardless of the back pressure in the permeate line.
The net result is that because the permeate flow rate from the membrane element is reduced, and the concentrate flow rate remains unchanged, the recovery will be reduced. For example, for a membrane element producing 25 gpd and operating at 25 percent recovery, the total feed rate would be 100 gpd, and the concentrate rate = 100 – 25 = 75 gpd. If the line pressure is 50 psi and the shut-off valve opens at 16.5 psi, the pressure at that moment available to produce permeate is 50 – 16.5. = 33.5 psi, and the permeate rate is now 33.5 ÷ 50 x 25 = 16.75 gpd. Therefore, the recovery has been reduced to 16.75 ÷ (16.75 + 75) = 18.3 percent, almost a 27 percent reduction over the initial 25 percent.
As a result of this phenomenon, the Water Quality Association (WQA) has proposed a revision to ANSI/NSF Standard 58 to include an “efficiency rating” measurement to reflect the recovery at the point when the shut-off valve opens and the storage tank is full. This efficiency rating would be listed as part of the system data, along with the recovery rating, calculated from flows when there’s no back pressure on the permeate line.
The dominant membrane polymer used in POU/RO membranes today is the thin-film composite. Although the degree of salt rejection of RO membranes is a function of applied pressure (the higher the pressure, the greater the rejection), the rejection of the thin-film composite membrane polymer is generally unaffected by pressures above 30 psig. In other words, down to 30 psig, the rejection characteristics of the membrane remain unchanged.
Another effect on permeate quality is the phenomenon known as “TDS creep.” When there’s no pressure differential across an RO membrane, salts will migrate through the membrane as the result of dialysis. Although only a small volume of water will be affected at any one time, over a period of a couple of weeks of this low-usage pattern, the overall water quality in the tank may show 85-90 percent rejection, as opposed to the expected 90-95 percent.
So what can be done to minimize permeate back pressure and TDS creep? For the former, it’s possible to utilize “water on water” storage tanks that use feed water flow to create pressure in the storage tank only when the faucet is opened. Also, there’s a “permeate pump” available that uses concentrate pressure to force permeate into the storage tank. On the issue of TDS creep, the most practical approach is to recommend that the consumer completely empty the storage tank at least once a week. In reality, TDS creep effect is so small that the slightly higher TDS will likely go unnoticed anyway.
The authors wish to acknowledge the following individuals who provided input for this article—Mel Hemp, research and development manager at Clack Corp., and Joseph F. Harrison, P.E., WQA technical director.
About the author
Peter S. Cartwright, president of Cartwright Consulting Co., Minneapolis, is a registered professional engineer. He has been in the water treatment industry since 1974 and has published more than 100 papers and articles on related issues. Cartwright has been chairman of several WQA committees and task forces, received the WQA Award of Merit and holds its Certified Water Specialist, Level 6, designation. A member of the WC&P Technical Review Committee since 1996, he can be reached at (952) 854-4911, (952) 854-6964 (fax) or email: [email protected].
Tom Cartwright is business manager of drinking water products with Osmonics Inc.’s Household Group in Milwaukee, Wis. He holds the WQA’s Certified Water Specialist, Level 1, designation and can be reached at (262) 238-4400 or email: [email protected].