RO Efficiency and Recovery
By Rick Andrew
Efficiency and recovery ratings for POU RO systems are becoming more and more important as focus on water conservation increases. Systems that have higher efficiency ratings use less water than systems with lower efficiency ratings, although there are diminishing returns because once efficiency becomes too high, the membrane element can be damaged by hardness and resulting scale in the water. Bearing in mind the importance of these measurements, NSF/ANSI 58 includes very specific test procedures for measuring the efficiency and recovery ratings for POU RO systems. Let’s take a look at these two very important RO system characteristics, how they are defined and how they are determined.
Recovery rating is the percentage of water introduced to the RO system that becomes available as RO water when the faucet is open and water is not flowing into the storage tank. If a system has a recovery rating of 50 percent, that means that for every two gallons of water flowing into the system, one gallon will flow out as RO water when the faucet is left open. The other gallon ends up as reject water going to drain.
Efficiency rating is the percentage of water introduced to the RO system that becomes available as RO water when the system is operating under specific conditions identified under NSF/ANSI 58. So if a system has an efficiency rating of 50 percent, that means that for every two gallons of water flowing into the system, one gallon will flow out as RO water when the system is operated as indicated in the standard. The other gallon ends up as reject water.
Note that for tankless RO systems, recovery rating and efficiency rating are the same. Also note that the efficiency rating cannot be higher than the recovery rating. For systems with pressurized storage tanks, operation of the system involves filling the tank against back pressure, which reduces the flowrate of RO water into the storage tank, while the flowrate of the reject stream is not affected. Therefore, the recovery rating will be higher than the efficiency rating.
Challenges with measurements
On the surface, it might seem straightforward to measure the amount of inlet water to a POU RO system that becomes RO water. Below the surface, however, there are some factors that complicate these measurements, especially if the measurements are to be reproducible for systems with pressurized storage tanks.
The main factor that complicates the measurements is the back pressure resulting from a pressurized storage tank. This reduces the system net driving pressure and therefore reduces the rate of production of RO water. As the storage tank fills, the back pressure increases because the air behind the diaphragm is compressed further. So, as the tank becomes more full, the back pressure increases, the system net driving pressure decreases and the production rate of RO water is reduced. The fuller the storage tank becomes, the more the production rate decreases.
The production rate of the system is a function of the amount of RO water in the storage tank. Because the production rate varies depending on fullness of the storage tank (whereas the reject flowrate remains constant), the efficiency rate also varies depending on fullness of the storage tank. As the storage tank becomes more full, the efficiency rate of the system is reduced. As such, a specific protocol for measuring efficiency rating is necessary to obtain reproducible results. Otherwise, the measurement would be variable, depending on how full the storage tank was when the measurement was made.
Measurement of recovery and efficiency ratings
The procedure in NSF/ANSI 58 spells out that recovery rating is measured with the RO system faucet open and the system producing water at steady state. The product water and reject water are both collected and measured until the product water volume is 100 mL. The recovery rating is determined by dividing the volume of product water by the total volume of product water plus reject water, i.e. 100 mL ÷ 100 mL + volume of reject water = recovery rating.
For tankless RO systems, recovery rating is the same as efficiency rating because efficiency rating takes into account the impact of the storage tank. NSF/ANSI 58 specifies that efficiency ratings for systems with storage tanks are to be reported as an average value measured under two different operating conditions:
- A complete product water storage-tank fill cycle (start with empty tank, end with full tank as defined by the termination of the flow of water by the automatic shut-off valve [ASOV])
- A partial product water storage-tank fill cycle from the point where the ASOV initiates refilling of the storage tank to the point where the ASOV terminates flow
As the RO system is put through these two different operating conditions on the test stand, measurements are conducted:
- The volume of product water is measured (by emptying the tank after the cycle is completed and recording the volume).
- The volume of reject water that flows through the reject flow restrictor is measured.
These measurements are all that is required to calculate the efficiency of the RO system. Efficiency is calculated by adding up the volume of product water generated during both operating conditions and dividing by the total amount of product water and reject water generated during each operating condition.
The value of the standard
Product standards have multiple purposes, with one of the most important being reproducibility of measurements. For RO systems with pressurized storage tanks, the efficiency of the system is a function of how full the storage tank is and therefore, how much back pressure is resulting from the storage tank. Measuring efficiency with varying amounts of water in the storage tank will cause different results. Because this variation could lead to a lack of reproducibility, there is a need to standardize the efficiency-rating measurement protocol, such that the measurements are being performed specifically and consistently. Without standardization, it would not be possible to compare production rates or efficiency ratings because the measurements could vary dramatically depending on how they were performed.
Bearing this idea in mind, the NSF Joint Committee on Drinking Water Treatment Units developed a defined procedure for measuring production rate and efficiency of POU RO systems. This procedure is found in NSF/ANSI 58–Reverse osmosis drinking water treatment systems. The standard serves this purpose of allowing reproducible measurements of efficiency, allowing end users to compare one system to another and assuring the same results on any given system any time the test is conducted.
About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: Andrew@nsf.org