By Rick Andrew

Many people are familiar with the concepts of RO efficiency and daily production rate, but few know the details regarding how they are measured. Efficiency is a measure of how much of the inlet water is converted into RO water, and daily production rate is a measure of the amount of RO water the system can produce in a 24-hour period. These concepts are straightforward enough, but measuring them involves some nuances. Recognizing these nuances, NSF/ANSI 58 includes detailed protocols for measuring POU RO efficiency and daily production rate.

Nuances of measuring efficiency and daily production rate
On the surface, it Nuances of measuring efficiency and daily might seem straightforward to measure the amount of inlet water to a POU RO system that becomes RO water. It might also seem relatively simple to measure the amount of RO water produced in 24 hours. Below the surface, however, there are some factors that complicate these measurements, especially if the measurements are to be reproducible.

Typical POU RO systems utilize an air-filled diaphragm product water storage tank. The air-filled diaphragm is employed to provide pressure to force RO water from the tank to the faucet with an acceptable flowrate. The air behind the diaphragm creates system back pressure, which reduces the system’s net driving pressure and therefore reduces the rate of production of RO water. As the storage tank fills, back pressure increases because the air behind the diaphragm is compressed further. So, as the tank becomes fuller, the back pressure increases, the system net driving pressure decreases and the production of RO water becomes slower. This means that 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 how full the storage tank is, a specific protocol for measuring production rate is necessary to obtain reproducible results. Although the production rate varies according to storagetank level, the flowrate of reject water does not. Reject water flowrate is controlled by a reject flow restrictor. Because efficiency is defined as the amount of inlet water that becomes RO water, we can see that the efficiency of the RO system decreases as the storage tank fills with RO water. The result is that the efficiency of the system is a function of the amount of RO water in the storage tank, which means that a specific protocol for measuring efficiency is necessary to obtain reproducible results.

A standardized protocol
Because both production rate and efficiency vary depending on the amount of RO water in the storage tank, there is a need to standardize the measurement protocol so that the measurements are being performed 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.

The NSF Joint Committee on Drinking Water Treatment Units understood this concept and took it into consideration as they developed the procedure for measuring production rate and efficiency of POU RO systems. This procedure is spelled out in NSF/ANSI 58 – Reverse osmosis drinking water treatment systems.

The standard requires that efficiency and production rate measurements are to be conducted under two different operating conditions:

• A complete fill of the RO storage tank, starting with the tank empty and ending at the point where the flow of water is stopped by the automatic shut-off valve (ASOV).
• A partial fill of the RO storage tank from the point where the ASOV initiates refilling of the tank to the point where the ASOV stops the flow of water.

As these measurements are conducted, inlet pressure, water temperature and composition of the feed water are also specified by the standard. These factors can affect RO system production rates, so they must be specified so the test results are reproducible. Fifty psig is the inlet pressure, 25°C (77°F) is the feed water temperature and feed water is specified as deionized water, to which 750 mg/L of TDS as sodium chloride is added.

The standard requires that efficiency and production rate measurements are to be conducted under two different operating conditions:

  • A complete fill of the RO storage tank, starting with the tank empty and ending at the point where the flow of water is stopped by the automatic shut-off valve (ASOV).
  • A partial fill of the RO storage tank from the point where the ASOV initiates refilling of the tank to the point where the ASOV stops the flow of water.

As these measurements are conducted, inlet pressure, water temperature and composition of the feed water are also specified by the standard. These factors can affect RO system production rates, so they must be specified so the test results are reproducible. Fifty psig is the inlet pressure, 25°C (77°F) is the feed water temperature and feed water is specified as deionized water, to which 750 mg/L of TDS as sodium chloride is added.

As the RO system is put through the two operating conditions described above, the following measurements are required to calculate efficiency and daily production rate:

  • The amount of RO water from each cycle, which is measured by emptying the storage tank after the cycle is complete.
  • The amount of reject water from each cycle.
  • The time to complete each cycle.

With these measurements, RO system efficiency and daily production rate can be calculated. Efficiency is calculated by totaling the volume of RO water generated during each cycle and dividing by the total amount of RO water and reject water generated during each cycle. In other words, the total volume of RO water is divided by the total amount of feed water, and the total amount of feed water is determined by adding up the total amount of RO water and the total amount of reject water.

Daily production rate is calculated by totaling the volume of RO water generated during each cycle, dividing by the combined time of the two cycles and then normalizing to 24 hours. Figure 1 describes an example calculation. This calculation includes hypothetical numbers that are also typical. Some observations can be made from this typical data. Note that the partial tank fill produced a gallon of RO water in only half an hour. This would translate into a daily production rate of 48 gallons (181.6 liters) per day. The complete tank fill, however, produced two gallons (7.5 liters) in 2.5 hours, which would be a daily production rate of only 19.2 gallons (72.6 liters) per day. This shows the decrease in production rate as the tank fills. Note also that the amount of reject water from each cycle was proportional to the time required for the cycle. The complete fill took five times as long as the partial fill, resulting in five times more reject water generated during that cycle. From these numbers, it can be seen that the production rate and efficiency for the partial tank fill are much better than for the complete tank fill. This is due to the effect of increased back pressure from the air charge as the tank fills with product water.

A simple idea with nuances understood
Efficiency and daily production rate of POU RO systems are relatively straightforward concepts to understand. Actually measuring them in a way that is reproducible requires some attention to the nuances of RO system operation, taking into consideration the fact that production rate slows down as the typical air-filled bladder storage tank fills. The standardized protocol developed for NSF/ANSI 58 allows consistent measurements of POU RO efficiency and daily production rate despite the variability over the operational cycle of the RO storage tank filling.

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

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