Water Conditioning & Purification Magazine

Daily Production Rate and Efficiency Rating of POU Reverse Osmosis Systems

By Rick Andrew

When it comes to performance specifications on POU reverse osmosis (RO) systems, daily production rate and efficiency rating are among the most popular. These ratings are included in various forms of product literature and sales sheets and are frequently cited in kitchen table sales presentations.

Although they are often referenced, the details of their measurement methods are not very well understood. Most industry insiders know, in general terms, that daily production rate has to do with how much water the system will make and that efficiency rating has to do with how much water put through the system becomes product water as opposed to reject water.
But few could accurately explain how daily production rate or efficiency rating is actually measured. This column will walk through these measurements step by step, to help remove some of the mystery from these commonly discussed but poorly understood ratings.

Why is this complicated?
At first glance, the concept of determining how much water a POU RO system makes seems pretty straightforward. So does the concept of measuring how much water put through the system becomes product water. But there are complicating factors.

Typical POU RO systems incorporate an air-filled diaphragm product water storage tank (Figure 1). The reason for the air-filled diaphragm is to provide pressure to discharge product water from the tank to the faucet with a reasonable flow rate.

However, the air-filled diaphragm creates system backpressure. This backpressure reduces the flow rate of product water through the membrane.

As the storage tank fills, backpressure becomes greater because the air charge is compressed further. The more full the storage tank becomes, the more backpressure and the slower the flow rate of product water through the membrane.

This means that the production rate of the system is a function of the amount of product water in the storage tank. Measuring the production rate is likewise not straightforward.

Effects on efficiency
Although product water flow rates through membranes vary with how full the storage tank is, the flow of reject water through the reject water flow restrictor does not. In other words, although the flow rate of product water into the storage tank decreases as the tank becomes fuller, the reject flow rate remains constant.

With efficiency being expressed as the percentage of the feed water that is available as product water, it then follows that the efficiency of the system decreases as the product water storage tank becomes fuller. This means that the efficiency of the system is a function of the amount of product water in the storage tank, which means that measuring the efficiency is not straightforward.

The value of the standard
With different measurements of production rate and efficiency being possible depending on how the measurement is conducted (i.e., with a full product water storage tank or an empty storage tank), there is a need to standardize the measurement protocol such that measurements are being performed the same way. Without standardization, it would not be possible to compare production rates or efficiency ratings because 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.

Measurement of production rate and efficiency
The procedure spells out that production rate and efficiency measurements are to be reported as an average value. This should be measured under two different operating conditions:

  1. 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]).
  2. 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.
    During these two cycles, RO systems are installed on a laboratory test stand to facilitate the testing and measurements (Figure 2).

Because inlet pressure, temperature and composition of feed water can also influence RO system production rates, they are defined by the standard. The inlet pressure is set at 50 psig.
The temperature of the water is 25°C (77°F). Test water consists of deionized water that has 750 mg/L of TDS as sodium chloride added to it.

As RO systems are put through the two cycles on the test stand, various measurements are conducted:

  1. The amount of product water is measured by emptying the tank after the cycle is completed.
  2. The amount of reject water that flows through the reject flow restrictor is measured.
  3. The time to complete each cycle is measured.

These measurements are all that is required to calculate daily production rate and efficiency of the RO system. Daily production rate is calculated by adding the volume of product water generated during each cycle, dividing by the combined time of the two cycles and then normalizing to 24 hours.

Efficiency is calculated by adding the volume of product water generated during each cycle and dividing by the total amount of product water and reject water generated during each cycle. An example calculation is noted in Figure 3.

Although the example calculation includes hypothetical numbers, they are somewhat realistic, which allow some real-world observations to be made. For example, note that the partial product water tank fill cycle resulted in only 20 percent of the product water of the complete tank fill cycle (0.5 gallons compared to 2.5 gallons), but took 33 percent as long to complete.

It should also be noted that the amount of reject water from each cycle was roughly proportional to the time required for the cycle. From these numbers, it can be seen that production rate and efficiency for the complete product water tank fill cycle are better than for the partial fill cycle. This is due to the effect of increased backpressure from the air charge as the tank fills with product water.

Hopefully understanding has been expanded on daily production rate and efficiency to the extent that issues can now be grasped that complicated these measurements. This should also help ascertain the actual conditions under which daily production rate and efficiency are determined.

About the author
Rick Andrew is the Operations Manager of the NSF Drinking Water Treatment Units Program for certification of POE and POU systems and components. He enjoys leveraging his more than 10 years of experience in this area to help explain the complexities and details of the NSF/ANSI DWTU Standards. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at 1-800-NSF-MARK or email: Andrew@nsf.org.

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