By Rick Andrew
It sounds like a simple questiom: “How much water does it make?” But for POU RO systems, the question can be far from simple. In fact, the question is complex enough that a complete, detailed section of NSF/ANSI 58: Reverse Osmosis Drinking Water Treatment Systems is dedicated to the procedure required to obtain reproducible values for production rates of these systems.
Factors influencing daily production rate
Reverse osmosis systems use water pressure to force water through a semipermeable membrane, leaving behind most of the dissolved solids present in the incoming water. Along with this sophisticated technology comes a number of factors influencing just how much water will be forced through the membrane. In some ways, the situation is analogous to measuring fuel economy in automobiles. There are many factors that influence fuel economy—just a few of them include stopping and starting frequently, accelerating rapidly, operating air conditioning, properly maintaining the vehicle, and pulling a trailer. In light of these variables, US EPA has created a specific set of operating conditions under which the agency’s fuel economy values are determined. These operating conditions are representative, but do not cover extremes, so there are disclaimers associated with US EPA fuel economy ratings.
Transitioning this line of thinking to POU RO systems, one can consider the factors that influence water production rates:
- Inlet pressure
- TDS concentration in the inlet water
- Inlet water temperature
- Membrane active area
- Membrane type and specific composition and manufacturing
- Age/condition of membrane
- Reject flow restrictor
- Type of storage tank
- If an air bladder storage tank:
- Air pressure precharge on bladder
- Degree of fullness of tank
Fixing the variables for consistent measurements
As we can see from our analogy to US EPA fuel economy ratings, when measured values are influenced by variables, it is important to fix
these values to obtain reproducible measurements. Some of the variables in question are fixed by the system itself. Others are not, so they must be fixed by the requirements in the standard. When considering the specific values at which these variables are fixed, it is important to consider that they are representative of typical conditions, or possibly representative of more extreme conditions if there are health considerations. In terms of daily production rate, it establishes comparability between various POU RO systems, so there are no health concerns here. For this reason, representative values are used. See Figure 1 for details regarding the variables mentioned above, whether they are fixed by the system or by the standard, and if they are fixed by the standard, then how.
Addressing storage tanks
The most complex variable to address in establishing daily production rates for POU RO systems is the degree of fullness of air-bladder type storage tanks. This is important because the fuller the tank, the more back pressure on the membrane and the slower the production rate. Conversely, an empty tank will have the least amount of back pressure on the tank and the highest production rate. With this in mind, the production rate measurement protocol in NSF/ANSI 58 requires measuring the rate of production when the tank is filled starting empty, as well as when the tank is filled starting at the point where the automatic shut-off valve (ASOV) turns on.
Note that there is quite a difference in these two operating conditions. This difference is clearly seen every time this type of measurement is taken in the laboratory. For example, values for these production rates may be as seen in Figure 2. In this case, if the standard were based on filling of an empty tank, the system production rate would be 120 liters per day. If the standard were based on filling from the point of ASOV activation to the point where the ASOV turns off, the system production rate would be 72 liters per day.
NSF/ANSI 58 specifies that the production rate of a POU RO system is defined based on the total volume filled and the total time elapsed during these two filling conditions. So, for the example in Figure 2, the production rate per NSF/ANSI 58 is 104 liters per day.
Actual performance may vary
Just as with actual fuel economy in automobiles, actual production rates obtained by consumers using POU RO systems may vary from the values determined under the methodology prescribed by NSF/ANSI 58. If consumers have TDS that is significantly different from 750 mg/L, or if they have water pressure that is not near 50 psi, or if they use only small amounts of water from the system during each use, they may have a production rate very different from that measured under the conditions in the standard. But the value provided by the standard method for measuring daily production rate—similar to the value afforded by US EPA’s method for measuring actual fuel economy in automobiles—is that it provides a production rate that can be reproducibly measured and used to compare one system to another in a standardized way. This can allow consumers to make more informed choices when selecting a POU RO system, just as it helps consumers to make more informed choices in automobiles.
About the author
Rick Andrew is the General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols), and Biosafety Cabinetry Programs. He has previously served as the Operations Manager, and prior to that, Technical Manager for the pro- gram. 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: [email protected]