By Rick Andrew
Residential point of use (POU) reverse osmosis (RO) systems continue to grow in popularity, in large part due to their contaminant reduction and overall water treatment capabilities. The RO elements in these systems can be effective at reducing the concentration of dissolved ionic contaminants and can also serve as a mechanical barrier.
The crossflow nature of RO, although potentially highly effective in treating these types of contaminants leads to product designs in which some of the inlet water typically is sent to drain. This leads to other aspects of the system that must be evaluated, such that characteristics of the system are fully understood, including the following:
- Recovery rating: The percentage of inlet water available as product water when the system is operated without a storage tank, or when the storage tank is bypassed.
- Efficiency rating: For RO systems with an automatic shut-off valve and storage tank, the percentage of inlet water available to the consumer under operating conditions that approximate typical daily use.
- Daily production rate: The volume of water produced by the system per day.
- Total dissolved solids (TDS) rejection: An evaluation of rejection characteristics – TDS composition, concentration, pressure – of the system under defined conditions and under operating conditions that approximate typical daily use.
For a given system, these characteristics vary depending on operating conditions. Changes in TDS composition or concentration, inlet pressure, temperature, or the system usage pattern can all affect each of these characteristics. In order to understand the meaning of any reported values for these characteristics, it is critical to understand the conditions under which the data was generated.
Recognizing the need to evaluate these system characteristics under standardized conditions so that data is directly comparable, the NSF Joint Committee on Drinking Water Treatment Units developed requirements and methodologies to evaluate these characteristics for POU RO systems and incorporated them into a test under NSF/ANSI 58. It is a baseline evaluation test required for all systems conforming to the Standard – the TDS reduction test.
TDS reduction testing
TDS reduction testing under NSF/ANSI 58 involves a seven-day test of two test systems under a variety of operating conditions designed to simulate typical product usage patterns:
- Full tank draws involving emptying the storage tank completely at the sample point. This is the optimal operating scenario for RO systems, because for systems with an air pressurized storage tank, an empty storage tank causes the least amount of backpressure and therefore allows the best net driving pressure and the best system performance. As the tank fills, the backpressure increases, net driving pressure decreases, permeate flow decreases and the ionic concentration of the product water increases.
- Partial tank draws involving emptying the storage tank only to the point where the automatic shut-off valve turns on. This is a less favorable operating scenario for RO systems. The partial tank draw may or may not deplete the tank to the point where the automatic shut-off valve turns on and allows the system to begin processing water. In any case, systems with air pressurized storage tanks will be operating under poorer average net driving pressure conditions than under the full tank draw scenario described above.
- A two-day stagnation period. This condition assesses the susceptibility of the system to membrane ‘creep’, which is a term that describes the phenomenon of RO systems allowing a diffusion of ions across the membrane that can cause significant concentrations of ions to move into the permeate under stagnant conditions.
The inlet pressure for TDS reduction testing under NSF/ANSI 58 is specified as 50 psig, because 50 psig is on the lower end of typical pressures in public water supplies. The lower end is selected because net driving pressure for the membrane is directly reduced by reducing the inlet pressure. Because permeate flow is proportional to net driving pressure, yet ionic passage through the membrane is independent of pressure, TDS rejection performance is poorer with lower net driving pressures, resulting in a more conservative test.
TDS reduction water chemistry
TDS reduction test water chemistry under NSF/ANSI 58 is defined as RO, deionized (RO/DI) water with 750 mg/L of sodium chloride (NaCl) added. This chemistry allows for a defined TDS concentration and composition, as well as a challenging composition consisting solely of monovalent Na+ and Cl– ions. Monovalent ions have poorer rejection characteristics than divalent ions such as calcium (Ca++), magnesium (Mg++), or sulfate (SO4—), which means that assessing rejection of TDS using NaCl will result in a conservative measurement compared to naturally occurring waters which include some divalent ionic species.
Measurement of recovery rating
On Day one and Day seven of the TDS reduction test, the recovery rating of the two test systems are measured by operating the systems with the faucet open for a time period long enough to allow permeate and reject flows to stabilize. 100 mL samples of permeate are collected, along with a measured sample of the corresponding amount of reject flow. The recovery rating is determined by calculating the percentage of total water put through the system (permeate plus reject) that becomes permeate.
Measurement of daily production rate and efficiency rating
Daily production rates and efficiency ratings for systems with automatic shut-off valves and storage tanks are determined under conditions approximating typical daily usage. This essentially means under two different conditions:
- A complete tank fill (the tank is filled from completely empty until the point where the automatic shut-off valve stops the flow of water through the membrane).
- A partial tank fill (the tank is first emptied to the point where the automatic shut-off valve turns on to allow filling of the tank, and then the measurements begin as the tank fills to the point where the automatic shut-off valve stops the flow of water through the membrane).
Under these conditions, the amount of water filling the tank is measured, as well as the amount of reject water generated as the tank fills. The efficiency rating is calculated as the percentage of total water (permeate plus reject) that becomes permeate. Note that the efficiency rating is equal to or lower than the recovery rating, and typically it is about half of the recovery rating.
The daily production rate is determined by measuring the volume of permeate, as well as the time required to generate the permeate, under both of these operating conditions. The results are then normalized to a 24 hour day to calculate the daily production rate.
Unlike TDS reduction, there are no minimum values required for recovery rating, efficiency rating, or daily production rate. These characteristics are simply measured and reported to allow comparisons from one system to another under standardized conditions. For the sake of consumer awareness, efficiency rating and daily production rate must be reported in the manual and performance data sheet of systems conforming to NSF/ANSI 58.
A standard method to evaluate system characteristics
POU RO systems are highly sophisticated, complex devices that may be evaluated for a number of characteristics. Since these characteristics can vary depending on operating conditions, standardized test conditions must be used to develop comparable data. This is one of the purposes served by NSF/ANSI 58, specifically with respect to the TDS reduction test.
Much like the automotive industry uses standardized procedures to measure engine horsepower and torque, NSF/ANSI 58 provides specific procedures to evaluate POU RO systems for recovery, daily production rate, efficiency and TDS rejection. These values, generated under standardized conditions, allow buyers to make true ‘apples to apples’ comparisons of systems.
About the author
Rick Andrew is the Operations Manager of the NSF Drinking Water Treatment Units Program. Prior to joining NSF, his previous experience was in the area of analytical and environmental chemistry consulting. 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: [email protected].