By Rick Andrew
Residential POU RO systems continue to be one of the primary technologies used for residential water treatment. One of the main reasons for their popularity is that RO technology can be effective at reducing both TDS and suspended particles. When evaluating these systems with storage tanks and automatic shut-off valves, which is the primary configuration sold for residential applications, there are four main relevant performance characteristics to be considered:
- TDS reduction. An evaluation of rejection of TDS under defined conditions of TDS composition, concentration, temperature and inlet pressure, and under operating conditions that approximate typical daily use.
- Daily production rate. The volume of water produced by the system per day under defined operating conditions that approximate typical daily use.
- 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. The percentage of inlet water available as product water under defined operating conditions that approximate typical daily use.
It is important to define the operating conditions for these evaluations because changes in TDS composition or concentration, inlet pressure, temperature or the system usage pattern can all affect these characteristics. In order to understand the significance of any reported values for these characteristics, it is critical to understand the conditions under which they were measured.
The NSF Joint Committee on Drinking Water Treatment Units recognized the need to evaluate these system characteristics under standardized conditions so that data is directly comparable. The committee developed test methodologies to evaluate these characteristics for POU RO systems and incorporated them into the TDS reduction test under NSF/ANSI 58. This TDS reduction test is the baseline evaluation test required for all systems conforming to the standard.
TDS reduction test
The TDS reduction test for systems with storage tanks and automatic shut-off valves under NSF/ANSI 58 is conducted according to a seven-day test protocol requiring evaluation of two test systems under a variety of operating conditions designed to simulate typical product usage patterns:
- Complete emptying of the storage tank at the sample point. This is the optimal operating scenario for systems with air-pressurized storage tanks, because an empty storage tank creates the least amount of back pressure and hence, allows the highest net driving pressure and the best system performance. As the tank fills, the back pressure increases, net driving pressure decreases, permeate flow decreases and the TDS concentration of the product water increases.
- Partial emptying the storage tank. This is a less favorable operating scenario than complete emptying of the storage tank, although one that occurs in real-world usage when consumers are drawing single servings of drinking water. It may require more than one partial tank draw to empty the storage 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 lower average net driving pressure with partial emptying of the storage tank compared to complete emptying of the storage tank.
- A two-day stagnation period. This condition assesses the susceptibility of the system to membrane ‘creep.’ Creep is a term that describes diffusion of TDS across the membrane from the reject side to the product side. Creep can cause significant concentrations of TDS to move into the product water under stagnant conditions, which can affect the overall TDS reduction of the system when measured according to the protocol in NSF/ANSI 58.
See Table 1 for a complete description of the sample points collected during the seven-day test protocol. NSF/ANSI 58 requires a minimum average 75-percent reduction of TDS.
Inlet pressure of 50 psig is used for TDS reduction testing under this standard. It is important to have specified and closely controlled inlet pressure because product water flow is proportional to net driving pressure. Passage of TDS through the RO membrane, however, is independent of flow. So not only is production rate affected by inlet pressure, but also TDS reduction. The test water for TDS reduction is RO/DI water with 750 mg/L of sodium chloride (NaCl) added. This specification allows for a defined TDS concentration and composition, and one that includes only monovalent Na+ and Cl– ions. Monovalent ions have poorer RO rejection characteristics than divalent ions such as calcium (Ca++), magnesium (Mg++) or sulfate (SO4–). So measuring TDS reduction using NaCl results in a conservative measurement compared to real-world drinking water, which includes some divalent ionic species.
Recovery rating
The recovery rating of the two test systems is measured on day 1 and day 7 of the TDS reduction test by operating the systems with the faucet open for a time period long enough to allow permeate and reject flows to stabilize. One hundred 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.
Daily production rate and efficiency rating
Daily production rate and efficiency rating for POU RO systems with automatic shut-off valves and storage tanks are determined under conditions approximating typical daily usage. This approximation is achieved by testing under two different conditions:
- A complete storage tank fill (the tank is filled from as empty as possible until the point where the automatic shut-off valve stops the flow of water through the system).
- A partial tank fill, whereby the tank is emptied to the point where the automatic shut-off valve turns on to allow filling of the tank and then measurements are conducted as the tank fills to the point where the automatic shut-off valve stops the flow of water through the system.
The total amount of product water filling the tank under both operating conditions is measured, as well as the total amount of reject water generated as the tank fills under both operating conditions. The efficiency rating is calculated as the percentage of total water (product water plus reject) that becomes product water. Note that the efficiency rating is equal to or lower than the recovery rating and typically about half of the recovery rating. The daily production rate is determined by measuring the total volume of product water under both operating conditions, as well as the time required to generate the product water. 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 among various RO systems 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 standardized testing method to allow comparisons
POU RO systems are highly sophisticated, complex systems that may be evaluated for a number of characteristics. And because 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. NSF/ANSI 58 provides standardized procedures to evaluate POU RO systems for recovery, daily production rate, efficiency and TDS reduction. These values, generated under standardized conditions, allow consumers or other product users to make true apples-to-apples comparisons of systems.
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: [email protected]