POU RO Systems TDS Reduction Testing
By Rick Andrew
The popularity of POU RO systems continues to grow. There has been widespread usage in North America for many years, with significant growth in Asia recently. In the US, sales occur through big-box stores and water treatment dealers. The dealer channel predominates sales in Europe, the Middle East and Asia.
RO was first commercialized in the 1960s. By the 1980s, its use in the US residential water treatment market was becoming established. Consumer acceptance of residential RO increased with the adoption of NSF/ANSI 58 – Reverse osmosis drinking water systems in 1981. The development of automatic shut-off valves also helped propel acceptance of residential RO.
POU RO systems incorporate a number of components due to the nature and complexity of the technology. Among them are premembrane and postmembrane filters, the reverse osmosis membrane itself, an automatic shut-off valve, a product water storage tank and a faucet. This combination of components working in concert provides sophisticated water treatment capabilities. A diagram of a typical POU RO system can be found in Figure 1.
The test methodology and requirements developed for evaluation of contaminant reduction capabilities of POU RO systems is derived from the strengths and potential shortcomings of the technology. This has been the approach in the NSF/ANSI DWTU standards for all technologies utilized for POU/POE systems.
Figure 1. POU reverse osmosis system
RO membrane function
RO membranes allow water to pass through, but reject any ionic contaminants that may be dissolved in the water. They also form a mechanical barrier to any suspended particles that may be present. The efficiency of rejection is influenced by several factors, including composition of the membrane, net driving pressure of the RO system, amount of crossflow to drain (i.e., the recovery), electrical charge of the ions, size of the ions, water temperature, concentration and composition of dissolved ions in the water, and other factors.
There are some general principles of reverse osmosis. In general, the greater the electrical charge, and the larger the ionic size, the better the membrane is able to reject the ions. Conversely, the lesser the electrical charge and the smaller the ionic size, the more easily ions can pass through the membrane and therefore the poorer the ionic rejection. The fundamental function of reverse osmosis membranes is ionic rejection. For this reason, NSF/ANSI 58 requires that all RO systems must be evaluated for ionic rejection. The standard specifies that test water consisting of 750 mg/L sodium chloride in deionized water be used for this purpose. It requires the use of sodium chloride, because sodium in water is a monovalent ion (Na+), as is chloride (Cl–). Additionally, these ions are relatively small in size compared to other ions, such as calcium (Ca2+) and sulfate (SO 2–). Testing with sodium chloride, which yields small, monovalent ions, leads to confidence that the system will be at least as efficient in rejecting ions under a variety of real-world usage conditions. This basic evaluation test with deionized water containing only sodium chloride is known as TDS reduction. All RO systems conforming to Standard 58 must demonstrate at least 75-percent reduction of TDS when tested.
Conditions for TDS reduction testing
RO membranes are durable when protected by prefilters in a properly maintained system; they can last for several years. This durability means that laboratory testing of an RO system through the entire replacement cycle of the membrane is impractical. With the idea that complete life- cycle testing is impractical, the standard instead requires testing according to a seven-day proto- col that evaluates system performance under a variety of usage patterns. System performance
with complete draw-down of the product water storage tank, partial tank draws and a two-day stagnation period is evaluated through 14 different sample points over the course of the week-long protocol. Figure 2 includes details of the test protocol. There is a very similar protocol for systems that do not include a storage tank. Figure 3 shows a typical POU RO system on the test stand.
All prefilters and postfilters are removed from the system during TDS reduction testing because these filters can also perform contaminant reduction functions, but the purpose of the test is to evaluate the membrane only. The simplest way to ensure that only the membrane is responsible for contaminant reduction performance is to remove all water treatment components except the membrane.
Figure 3. POU RO system on test stand
TDS reduction testing is conducted at 50-psig inlet pressure. This pressure is lower than the 60 psig used for contaminant reduction testing of active media filters and other mechanical filters under NSF/ANSI 42 and 53. Because higher inlet pressure leads to higher net driving pressure and improved RO ionic rejection, RO systems are tested with a lower inlet pressure to represent a more conservative analysis of system performance.
The rate of product water production is measured under two different operating conditions:
- complete filling of the storage tank starting from empty; and
- partial filling of the storage tank from the point where the automatic shut-off valve first turns on.
The daily production rate as defined by NSF/ANSI 58 is an average value based on these two operational conditions, expressed as volume per day (volume per 24 hours). The procedure for systems that do not include a storage tank is similar.
Test methods appropriate for the technology
RO is an impressive water treatment technology, able to largely separate pure water from dissolved ions and suspended particles. It also continues to enjoy growing popularity with consumers worldwide. The NSF Joint Committee on Drinking Water Treatment Units recognizes the capabilities and potential shortcomings, such as low net driving pressure, and takes all of this information into consideration as seen in the requirements of the test protocols in NSF/ANSI 58. By carefully selecting and specifying inlet pressures, operational requirements, product configurations, and sampling schemes, the Joint Committee has created conservative tests of these systems to verify their performance and help ensure quality RO systems for a growing number of consumers around the world.
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: Andrew@nsf.org.