By Rick Andrew
Reverse osmosis is one of the most widely employed water treatment technologies. The process utilizes a semipermeable membrane that allows water to pass through, but rejects particles and ions. Pressure is applied to drive equilibrium toward producing treated water, the opposite from the natural osmotic pressure-driving force, which is why the treatment technology is called reverse osmosis (RO).
RO is used in systems of various sizes and scale, with the scale varying widely. Most of the largest RO water treatment plants are focused on desalination of sea water. The Ashkelon desalination plant in Israel has a reported production capacity of 330,000 m3 per day (about 87,000,000 gpd), providing about 13 percent of Israel’s water.(1) The Carlsbad Desalination Plant in California is the largest in the US, producing almost 50,000,000 gpd, or about one-third of the water for San Diego County.(2) These systems use hundreds of large RO elements, typically 40 inches long and four inches in diameter.
On the other end of the spectrum, RO has long been used in POU applications for homeowners. These typical POU systems are found in many homes across North America and Asia, providing families with water for drinking and cooking. They typically use a single RO element that is about a foot long and two inches in diameter.
Different end uses
The large elements used in municipal water treatment plants have constant or nearly constant flow through them throughout their operational lifetimes. They also may be installed with multiple stages, such that the second stage uses reject or concentrate water from the first stage as its feed and so on.
POU RO has quite different usage characteristics. Typically there are long periods of stagnation when no water is being withdrawn from the RO system and no water is flowing through the RO element. Some systems may be designed to periodically flush the RO element when no water is being withdrawn. And usually there is just one RO element with one feed source, as opposed to any type of staging or feeding reject/concentrate from one RO element into another.
Somewhere in the middle of these two end uses is POE RO. While POE RO is not nearly as common as municipal or POU RO, there are POE systems in the marketplace. POE RO systems tend to have some periods of stagnation with no flow, but they are not as frequent nor as long as POU RO. RO elements used for POE also tend to be somewhere in the middle in size, smaller than the large municipal RO elements but larger than POU RO elements.
Because of the differences in end uses, as described above, these three varying types and end uses of RO elements (municipal, POE, POU) are addressed by three different test methods in two different standards. Each of these standards addresses material safety for these RO elements, which means that an extraction or leaching test is conducted. The elements are conditioned and exposed to the extraction water and then the extraction water is analyzed for possible contaminants that may have leached from the RO element. Any contaminants that are detected are assessed for their relative toxicity to determine if the amount that was detected could be harmful to people drinking the water.
Figure 1 summarizes some of the relevant similarities and differences among these protocols as they apply to municipal, POE and POU end uses for RO elements. Note that there are differences in conditioning of the element prior to the test being conducted; there are sometimes differences in whether the element is simply soaked in the extraction water or whether extraction water is flowed through the element (and only permeate is analyzed), and there may also be differences in the test-water composition.
Municipal and POE RO elements are evaluated only for material safety and contaminant leaching under NSF/ANSI 61. POU RO elements, however, can be evaluated for TDS reduction and reduction of other contaminants under NSF/ANSI 58. Highly detailed TDS reduction and contaminant reduction test protocols are described in NSF/ANSI 58, mainly focusing on testing of complete POU RO systems, but also including specific criteria for transferring contaminant reduction test data from one POU RO system to other POU RO systems that use the same RO element.
Sorting out the details
Obviously, there are quite a few details to consider when thoroughly investigating the standards applicable to various types and end uses of RO elements. Issues such as exposure protocols, composition of test waters, conditioning periods, etc., can all make a difference as conformance to these standards is tested. When looking at the big picture and the most important point to keep in mind, however, there is one key takeaway: the standards all focus on making sure that RO elements used to treat drinking water are evaluated very thoroughly and rigorously to make sure they are constructed of materials that are safe for drinking-water contact. This is indeed the cornerstone of all of the NSF/ANSI drinking water standards: a continuing focus on safety and human health.
- Ashkelon Seawater Reverse Osmosis (SWRO) Plant, Israel. https://www.water-technology.net/projects/israel/
- 50 Million Gallons A Day: The Pacific is Now on Tap. https://www.carlsbaddesal.com.
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: Andrew@nsf.org