Reverse osmosis (RO) filter systems were introduced to the water-treatment industry in the late 1950s, and, over time, they emerged as an effective solution to address the escalating concerns about water quality and safety. A 2023 study conducted by NSF found that across the United States, only 52 percent of the filter owners surveyed trust their tap water at home, revealing mistrust in water quality and consumers’ desire for trusted solutions1.

RO systems, in addition to other filtration technologies, including activated carbon, sand water, under-sink systems, and pitcher filters, offer consumers options to combat these concerns. With increasing environmental degradation, industrial pollution, and aging infrastructure jeopardizing water sources nationwide, technological solutions like RO systems help safeguard public health.

RO Technology Overview

RO systems are widely used within households to treat residential water and have become popular due to their efficiency in removing contaminants from drinking water. In contrast to a regular carbon filtration system, which uses a filter to draw, trap, and retain contaminants, RO systems use a semipermeable membrane to remove impurities in water. They draw water through the membrane and use pressure to force it through. Certain contaminants and minerals cannot pass through and are held there until “the reject stream” washes away the impurities from the membrane.

Considering that semipermeable membranes cannot distinguish between contaminants and minerals, RO systems commonly incorporate a remineralization process, in which minerals are mixed back into the water. They may be added back into the treated drinking water for a variety of reasons, whether for personal taste or health-related preferences. This option is left to the consumer. Multiple RO system designs allow the treated water to be delivered to the consumer in many ways.

The most common household RO system is stored under the sink or on the counter and has a storage tank attached. As technology has advanced for RO system manufacturers, more options for in-house RO systems have emerged. Countertop RO systems are now common, as well as RO systems that can be plumbed directly into the cold-water line and disperse the treated water through the faucet. Depending on the system design, other water-treatment technologies are regularly used in coordination with the RO membrane. Most commonly, they include sediment filtration cartridges and carbon filtration cartridges. The different options allow consumers to decide what is best for them.

Certifying Chemical and Mechanical Reduction Claims

Counterfeit water filters are becoming more prevalent, increasing consumer concerns. In fact, NSF’s survey found that 53 percent of water filter owners surveyed are cautious of counterfeit water filters, and a significant majority— 83 percent—believe third-party certification should be mandatory for all water filters sold in stores1. Product claims for point-of-use (POU) RO systems can be verified through third-party certification under NSF/ANSI 58: Reverse Osmosis Drinking Water Treatment Systems based on the performance of the RO membrane within the system.

For all inorganic chemical reduction claims, the reduction is based on the RO membrane, whether an RO system has pre- and post-filters or not. To ensure pre- and post-filters do not affect testing, those cartridge positions are plugged during inorganic chemical testing. The testing method lasts for seven days and considers a variety of testing scenarios that follow NSF/ANSI 58. Given that inorganic chemical reduction claims are derived from the ionic rejection potential of the membrane, the testing does not assess treatment capacity. A summary of the inorganic claims and reduction requirements can be found in Figure 1.

Figure 1. Inorganic chemical reduction claims under NSF/ANSI 58.

Note that the only claim required under NSF/ANSI 58 is total dissolved solids (TDS) reduction. TDS reduction is the minimum performance evaluation that RO systems must comply with, while all other reduction claims are optional to the manufacturer.

Unlike inorganic chemical reduction claims, mechanical filtration claims on RO systems are established by the performance of the complete RO system. The testing procedure for mechanical reduction claims follows the same seven-day testing procedure as inorganic chemical testing. See Figure 2 for mechanical filtration claims and reduction requirements.

NSF/ANSI 58 and PFAS Reduction

Recently, there has been significant public health concern regarding perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) contaminants. These synthetic organic molecules break down very slowly and are thus referred to as forever chemicals. PFAS exposure has been linked to harmful health effects in animals and humans. As a result, filtration manufacturers have prioritized elevating technology to better reduce PFAS.

Figure 2. Mechanical filtration claims under NSF/ANSI 58.

As filtration technology has advanced and knowledge within the public health field about different compounds associated with PFAS has increased, a new reduction claim has been adopted by NSF/ANSI: total PFAS reduction. This broad reduction claim has significant value because it provides the public with confidence that a given water-treatment technology will be effective in the reduction of specific PFAS compounds that are outlined in the standard.

In 2019, the testing protocols for evaluating PFOA/PFOS reduction for POU RO systems were integrated into NSF/ANSI 58. A task force continuously worked to develop the testing methodologies and, through comprehensive evaluations, was able to produce a testing methodology to support a broad PFAS reduction claim. In 2022, to incorporate all PFAS compounds into a single reduction claim, the chemical structure and properties of PFAS compounds were evaluated, and a new testing protocol was established within NSF/ANSI 58 for the reduction of total PFAS. The evolution of NSF/ANSI 58 exemplifies the work the industry is putting into reducing these harmful contaminants and safeguarding the public.

Evaluating PFAS

Under NSF/ANSI 58, POU RO systems make total PFAS reduction claims as they do other chemical reduction claims. To test these claims, a weeklong test is completed on two RO systems. Thirteen samples are taken to analyze the challenge water and treated water collected from the systems. The samples are collected throughout varying operating conditions to simulate the real-world operation of the systems. They include a 48-hour period of stagnation, allowing confidence in the system’s operating ability to provide treatment consistently. Per the standard, PFAS is added to the TDS reduction’s test water, along with sodium chloride, to complete the required test-water concentration. If the systems successfully reduce the PFAS concentration in the influent challenge water to the specified reduction totals, then the RO system can make a claim of total PFAS reduction.

To increase confidence in an RO system’s ability to reduce PFAS, consumers need to be knowledgeable about their system. NSF/ANSI drinking-water treatment units standards require user instructions to be provided with RO systems to ensure proper installation, operation, and maintenance of the systems. Additionally, the user instructions must include performance data sheets associated with different RO systems. Data sheets provide added information on the total PFAS reduction claim, laying out the influent challenge concentrations, substance, and maximum permissible effluent concentrations to provide consumers with all the information they need to feel confident in the performance of their system.

Since the U.S. Environmental Protection Agency established the health advisory for PFOA and PFOS, the understanding of PFAS contamination and regulatory levels has continuously advanced. Some states have been more aggressive than others in developing their own acceptable drinking-water values related to PFAS compounds. To reflect the evolving landscape around PFAS chemicals, task groups have been assembled to expand the scope of NSF/ANSI 58 and NSF/ANSI 53: Drinking Water Treatment Units – Health Effects to include additional requirements for PFAS compounds.

PFAS contamination in drinking water will continue to be a major issue for years to come. The hope is that more attention given to the subject will lead to more consumer knowledge on protecting against ingesting contaminated water. The continual advancements of the third-party standards, such as NSF/ANSI 58 and NSF/ANSI 53, are additional tools for the industry to help increase confidence in drinking-water safety.


“New Research Reveals 92% of Water Filter Owners Are More Likely to Trust an Independently Certified Water Filter,” News, NSF,

About the author

Mike Sheffield is the senior technical reviewer of NSF’s Global Water Filtration Division. He has 13 years of experience in the water industry and holds bachelor’s degrees in psychology and criminal justice from New Mexico State University and a master of science degree in environmental science from American University.


Comments are closed.