Comparing RO Filter Quality and Performance

July 13, 2011

Quality tests and certifications ensure reliable filter performance

Consumers who choose to use a residential point-of-use (POU) reverse osmosis (RO) system in their home need to feel secure that their system is producing high quality, reliable water for their family. Because most of us lack sophisticated equipment at home to make this determination, many homeowners rely on trusted brands to deliver the water quality they expect.

Brand owners, OEMs and component suppliers seek certification from recognized experts to communicate to consumers that their system provides acceptable performance. Organizations like NSF Intl. and the Water Quality Assn., along with geographically specific agencies like KIWA, provide this certification service. Along with proper certification, consumers expect the components they use to provide consistent, reliable performance throughout the product’s designed lifetime.

NSF/ANSI 58 - Reverse Osmosis Drinking Water Treatment Systems provides criteria for evaluating the quality of a POU RO system. Systems must meet strict requirements for material safety, structural integrity, contaminant reduction performance and product literature that explains functionality and maintenance requirements.

The engine behind a POU RO system is the RO element itself. In order for any component supplier to produce a product that is consistently reliable, it should be made with automated equipment using high-quality components and rigorous quality tests throughout the process. For an RO element, this includes first-in-class manufacturing for both the membrane and the filter.

How does one assess whether an RO element supplier produces reliable, consistent products? First, there is certification. NSF/ANSI 58 provides criteria for certification of POU RO elements. Larger elements can be certified to NSF/ANSI 61. Both of these standards require rigorous extraction testing to ensure the safety of the construction materials for contact with drinking water.

Beyond the material safety of elements, there are several tests to look into. The most important compares the mean normalized flow and rejection across different water conditions for various RO element suppliers. The actual performance should be compared with the stated performance in the data sheets. Some RO element providers state their elements’ performance at beneficial conditions (higher feed pressures) so that the elements appear to have higher flow and rejection. Because there are no industry standards for test conditions, RO suppliers may also choose to publish their elements’ performance at recoveries or feed waters that represent what they have available in their factories, which usually does not represent the feed water and operating conditions in the field. Because of this, it is critical to test elements using the same feed water and test setup as in the field.

Once elements from different RO suppliers have been tested, the standard deviations for flow and rejection should be compared to see which has the most consistent performance. In order to see a significant comparison, sample sizes for each manufacturer should be at least 50.

After completing this testing, you will notice that there is a tradeoff between flow and rejection. Some elements achieve higher flow but lower rejection and vice versa. Assessing which is the optimal combination of flow and rejection may be difficult. A useful test is to compare the ratio of permeate water flow rate to permeate conductivity. The manufacturer that demonstrates the highest ratio of flow rate to conductivity should be considered superior because it demonstrates that for each unit of conductivity that passes through the membrane, the element is providing the highest flow rate. Figure 1 (Page 14) shows the results of tests performed by Dow Water and Process Solutions (DW&PS).

Any elements that demonstrate poor rejection or flows that are very low or high should be dyed and autopsied. This service is provided by some RO manufacturers, or it can be managed directly. When autopsying or dying an element, check whether the membrane has patches on it. Some inferior membranes require suppliers to fix defects in their products by patching them, which reduces the active area and therefore the flow. This is also a potential area for salt passage.

Inspect the glue lines along the edges of the leaves to ensure that they are straight and that corners do not have excess glue. Figure 2 (Page 14)shows a corner on which excess glue is applied, which reduces the active area. Figure 3 (Page 14) shows an element that has precise, automated glue lines.

Other aspects of precision fabrication may be noticed by comparing elements side by side. For multi-leaf elements (usually 4 in. in diameter or larger), even leaf placement along the product water tube can be assessed visually as shown in Figures 4 and 5. More leaves and even leaf placement around the tube allow for more efficient water production, balance the water flow throughout the filter and are a sign of quality fabrication.

Besides the product itself, OEMs and brand owners must consider the business characteristics of their component suppliers to ensure that they are consistent with their own growth strategies. Questions to consider are:

  • Can my component suppliers meet my expected growth demands without having product shortages?
  • Do they have diversified manufacturing assets to meet my geographic needs?
  • Do my suppliers have a full range of products with the performance I need?
  • Do my suppliers have continuous improvement product development programs and ISO certification?
  • If customization is important, can my RO element suppliers provide me with the proper form factors for my products?

While it may be difficult to assess quality and consistency of RO elements without testing many elements, you can try to ascertain an RO supplier’s warranty rate of return. For example, Dow Water & Process Solutions declares return rates of less than 0.03% annually in order to demonstrate consistent product performance.

Diego Bonta is an applications development specialist for Dow Water & Process Solutions and a member of the WQP Editorial Advisory Board. Bonta can be reached at dbonta@dow.com.

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