By Jon Goodman

Summary: Once a customer has narrowed their choice to a particular type of water purification unit—RO membranes—for their home, it only stands to reason they’ll want to make sure they’re getting the most for their money. As a dealer, you want to assure them the product is backed with a good reputation for performance as well as affordability. Some factors for obtaining the right equipment to provide your customers are outlined here.

There are many different water treatment technologies for consumers to choose from to purify drinking water, ranging from simple filters that remove sediment and chlorine to reverse osmosis (RO) membranes that remove salts and other dissolved solids. Of all the technologies available, RO offers the “tightest” filtration available today, and is the technology of choice for a growing number of customers who want the added security of purified water in their homes.

As the RO market has grown, so have the number of membrane suppliers and product choices. Of course, there are options with different prices and quality. Some of the important considerations one should use to select an appropriate RO membrane for a system include third-party certification, system supplier, membrane size, membrane type, flow rating and salt rejection.

Meeting regulations
Third-party certification is a very important issue to consider. A few states have laws requiring certification of drinking water treatment equipment and components sold to consumers, and it’s essential to make sure the membranes comply with all regulations. Many RO systems, as well as RO membranes, have been component and performance certified by accredited labs or agencies, such as the Water Quality Association, NSF International, Underwriters Laboratories, National Testing Laboratories or Spectrum Labs.

When deciding which RO system to sell, it’s important the system is certified (not just tested to a standard). This helps protect consumers from overstated and/or misleading claims and strengthens the industry’s perception in the minds of the general public. When selecting a replacement membrane, it’s equally as important to ensure that the system certification isn’t impacted. The safest way to ensure this is to replace the original membrane with the same model.

Choosing a system supplier is also an important step when selecting RO equipment. Some questions to ask include: Is the company experienced with RO? Since well-established companies may offer services others don’t, how many years has it been in business? Available resources may also come into play, so is it a large or small operation? Is the company an industry leader and well-recognized by customers? These are all basic considerations, and it’s also important to consider the range of systems offered and service levels. Are products available from stock? Is technical assistance available? Are sales tools available?

The system supplier is also very important when selecting replacement RO membranes since many companies utilize custom components with particular tolerances for optimal fit and performance. This is where getting the right size membrane becomes imperative for proper system operation.

Similar to system suppliers, the specific membrane element supplier is also important to consider when selecting RO membranes. Different companies have different strengths and weaknesses, and it’s important to ask questions of the supplier. For example, is the membrane supplier stronger with standardized products or custom products? Does the membrane supplier stand behind their products? Does the supplier have automation and other leading technology that help assure performance, reliability, consistency, etc.?

Once a system supplier and specific membrane element supplier are identified, it’s time to start analyzing the actual membranes.

Membrane properties
Size is important because membrane elements used for home drinking water purification have very similar dimensions. For instance, spiral-wound construction may produce elements that look alike—but how similar are they? Generally, membranes fit into one of two categories—standard or custom.

Standard membranes have different body diameters (often referred to as scroll diameter) ranging from 1.3-1.9 inches and a seal (commonly known as a “brine seal”) designed to fit in 2-inch Schedule 40 pipe. Standard membranes have one or two rubber o-rings at one end of a 0.68 inch outside diameter permeate tube. The nominal length of a standard membrane element is 11.74 inches. These products are often referred to as “1512,” “1812” or “2012” elements.

Custom membranes are specifically designed for individual system suppliers. One advantage of using custom parts is it ensures the right membrane is being used in a system; this is especially true for certified equipment. When selecting replacement membranes, it’s very important to know if a custom membrane is required because its size differs from that of standard membranes.

Types of membranes
In addition to determining the size used, it’s necessary to select a membrane type as well. There are two primary RO membrane types to consider—polyamide thin-film composites and cellulosic (or cellulose acetate). Although cellulose acetate technology pre-dates other membrane types, use of these membranes has been largely displaced by thin-film composite membranes offering superior operating performance, service life and overall economy. In the past, thin-film composite membranes cost significantly more than cellulose acetate products. During the past few years, however, the prices of the two membrane types have become very similar and this is no longer a major issue.

One convenience factor often used when purchasing membranes is availability of dry products. Dry products offer a longer shelf life and reduce the possibility of biological contamination when compared to products shipped wet. This is very important when one considers the number of membranes inside service vehicles during warm months.

Performance criteria
While there are other differences between the two membrane types to consider for larger systems, the biggest differences for home drinking water systems are performance and chlorine tolerance. Thin-film composite membranes have documented performance advantages of higher flow rates and better salt rejection. Cellulosic membranes have the advantage of being tolerant to the chlorine in feed water. In fact, these latter membranes are generally only used on water sources containing chlorine. RO systems containing thin-film membranes protect the membrane from chlorine damage with a carbon pre-filter.

Each criterion used for selecting a membrane is important, but flow rating is probably the most commonly used. Flow rates are often included in the product names. These are usually the most visible means of differentiating between products.

Going with the flow
When considering flow, there are two important things to keep in mind. First, most households use less than five gallons of purified water per day for drinking, cooking, making coffee and other common applications. So while a membrane element may be rated to purify 50, 75 or even 100 gallons of water per day, in practice, they supply much less water. The value of high-flow membranes is they produce more water at lower line pressure. This is very important for many overseas customers who live in apartment buildings with very low water pressure.

In the United States, with line pressure commonly in the 40-60 pounds per square inch (psi) range; and, with under-sink systems that include a holding tank, high flow membranes simply fill the tank faster. This is why most membranes sold in the U.S. are rated at 36 gallons per day (gpd) or less.

The second thing to keep in mind about a membrane’s flow rating is the pressure required to produce the specified amount of permeate or product water. Different membrane suppliers use different pressure conditions when rating membranes, with pressure ranging from 50 to 65 psi. For example, a 36 gpd membrane rated at 50 psi will produce over 43 gpd at 60 psi and over 47 gpd at 65 psi. As this example shows, a 36 GPD product from one supplier isn’t always the same as a 36 gpd product from another. Keep in mind that temperature and total dissolved solids (TDS) can also affect actual performance.

The other common performance criteria used to differentiate RO products is salt rejection. Once again, the manufacturers’ test conditions, especially the feed water, have a major impact on rated salt rejection. Because so many home drinking water systems and membranes are certified to reduce specific contaminants, certification may be a more useful means of differentiating one membrane from another than a manufacturer rated
salt rejection.

While flow and rejection are measured performance criteria, product reliability and consistency are highly valued by most companies managing large sales volumes. Every service call costs time and money and every problem may damage credibility in the eyes of the customer. Though consistency and reliability are hard to measure, they are extremely important criteria to keep in mind as they have a direct impact on a company’s reputation.

Of course, no list of selection criteria is complete without mentioning price. But like any product, price doesn’t always equal value when it comes to RO membranes. While price is always important, the issues discussed above may deliver more value or cost than the entire price of the membrane element.

With the market for water treatment continuing to grow, sales of new RO systems, replacement membranes and components, and the servicing of existing equipment continue to grow as well. When selecting a RO membrane, it’s important to keep in mind the many ways to differentiate one product from another. In fact, knowing these criteria will probably help make the sale.

About the author
Jon Goodman is global marketing manager at Dow Liquid Separations. He can be reached at (952) 897-4386 or email: [email protected].


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