Summary: There's been much talk about movement toward a "tankless" RO system. And, while advances in technology and design have meant less cramped space under homeowners' sinks, limitations of high output membranes still make that a far flung notion. The following article discusses the state of the RO industry today.

The point-of-use (POU) water treatment industry has changed and grown immeasurably in the last 20 or so years. This is especially true for reverse osmosis (RO) systems that today do more and are better and faster than the RO systems commercially available 10 or even five years ago. Advances in membrane and filtration technologies, coupled with refinements in RO control systems have spawned an era of truly affordable, high quality drinking water systems for homes and small businesses.

Residential RO systems
Since introduction of the first RO system for home use in the mid-1960s, refinement and better affordability have been the mission. RO membranes separate many contaminants from water by forcing the water molecules, under pressure, through a semi-permeable membrane. The resulting "product water" or permeate can contain as low as 5 percent of the original dissolved solids content (depending on the feed water makeup and membrane properties). This product water is usually stored in a pressurized storage tank for use by the customer and delivered through a dedicated faucet on the sink-top. Incorporation of high capacity prefilters and postfilters and design refinements complete the product to ensure customer satisfaction.

Cellulosic membranes
The first home ROs were based on cellulosic membrane materials (cellulose acetate or CA; followed later by cellulose tri-acetate or CTA). CA was the first version of an RO membrane, but is virtually unused today. More advanced CTA membranes became the mainstay of the home RO channel since they were readily available and fairly inexpensive. They also exhibit a fairly high tolerance toward oxidizing chemicals such as chlorine. Since most U.S. municipal water supplies are chlorinated to control bacteria levels, this chlorine tolerance was an important contributor to membrane life.

However, CTA as a material exhibits some distinct disadvantages as well. It's a comparatively low flux membrane material. Flux is defined as the amount of water in gallons that can be passed through one square foot of membrane and is represented by gallons per square foot per day (GFD). This results in more square inches of membrane in a roll required to achieve the same flow rate as that of a comparable thin film membrane element. Also, although some recent advances by membrane manufacturers have improved CTA's pH tolerance, they're still limited in application. If feed water pH is higher than 8.5, CTA membranes begin to quickly degrade and lose total dissolved solids (TDS) rejection performance. When this happens, the CTA membrane is said to hydrolyze, a condition characterized by high output and poor rejection. Lastly, CTA is more sensitive to high feed water temperatures (see Figure 1). A typical CTA-RO system has an upper limit of 85°F for feed water temperature. And feed water temperatures approaching 100°F (38°C) aren't uncommon in many parts of the world.

Figure 1. Production rate vs. feed water temperature

SOURCE: WQA RO Manual

In spite of these disadvantages, CTA dominated the point of use RO industry for the first 15 years or so. Then in the early '80s, the first thin film membrane--which is made of an ultrathin active layer of polyamide polymer coated on a much thicker polysulfone polymer support layer-appeared on the market for home use. CTA still had a price advantage over thin film and, therefore, continued to enjoy a good percentage of the market. The advantages of thin film-as well as advances resulting in lower, more comparable pricing-however, have resulted in what is, for all practical purposes, a full swing to thin film membranes for home RO systems.

Thin film composite membranes
Thin film membranes enjoy multiple performance advantages over their cellulose-based cousins. Thin film membranes actually "reject" or exclude a higher percentage of many common water contaminants resulting in a more "pure" finished product (see Figure 2 & 3). They're more tolerant of higher feed water temperatures as standard thin film RO products can operate up to 100°F. Thin film membranes also exhibit a higher tolerance to extremes of pH, operating normally within a pH range of 3 to 11. The main disadvantage of thin film membranes is that they have almost no tolerance for chlorine or other oxidizers.

Figure 2. Percent rejection vs. feed water TDS level

SOURCE: WQA RO Manual

Systems manufacturers have compensated for this by inclusion of an activated carbon filter as a prefilter. However, this is the single biggest weakness in RO system design because the RO membrane will only remain effective as long as the prefilter is removing a very high percentage of the feed water chlorine concentration. Make sure the manufacturer includes a high quality, high capacity carbon filter before installing any thin film RO system on chlorinated water.

Figure 3. Percent rejection of salts vs. net pressure

SOURCE: WQA RO Manual

Thin film membranes have a higher flux rate, allowing manufacturers to use less membrane material to achieve the desired flow rate from a given sized element. Advances in thin film technologies have resulted in large flux increases. Elements that were limited by their size (2-inch x 12-inch) to a maximum of 20 gallons per day (gpd) a decade ago can now be manufactured to provide up to 75 gpd. A 100-gpd residential elements appear to be on the near horizon with at least one membrane manufacturer test marketing the design.

Flux and flow rates
In general, membrane materials have a maximum flow rate, which results in premature fouling of the membrane if exceeded. Typical under sink RO designs are equipped with some form of reject water flow control. By controlling the amount of flow to drain, manufacturers of residential RO systems can walk a balance between premature fouling (by allowing some amount of water to exit the system, constantly flushing the membrane surface of contaminants) and efficiency.

Higher flux thin film membrane materials have been developed for industrial applications where much attention to prefiltration and water treatment (i.e. softening, chemical feed and bed/depth filtration) offers some amount of added protection not available in a standard residential application. These higher flux membranes allow more gallons to permeate per square foot, but this is compensated for by the increased attention to prefiltration. The end result is new thin film membrane materials that can almost double standard thin film GFD flow rates. This allows the industrial user to operate at lower feed pressures to produce comparable amounts of water. In addition, this concentration on composite polyamide chemistry-thin film membranes-has helped in the development of ranges of both RO and nanofiltration (NF) membranes, all having their own unique rejection and flux characteristics. For example, a standard 2-inch diameter by 12-inch long membrane designed for use in a residential system could be either an RO or NF membrane. Containing about 4.8 square feet of active membrane area, the high rejection polyamide element (RO) will produce around 35 gallons a day, while the NF element-containing the same amount of active membrane area-will produce about 80 gallons a day under similar operating conditions.

Undersink installations
The natural tendency would be to apply these materials for home use. However, the general application of high flux elements must be considered very carefully. Undersink RO designs can be susceptible to a phenomenon known as "TDS creep." This is the tendency for the product water in the storage tank to slowly increase in TDS over time. RO membranes take a little bit of time to return to optimal rejection level after sitting idle for some time. This is because, without forward pressure applied by the feed water driving water molecules through the membrane (RO), the natural phenomenon of diffusion will apply. That means, when the membrane is idle, the concentration of salts on the feed and product sides will move toward equilibrium (i.e., the same TDS on feed as on product side). When the system restarts, this "slug" of high TDS water will enter the tank. Secondly, thin film membranes increase in rejection over the first five to 15 minutes of operating time at full driving pressure. So, although they're not operating optimally, they still deliver this first water to the tank. The faster the membrane can refill the tank (i.e., the higher output membrane with more surface area), the worse this TDS creep can be. A well designed RO system will take this phenomenon into account by balancing RO membrane output with shutoff valve characteristics. A poorly designed RO or one with too much membrane for the application will result in lower quality product water.

Air gaps
A properly installed RO system will make use of some form of an air gap device. This is usually incorporated into the RO spigot. Since proper system design dictates a reject water to product water ratio of anywhere from 3:1 to 5:1 or more, the air gap device must be able to handle the reject flow rate. However, since a common consumer complaint is "air gap noise," one should be careful when considering very high output membranes. Basically, this translates to the higher the membrane rating, the more noise you can expect at the RO system air gap and/or drain connection.

Storage tanks
Undersink RO designs currently all have a storage tank. The tank is typically a captive air design (air charged with a bladder or diaphragm to hold the treated water). Most undersink RO systems are controlled with some form of water conserving automatic shutoff valve. These valves are intended to maintain available water in the storage tank at some volume that maximizes customer satisfaction. In other words, the shutoff valve must reopen the feed supply before the tank runs out of water. The most efficient designs will turn off at a pressure high enough to store a satisfactory amount of water (but not so high that finished product quality suffers from TDS creep) and turn back on before the tank reaches a critically low level of storage. A well designed RO system optimizes this balancing act by keeping water in the tank fresh and low in TDS while ensuring that, in all but the highest use scenarios, the customer has water left in the tank when needed.

Quicker servicing options
Certain RO system manufacturers have also developed disposable, encapsulated elements or modules for easier servicing of the system. The RO element is permanently sealed inside a plastic housing with easy-threaded or other convenient end connection fittings so the user can easily replace the spent module. A simple hand twist slides the fitting into the bracket. Change-out of modules is quicker, cleaner and more efficient with an encapsulated element. Since there's no actual contact with the spent membrane, servicing of the RO system is also more sanitary.

Conclusion
So what does the future hold? We feel new efforts are to simplify membrane systems in order to reduce size and cost. A chlorine resistant thin film composite membrane would greatly reduce prefiltration while extending membrane life. Membrane manufacturers continue to drive more water through small membrane elements. Could this eventually result in "tankless" RO for the home? Perhaps. So far, even a 100-gpd membrane can only deliver 0.07 gallons per minute (gpm) to the faucet with fairly warm water and good pressure. This drops significantly as temperature and pressure drop. This wouldn't be acceptable to the standard consumer, so a tank is still required. When a small, low cost element can deliver 0.5 gpm of reduced TDS water, we may see a new system on the market. In the interim, Figure 4 offers a good comparative chart to use when weighing your options between membrane choices.