By Niklas Pahlman and Dr. Tibor Nemeth

Tankless? Yes! Technological advances have made it possible to produce convenient reverse osmosis (RO) point-of-use (POU) equipment that delivers a continuous flow of pure water on demand—without a storage tank. Let’s call them “tankless” RO systems.

Such systems are currently in production, contrary to claims in the article, “RO: An Overview on Advances in POU Technology” (WC&P, July 2000). “Undersink RO designs currently all have a storage tank,” the article states. This is incorrect due to the latest technological advances, and thus the way a tankless domestic RO system can be designed. This article also demonstrates how higher than normal recovery rates can be achieved, which saves customers from large amounts of wasted feed water. Finally, the article explains why a tankless RO system produces purer water than a conventional RO system for home use.

Tankless option
Why strive to get rid of tank storage? The advantages are both numerous and important.

Better capacity: A direct flow of water on demand ensures delivery of a desired amount of pure water for drinking, cooking and rinsing purposes—without much wait.

More convenient: You’ll always obtain water on demand. Moreover, since water isn’t stored in a tank but flowing directly from the top, it will be cooler.

Purer: A direct flow, tankless RO system should require a pressurized pump that squeezes as much water as possible through the membrane, in order to maximize production capacity of permeate (purified water) flow. The RO process is thus optimized (see Figure 1). This will result in the highest possible rejection, because achieving the fastest possible flow through the membrane would only allow a minimized amount of impurities to have time to pass through the membrane and mix with permeate water.

Furthermore, a tankless solution avoids the first amount of permeate water from a tank that’s never fully purified. It’s a phenomenon referred to as “TDS creep,” where during standby periods there’s ongoing mass flow across the membrane until the concentration equalizes on both sides of the membrane—known as diffusion. Since there’s no escaping this, the quality of water collected in a tank is inevitably affected. If a tankless system is equipped with a quality monitoring system, however, it will signal when water coming out of the faucet has reached high levels of purity and, thus, you can let the first amount run to the drain.

The tankless system described here rejects more than 99 percent of most impurities. It has high refection rates for many substances including arsenic, lead, Cryptosporidium and specific families of bacteria such as E. coli, Heterotrophic bacteria, coliforms, Enterococcus fecalis and Pseudonomas auruginosa.

Safer: There would be no risk of bacteria build-up via biofilm, which is associated with storing water in a tank. Bacteria colonization on bladders is a well known and common problem in even larger systems. To ensure water quality after it’s been stored for some time, one should flush the tank until fresh water can be used for drinking purposes. Low performing systems cannot handle this without causing some major inconvenience for the customer, because it would be a while for purified water to fill up the tank.

Taste: There’s no risk of water obtaining unwanted taste from the inside of a pressure tank. The bladder may leach contaminants into water resulting in bad taste. To improve taste, a post-carbon filter is utilized in many cases. In a non-backwashable post-carbon filter there will be a risk of bacteria growth and, in some countries, a post-ultraviolet or other disinfection treatment is required. This results in added costs as well as unreliable solutions.

Once you’ve decided tankless is the best alternative, the next question becomes, how to get rid of the tank?

The determining factor
Membrane size is a key determinant in the feasibility of tankless RO. The spigot-flow, or capacity, is determined by the performance of the membrane surface. It’s referred to as “flux,” which is the amount of water produced on a surface spot of the membrane over a given time. Advances in membrane technologies and refinements in control systems increase the capacity, but this increase—even if doubled—is limited.

It’s the size of the membrane surface area that determines the quantity of pure water produced within a given time. Of course, the quality and characteristics of membranes as well as the design of RO systems are significant factors, but when it comes to permeate flow capacity, choosing membrane size is what really matters.

The tankless system described in this article delivers as much as four liters (one gallon) per minute. One model now available delivers as much as four liters per minute. In order to provide such a capacity, a spiral wound thin film composite (TFC) membrane was chosen with a surface area of 8 square meters. Such a membrane is much larger than most, but it’s still been possible to include it in a compact RO system that fits under the sink.

Is that all that’s required? Well, some other components need to be considered and some engineering skill is required to make it come to life.

Squeezing
Since the membrane is large, relying on the pressure that comes from the tap won’t be enough. In order to squeeze water effectively through such a large membrane, the unit must be equipped with a pressurizing pump to carry out the work.

Membrane cleaning
It’s very important to provide a system that keeps the membrane surface clean in order to prevent scaling and fouling of the membrane. This is essential for maintaining membrane performance and allowing for a longer lasting life. In particular, this is very important when using a large membrane, since replacing it will cost a considerable amount of money. A membrane that’s effectively cleaned should last five to seven years under normal conditions and use. This is possible by using a system that automatically flushes the membrane after operation.

When pure water has been tapped from the faucet and shut off, a two minute long flushing of the membrane will start automatically. Moreover, at regular intervals of twice a day, an automatic flushing is carried out in order to combat diffusion and keep the membrane clean. Using a pump for this purpose isn’t essential, but it would allow for a very effective flushing of the membrane. The importance of using a pump increases with the size of the membrane, in order to guarantee enough pressure to flush the entire surface.

Lastly, in order to guard the membrane from strain and impurities, it’s essential to have prefilters that take care of particles of sediment as well as chlorine, because TFC spiral wound membranes are sensitive to chlorine.

Recovery rate
Installing a pump can play another very important role. On the system described here, a “loop system” has been designed to effectively recover intake water. The pump is the driving force and ensures there’s intense circulation of water in the vessel where the membrane sits while supporting mechanisms limit the escape of reject water. This “loop system” allows it to reach a uniquely high recovery rate of 80 percent, which makes it more economically and environmentally sound.

Managing tough intake
Using a considerably sized membrane in combination with an effective method of membrane cleaning not only brings advantages for tankless applications, but such a product will also manage tougher water conditions, i.e. higher contents of impurities. The tankless system discussed here will take care of brackish water with a salinity of up to 7,500 parts per million (ppm)—measuring the amount of total dissolved solids (TDS).

Price vs. performance
As described, technological advances allow a tankless system to fit under the kitchen sink. But will it cost a fortune? No! For an obvious reason, membranes represent the lion’s share of an RO unit’s expense. Thus, a direct flow RO purifier will cost more—but the added value outperforms the conventional RO unit by far.

Furthermore, it shouldn’t be forgotten that a very effective membrane saves you from the cost of a tank and a number of filter stages as well as other mechanisms included in a conventional system with tank storage. This includes an automatic shutoff valve, air gap and post-treatment filters. These and other shortcomings are comprehensively described in the article cited earlier.

The article argues that product development will be toward lower cost and smaller sized membranes. Still, many people believe customers will demand better performance and convenience. This requires membranes of considerable sizes. Customers must be convinced the benefits of a tankless system are affordable.

Many are hesitant about the tankless RO system because of the price gap between it and a conventional RO unit. As a result, a new tankless model will be produced later this year. The lighter and more compact version will have a smaller membrane with a surface area of 5 square meters. It delivers a continuous supply of 2.5 liters (0.7 gallons) per minute.

Conclusion
The tankless RO purifier is what the modern household demands. Technological advances have made it possible to produce tankless RO purifiers that are attractive in terms of design and capability as well as affordability.

About the authors
Niklas Pahlman is sales manager at Electrolux Water Purification, a product line of the Electrolux Group based in Stockholm, Sweden. Electrolux is a leading manufacturer of kitchen, indoor and outdoor appliances with brand names that include Frigidaire, Westinghouse, Eureka and Electrolux. Pahlman holds a master’s degree in business administration. He can be reached at +46 31 738 77 82 or email: [email protected]

Dr. Tibor Nemeth is a water chemistry specialist at the Water Purification Centre, located at the Electrolux factory in Torsvik, Sweden. He holds a doctorate and a master’s degree in chemical engineering. He can be reached at +46 8 318 006 or email: [email protected]

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