By Sanghyeon Kang, PhD, CWS-VI

Several technologies (such as membrane, filtration, chemical treatment, etc.) are widely used for purification of drinking water. Reverse osmosis is considered the best technology for contaminant reduction, as it can remove heavy metals, pesticides, microorganisms and even some VOCs. Its production rate of purified water, however, is too low, requiring most water purifier systems to have a storage tank for the purified water. This makes the product bulky and increases the potential for secondary  contamination by foreign substances. RO systems with a storage tank have a higher risk of contamination by general bacteria or VOCs from the air, even if periodic service is supplied. Another limitation is low recovery; that is, a large quantity of water must be discharged as waste for prolonging the RO membrane life. The recovery shows different values depending on product types.

There are typically two types of purification systems with RO membranes. Under-the-sink (UTS) types have a pressurized storage tank for the purified water and to dispense it at a high flowrate. Counter-top (CT) or standing types (ST) have a reservoir to store the purified water and dispense it by the force of gravity. There is also a third category, the direct RO product (without a storage tank), which has a large surface-area membrane, such as a 3012 or 3512 module (general manufacturer terms to express RO dimensions). It doesn’t need a storage tank because it can produce a high flowrate of purified water. Each system has its own advantages and disadvantages.

UTS RO saves counter space and the pressurized tank supplies purified water from the bottom to a faucet. The pressurized tank, however, has its own drawbacks. The back pressure from the tank to the membrane increases as the tank is filled up. This results in the reduction of operating pressure of RO (the flux decline), as well as longer fill-up time, lower recovery and smaller TDS reduction, as shown in Figure 2.

On the other hand, these limitations are eliminated in CT/ST RO systems, although they supply purified water at a relatively low flowrate. CT/ST RO can produce purified water to fill the tank at a constant rate and dispense it to the customer at almost a constant rate. This system has a physical limitation, however; the reservoir should always be placed over the faucet to utilize gravitational forces to deliver the flow. Therefore, it requires a relatively bigger space on the sink. The direct RO system can be the best solution because it can continuously produce purified water and minimize secondary contamination. The bigger membrane, however, may result in an expensive replacement cost.

 Figure 3. Schematic diagram of water-on-water system

UTS-RO system with water-on-water tank

A differentiated UTS RO system to overcome the limitations of typical UTS RO systems uses a water-on-water tank instead of air-on-water tank. A water-on-water system uses feed water pressure to dispense purified water, as shown in Figure 2.

Using water to generate dispensing pressure has many advantages in comparison with a typical pressurized tank. First, it does not generate backpressure on the membrane when the tank is filled up. Second, the air-on-water tank cannot use a certain volume occupied by air gas but the water-on-water tank can use its whole volume, making the system size more compact. Typically, the system size is 50-percent smaller than a UTS RO. Finally, it produces and dispenses the purified water at the same flowrate as CT/ST RO. Figures 4 and 5 show a comparison of performance of each system.

One drawback to a water-on-water system is that it is more expensive than a typical UTS RO without a pump because it uses a complicated valve system to manage water. It can have a price position similar to UTS RO with a pump, which makes it difficult to compare costs because a UTS RO with a pump offers almost the same or even better performance than the water-onwater system.

Water purifier with electrodeionization technologies
All systems mentioned earlier have their advantages and disadvantages but they have the same limitation: low recovery because they all use RO membranes. RO recovery cannot be over 50 percent because the concentrate concentration is theoretically double the feed concentration at 50-percent recovery. The membrane fouling begins to accelerate after the 50-percent recovery rate, as shown in Figure 6.

An ion exchange membrane (an alternative to RO) is a strong candidate. It hasn’t been feasible to be applied to water purification, however, because membrane prices have been too expensive, until now. A new electro-adsorptive media has been developed by a US company that makes membrane prices cheaper, enabling their use in water purifiers. The new electro-adsorptive media and high electric voltage reduce ionic contaminants in the feed and periodically regenerate saturated media by reverse electric current, as shown in Figure 7.

It is possible to achieve over 50- to 80-percent recovery, depending on water quality. It is thought that this technology could be the most advanced technology to overcome typically low recovery RO rates. This technology has some limitations, however. It cannot reduce non-polar  contaminants as ion exchange resin and it uses high-voltage electric power.

Figure 5. Comparison of time to fill up the 6L tank

Figure 6. Flux decline with recovery in RO membrane (200 ppm TDS and 40 psig operating pressure with 2008 RO membrane)

Figure 7. Principles of purification (left) and regeneration (right) process by a new electro-adsorptive media




Each water purifier with RO technology has its own advantages and disadvantages, such as size, efficiency, price, etc. Typical UTS RO is the most popular water purifier system, with a competitive price and the ability to dispense purified water at a high flowrate, although it shows relatively bad performance, such as low efficiency and daily production capacity, especially in the case of a system without a pump. CT/ST RO systems have the advantage of offering cold and hot functions inside a small system, although it has a design limitation and risk for secondary contamination. Some innovative products like water-on-water, deionization technology, etc., have been introduced to the market, although they have yet to achieve broad acceptance. Whatever the next technology emerges as an alternative to traditional RO membranes, water purifiers should save more water and energy and supply safe water to customers. To help facilitate these changes in technology, the industry will have to prepare the right standards to evaluate the performance of novel systems that are emerging in the market.


  2. H. Martin Jessen. Evaluation of a New Ion Exchange Drinking Water System. Water Conditioning & Purification International, April 2011.

About the author

Dr. Sanghyeon Kang, CWS-VI has 15 years of experience in water treatment and bioseparation technologies. He received his BS, MS and PhD in chemical engineering from Korea Advanced Institute of Science and Technology (KAIST). Kang is Principle Research Manager in charge of development of water purification technologies at Coway. He may be reached via email,

About the company
Coway ( is the largest home-appliance manufacturer headquartered in Seoul, Korea. The company manufactures water and air purifiers, digital bidets, water softeners and other well-being home appliances. The company’s industry-leading market share in Korea ranges from over 50 percent for water purifiers to 40 percent for air purifiers.



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