RO/EDI: The Preferred Water Purification Technology for Food and Beverage Laboratories

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Please check out our RO Water System / Reverse Osmosis Water Purification System and Deionized Water Systems / DI Water System sections for more information or to find manufacturers that sell these products.

The instrumentation used and analyses conducted by food and beverage industry quality assurance and quality control (QA/QC) laboratories have improved significantly. The increasing sensitivity of this testing has led to stricter quality requirements for purified water. Manufacturers of water purification systems have developed new technologies, such as reverse osmosis/electrodeionization (RO/EDI) systems, that can meet these stricter requirements while integrating the necessary system monitoring and control capabilities. Even with the advancements in technology, many QA/QC laboratories continue to rely on distillation systems for producing purified water, despite the need for frequent maintenance, lower purity of water produced, and higher running costs. This article compares and contrasts distillation water purification apparatus (also known as stills) with the newer RO/EDI technology as they impact the food and beverage quality control laboratory.

Purification technology: Stills versus RO/EDI

A still uses the process of distillation to purify water. The technique of distillation was developed more than 500 years ago and has changed little since. Water is heated in a boiling pot to create steam that flows through a distillation column. The contaminants with boiling points higher than water are left behind in the boiling pot. The steam is then cooled in a condensing column and the purified water is collected.

Figure 1 - Elix^® 5 UV RO/EDI system (Millipore Corp., Billerica, MA). The systems are available in 3–100 L/hr capacities.

Figure 2 - Electrodeionization combines electrodialysis and ion exchange technologies to purify water. Comprised of sequential purification and concentration (waste) channels, an Elix module contains cation and anion exchange resins in a mixed bed that are separated by ion exchange membranes. An electric flow is passed across the module, which migrates ions toward the concentration channels and regenerates the resins through the electrolysis of water.

After repeated use, the boiling pot will be coated with organic and inorganic contaminants including scale, which is left behind in the distillation process. These contaminants must be removed by using a strong acid (~5% HCl) and a wire brush. Maintenance and cleaning of even a small still can take more than 1 hr per week. This is valuable time that can be used more productively running assays and documenting results to ensure that the products being produced and released meet the highest safety and quality standards.

Alternatively, RO/EDI combines multiple technologies to consistently produce very high water quality (see Figure 1). The first step is reverse osmosis, which involves passing water across an ultrathin membrane at high pressure. Reverse osmosis can effectively remove greater than 97% of all feedwater contaminants. The ability to broadly remove all forms of contamination, including ionic, organic, and particulate contamination, makes RO an ideal technology for purifying feedwater from a wide range of sources. The second stage is EDI, a technology developed in the mid-1980s for the removal of charged contaminants (mostly ions) from water (see Figure 2).

Use of an electrical charge to purify water produces very pure and consistent water quality because resins are not exhausted and exchanged as with traditional deionization techniques. The consistently pure water produced through the RO/EDI method removes the quality fluctuations seen with traditional water purification technologies.

The well-designed RO/EDI system has easy-to-change filtration packs to pretreat the water before the RO membrane. The most advanced systems monitor usage and alert the end user to exchange these packs. Incorporating system sanitization into the design of the pretreatment pack eliminates the need for an additional maintenance step. The maintenance can be done in an operation that takes less than 5 min and normally every 3–6 months, depending on the quantities of water produced. This is significantly less time than is spent maintaining the average distillation system.

Higher levels of purity with RO/EDI

The RO/EDI method not only requires less maintenance, but also produces a higher quality of water than distillation. A study¹ was conducted comparing single- and double-distillation systems to the RO/EDI method. The feedwater used in the study contained 1410 ppb of total oxidizable carbon (TOC). The single-distillation process produced water with a purity of 104 ppb TOC, and the double-distillation method improved this to 76 ppb. The RO/EDI method, which produced water with only 27 ppb of TOC contamination, is more efficient at removing organics because the boiling point of water is not a limiting factor to the purification technology. Using distillation technology for the removal of volatile organics (such as alcohol) is especially difficult due to their boiling points, which are lower than or near the boiling point of water. These compounds can only be separated through the use of more sophisticated (and therefore expensive) fractionation techniques. The removal of organic compounds is important when employing today’s sophisticated laboratory equipment.

In terms of ionic contamination, the three methods (single distillation, double distillation, and RO/EDI) produced water with similar purity levels. All three methods were tested using the same feedwater source. The major species of ions in the feedwater were measured at 176 ppm. Distillation purified water to a level of 18.6 ppb of ionic contamination with a single distillation and 5.9 ppb with double distillation. The RO/EDI method produces water with 1.7 ppb of ionic contamination.

When examining the levels of bacterial contamination, the RO/EDI unit also performed better. For the double-distillation system, samples were taken at both the end of the condensing column and the outlet of the system. The samples taken from the end of the condensing column showed no bacterial contamination. However, samples taken from the outlet of the system had bacterial counts as high as 31 cfu/mL. The RO/EDI system was also tested for bacterial contamination. The highest bacterial count from the outlet of the RO/EDI system was 1.3 cfu/mL.

Analyzing water purification costs

Stills are deceptively expensive to run. When purchasing a still, the consumer typically looks at the up-front capital costs and does not factor in the expenses of running the system. The greatest expense is the electricity needed to heat the water in a still. Even in a small still (~6 L/hr), the electricity to produce 1 L of water can cost between $0.04 and $0.15, depending on local electricity costs. Therefore, producing 50 L per day over a fiveday week can add up to $520–$1950 per year in electricity expenses. Stills also use an enormous amount of water. To produce 1 L of water, the typical double distillation requires about 50 L of water. The majority of this water is used in the condensing columns. This also can become quite costly in areas with high water costs. Combining the capital costs and utility expenses (water and electricity), it may cost between $0.15 and $0.25 to produce a single liter of purified water.

In comparison, a typical RO/EDI system (including hardware and consumables) should produce water for approx. $0.10–$0.15/L. In addition to the financial comparison, the lower maintenance requirements for an RO/EDI system will lead to increased productivity. This allows laboratory technicians to focus on their core mission rather than on equipment maintenance. A system that produces a higher water quality consistently will also lower costs by reducing the number of assays rerun due to inconclusive results.

Conclusion

The RO/EDI method is a major advancement in water purification technology. The method produces water with levels of ionic and organic contamination that today’s food quality laboratories require to run sensitive assays. Compared with traditional distillation methods, the RO/EDI system produces higher water quality, lowers operating costs, and reduces maintenance time.

Please check out our RO Water System / Reverse Osmosis Water Purification System and Deionized Water Systems / DI Water System sections for more information or to find manufacturers that sell these products.

Reference

1. Mabic S, Castillo E, Kano I. Comparison and assessment of water purification technologies using ion chromatography. Millipore Corp. Session 1090, Pittcon^® 2005, Orlando, FL.

Mr. Crescenzi is Business Development Manager, and Mr. Norris is Product Marketing Manager, Millipore Corp., Bioscience Div., 290 Concord Rd., Billerica, MA 01923, U.S.A.; tel.: 978-762-5312; fax: 978-762-5112; e-mail: [email protected].