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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
-
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].