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Numerous facilities operating secondary water distribution
systems are searching for the appropriate point-of-entry (POE) disinfection
technology to reduce the occurrence of Legionella and other pathogens at distal
sites. POE disinfection technologies are relatively new. Therefore, some uses
have little field data available to support vendor claims regarding
applicability and effectiveness when used outside traditional water treatment
Chlorine dioxide as a disinfection method to control Legionella
has been used effectively in the United Kingdom and Europe for several
years.1?5 New and safer chlorine dioxide generation methods, increasing
concern over pathogens in secondary distribution systems and marketing by
chlorine dioxide vendors may influence an increased use of chlorine dioxide in
the United States.
Chlorine dioxide is not a new technology for public drinking
water facilities or pulp and paper producers, but its use as a secondary
treatment system for small-scale applications is new. Beyond the chemistry and
microbiology, potential small-scale operators want answers to a few simple
questions: Should I use it? How does it work? What extra work is it going to
make for me?
Should I Use Chlorine Dioxide?
This question is not answered as easily as one would hope.
There are numerous POE disinfection technologies available. Some are proven
while others are not. Selecting the appropriate technology for a specific
system and situation should be done with care. Table 1 compares chlorine dioxide
with three other technologies used to control Legionella in potable water.
Table 1 does not indicate a clear technological superiority.
This is where the facility manager has to make informed decisions on how the
POE system is going to be applied. Heat and flush and hyperchlorination are
disinfection methods that should not be applied on a regular basis due to
direct and indirect costs associated with implementing these strategies,
including corrosion and man-hours. To provide continual disinfection a system
requires a technology that provides a residual, without adversely affecting the
distribution system. The choice between chlorine dioxide and copper-silver
ionization may be based on water quality parameters, point of application and
budget. Benefits of chlorine dioxide include effectiveness over a broad pH
range, easily measured concentrations and approval by the EPA as a primary
drinking water disinfectant. Currently there is more published information on
the efficacy of copper/silver against Legionella6?9?chlorine
dioxide efficacy has been shown in Europe and is under investigation in the
United States?and copper-silver is EPA-approved as a device rather than a
disinfectant. No matter what technology a facility selects, a monitoring
strategy always is recommended to ensure proper disinfection is accomplished.
How Does Chlorine Dioxide Work?
Chlorine dioxide possesses several chemical traits that make
it perform well as a disinfectant. Chlorine dioxide?s oxidation reduction
potential (0.95V) is much lower than chlorine (1.36V) and its oxidation
capacity (5) is much greater than chlorine (2).10,11 The oxidation reduction
potential (ORP) measures an oxidizer?s strength or speed at which it
reacts with an oxidizable material. Although chlorine dioxide has a low ORP, it
is more selective as to the types of oxidizable materials with which it reacts.
Chlorine dioxide targets specific organic molecules including cysteine,
tyrosine, methoionyl, DNA and RNA. Chlorine and ozone have much broader
reactions.12?15 The oxidation capacity indicates that on a molar basis
chlorine dioxide has a greater capacity for disinfecting over chlorine. The
selectivity and oxidation capacity of chlorine dioxide makes it a stronger
oxidative disinfectant than chlorine.10,16
On-site generation is the only economically viable method to
implement chlorine dioxide. Old generation methods were large in scale,
required numerous hazardous chemicals and produced dangerous concentrations of
chlorine dioxide with uncertainty as to the purity of the final product. Until
recently, these disadvantages have excluded small system operators from using
chlorine dioxide. New methods offer the ability to produce controlled
quantities of chlorine dioxide at safe concentrations with less equipment. Current
generation methods involve the oxidation of sodium chlorite to chlorine dioxide
and various byproducts. The three methods used to produce chlorine dioxide are
summarized in Table 2. All the methods offer the advantage of having a small
footprint and easy integration into existing distribution systems.
The newest generation method utilizes an electrical source
and membrane technology to directly oxidize sodium chlorite. This new
generation technology offers easily scalable generators without the need for
chlorine gas or strong acid handling and storage. These generators typically
offer the flexibility to provide 5 grams/hour to 2.4 kg/day of chlorine
dioxide. The chlorine dioxide can be fed into the water system at various
points (e.g., cold water supply, hot water supply and reservoir) depending on
where disinfection is desired.
What Extra Work Will This Create?
This study evaluated a DIOX generation system installed in a
Pennsylvania hospital (Table 2). The system was evaluated during an on-going
field study to analyze the effectiveness of chlorine dioxide for control of
Legionella. Table 3 outlines the conditions of the installation. This
installation has three chlorine dioxide generators designated to cycle through
rolls as lead, lag and stand-by. The redundancy allows a unit to be off-line
for maintenance or provides multiple generation units during high demand
periods. The chlorine dioxide is fed directly into a reservoir prior to being
distributed to the hospital water distribution system.
Chlorine dioxide generators can be as automated as the user
desires. The units can operate on flow-paced and constant production modes as
well as being controlled through external inputs such as ORP meters or in-line
chlorine dioxide analyzers.
Table 4 provides a breakdown of the typical operation and
maintenance costs associated with operating the generators at this
installation. While maintenance has a larger number of line items to address,
operation is the most time intensive activity due to regulatory requirements
(typically less than 1 hour per day).
Maintenance on the generators consists of required and
preventative items. The required maintenance involves changing the membrane
containing cartridges. As chlorine dioxide is generated, these cartridges slowly
lose their oxidizing ability and require replacement (typically after 2,000
operating hours). Preventative maintenance includes replacing various filters
and tubing. The system is driven with a peristaltic pump, so pump line
replacement will provide consistent flow through the generator. Other tubing
and filter replacement is suggested to keep the system operating smoothly. If
placed on a routine maintenance schedule, these activities do not prove
challenging and are not time consuming.
Operating the generators requires daily observations. A
brief daily inspection includes verifying that the units are operating,
checking the volume of sodium chlorite available and checking the softener
brine salt tank. The time consuming part of operating the generators involves
complying with regulatory guidelines. Table 5 outlines the requirements set by
the U.S. Environmental Protection Agency (EPA) for those systems that use
chlorine dioxide as their primary disinfectant. Installations using chlorine
dioxide as a supplemental disinfectant may not be required to implement the
same monitoring programs as primary operators, but should check with local
regulators for confirmation.
Proper monitoring of chlorine dioxide and chlorite (a
disinfection byproduct) can be challenging. Currently, there is debate as to
the best measurement technique for chlorine dioxide.17?22 The EPA
approved methods include DPD, Standard Method 4500-CLO2 D or Amperometric
Method II, Standard Method 4500-CLO2 E.13 The study facility uses the N,N-Diethyl-Phenylenediamine
(DPD) method with glycine to mask interferences. The test is consistently used
and the results easily can be compared and used to make operational adjustments
as necessary. The DPD test for chlorine dioxide is similar to that for free chlorine
and takes only several minutes to perform. In order to analyze color change the
DPD test requires a hand-held field colorimeter or bench top spectrophotometer,
ranging in cost from $300 to $2,000. Chlorite analysis is commonly performed
using Amperometric Titration or Ion Chromatography (IC).13 Amperometric
Titration may be the most practical for facilities that do not have access to
IC equipment. Overall, monitoring the operation of the generators may take from
15 minutes to an hour each day depending on the size of the facility served,
location of the generators and previous violations requiring increased
Automated analysis of chlorine dioxide at concentrations
found in potable water is a developing technology. These technologies are not approved
EPA methods but may offer assistance in providing a consistent chlorine dioxide
residual. The two most prominent automated technologies are direct in-line
chlorine dioxide measurement and indirect ORP measurements. The direct
measurement of chlorine dioxide in a potable water stream is a very new
technology with only a few companies offering these special sensors. The
facility evaluated in this study does not utilize in-line measurement but
rather indirect control by ORP. ORP meters monitor the bulk oxidation-reduction
potential of water. Chlorine dioxide exerts an ORP that can be measured by
these instruments. The ORP measurement (mV) theoretically can be related to
chlorine dioxide concentration and used to control the lag generator?s
production. However, many other constituents in potable water exert an ORP
including free chlorine, organics and pH. Since the bulk of the supply water to
the hospital facility is from a chlorinated municipal surface water source, the
ORP is highly variable. Even with dual ORP meters comparing incoming water with
the reservoir, this monitoring system has not provided a reliable method for
controlling the lag generator. Figure 1 shows the DPD measured chlorine dioxide
residual in the reservoir compared with the absolute ORP measurement. Clearly,
in this facility, ORP does not correlate with residual chlorine dioxide levels.
Despite this result, the operators have been able to consistently control the
chlorine dioxide concentration at their facility. This may in part be due to
the buffer of a 520,000-gallon reservoir that can absorb any spikes or dips in
The facility manger is pleased with the operation of the
chlorine dioxide generators. The system has been online since June 2000 without
incident. In addition to installing the chlorine dioxide system, the facility
has implemented an environmental monitoring program to evaluate the efficacy of
chlorine dioxide against Legionella. The study is ongoing with results to be
published during 2002.23 Currently, the evaluation has seen a significant
decrease in the number of Legionella positive distal sites since the
introduction of chlorine dioxide. Adequate chlorine dioxide residuals can be
maintained in the main distribution system and at cold water distal sites.
Difficulties have been encountered in maintaining an adequate chlorine dioxide
residual in the hot water system. This is attributed to the distance between
the chlorine dioxide injection point at the reservoir and the hot water systems
and higher water temperatures leading to increased decay rates and loss of
chlorine dioxide gas in boiler head space. This may not be a problem for
systems installed on the hot water return lines or hydraulically closer to the
hot water make-up.
As is the case with any POE treatment technology, good
engineering and maintenance practices are critical to success. Maintaining a
consistent residual, increasing flow in low demand areas (flushing) and
removing dead legs have been shown to result in better disinfecting performance.24,25,26
While every facility has different requirements, chlorine
dioxide appears to be a valid disinfectant for consideration by facilities
looking to install an effective and easy-to-operate POE disinfection
For a list of references, visit our website at
This article represents information collected during
completion of graduate research by Frank Sidari. The research has been
conducted in conjunction with Carnegie Mellon University and the Pittsburgh VA
Medical Center. Information and data collection has been provided in part by
hospital and vendor personnel.