Using chlorine as secondary treatment in small-scale applications
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 plants.
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 scrutiny.
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 production.
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 technology.
For a list of references, visit our website at www.waterinfocenter.com.
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.