disinfection Reprinted from Journal AWWA, Vol. 106 (10), by permission. Copyright © 2014, American Water Works Association. Permission to reproduce this document is granted for informational purposes only and does not represent or imply approval or endorsement by AWWA of any particular product or service. FRA NK P. SIDA RI III, JA NE T E . S T O U T, S C O T T D U D A, D O U G G R U BB, AN D AL AN N EU NER Maintaining Legionella control in building water systems THIS ARTICLE REVIEWS HOW LEGIONELLA AND OTHER WATERBORNE PATHOGENS CAN PRESENT A RISK TO CONSUMERS OF POTABLE WATER, SECONDARY DISINFECTION OPTIONS, AND A CASE STUDY ON CHLORINE DIOXIDE. L egionella and other waterborne pathogens can present a risk to consumers of potable water. In particular, building hot water systems have been established as the primary reservoir for bac teria linked to cases of Legionnaires’ disease (LD). These systems provide ideal conditions for Legionella proliferation because of their elevated temperature and lack of disinfection residual. Control of Legionella in potable water systems has become a focus for health care facilities because they serve a population that is particularly susceptible to LD from underlying health conditions, such as suppressed immune systems. In addition, the potential for disease transmission, associated liability concerns, and anticipated new standards from professional organizations are raising awareness of the importance of maintaining building water quality in non– health care facilities. UNDERSTANDING THE ISSUES The US Environmental Protection Agency’s (USEPA’s) National Primary Drinking Water Regulations establish maximum contaminant levels (MCLs) for several potentially harmful constituents, including microorganisms. The MCLs include using treatment techniques to achieve removal of Giardia lamblia, Cryptosporidium, enteric viruses, total coliforms, and Legionella. The regulations make the assumption that if Giardia and viruses are removed according to the treatment techniques in the Surface Water Treatment Rule, then Legionella will also be controlled in the potable water supply. The key word in this regulation is “controlled,” which suggests that Legionella may remain present but at quantities that do not present negative health effects from exposure. Legionella may be controlled through municipal water treat ment processes, not because treatment techniques are successful in removal of Legionella, but because the physicochemical conditions of the treated cold 24 OCTOBER 2014 | JOURNAL AWWA • 106:10 | SIDARI ET AL 2014 © American Water Works Association TABLE 1 Four technologies providing residual disinfectant to control Legionella Supplemental Chlorination Chlorine Dioxide Copper–Silver Ionization Monochloramine Typical concentration 2–4 mg/L free residual chlorine 0.1–0.8 mg/L ClO2 2.0–4.0 mg/L as Cl2 Copper 0.2–0.8 ppm Silver 0.02–0.08 ppm Distribution system residual Yes Yes Yes Yes pH pH > 8 affects efficacy No impact in 6.0–10.0 range No impact in 7.0–9.0 range pH > 8.5 may affect efficacy Temperature Elevated temperatures accelerates decay Elevated temperatures accelerate decay Minimal impact by elevated temperature No impact by temperature Disinfection by-products THM and HAA5 Chlorite Reduced THM and HAA5 compared with chlorine No chemical reaction to form by-products Drinking water standard Chlorine < 4.0 mg/L Chlorine dioxide < 0.8 mg/L Chloramine < 4.0 mg/L as Cl2 Copper < 1.3 mg/L Silver < 0.10 mg/L Cl2—chlorine, ClO2—chlorine dioxide, HAA5—haloacetic acids, THM—trihalomethane water are not conducive to amplifi cation of Legionella. Water tempera tures exceeding 77°F (25°C) have been observed as necessary to sup port the growth of Legionella (Yee & Wadowsky, 1982). Typical munic ipal treated water temperatures are below 77°F (25°C), allowing Legionella to persist in the water but not to amplify to detectable quantities. To maintain microbiological water quality in the distribution system, water suppliers provide a disinfec tant residual, most commonly in the form of free chlorine. Drinking water regulations require water suppliers that use surface water or groundwa ter under the influence of surface water to maintain a residual disinfec tant concentration in the water entering the distribution system of not less than 0.2 mg/L for more than 4 hours (40 CFR 141.72(a)(3) and (b)(2)). The residual disinfectant in the distribution system cannot be undetectable in more than 5% of the samples from the distribution system each month for any two con secutive months (40 CFR 141.72(a) (4) and (b)(3)). Although the au thors believe most public water sup pliers make their best efforts to maintain a detectable disinfectant residual in all delivered drinking water, maintenance of a detectable disinfectant residual throughout an entire distribution system may not always be possible. Although maintenance of the min imum disinfectant level may provide some control over growth of general heterotrophic plate count bacteria, 0.2 mg/L of free residual chlorine is not sufficient to control regrowth of all bacteria, including Legionella. A study of the susceptibility of Legionella to chlorine in tap water con cluded that Legionella can survive in the presence of low levels of chlorine for relatively long periods of time (Kuchta et al, 1983). Other studies have shown that Legionella bacteria can survive in the presence of chlo rine concentrations up to 50 mg/L because of their association with other organisms in the biofilms that exist throughout the treatment and distribution system. (Cooper & Han lon, 2010; Kilvington & Price, 1990; King et al, 1988). Researchers also conducted studies during several years and found that the potential exists for Legionella growth within municipal drinking water systems and that public water supplies may contaminate the plumbing systems of hospitals and other large buildings (States et al, 1987). Once Legionella enter a building water system, the bacteria can amplify (particularly in hot water systems where temperatures are optimum), nutrients accumulate, and any disinfectant residual in the cold supply is consumed. However, Legionella are not found in all buildings, with studies reporting that 12–70% of surveyed water sys tems have some level of Legionella colonization (Lin et al, 1998). Where Legionella is found in build ing water systems, an assessment of risk should include identifying the species and Legionella serogroups present, the extent of colonization, and the susceptibility of occupants to acquiring LD. Using this information, a building operator can determine if secondary disinfection is warranted to reduce the risk associated with the transmission of Legionella from building water systems. EXPLORING LEGIONELLA DISINFECTION OPTIONS To reduce the amplification of Legionella in building water sys tems, particularly those serving a susceptible population such as health care facilities, secondary dis infectants are often necessary. Table 1 shows four technologies that pro vide a residual disinfectant that have been considered for systemic disin fection of building water systems to control Legionella: supplemental chlorination, chlorine dioxide, monochloramine, and copper–silver SIDARI ET AL | 106:10 • JOURNAL AWWA | OCTOBER 2014 2014 © American Water Works Association 25 USEPA guidelines on use and monitoring of chlorine dioxide as the primary disinfectant TABLE 2 Requirement* Parameter Chlorine dioxide MRDL 0.8 mg/L as chlorine dioxide Chlorine dioxide MRDLG 0.8 mg/L as chlorine dioxide Chlorine dioxide residual monitoring Daily at entrance of distribution system Chlorite MCL 1.0 mg/L as chlorite Chlorite MCLG 0.8 mg/L as chlorite Chlorite monitoring Daily at entrance of distribution system and monthly at three locations in distribution system MCL—maximum contaminate level, MCLG—maximum contaminate level goal, MRDL—maximum residual disinfectant level, MRDLG—maximum residual disinfectant level goal *USEPA, 1998. ionization. Evaluation of disinfec tion methods to demonstrate their efficacy should be evidenced-based and follow a four-step approach: (1) demonstrate in vitro efficacy, (2) anecdotal experience of efficacy in individual hospitals, (3) peerreviewed controlled studies of pro longed duration documenting effi cacy and prevention of LD, and (4) confirmatory reports from multiple hospitals with a prolonged duration of follow-up (Stout & Yu, 2003). There is not a single disinfection technology that is applicable to all water systems, with anecdotal and published reports of underperfor mance or failure of disinfectants for various reasons. The successful application of a secondary disinfec tant is dependent on several factors, including the ability to maintain a disinfectant residual, configuration of the water system, cost of consum ables, operation and maintenance, source water quality, and permitting requirements. Most important, the selected secondary disinfectant should demonstrate efficacy against Legionella without negatively affect ing the water distribution system. Supplemental chlorination in volves the injection of sodium hypo chlorite into the potable cold and/or hot water system to achieve free chlorine residuals ≤ 4 mg/L at the building outlets. A review of 17 hos 26 pitals that used supplemental chlori nation for Legionella disinfection was presented in 1990, and it was reported in 2011 these facilities were no longer using supplemental chlori nation (Lin et al, 2011; Muraca et al, 1990). Concerns with supplemental chlorination in building water sys tems include accelerated corrosion of the water system, formation of dis infection by-products, and poor bio film penetration resulting in persis tence of Legionella (Giao et al, 2009; Garcia et al, 2008; Morris et al, 1992; Helms et al, 1988). For these reasons, along with previously dis cussed peer-reviewed studies docu menting Legionella resistance to chlorination, supplemental chlorina tion is not considered an effective permanent disinfection method for Legionella in building water systems. The successful use of copper–silver ionization for Legionella control in building hot water systems has been well-documented in the peerreviewed literature (Lin et al, 2011; Stout & Yu, 2003; Colville et al, 1993). Target concentrations vary between vendors but typically fall within a range of 0.2–0.8 mg/L for copper and 0.01–0.08 mg/L for sil ver. Ionization systems consist of a flow cell with metallic copper and silver anodes connected to an elec tronic controller. The release of ions into the water is controlled by OCTOBER 2014 | JOURNAL AWWA • 106:10 | SIDARI ET AL 2014 © American Water Works Association adjusting the amperage of current that is applied across the anodes. Monitoring of copper ions can be performed in the field using a handheld colorimeter, but laboratory analysis is required for measurement of silver ion concentration and to confirm field-measured copper con centrations. Water system operators considering ionization for secondary disinfection should be aware of regu latory agencies’ unfamiliarity with ionization because it is not consid ered a primary disinfectant in the municipal treatment process and is regulated under the Safe Drinking Water Act as a primary metal con taminant (copper) and secondary contaminant (silver). Monochloramine is used as a dis infectant by municipal drinking water providers because it is more stable than chlorine and produces fewer regulated disinfection by-prod ucts. In areas where monochlora mine is used at the municipal level for disinfection of drinking water, a lower incidence of LD has been doc umented compared with municipal water systems that use chlorine for disinfection. (Heffelfinger et al, 2003; Kool et al, 1999). Using monochloramine to treat secondary distribution systems is made possible through small-scale monochlora mine generators. Monochloramine for injection into building water sys tems is produced from a combina tion of a stabilized chlorine solution and a buffered ammonium salt solu tion. Target concentrations for monochloramine are 2–4 mg/L as total chlorine. Operators should also monitor free ammonia in the water systems receiving monochloramine treatment to ensure the correct pro portion of chemical precursors is being applied. The first evaluation of the efficacy monochloramine for Legionella control in a building hot water system was performed in an Italian hospital; the results of this evaluation showed a dramatic reduc tion in Legionella colonization (Marchesi et al, 2012). The first US evaluation of monochloramine application in a hospital water sys tem has reported similar results documenting Legionella efficacy (Stout et al, 2012). Although these two evaluations show promising results, continued long-term evalua tion of monochloramine should be performed to understand any limita tions that may be experienced in dif ferent water systems. Chlorine dioxide may also be con sidered by water system operators for Legionella disinfection. The case study that follows provides a review of chlorine dioxide and a long-term evaluation of one facility that has used chlorine dioxide for Legionella control during the past 13 years. CONSIDERING CHLORINE DIOXIDE DISINFECTION Chlorine dioxide has been used as a water treatment chemical in the United States since 1944, when it was used as an oxidant to remove phenol-related compounds from drinking water. Chlorine dioxide is now primarily used by municipal water providers for disinfection and control of tastes, odor, iron, manga nese, hydrogen sulfides, and phenolic compounds. USEPA reported in 1999 that an estimated 700–900 public water systems in the United States use chlorine dioxide as a municipal water treatment technol ogy (USEPA, 1999). Chlorine diox ide disinfection for the control of Legionella in building and secondary distribution systems has been reported in facilities throughout the United States and Europe (Zhang et al, 2009; Sidari et al, 2004). USEPA has established regulations governing the use and monitoring of chlorine dioxide in water systems using it as a primary disinfectant (Table 2). Treatment of potable water with chlorine dioxide results in the formation of chlorite, which is a disinfection by-product regulated by USEPA. Table 2 also provides details on the maximum contami nant levels and monitoring require ments for chlorite in municipal drinking water systems. USEPA lists the following health concerns regarding chlorite: “some people may experience anemia, some infants and young children who drink water containing chlorite in excess of the MCL could experience nervous system effects, and similar effects may occur in fetuses of pregnant women who drink water containing chlorite in excess of the MCL” (USEPA, 2014). Chlorine dioxide possesses several characteristics that allow it to per form well as a disinfectant. Chlorine dioxide’s oxidation reduction poten tial (0.95 V) is lower than that of chlorine (1.36 V), whereas its oxida tion capacity—5—is greater than chlorine—2. The oxidation reduc tion potential (ORP) measures an oxidizer’s strength or the speed at which it reacts with an oxidizable material. Although chlorine dioxide has a low ORP, it is more selective about the types of oxidizable materi als with which it reacts. Chlorine dioxide targets specific organic mol ecules, including cysteine, tyrosine, methionyl, DNA, and RNA, unlike the broad reactions of chlorine and ozone. The oxidation capacity indi cates that, on a molar basis, chlorine dioxide has a greater disinfection capacity than chlorine. The selectiv ity and oxidation capacity of chlo rine dioxide makes it a stronger oxi dative disinfectant than chlorine. Chlorine dioxide has also been shown to have superior biofilm pen etration and biofouling control com pared with chlorine (Simpson et al, 1993; Mayack et al, 1984). Chlorine dioxide disinfection in building water systems is performed by injecting a concentrated chlorine dioxide solution into the potable cold and/or hot water systems. Chlo rine dioxide can be supplied for small-scale applications as a stabi lized solution or generated onsite by means of direct oxidation (electro chemical) or a chemical blending process, typically using a strong acid solution and sodium chlorite. The various generation methods produce differing levels of solution purity and production yields depending on water system disinfection require ments and operating conditions. LONG-TERM CASE STUDY OF CHLORINE DIOXIDE In January 2004, the results of the first large-scale evaluation of chlorine dioxide efficacy for Legionella disinfection in a US secondary water distribution system were published in Journal AWWA (Sidari et al, 2004). The study con cluded that chlorine dioxide was a promising alternative disinfectant for Legionella, and long-term stud ies of its efficacy were warranted. This case study provides a longterm evaluation of the chlorine dioxide system after 13 years of operation to determine if chlorine dioxide has continued to produce acceptable results in controlling Legionella in the facility’s water system and the prevention of LD. Background. The study hospital in Pennsylvania identified 13 cases of LD between 1994 and 1999, with three cases in an 18-month period that were confirmed as hospitalacquired. Responding to these cases, the hospital began environmental monitoring for Legionella in its drinking water system in 1998. Interventions to control Legionella included maintenance of a free chlo rine residual throughout the distri bution system and performance of thermal eradication (heat and flush) in identified Legionella-positive areas of the distribution system. Although temporarily effective, the hospital did not want to use thermal eradication as a permanent disinfec tion approach because of short-term efficacy, logistical difficulties, and cost. Alternative disinfection ap proaches evaluated by the hospital in 1998 included supplemental chlo rination, copper–silver ionization, and chlorine dioxide. Supplemental chlorination was ruled out because of corrosion and efficacy concerns. Copper–silver ionization was ini tially considered as an option because of its effective performance SIDARI ET AL | 106:10 • JOURNAL AWWA | OCTOBER 2014 2014 © American Water Works Association 27 Campus water distribution system and sampling locations at study hospital Reservoir Distribution loop Sampling location Parking and roads Pubic buildings Staff buildings at other facilities; however, because each of the hospital’s 23 buildings would have required installation and maintenance of a separate ionization system, this option was determined to be cost-prohibitive. Monochlora mine was not considered during the initial selection because generation technology had not been developed or evaluated at the time. On the basis of the favorable literature review of laboratory trials, reports of success ful European applications, and cost considerations, the hospital decided to install a chlorine dioxide disinfec tion system for treatment of its cam 28 pus water distribution system. Sys tem installation was completed in June 2000. Installation and operation. The hos pital operates a sizeable secondary distribution system on its 140-acre campus (see the map above). The facility included 23 buildings provid ing 437 patient beds during the initial study; however, after several demoli tion and construction projects, the campus now incorporates 20 build ings providing 478 patient beds. The distribution system consists of a 520,000-gallon covered, above-grade concrete reservoir and ~ 10,000-foot OCTOBER 2014 | JOURNAL AWWA • 106:10 | SIDARI ET AL 2014 © American Water Works Association distribution loop of 6- and 8-inch pip ing. The distribution loop has re mained largely unchanged since completion of the initial study, with only minor modifications to accom modate new construction. Each building is supplied water from the distribution loop. Cold water is dis tributed in each building for potable and utility uses and separate hot water generation systems supply hot water within each building. The hospital receives the majority of its daily water supply (approxi mately 80%) from the local munici pality, which provides conventional treatment, including chlorination of a surface water source. The remain ing daily demand is met by the hos pital using onsite well water that is chlorinated before being blended with the municipal supply in the res ervoir. Average daily water use dur ing the initial study was 400,000 gpd in the summer and 250,000 gpd in the winter; by 2013, this was reduced to 232,000 gpd in the summer and 222,000 gpd in the winter. The reduction in water usage is attrib uted to reduced utility water demands from energy-efficiency improvements. The chlorine dioxide system was installed in June 2000 and consisted of three electrolytic generators.1 In 2009, one unit was replaced with a 2000 series generator and is now used as the primary generation unit. The generators directly oxidize a 25% active sodium hypochlorite solution across a membrane system, producing a concentrated 500-mg/L chlorine dioxide solution. The initial operation of the generators was in a flow-paced lead, ORP-controlled lag mode, allowing for continual dosing of chlorine dioxide into the reservoir. In March 2003, the generator opera tion was changed to a time-paced application with the generators pro ducing chlorine dioxide for approxi mately 45 minutes per hour based on measured chlorine dioxide residual in the reservoir. The application rate is adjusted manually by operators to target a chlorine dioxide range in the reservoir of 0.3–0.5 mg/L. The chlorine dioxide system dur ing the initial study operated con tinuously, seven days per week. In February 2003, the operational mode changed because of staffing availability at the hospital and the need to have a water treatment oper ator onsite during chlorine dioxide application. Currently, the genera tors are typically turned off from 23:00 on Friday night until 01:00 Monday morning each week. The hospital’s water system is per mitted as a nontransient, noncom munity water system, and the addi tion of the chlorine dioxide treatment system required a permit modifica tion. The permit from the regulatory agency limited chlorine dioxide con tractions to 1.0 mg/L at the reservoir and 0.8 mg/L at the distal outlets. The hospital has regularly performed monitoring of the chlorine dioxide system to comply with permitting requirements and to ensure that ade quate chlorine dioxide residuals for maintenance of Legionella control are present within the distribution system. The hospital has reported no permit violations associated with the chlorine dioxide system since opera tion began. Chlorine dioxide is mea sured at the entry point to the distri bution system on mornings when disinfectant is being applied (typi cally Monday–Friday). Chlorine dioxide concentration in the reser voir is also checked three times per day (once per shift) during operation to check on appropriate application rates. Chlorine dioxide is measured by the hospital using an N,N-diethylp-phenylenediamine method and field colorimeter. Chlorite measure ment using a titrator2 is performed by the hospital at the entry point to the distribution system each day that disinfectant is applied. The hospital also performs third-party laboratory testing of chlorite quarterly at the distribution system entry, point of medium residence time, and point of maximum residence time. Figure 1 shows 2013 average daily chlorine dioxide and chlorite measurements at the distribution system entry point (closest distal outlet to the reservoir). Chlorine dioxide concentrations decline during the weekend when chlorine dioxide application is sus pended and then recover during the week (Figures 1 and 2). System monitoring. System moni toring to support this long-term evaluation of chlorine dioxide has been performed since 1998. The monitoring periods are pre-chlorine dioxide (pre-ClO2; April 1998–May 2000), phase 1 (June 2000–January 2001), phase 2 (February 2001– April 2002), phase 3 (October 2003–August 2012), and phase 4 (October 2013). Figure 3 shows a summary of the hot water distal outlet monitoring completed during this long-term evaluation of chlorine dioxide at the hospital. The results of pre-ClO2, phase 1, and phase 2 monitoring were reported in the initial study. Phase 3 consisted of sampling from hot water distal outlets across the campus and was conducted 27 times by the hospital between October 2003 and August 2012. The sam pling locations and methods were generally consistent with those from the initial study, with adjustments made for patient occupancy and con struction. Phase 4 consisted of a single sampling event conducted in October 2013 by the authors to independently evaluate the system using sampling methods correspond ing to the initial study protocol. During phase 3, the hospital col lected 304 samples from hot water outlets for Legionella culture with four samples positive for Legionella (1.3% distal positivity). In July 2004, three positive samples were identi fied, and in a 2011 sampling event, one positive sample was identified. Phase 4 sampling found one of 20 (5% distal positivity) hot water out lets positive with Legionella pneumophila, serogroup 5. The mainte nance of very low Legionella positivity has been completed in the presence of average chlorine dioxide residuals at the hot water distal out lets of 0.08 mg/L (phase 3), which is not different from the average con centration observed during the initial study (phases 1 and 2; Figure 3). During the pre-ClO2 period, Legionella was detected in 9% (2/22) of samples from the building cold source water. After application of chlorine dioxide, no Legionella was detected in phase 1 (0/14, 0.26 mg/L average chlorine dioxide) and phase 2 (0/80, 0.50 mg/L average chlorine dioxide). Legionella in the building cold source water was not monitored by the hospital during phase 3. In phase 4, water samples were col SIDARI ET AL | 106:10 • JOURNAL AWWA | OCTOBER 2014 2014 © American Water Works Association 29 FIGURE 1 2013 average daily chlorine dioxide and chlorite measurements at the distribution system entry point 1.1 Chlorite MCL Clorine dioxide MRDL Chlorite Cold water chlorine dioxide Hot water chlorine dioxide 1.0 Point of Entry Residual Concentration—mg/L 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Monday Tuesday Wednesday Thursday Friday Day of Week Sampled MCL—maximum contaminant level, MRDL—maximum residual disinfectant level lected from the cold water supply to five buildings and no Legionella was detected (0/5, 0.22 mg/L average chlorine dioxide residual). Another indication of successful application of a secondary disinfec tant is if the application results in reduced rates of infection. The hos pital had experienced 13 cases of LD in the five years before installa tion of the chlorine dioxide system, and the initial study reported that no cases of LD had been detected between June 2000 and April 2002. The hospital has used the urinary antigen test for clinical screening of suspected cases of LD since 2003, and no cases have been detected in the 13 years since application of chlorine dioxide started. Initial study review. The initial study provided several conclusions drawn from an evaluation of the data collected after approximately two years of chlorine dioxide appli cation (phases 1 and 2). To evaluate and support the long-term efficacy of chlorine dioxide for Legionella con trol, the conclusions from the initial study can be reviewed in light of the 30 data collected (phases 3 and 4) dur ing the long-term operation of the chlorine dioxide system. Conclusion 1. A significant reduc tion in Legionella positivity was observed in cold building source water and distal hot water outlets after application of chlorine dioxide, and chlorine dioxide provided better Legionella control at distal outlets than thermal eradication and chlori nation. The data collected during phases 3 and 4 demonstrated that Legionella positivity in the distal hot water outlets has decreased further from the initial study and the preClO2 levels (Figure 3). Also, after application of chlorine dioxide, Legionella has not been detected in the cold building source water during monitoring in phases 1, 2, and 4 (cold source water monitoring was not per formed during phase 3). The results of the long-term study indicate that chlorine dioxide can maintain a last ing reduction in Legionella positivity in a secondary distribution system. Conclusion 2. Complete eradica tion of Legionella from hot water distal sites was not realized after OCTOBER 2014 | JOURNAL AWWA • 106:10 | SIDARI ET AL 2014 © American Water Works Association 1.75 years of chlorine dioxide treat ment; however, a trend of declining distal outlet positivity was observed. The trend of decreasing Legionella positivity has continued with only one positive Legionella sample col lected during phase 3 and one posi tive sample during phase 4. Overall percent positivity has continued to decrease since the initial study. The positive samples indicate complete eradication of Legionella in the hot water systems may not be possible, although maintenance of a very low level of Legionella positivity can be achieved with chlorine dioxide. It may be unreasonable to achieve complete eradication of Legionella from a distribution system, and complete eradication may not be necessary to prevent disease (see Conclusion 4). Conclusion 3. Chlorine dioxide concentrations of 0.3–0.5 mg/L were effective in reducing or eradi cating Legionella. However, a sig nificant reduction in chlorine diox ide residual was observed between the reservoir and distal hot water outlets in the buildings. Average 2013 chlorine dioxide concentra tion (Monday–Friday during opera tion of the chlorine dioxide genera tors) was 0.35 mg/L in the reservoir, 0.36 mg/L at the cold water point of entry, and 0.09 mg/L at the hot water point of entry. A reduction in chlorine dioxide residual is still observed between the reservoir/cold water and distal hot water outlets. This is to be expected because of long residence time in the distribu tion system and elevated tempera tures in the hot water systems, which result in the accelerated decay of chlorine dioxide. The chlo rine dioxide residuals being main tained by the hospital are providing effective control of Legionella in the distribution system, as evidenced by the environmental sampling results. Conclusion 4. No cases of LD were detected at the hospital after the use of chlorine dioxide began, despite sporadic isolation of Legionella at distal hot water outlets. Sam ABOUT THE AUTHORS Frank Sidari (to whom correspondence should be sent) is vice-president of consulting at SPL Consulting Services, 1401 Forbes Ave., Suite 209b, Pittsburgh, PA 15219; [email protected] He is a registered professional engineer, board-certified environmental engineer, certified construction document 0.40 Chlorine dioxide generation stopped Friday afternoon Chlorine dioxide generation started Monday morning 0.35 0.30 0.25 0.20 Sunday Monday Tuesday Wednesday Thursday Friday Saturday Day of Week Sampled FIGURE 3 Hot water distal outlet monitoring completed during the long-term evaluation of ClO2 at the hospital 0.14 Distal site positivity Hot water chlorine dioxide 25 43/186 0.12 20 0.01 15 0.08 15/124 32/257 0.06 10 0.04 1/20 5 0.02 4/304 0.00 Phase 1 Phase 2 Pre-ClO2 (April 1995– (June 2000– (Feb. 2001– May 2000) Jan. 2001) April 2002) Phase 3 (Oct. 2003– Aug. 2012) Phase 4 (Oct. 2013) 0 Hot Water Distal Site Legionella Positivity—% The long-term sampling conducted by the hospital and the independent sampling performed by the authors demonstrates that, after 13 years of chlorine dioxide application, Legionella continues to be successfully controlled at the hospital. Using the chlorine dioxide system to apply a residual at the point of entry to the campus distribution system, the hos pital is able to provide a chlorine dioxide residual in its building water systems. Legionella in the cold water has not been detected since applica tion of chlorine dioxide began. Legionella positivity at the hot water distal outlets has continued to decrease since application began and was only detected in one sample dur ing long-term monitoring by the hos pital. Most important, the applica tion of chlorine dioxide has prevented LD at the hospital, dem onstrating the long-term efficacy of chlorine dioxide in a secondary dis tribution system. 0.45 Reservoir Chlorine Dioxide Residual—mg/L SUMMARY FIGURE 2 2013 average daily chlorine dioxide residual in reservoir, showing impact of suspended application on weekends Average Hot Water Chlorine Dioxide Residual—mg/L pling conducted during phases 3 and 4 detected Legionella at distal hot water outlets, but at a reduced level from phases 1 and 2. The hospital has continued to perform clinical screening for suspected cases of LD and no cases have been detected after 13 years of chlorine dioxide applica tion. This is perhaps the best evi dence to demonstrate the long-term efficacy of chlorine dioxide in a sec ondary distribution system. Monitoring by Phase ClO2—chlorine dioxide technologist, and a certified Hazard Analysis and Critical Control Points auditor. Sidari holds a master’s of science degree in civil and environmental engineering from Carnegie Mellon University and a bachelor’s of science in forest engineering from the State University of New York Environmental Science and Forestry. His project work has focused on distribution systems, pumping, storage, treatment, and disinfection on systems ranging in size from 10 to 70 mil gal; he specializes in engineering assessments of water systems affected by Legionella. Janet E. Stout is president of Special Pathogens Laboratory, Pittsburgh, SIDARI ET AL | 106:10 • JOURNAL AWWA | OCTOBER 2014 2014 © American Water Works Association 31 Pa. Scott Duda is a project engineer for SPL Consulting Services, Pittsburgh, Pa. 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