Management of Legionella in Water Systems (2020) / Chapter Skim
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4 Strategies for Legionella Control and Their Application in Building Water Systems
4 Strategies for Legionella Control and Their Application in Building Water Systems
Pages 165-232
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From page 165... ...
The chapter also discusses emerging issues, such as potential conflicts among strategies for green building design, water and energy conservation, and more prospective Legionella control strategies. As detailed in the following sections, factors known to influence Legionella growth in water systems include temperature, disinfectant type and levels, hydraulic conditions (particularly avoiding stagnation)
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From page 166... ...
(page 207) Temperature Control ✓ ✗ ✓ ?
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From page 167... ...
Although any of these water systems has the potential to grow and transmit Legionella, this discussion is limited to the premise plumbing of buildings, cooling towers, humidifiers, hot tubs, and corresponding water supplies, though some of the basic principles apply to other systems as well. The precise target for Legionella control can be quite complex in terms of species, serotypes, strains, and corresponding virulence factors.
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From page 168... ...
This helps provide hot water on demand in distal reaches of the building and also keeps the water lines sufficiently hot to deter Legionella growth. Recirculation lines cannot reach each point of use, such that the volume of water between the recirculating pipe and the faucet or showerhead will remain stagnant between uses.
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From page 169... ...
. In a survey of 40 Italian hotels, hot water above 60°C in the drinking water system and above 55°C in the outlet water was protective from legionellae (Borella et al., 2005)
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or individual hot water supply (26)
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Hydraulic distribution of hot water was improved by balancing return loops and removing dead ends and faulty mixing valves. • Corrective action reduced positivity much more drastically than it did the range of concentration at positive sites.
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From page 172... ...
In residences, electric or fuel-heated tanks and on-demand water heaters are commonly used, with a possibility of in-tank recirculation. Balancing the thermal and sanitary performance of domestic hot-water storage is a growing concern as energy stored in sanitary hot-water systems represents about 14.8 percent of total residential energy consumption in the United States2 and 19 percent of residential energy consumption in Canada.3 The type of water heater and the presence of storage and recirculation are critical features in determining the risk of Legionella spp.
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pneumophila. In a survey of 343 German residential water heaters with a water tank and, in some cases, recirculation, 94 percent of sites were positive for Legionella spp.
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Scalding The higher water temperatures (greater than 140°F/60°C) that prevent Legionella growth are associated with an increased risk of scalding and burns.
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Although most scalding and burn injuries in the homes are related to exposures other than hot water, such as food, cookware, and microwaved items, the risk of scalding from home premise plumbing remains important. It is difficult to tell from CDC (2009)
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From page 176... ...
Chemical Disinfection Chemical disinfectants, particularly oxidizing agents such as chlorine, chlorine dioxide, chloramine, and ozone, are widely used to control Legionella spp. and protozoa -- both as disinfectants in drinking water distribution systems and as secondary disinfectants within buildings.
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From page 177... ...
pneumophila at 30°C, but not at 50°C. Guidelines for the maintenance of continuous chlorine residuals in building premise plumbing to prevent amplification of Legionella tend to recommend residual concentrations similar to those required in drinking water distribution systems.
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From page 178... ...
outbreaks of Legionnaires' disease from 1979 to 1997 where drinking water was implicated. They found that the odds of a nosocomial Legionella outbreak were 10.2 times higher in hospitals supplied by a water system that maintained free chlorine than in those supplied by a water system using a chloramine residual.
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Legionella occurrence was also reduced in 96 buildings in Pinellas County, Florida, when the drinking water distribution system converted from chlorine to monochloramine disinfection (Moore et al., 2006)
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. Because UV does not provide a residual, it is only effective at the point of treatment and is typically combined with a chemical disinfectant for distributed water to effectively control Legionella spp.
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ionization to control Legionella in building water systems is widespread, partly because it is relatively low cost and low maintenance compared to other controls. Copper (Cu)
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have been widely used in cooling towers to control Legionella i spp.
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Flushing to Control Distal Growth Flushing of water can have significant benefits in terms of water quality and more specifically Legionella levels. Flushing can reduce total cell counts in premise plumbing by dislodging loose deposits and biofilm, which tend to harbor higher levels of heavy metals, Aeromonas, adenosine triphosphate or ATP (indicator of biological activity)
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. A study in which biofilms were first established under laminar or turbulent flow looked at the effect of unsteady hydraulic conditions on the biological quality of the drinking water (Manuel et al., 2010)
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pneumophila levels in Dutch drinking water distribution systems (van der Kooij and van der Wielen, 2014)
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Plumbing Materials Plumbing materials are an important factor to consider in Legionella control. Common plumbing materials in buildings include copper, iron, and numerous plastics, with crosslinked PEX and cross-linked polyvinyl chloride (PVC)
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From page 187... ...
. In the meantime, awareness of the presence of iron pipes and other components and practicing appropriate corrosion control, e.g., through orthophosphate addition federally mandated by the Lead and Copper Rule, are key to reducing this potential risk factor for Legionella growth in premise plumbing.
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From page 188... ...
Biofilm growth and Legionella proliferation at distal sites can be prevented through various actions. Small diameter piping in the distal portion of premise plumbing can minimize water volumes and their age.
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Their failure results in the mixing of cold and hot water in the piping, which leads to poor service and temperature conditions favorable to the growth of Legionella (Boppe et al., 2016)
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In a cancer center in Pennsylvania, a new extended-life faucet filter ensured total removal of Legionella spp. for 12 weeks, exceeding the recommended period of use of 62 days, while mean concentrations at control faucets ranged from non-detect to more than 600 CFU/mL (Baron et al., 2014)
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; and (3) other devices (cooling towers, humidifiers, hot tubs)
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From page 192... ...
The underlying statutes of these regulations and practices were developed to provide protection from a wide range of chemical and microbiological hazards. Because of cold water temperatures and the presence of a disinfectant residual, public water distribution systems are generally thought to harbor low levels of Legionella, although there are few data to support this assumption.
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Legionella spp. have been detected by qPCR in 66.7 percent of municipal drinking water storage tank sediments from 18 sites (Lu et al., 2015)
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(2018a) reported that risks of Legionella exposure from reclaimed water used for irrigation or cooling towers could exceed 10−4 annual risk of infection for various scenarios.
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For example, a 2 million gallon per day direct potable reuse plant in Big Spring, Texas, treats wastewater to drinking water standards via microfiltration, reverse osmosis, and UV disinfection before blending with raw drinking water sources and routing to a conventional drinking water treatment plant (Trussell et al., 2015)
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From page 196... ...
Legionella is inherently more difficult to manage in larger building water systems because the plumbing networks are correspondingly larger and subject to more variability, making it more challenging to ensure that controls are adequately supplied throughout the building. The extended stagnation periods experienced by water in large building premise plumbing place these systems at further risk.
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In this section the emphasis is on piped potable water used for drinking and bathing, though general principles apply to other piped water systems. Cooling towers, humidifiers, and hot tubs are discussed in separate sections.
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30-min 70°C heat Legionella spp. and Frequent heat-shock treatment (65°C)
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Hospital with ClO2 dose of 0.3 mg/L Superheat (>60°C ) Legionella spp.
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From page 200... ...
However, as a consequence of their lower flow, these fixtures, primarily faucets but also showers, increase water age and restrict disinfectant levels, including the disinfection provided by elevated water temperatures. As such, low-flow fixtures present a greater risk for Legionella development in the distribution systems that feed them.
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From page 201... ...
, dead legs, and low-use locations can all provide the opportunity for Legionella growth. Additionally, most home hot-water heaters are set at temperatures to limit the risk of scalding but are within the range for Legionella growth (see Table 4-3)
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that developed materials for water utility websites.9 Because smaller-diameter pipes are found in buildings and homes, premise plumbing is particularly prone to growth of biofilm bacteria and resulting water quality problems. Although there are no ways to reduce the nutrient content of water entering premise plumbing, other strategies can be employed to control biofilm growth, including flushing pipes to reduce water age and deliver disinfectant residuals throughout the home.
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From page 203... ...
Plume formation can be important in open and closed wet cooling towers when air with a high moisture content leaves the cooling tower, mixes with the atmosphere and begins to cool down. Both wet and wet/dry device types can be sources of Legionella infections due to their large use of water, their operating temperature, and their capacity to generate aerosols.
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. A variety of microorganisms can grow in cooling towers during the course of normal operation, which involves water temperatures ranging from 29°C to 35°C (ASHRAE, 2000)
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to control Legionella, the Committee suggests that refrigeration, HVAC, and cooling tower manufacturers collectively design and develop new systems that can operate at condenser water temperatures whereby the temperature going to the cooling tower will be greater than 60°C. In this proposed conceptual system, the condenser water temperature coming from the refrigeration equipment or chiller would be 65°C to 70°C and travel first to a reheat heat exchanger.
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From page 206... ...
When their source water comes directly from the building's cold-water supply or if the source water has been sent through reverse osmosis, these humidifiers can be used safely. However, when the source water is in holding tanks or in the pipes exposed to heat, the temperature of the water can reach 25°C to 43°C, a range that supports Legionella growth.
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. Indeed, hot tubs were the third leading cause of legionellosis outbreaks among 27 investigations reported between 2000 and 2014, following potable water and cooling towers (Garrison et al., 2016)
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. The following examples illustrate some of the complex and untested scenarios that can have unintended consequences and increase risk of Legionella growth in building water systems (Rhoads et al., 2015b)
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. One survey found the premise plumbing water age in a typical LEED-certified healthcare suite to be eight days; it was more than six months in an off-grid office suite (Rhoads et al., 2016a)
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As water stagnates, it is also more often within an optimal temperature range for Legionella growth. In the LEED-certified healthcare suite, Legionella spp.
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Finally, the water savings incurred by rainwater harvesting can indirectly increase the water age within potable water plumbing. One study found that using rainwater to flush toilets resulted in a 58 percent to 80 percent reduction in potable water use, with premise plumbing water age at some taps exceeding three weeks (Nguyen et al., 2012)
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Unintended Consequences of Energy Conservation As noted above, elevated water temperature is a master variable for Legionella control in buildings. Incentives in green buildings that encourage lowering this temperature to achieve energy savings can create conditions conducive to Legionella growth (Brazeau and Edwards, 2013b)
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Further, due to cloudy days, the solar pre-heat tank may essentially end up in the optimal temperature range for Legionella growth. Legionella spp.
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. Potential Trade-Offs with Other Microbial Risks Finally, it is important to consider whether recommendations herein intended for Legionella control could potentially have unintended consequences by favoring survival of other pathogens that are problematic in premise plumbing.
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are essential to Legionella Re acti Plumbing on Materials s, D y iss ,D eca olu tio cti on s Faste n ,L Rea Deca y r ea ch ing Large Engineered Cooling Towers Building Water Water Systems and Other Devices Systems Disinfectant Elevated Temperature Nutrient Aerosol Distal Design De Limitation Control liv r y er y ve li De Hydraulic Design FIGURE 4-5 Interactions between Legionella controls in different water system types. Disinfection, hydraulic design, and aerosol conrol apply to all major systems.
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The conclusions and recommendations below highlight key lessons regarding Legionella control strategies for the building and device types discussed in this chapter. For all types of buildings, hot-water heater temperature should be maintained above 60°C (140°F)
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From page 217... ...
These studies should examine, in full-scale drinking water systems, the impact of nutrient reduction on the concentration and composition of the microbiome in biofilms and water including amoebae growth and life stages and the subsequent effect on occurrence and decrease of pathogenic Legionella species. New NSF/ANSI standards regarding microbial growth potential of materials are needed so that water utilities, plumbers, and building contractors can include Legionella control when making decisions about pipe material usage.
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From page 218... ...
and lowering hot-water temperatures in premise plumbing. Criteria for certifying green buildings, energy-conserving features, and water-conserving features should be modified to take into account risk factors for growth of Legionella and other water-based pathogens in building water systems.
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1991. Risk factors for contamination of domestic hot water systems by legionellae.
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2015. Temperature diagnostic to identify high risk areas and optimize Legionella pneumophila surveillance in hot water distribution systems.
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2019. Controlling Legionella pneumophila in water systems at reduced hot water temperatures with copper and silver ionization.
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2015. Guidelines for Legionella control in the operation and maintenance of water distribution systems in health and aged care facilities.
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2016a. Technologies for Legionella control in premise plumbing systems: Scientific literature review.
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2018a. Health risks from exposure to Legionella in reclaimed water aerosols: Toilet flushing, spray irrigation, and cooling towers.
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2019. Legionella growth potential of drinking water produced by reverse osmosis.
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2017. Hotspots for selected metal elements and microbes accumulation and the corresponding water quality deterioration potential in an unchlorinated drinking water distribution system.
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2010. Legionella species colonization in cooling towers: risk factors and assessment of control measures.
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1983. Relationship between colonization of hospital build ing with Legionella pneumophila and hot water temperatures.
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2016a. Survey of green building water systems reveals elevated water age and water quality concerns.
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hospital: interaction of treated drinking water with plumbing materials, aesthetics and other considerations. Water Research 102:1-10.
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2016. Efficacy of copper-silver ionization in controlling Legionella in complex water distribution systems and a cooling tower.
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2009. Legionella control by chlorine dioxide in hospital water systems.
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