Legionella Prevention Plan Introduction

  • Date 03.06.2015

Legionella Prevention Plan Introduction

Legionella 101

Legionellae are rod-shaped, gram negative bacteria. Over 40 species of Legionella have been identified; L. pneumophila appears to be the most virulent and is associated with approximately 90% of cases of Legionellosis. The risk of acquiring Legionnaires' disease is greater for older persons and for those who smoke tobacco or have chronic lung disease. Persons whose immune system is suppressed by certain drugs or by underlying medical conditions appear to be at particularly high risk.

Legionellae bacteria are commonly present in natural and man-made aquatic environments. The organism is occasionally found in other sources, such as mud from streams and potting soils. In natural water sources and municipal water systems, Legionellae are generally present in very low or undetectable concentrations. However, under certain circumstances within manmade water systems, the concentration of organisms may increase, a process termed "amplification." Conditions that are favorable for the amplification of legionellae growth include water temperatures of 25-42 degrees Celsius (°C) (77-108 degrees Fahrenheit), stagnation, scale and sediment, biofilms, and the presence of amoebae. Legionellae infect and multiply within several species of free-living amoebae, as well as ciliated protozoa. The initial site of infection in humans with Legionnaires' disease is the pulmonary macrophage. These cells engulf Legionellae, provide an intracellular environment that is remarkably similar to that within host protozoa, and allow for multiplication of the bacterium. Growth in nature in the absence of protozoa and/or in the absence of complex microbial biofilms has not been demonstrated.

There is an indication that growth of Legionella is influenced by certain materials. Natural rubbers, wood, and some plastics have been shown to support the amplification of Legionella, while other materials such as copper inhibit their growth. Generally, Legionella thrive in diverse, complex microbial communities because they require nutrients and protection from the environment. Controlling the populations of protozoa, and other microorganisms may be the best means of minimizing Legionella.

Transmission of Legionnaires' Disease

Most data on the transmission of Legionnaires' disease are derived from investigations of disease outbreaks. These data suggest that, in most instances, transmission to humans occurs when water containing the organism is aerosolized in respirable droplets (1-5 micrometers in diameter) and inhaled by a susceptible host. A variety of aerosol-producing devices have been associated with outbreaks of Legionnaires' disease, including cooling towers, evaporative condensers, showers, whirlpool spas, humidifiers, decorative fountains, and a grocery store produce mister. Aspiration of colonized drinking water into the lungs has been suggested as the mode of transmission in some cases of hospital-acquired Legionnaires' disease. Numerous investigations have demonstrated that cooling towers and evaporative condensers have served as the sources of Legionella-contaminated aerosols causing outbreaks of community- and hospital- acquired infection. A number of outbreaks of Legionellosis associated with cooling towers and evaporative condensers have occurred after these devices have been restarted following a period of inactivity. Shower heads and tap faucets can also produce aerosols containing legionellae in droplets of respirable size.

Common amplifiers (growth factors) associated with building water systems, including the treatment recommended to minimize the risk of Legionellosis, are discussed below.

Potable Water Systems

Factors associated with the plumbing system that may influence the growth of legionellae are as follows:

  • Chlorine concentration;
  • Temperature; and
  • Plumbing system design and materials

Municipal potable water supplies are generally chlorinated to control the presence of microorganisms associated with sewage. Legionellae are more tolerant of chlorine than many other bacteria, and may be present in small numbers in municipal water supplies. Potable water can also support legionellae growth if the water temperature is in the range of 77-108°F. Plumbing design and materials also influence the growth of legionellae.

Growth of legionellae may occur in portions of the system with infrequent use, in stagnant water, and in portions of the system with tepid temperatures. Growth may also occur in dead-end lines, attached hoses, shower nozzles, tap faucets, hot water tanks, and reservoirs. Rubber washers and fittings, including water hammer arrestors and rubber hoses with spray attachments, have been shown to provide sites for growth of legionellae. Organic compounds leached from plumbing materials may contribute to growth of heterotrophic bacteria, including legionellae.

Contaminated potable water sources present the greatest risk when dispersed into the air in a very small droplet size (less than 5 micrometers) that can be inhaled deeply into the lungs. Actions that may generate small droplets are those that break up the water stream, i.e., shower nozzles, aerators, spray nozzles, water impacting on hard surfaces, and bubbles breaking up. Both dead and living microorganisms, biofilms, and debris may provide nutrient sources for legionellae growth. When legionellae are found in plumbing systems, it is common to detect the microbes in the sediment in hot water tanks, and in peripheral plumbing fixtures that accumulate sediment.

Where practical in high-risk situations, cold water should be stored and distributed at temperatures below 20°C (68°F), while hot water should be stored above 60°C (140°F) and circulated with a minimum return temperature of 124°F. However, great care should be taken to avoid scalding problems. One method is to install preset thermostatic mixing valves. Where buildings cannot be retrofitted, periodically increasing the temperature to at least 66°C (150°F) or chlorination followed by flushing should be considered. Systems should be inspected annually to ensure that thermostats are functioning properly. Where practical in other situations, hot water should be stored at temperatures of 120°F or above. Those hot or cold water systems that incorporate an elevated holding tank should be inspected and cleaned annually. Lids should fit closely to exclude foreign materials.

Where decontamination of hot water systems is necessary (typically due to implication of an outbreak of Legionellosis) the hot water temperature should be raised to 160~170°F and maintained at that level while progressively flushing each outlet around the system. A minimum flush time of five minutes has been recommended by the Center for Disease Control. However, the optimal flush time is not known and longer flush times may be necessary.

Emergency Water Systems-safety Showers, Eye Wash Stations, And Fire Sprinkler Systems

These systems are generally plumbed to the potable water system, have little or no flow with resulting stagnant conditions, and may reach temperatures warmer than ambient. The presence of legionellae, heterotrophic bacteria, and amoebae in these systems has been documented. When the devices are used, aerosolization is expected.

Safety shower and eye wash stations should be flushed at least monthly. In the case of fire sprinkler systems, it is recommended that fire-fighting personnel wear protective respiratory gear and that non- firefighting personnel exit the burning area. Appropriate precautions should be taken when checking the operation of fire sprinkler systems.

Architectural Fountains And Waterfall Systems

In these systems, water is either sprayed in the air or cascades over a steep media such as rocks, and then it returns to a man-made pool. These systems are sometimes operated intermittently with on-time often scheduled only during certain time periods. Systems that are operated intermittently may encourage greater biocontamination.

Because of the high temperature ranges needed for proliferation of legionellae bacteria, outdoor fountains and pools in hotter climates, and indoor fountains and pools subject to sources of heat may be susceptible to legionellae growth. Temperature increases may be facilitated by heat from pump/filter systems themselves. Intermittent operation may also create situations where temperature increases occur in isolated areas of the system. Fountains are subject to contamination from a wide variety of potential nutrient sources, including materials scrubbed from the air and returned to the pool with the falling water droplets as well as organic and inorganic materials dropped, thrown, or blown into the pool.  

The recommended treatment for fountains includes:

  • Regular cleaning is recommended; and
  • Use of filters should be considered; however, systems with a small water volume may be drained, and refilled with fresh water every few weeks in lieu of filtering.

Microbial fouling control is important, especially where the conditions are such that there are significant periods of time when the temperature of the fountain water is in the range that is favorable for the amplification of legionellae growth. When biocidal treatment is employed for microbial fouling control, the biocide must be registered with the United States Environmental Protection Agency (USEPA) for use in decorative fountains.

Cooling Towers Including Fluid Coolers (closed-circuit Cooling Towers) And Evaporative Condensers

Evaporative heat rejection equipment such as cooling towers and evaporative condensers have been implicated in numerous outbreaks of Legionnaires' disease, and studies have shown that detectable levels of legionellae are present in many of these devices.

A cooling tower is an evaporative heat transfer device in which atmospheric air cools warm water, with direct contact between the water and the air, by evaporating part of the water. Air movement through such a tower is typically achieved by fans, although some large cooling towers rely on natural draft circulation of air. Cooling towers typically use some media, referred to as "fill," to achieve improved contact between the water and the cooling air. The typical temperature of the water in cooling towers ranges from 85°F to 95 °F although temperatures can be above 120 °F and below 70°F depending on system heat load, ambient temperature, and system operating strategy.

Closed-circuit cooling towers and evaporative condensers are also evaporative heat transfer devices. Both are similar to conventional cooling towers, but there is one very significant difference. The process fluid (either a liquid such as water, an ethylene glycol/water mixture, oil, etc., or a condensing refrigerant) does not directly contact the cooling air. Rather, the process fluid is contained inside a coil assembly. Water is drawn from the basin and pumped to a spray distribution system over the coil assembly while the cooling air is blown or drawn over the coil by fans. Removal of heat is achieved by evaporating part of the water. Water temperature in closed-circuit cooling towers and evaporative condensers is similar to that in cooling towers.

Cooling towers and evaporative condensers incorporate inertial stripping devices called drift eliminators to remove water droplets generated within the unit. While the effectiveness of these eliminators can vary significantly with the design (new state-of-the-art eliminators are significantly more efficient than older designs) and the condition of the eliminators, it should be assumed that some water droplets in the size range of less than 5 micrometers leave the unit. In addition, some larger droplets leaving the unit may be reduced to 5 micrometers or less by evaporation.

Because cooling towers and evaporative condensers are highly effective air scrubbers and because they move large volumes of air, organic material and other debris can be accumulated. This material may serve as a nutrient source for legionellae growth. Diverse biofilms, which can support the growth of legionellae, may be present on heat exchanger surfaces, structural surfaces, sump surfaces, and other miscellaneous surfaces.

The key recommendations are that the system be maintained clean and that a biocidal treatment program be developed and implemented. It is also recommended that the services of a qualified water treatment specialist be used to define and oversee the treatment. Keeping the system clean reduces the nutrients available for Legionella growth. Regular visual inspections should be made for general cleanliness. The cold water basin of the unit should be cleaned when any buildup of dirt, organic matter, or other debris is visible or found through sampling. Mechanical filtration may be used to help reduce these solids. Strainers, cartridge filters, sand filters, centrifugal-gravity-type separators, and bag-type filters can be used to assist in removal of debris. The drift eliminators should also be inspected regularly and cleaned if required or replaced if deteriorated or damaged.

An effective water treatment program allows more efficient operation due to lower fouling, a longer system life due to decreased corrosion, and safer operation of the system due to reduced chances of microbial exposure to the public.

Control of scaling and corrosion is necessary in many water treatment programs. Scale such as calcium carbonate and/or other minerals containing silica, magnesium, and phosphate may precipitate onto heat exchanger and piping surfaces. Scaling can be minimized by use of inhibitors containing phosphonates, phosphates, and polymers to keep calcium and carbonate and other minerals in solution. Corrosion can be minimized by the use of inhibitors such as phosphate, azoles, molybdenum, and zinc. Scale and corrosion inhibitors are effective if microbial fouling and biofilm development are properly controlled. Microbial fouling can influence scaling and corrosion processes and can affect the performance of inhibitors. Microbial biofilms on surfaces can consume certain inhibitors (such as phosphates, phosphonates, and azoles), prevent access of inhibitors to surfaces, create localized oxygen-depleted zones, change the pH near surfaces, and accumulate or trap deposits onto surfaces.

Microbial fouling is controlled by the use of biocides, which are compounds selected for their ability to kill microbes while having relatively low toxicity for plants and animals. In the USA, the Environmental Protection Agency has regulatory authority for biocides and requires registration of all biocides. In addition, registration is required in each state where the biocide will be distributed. Non-oxidizing biocides include many organic compounds registered with the USEPA for cooling water applications. These biocides function in a number of ways, including reacting with intracellular enzymes, solubilizing cell membranes, and precipitating essential proteins in microbial cell walls. Properly used, non-oxidizing biocides are effective for control of the microbial fouling process in cooling water systems. It is generally good practice to regularly alternate the biocides used for a cooling water system to avoid the selection and growth of resistant strains of microbes. The alternating biocide approach has been emphasized with the rationale that the population that survives the biocide treatment one week is susceptible to the alternate biocide a week or two later. Alternating the dose and frequency of the same biocide is also used to achieve this goal.

Equally important to controlling scale and corrosion is keeping the system clean and free of sediment. Common sources of sediment include materials scrubbed from the air (dirt, leaves, paper, kitchen or other organic exhaust), precipitated solids (calcium, magnesium, carbonate silica), and corrosion products (rust). Microbes including bacteria, protozoa, algae, and (infrequently) fungi can grow in cooling systems and use the above materials as nutrients. Consequently, it is desirable to either prevent the entry of the material into the system or to remove it from the system.  

When the system is to be shut down for a period of more than three days, it is recommended that the entire system (cooling tower, system piping, heat exchangers, etc.) be drained to waste. When draining the system is not practical during shutdowns of short duration, the stagnant cooling water must be pretreated with an appropriate biocide regimen before tower start-up.


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