By Mark LeChevallier, principal and manager, Dr. Water Consulting, LLC
By American Water
Recently, Dr. Mark LeChevallier, vice president and chief environmental officer for American Water, participated in a webinar hosted by the WateReuse Pacific Northwest and the WateReuse Research Foundation. He presented findings on the prevalence of Legionella in recycled water.
Legionella pneumophila is on the U.S. EPA’s Candidate Contaminant List (CCL) as an important pathogen. It is commonly encountered in recycled water and is typically associated with amoeba, notably Naegleria fowleri (also on the CCL) and Acanthamoeba sp. No legionellosis outbreak has been linked to recycled water and it is important for the industry to proactively keep things that way.
Legionella have a ubiquitous occurrence in diverse water environments, including water systems where their presence could be an issue because human infections may be acquired through inhalation of contaminated droplets. Drinking water and reclaimed water distribution systems have been shown to be reservoirs for Legionella. Factors contributing to Legionella growth include elevated ambient temperatures, scale and sediment accumulation, corrosion, absence of disinfectant residuals, and presence of high levels of biodegradable and assimilable organic carbon (i.e., BDOC and AOC).
During the presentation, Dr. LeChevallier gave an overview of the Legionella organisms, occurrence in water, factors contributing to growth and survival, sensitivity to disinfection, and their relationship to free living amoebae. Factors contributing to increased risk and best management practices were also summarized.
Dr. LeChevallier referenced a comprehensive literature review on Legionella to understand the occurrence in environmental matrices, impacts of various treatment processes and status of Legionella detection methods, and human health risks. Quarterly monitoring of six reclaimed water utilities (representing different treatment processes, storage conditions, and distribution system sizes) in four states (California, Florida, Texas, and Arizona) was performed where grab samples from each utility were taken at five locations (i.e., plant effluent, reservoir, and three points within the distribution system).
The systems produced water with varying levels of organic carbon and other nutrients, which allowed for assessment of each parameter’s impact on occurrence of Legionella and their associated protozoa hosts. Protection of Legionella in biofilms was also measured at two utilities using experimental pipeloops configured in a manner to allow for evaluation of several disinfectants in parallel. A period for biofilm development was followed by treatment of three loops with one of the test disinfectants; free chlorine, monochloramine, or peracetic acid, while the fourth loop was used as an untreated control. Forty-eight hours after treatment, bulk water and biofilm samples were analyzed for Legionella, protozoa, disinfectant residual, and a variety of other water quality parameters.
In addition to conventional microbial and water quality analyses, differentiation of viable and membrane-compromised Legionella cells was evaluated with the aid of a molecular assay requiring pre-treatment with an intercalating dye (ethidium monoazide).
Summary of Results
Data indicated Legionella occurrence appears to be dependent on complex ecological interactions between Legionella, their protozoa hosts, and the dynamic reclaimed water environment.
- species were found frequently (50 percent and 80 percent of samples using the culture and quantitative PCR (qPCR) methods, respectively) in systems where they were monitored, though their concentrations were generally low. Overall, 16 species of Legionella were identified. L. pneumophila was most frequently detected and was the only species found in all six reclaimed water utilities.
- Mesophilic (100 percent of samples) and thermophilic (97 percent of samples) protozoa were regularly detected throughout the reclaimed water systems and their numbers increased as the water aged in the distribution system.
- Disinfectant residuals, when present, were effective at controlling Legionella occurrence in reclaimed water distribution systems.The system that was able to maintain a 0.2 mg/L free chlorine residual (through biological nutrient removal) has the lowest occurrence and concentrations of Legionella.
- In pipe loop studies, free chlorine was more effective than monochloramine and peracetic acid at inactivating Legionella, heterotrophic bacteria, and total coliforms.The disinfectants had different effects on the populations of protozoa. Free chlorine eliminated protozoa entirely, while monochloramine and peracetic acid transformed the trophozoites into cysts which are persistent.
- AOC, TOC, and ammonia impacted Legionella occurrence.Over 70 percent more Legionella were detected when AOC was above 1,000 µg/L and/or TOC was above 10 mg/L. The reclaimed system with the lowest occurrence of Legionella also had the lowest average concentrations of AOC, TOC, and ammonia.The two systems with the most Legionella also had the highest average concentrations of AOC, TOC, and ammonia.
- Quantitative microbial risk assessment (QMRA) models were developed for water reuse with toilet flushing, spray irrigation, and cooling towers. Although there are no public health guidelines for reclaimed water, the risk of Legionella infection for toilet flushing and spray irrigation was less than the 1/10,000 annual risk of infection guideline used for drinking water.Cooling towers posed a greater risk in part due to the fine mist generated by these facilities.In these cases, additional treatment would necessary to further reduce Legionella concentrations.The research is one of the first to apply QMRA models to reclaimed water for Legionella, and was useful in establishing guidelines for treatment.
- occurrence can be influenced by complex ecological interactions and managing its risk must be informed by understanding the interactions between free living amoebae and Legionella. The system that maintained a chloramine residual had the lowest concentration of trophozoites (the free living form of amoebae). This could be an important management strategy as it is thought that Legionella primarily grow in the amoebae trophozoites.
To manage the risk of Legionella, utilities should follow best management practices for distribution system management. These practices include cleaning and flushing the system, as well as maintaining disinfectant residual to inactivate Legionella and protozoa trophozoites. In addition, minimizing occupational exposure can be achieved by irrigating at night when fewer workers are likely to be in the area.