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Management of Legionella in Water Reclamation Systems

Project: 12-05
Estimated Release: 2015
Type: Report

Program: Principal
Funding Partner: Singapore PUB
Total Budget: $355,164 (Cash: $200,000, In-Kind cash and service: $155,164)

Principal Investigator: Mark LeChevallier, American Water


Members of the genus Legionella are common inhabitants of potable water distribution systems, often proliferating in biofilms and in the presence of free-living protozoa. In WateReuse Research Foundation projects Guidance Document on the Microbiological Quality and Biostability of Reclaimed Water Following Storage and Distribution (WateReuse-05-002 published 2010) and Approaches to Maintain Consistently High Quality Reclaimed Water in Storage and Distribution Systems (WateReuse-08-04 expected publication date March 2012), Legionella were observed in recycled water distribution systems. Growth was attributed to the rapid loss of the disinfectant residual in recycled water and was associated with nutrient levels, particularly BDOC. However, these and other studies have not discerned the incidence of the Legionella serogroups that are pathogenic to humans, or determined the significance of pathogenic serogroups in water systems and end user sites. Recycled water uses such as spray landscape irrigation, water features (e.g. decorative fountains), cooling towers, and other public access uses can aerosolize water. Therefore, it is important to understand the role of Legionella transport in water droplets because inhalation of pathogenic Legionella spp. attached to water aerosols is the primary route of infection.

Despite the ubiquitous nature of Legionella in water, the public health importance of water-associated Legionella in drinking water and recycled water is not known. In order to ensure that that risk is minimized and to further build public confidence for using recycled water for these applications, it is important that the WateReuse Research Foundation investigate the incidence, relative risk (compared to drinking water), and develop management recommendations for the end users of recycled water. While L. pneumophila Serogroup 1 is an important human pathogen, the occurrence of this species and Serogroup versus other members of the genus and species in recycled water distribution systems is unknown. The effects of recycled water quality parameters on the growth (emphasis added), survival, and susceptibility of Legionella in distribution systems are also unknown. Data on the occurrence, survival, and disinfection susceptibility of Legionella spp in recycled water distribution systems are needed in order to formulate strategies for Legionella control in these systems.

Goals and Objectives

The project will demonstrate that there is a method for addressing health concerns related to Legionella. Since Legionella has been shown to be present in drinking water distribution systems, we want to assess the relative risk and provide an appropriate action to be taken. The project will give a clearer understanding of whether aerosols from recycled water are a risk to human health. The main audience includes water utilities supplying recycled water, building owners and operators (i.e. of devices that can proliferate Legionella), and health regulators.

The primary objectives of this project are to understand and communicate the risks of exposure to Legionella via recycled water and to develop management tools and techniques for Water Quality Managers to reduce these risks. Specific objectives include:

  • Summarize what is currently known about the incidence/occurrence of Legionella spp., especially for pathogenic species occurring in recycled water distribution systems and end user sites;
  • Characterize disinfection kinetics of Legionella spp. Emphasis should also be placed on recycled water treatment processes and include chemical and operational parameters which may be different from drinking water production facilities that could affect microbial water quality. Parameters should include type of disinfectant, disinfectant residual, residence time, temperature, etc.;
  • Characterize the chemical and operational parameters of Legionella persistence and growth in recycled water distribution systems and end user sites. Parameters could include total organic carbon (TOC), assimilable organic carbon (AOC), biodegradable dissolved organic carbon (BDOC), nutrients, conveyance materials, trace metals, etc.
  • Discuss whether the drivers characterized in objectives #2 and #3 are different in recycled water compared to drinking water or other water matrices, and describe management tools to minimize risk of Legionella spp. in recycled water;
  • Characterize the relative risk to human health from Legionella in aerosols derived from recycled water and assess management practices to reduce or minimize this risk;
  • Summarize and evaluate management techniques to control Legionella occurrence and/or concentration, such as flushing, reduced detention time, disinfection strategies, etc.

Research Approach

Task 1: Literature Review: Investigators should conduct an international literature survey of peer-reviewed publications and gray literature and summarize findings of what is currently known about the incidence and occurrence of Legionella spp., including serotype information, if available, in water and recycled water production facilities, distribution systems, and end user sites. Include any available risk information and risk management tools. Also include summary tables of indicator, pathogen, and other microbial data from these studies. Treatment processes, water quality characteristics, storage times, conveyance information, user sites, etc., should be included. A brief summary of Legionella spp. and serotype incidence, illness, and risk in drinking water and other exposure routes should be included.

Task 2: Characterize hazards, control conditions, and management strategies of Legionella in recycled water systems: This task will use published data to characterize and verify the main hazards and control conditions of Legionella spp. and serotypes in recycled water. Benchtop analysis should be used to validate the models generated.

  • Characterize and summarize the main hazards and control conditions during recycled water production. Characterize disinfection kinetics, chemical, and operational parameters of recycled water systems that affect Legionella persistence, growth, decay, etc. Investigators should select treatment strategies to be representative of a broad range of geography, size, etc. with the purpose of investigating what types of Legionella occur in distribution systems (pathogenic vs. non pathogenic) and how they are effectively managed and/or removed. The effects of temperature, seasonality, and other common factors which could affect growth should be included.
  • Characterize and summarize the main hazards and control conditions in distribution systems and end user sites. Examine occurrence and growth of Legionella spp in the distribution system water as a function of 1) recycled water characteristics in the distribution system, 2) residence time within the distribution system; 3) recycled water user sites.
  • Develop management strategies to control Legionella in recycled water production, storage, distribution, and use. Evaluations should include studies of pilot-scale or full-scale distribution systems. Effects of management practices on distribution system variables such as residence time, temperature, nutrients, AOC, BDOC, disinfectant residuals, and co-occurrence of other microorganisms (e.g. free living amoebae).
  • Validation of Management Model. Bench-scale testing will be carried out on a representative sample of Legionella (including L. pneumophila) to characterize the kinetics of inactivation in the proposed model.

Task 3: Characterize relative health risk posed by Legionella in recycled water distribution systems and end user applications. The results from the above work packages should be used to estimate the relative risk to human health from Legionella found in recycled water systems, relative (emphasis added) to drinking water and other exposure routes, particularly for the end user. The assessment should evaluate management recommendations to minimize and reduce public health risks (if necessary). When discussing best management practices, a range of end uses should be considered. Identify problems that could increase risk and provide tools to manage these conditions.

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