Quantitative microbial risk assessment for sustainable water resources
Authors:   , Hamilton Kerry Ann , Haas Charles Nathan 
Published: July 2021

This thesis evaluates the microbiological health risks of using the sustainable water resources roof-harvested rainwater and reclaimed water. This research was accomplished by performing six tasks: 1) prioritizing pathogens for exploration; 2) conducting a screening study of opportunistic pathogens in rainwater tanks identified in task 1 in Brisbane, Australia; 3) conducting a longitudinal study of opportunistic pathogens for contaminated tanks identified in task 2; 4) developing a series of new dose response models necessary to conduct a risk assessment; 5) conducting a quantitative microbial risk assessment (QMRA) for two index opportunistic pathogens in roof-harvested rainwater; and 6) conducting a QMRA for an opportunistic pathogen in reclaimed water in partnership with a large field and laboratory study conducted by American Water. Decentralized, alternative, or otherwise "sustainable" water resources are growing in popularity as the world's water resources are strained due to population growth, climate change, and water scarcity. These resources bring new challenges for the water industry in terms of maintaining water quality standards and minimizing adverse impacts. Increased attention has been devoted to opportunistic pathogens due to their growing importance as a portion of the waterborne disease burden in many countries and need to assess their associated health risks. The two systems investigated in this thesis are roof-harvested rainwater (RHRW) systems and reclaimed water. Roof-harvested rainwater systems are used in many parts of the world to collect rainwater that falls on roof surfaces in a tank or barrel. This rainwater is then used for a variety of potable or non-potable purposes. Reclaimed water is wastewater reused for beneficial purposes with treatment, where the level of treatment depends on the reuse application. Both types of water systems can foster environments that are conducive to the occurrence of opportunistic pathogens. Opportunistic pathogens are microorganisms that are pathogenic under certain sets of circumstances and typically affect children, the elderly, and immune-compromised hosts rather than healthy individuals. The two opportunistic pathogens that are the focus of this work are Legionella pneumophila and Mycobacterium avium complex (MAC). L. pneumophila causes infection and respiratory disease through inhalation of aqueous aerosols, while MAC can causes infection or disease through inhalation or ingestion. Due to the use of RHRW for both potable and non-potable purposes, and the use of reclaimed water for potable purposes that generate large-scale aerosols (such as cooling tower mists), the potential for public health risk exists as a result of human contact with these water sources. A QMRA is therefore needed to assess risk and prioritize risk management and data gathering needs for both of these pathogens under a variety of scenarios. This thesis presents a large-scale field study of RHRW in Brisbane, Australia, where RHRW is used on a large scale due to aggressive droughts in the region. Molecular biology methods (qPCR) and culture-based methods were used to screen tanks for opportunistic pathogens, and a subset of contaminated tanks was chosen for a six-month follow-up study. A survey of rainwater tank owner use and maintenance practice was conducted. Additionally, an analysis of the correlation among microorganisms and between microorganisms and meteorological factors was performed to inform risk management approaches. A major barrier to conducting a QMRA for MAC has been the development of a risk assessment framework and dose response models. These models have been developed for this work and as a result, a QMRA is performed for RHRW using Monte Carlo simulation and sensitivity analysis. A similar modeling approach is used for assessing risks from toilet flushing, spray irrigation, and cooling tower use for reclaimed water. As a result, appropriate uses for RHRW are designated and appropriate setback distances for cooling towers and spray irrigation systems are proposed for reclaimed water.