Review Quantitative microbial risk assessment and its applications in small water systems: A review Mohamed A. Hamouda a,b, ⁎, Xiaohui Jin c , Heli Xu d , Fei Chen d a Department of Civil and Environmental Engineering, UAE University, P.O. Box 15551, Al Ain, United Arab Emirates b National Water Center, UAE University, P.O. Box 15551, Al Ain, United Arab Emirates c Walkerton Clean Water Centre, 20 Ontario Rd., P.O. Box 160, Walkerton, Ontario N0G 2V0, Canada d QuantWave Technologies Inc., 50 Westmount Road North, Waterloo, ON N2L 6N9, Canada HIGHLIGHTS • QMRA is valuable for SWSs to enhance water safety and identify sources of risk. • Pathogen removal by typical small water system technologies is reviewed. • Knowledge gap for successful imple- mentation of QMRA in SWSs is discussed. • Framework of QMRA application in SWSs is proposed to deal with limited data. GRAPHICAL ABSTRACT abstract article info Article history: Received 7 March 2018 Received in revised form 7 June 2018 Accepted 16 July 2018 Available online xxxx Editor: Paola Verlicchi Quantitative microbial risk assessment (QMRA) has been mainstreamed in many large municipal water systems as part of a paradigm shift in the drinking water industry towards water safety planning and risk-based system assessment. Small water systems (SWSs) are generally more vulnerable to typical water system hazards, and consequently have a higher risk of waterborne disease outbreak. In this paper, a review of experiences in implementing QMRA in SWSs helps elaborate the sources of risks and highlights some of the challenges facing SWSs in developed countries. A critical review of the important elements for practical implementation of QMRA was conducted. The investigation focuses on aspects related to challenges in identifying relevant hazards to SWSs to create failure scenarios, acquiring monitoring data for pathogens' concentrations in source water, es- timating treatment efficiencies of typical small system technologies, and access to software tools to support suc- cessful implementation. The review helped outline ways through which SWSs can overcome the identified challenges in implementing QMRA. An adjusted framework for implementing QMRA for small water systems was formulated and discussed. © 2018 Elsevier B.V. All rights reserved. Keywords: QMRA Limited data Framework Knowledge gap Small water systems Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 1.1. Small drinking water systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 1.2. Quantitative microbial risk assessment and water safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 1.3. Challenges to QMRA in small water systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 995 Science of the Total Environment 645 (2018) 993–1002 ⁎ Corresponding author at: Department of Civil and Environmental Engineering, UAE University, P.O. Box 15551, Al Ain, United Arab Emirates. E-mail address: m.hamouda@uaeu.ac.ae (M.A. Hamouda). https://doi.org/10.1016/j.scitotenv.2018.07.228 0048-9697/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv