Practical Paper Spatial and seasonal evolution of trihalomethanes in water distribution systems in Lebanon Lucy Semerjian, John Dennis and George Ayoub ABSTRACT Lucy Semerjian (corresponding author) George Ayoub Department of Civil and Environmental Engineering, American University of Beirut, Beirut, Lebanon Tel.: +961 1 34 79 52 Fax: +961 1 74 44 62 E-mail: ls07@aub.edu.lb John Dennis Department of Community Health Sciences, University of Calgary, Calgary Alberta, Canada Spatial and seasonal changes in trihalomethane concentrations were investigated in various water distribution systems in Lebanon supplied by different water sources. A seasonal sampling program was initiated during which trihalomethanes and various contributing water quality and operational parameters were monitored at points along the distribution system. The majority of samples collected from points within the distribution network exhibited higher trihalomethane concentrations (1–13 folds) when compared to total trihalomethane levels detected in samples collected directly after chlorination. Recorded trihalomethane concentrations in Lebanon were highest in the spring, followed by the summer and winter. Moreover, total trihalomethane concentrations were higher when the abstraction sources were surface waters compared to ground water sources. Also, higher mean trihalomethane values were recorded in chlorinated waters originating from sources subjected to semi-conventional water treatment before the application of chlorine compared to mean trihalomethane values established in chlorinated waters originating from sources subjected to direct chlorination. Key words | chlorination, distribution networks, drinking water quality, Lebanon, trihalomethanes ACRONYMS AUB American University of Beirut DAC Directly after chlorination EERC Environmental Engineering Research Center NOM Natural organic matter THM(s) Trihalomethane(s) TOC Total organic carbon TTHM(s) Total trihalomethane(s) WTP(s) Water treatment plant(s) INTRODUCTION Traditionally, water chlorination has been a preferred disinfection strategy because of its proven efficiency and cost effectiveness in improving microbiological water quality until Rook (1974) and Bellar et al. (1974) revealed that chlorine reacts with naturally occurring organic matter in water, particularly humic and fulvic acids, to generate potentially harmful by-products such as trihalomethanes (THMs). The primary THMs of concern in drinking water are chloroform, bromodichloroform, dibromochloroform, and bromoform. Concerns about health risks associated with THMs have prompted environmental agencies in most countries to regulate THMs in drinking water to safeguard consumers from adverse health impacts. Past investigations have observed that the occurrence of THMs in chlorinated water may vary significantly according to season and geographical location due to changes in raw and treated water quality (fulvic and humic constituents, ammo- nia), as well as in operational parameters related to chlori- nation such as chlorine dose, water temperature, pH and travel time (Denne et al. 1984; Garcia-Villanova et al. 1997; Arora et al. 1997; Premazzi et al. 1997a; Rodriguez et al. 2000). doi: 10.2166/aqua.2007.029 263 Practical Paper Q IWA Publishing 2007 Journal of Water Supply: Research and Technology—AQUA | 56.4 | 2007