1. Waterworks Research Institute, Seoul Metropolitan Government, 130-1 Gui2, Gwangjin, Seoul 143-820, Republic of Korea (E-mail: anjchan@seoul.go.kr)
2. Department of Environmental Engineering, University of Seoul, 13 Siripdaegil, Dongdaemoon, Seoul 130-743 Republic of Korea (E-mail: jykoo@uos.ac.kr)