1. Researcher, Japan Water Resources Environment Technology Center, NK Bldg. 2-14-2, Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan (corresponding author). E-mail: umeda@wec.or.jp
2. Associate Professor, Dept. of Civil Engineering, Tokyo Metropolitan Univ., 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan.
3. Professor, Dept. of Environmental Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuda, Midoriku, Yokohama, Kanagawa 226-8502, Japan.