1. University in Tokyo, Department of Urban and Environmental Engineering, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan
2. Delft University of Technology, Faculty of Chemical Technology and Materials Science, Department of Biochemical Engineering, Julianalaan 67, 2628 BC Delft, The Netherlands
3. International Institute for Infrastructural, Hydraulic and Environmental Engineering IHE Delft, Department of Environmental Engineering, P.O. Box 3015, 2601 DA Delft, The Netherlands