1. Department of Environmental Science, Institute for Wetland and Water Research, Faculty of Science, Radboud University Nijmegen, The Netherlands; Institute of Environmental Engineering, ETH Zurich, Switzerland; ESU Services, Kanzleistrasse 4, 8610 Uster, Switzerland; Department of Mathematics, Faculty of Science, Radboud University Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen; and The Netherlands, Institute for Chemical- and Bioengineering, ETH Zurich, CH-8093 Zürich, Switzerland