1. Swiss Federal Institute for Environmental Science and Technology (EAWAG), Ueberlandstr. 133, CH-8600 Duebendorf, Switzerland (E-mail: wanner@eawag.ch)
2. Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue, Urbana, IL 61801, USA (E-mail: emorgenr@uiuc.edu)