1. Department of Environmental Technology, Wageningen Agricultural University, P.O. Box 8129, NL-6700 EV Wageningen, The Netherlands
2. Centre for Environmental Sanitation, University of Gent, Coupure Links 653, B-9000 Gent; current address: Delft University of Technology, P. O.Box 5057, NL-2600 GB Delft, The Netherlands