Johannes M Keegstra1, Francesco Carrara2, Roman Stocker3. 1. Institute for Environmental Engineering, ETH Zurich, Zurich, Switzerland. keegstra@ifu.baug.ethz.ch. 2. Institute for Environmental Engineering, ETH Zurich, Zurich, Switzerland. 3. Institute for Environmental Engineering, ETH Zurich, Zurich, Switzerland. romanstocker@ethz.ch.