1. School of Materials Energy Water and Environmental Sciences, Nelson Mandela African Institution of Science and Technology, P.O. Box 447, Arusha, Tanzania
2. Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
3. Department of Mechanical and Industrial Engineering, University of Dar es Salaam, P.O Box 35131, Dar es Salaam, Tanzania