1 Department of Chemical Engineering and Environmental Technology, School of Industrial Engineering, Universidad de Valladolid, Dr. Mergelina s/n, 47011, Valladolid, Spain.
2 Sub-department of Environmental Technology, Wageningen University, P.O. Box 17, 6700 AA Wageningen, The Netherlands.
3 KWR Watercycle Research Institute, P.O. Box 1072, 3430 BB Nieuwegein, The Netherlands.
4 Department of Environmental Engineering and Water Technology, IHE Delft Institute for Water Education, Delft, The Netherlands E-mail: p.vandersteen@un-ihe.org.