the following is part of a paper presented by Marianne Nyström of Lappeenranta University of Technology, Finland, at the recent '3rd International Conference on Effective Membrane Processes: New Perspectives,' held in Bath, UK, from 12–14 May 1993. Co-authors were Jutta Nuortilla-Jokinen and Lena Kaipia, also of Lappeenranta University of Technology, and Susana Luque of University of Oviedo, Spain.