The application of membrane bioreactors (MBRs) to treat effluents has grown significantly in recent years, and yet the technology was almost unknown in commercial terms ten years ago. The first applications were on a small scale. Since then efforts by membrane system suppliers and a growing demand from end users have resulted in the appearance of large-scale installations. Ross Severn, Aquator Group Ltd, UK, presents long term operating data from several MBR plant projects that the company has been involved in over recent years.