The Clay Lane Water treatment works of Three Valleys Water, in the south east of England, is currently the world’s largest ultrafiltration works with a capacity of 160 Ml/d. It utilises ultrafiltration membranes constructed as hollow fibres with a number of membrane elements in a pressure housing. The plant has been operating since spring 2001. The decision to install the system was made in anticipation of the introduction of tighter regulations on Cryptosporidium in water supplies in the UK. Once a decision was made to proceed with a membrane system the ability to monitor the integrity of the system and to repair problems became a crucial design parameter that was a critical part of membrane selection. The need to include a system affected the design of the filtration units offered by manufacturers. The available systems for integrity testing are reviewed and the reasons for selecting the system adopted are discussed. These include particle counting and the dosing of test particulate loads. The different forms of air passage integrity tests are discussed and the displaced air flow system used is described. Once a failure has been detected it must be traced so that the fault can be repaired. This procedure is described together with the techniques of pin repair of damaged fibres. At Clay Lane and other membrane filtration sites the backwash water from membrane cleaning is recovered using a secondary membrane system. Currently the secondary system operates in the same integrity testing regime as the primary system and the secondary filtrate is returned ahead of the primary membrane system. The relative merits of this system, or the alternative of adding the recovered water to the filtrate are discussed.