In spiral wound nanofiltration and reverse osmosis installations several fouling types may occur. Simultaneous screening of all fouling types could be carried out to establish the impact of each individual fouling type on membrane performance. In extensively pre-treated water biofouling is the major fouling type. Membrane manufacturers recommend to take corrective actions based on a 15% pressure drop increase criterion. In general this approach is not successful. For an adequate anti-biofouling strategy early warning monitoring plays an essential role. Early warning of biofouling requires (i) a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS to monitor the pressure drop increase, and (iii) a higher linear water velocity in the MFS compared to practical conditions to increase the biofilm formation rate and pressure drop, enabling earlier detection. Action based on this early warning monitoring system for biofouling control is more reliable and successful than the approach recommended by membrane manufacturers and the costs are a fraction only of the potential annual savings. (C) 2010 Elsevier B.V. All rights reserved.