Amsterdam Water Supply investigated the possibilities of increasing the capacity of the River Dune production plant from 70 to 83 million m3/year. Two possibilities concern the direct treatment of pretreated Rhine River water (C/S/F) by reverse osmosis, without soil passage. In scheme 1, the RO feedwater is pretreated by ozonation, biologically activated carbon filtration and slow sand filtration. In scheme 2 only slow sand filtration is applied as pretreatment of the RO feedwater. To avoid scaling problems with BaSO4 and CaCO3, the RO feedwater was acidified with HCl up to a LSI in the concentrate of 0.0 and the RO units were operated with a recovery of 85%. The use of an anti-scalant (Flocon 100) in combination with H2SO4 dosage (LSI in the concentrate 2.0) and a recovery of 90% was also successful to control scaling, but this operation mode resulted in severe biofouling as the anti-scalant acted as a nutrient for microbial regrowth in the membrane elements. Both pretreatment schemes resulted in an excellent RO feedwater quality with respect to MFI, DOC, AOC and BFR (biofilm formation rate) with an accompanying very stable operation of the RO units. Over a period of 11 months the MTC of the RO units only showed a decrease of 16% and 23% for pretreatment schemes 1 and 2, respectively, while the differential pressure remained constant. Pretreatment scheme 1 is preferred above pretreatment scheme 2 as it results in additional capacity related to disinfection and organic micropollutant removal. Also the RO concentrate is easier to be disposed of from an environmental point of view. Possibilities are mentioned to further optimize scheme 1 with respect to chemical costs and environmental impact.