By combining total water softening in the pretreatment stage with alkaline operation of the partial desalination reverse osmosis stage (LPRO), membrane fouling was prevented without using inhibitors or biocides. The retentates of this stage can be further concentrated by medium pressure reverse osmosis (MPRO) or electrodialysis reversal (EDR), thus gaining a higher product yield and possibilities for concentrate reuse. Due to the alkaline conditions under which the reverse osmosis membrane operates, scaling caused by silicates and fouling by disserved organic matter, e.g., humic substances are definitely prevented so that a raw feed water yield of up to 98% from river water can be obtained.