Nanofiltration of conventionally pretreated groundwater with elevated hardness and content of NOM (Ca2+: 115 mg/L; Me2+ 12 mg/L; DOC = 2.9 mg/L) was carried out in a German water treatment plant of the public works of Mainz. For the investigated water the employed membrane (NF200B, Filmtec/DOW) showed almost complete rejection (>95%) of NOM, determined by measuring DOC, UVA (254 rmi) and AOX- and THM-formation potential. Due to the high concentration of SO42- and presumably due to complexation of Ca2+ with humic substances the rejection of Ca2+ and Mg2+ was unexpectedly high (>74% and >86%, respectively) compared to a pure CaCl solution (R(CaCl2) <45%). Within an operation period of 4 weeks no significant fouling occurred (flux decline <2%). This was mainly due to the pre-treatment of the raw water (deferrization, demanganization, rapid sand filtration). The operating costs for a nanofiltration plant were calculated to about Euro 0.23/m(3) at a permeate output of 20,000 m(3)/d which means an increase of the price for drinking water of approximately 9%. By blending the permeate and the conventionally treated water at a ratio to yield a DOC concentration in the blend of 1 mg/L, the additional costs for nanofiltration would come to Euro 0.11/m(3).