Biological treatment of drinking water is a cost-effective alternative to conventional physico/chemical processes. A new concept was tested to overcome the main disadvantage of biological denitrification, the intensive post-treatment process to remove microorganisms and remnant carbon source. The biological reaction zone and carbon supply were separated from the raw water stream by a nitrate-permeable membrane. Denitrification takes place in a biofilm, which is immobilized at the membrane. In a series of bench-scale runs, different types of membranes and reactor configurations were investigated. The best denitrification rates achieved were 1230 mg NO3--N m(-2) day(-1). In one run, raw water containing 100 mg NO3- l(-1) was completely freed from nitrate. The membrane and the attached biofilm also represent a barrier against the passage of the C source and nutrients into the raw water. At concentrations of 20 mg l(-1) ethanol and 15 mg l(-1) phosphate in the bioreactor no diffusion through the membrane into the treated water was observed. Without any post-treatment, the effluent met nearly all the relevant criteria for drinking water; only the colony count was slightly increased.