A hybrid process combining electrodialysis and a membrane bioreactor was investigated to treat ground waters with excessive nitrate concentrations. Electrodialysis (ED) allowed the nitrate separation producing, on one hand, partially demineralized waters whose ion concentrations were in agreement with the norm, and on the other hand, brines that were treated by a membrane bioreactor. Experiments performed at laboratory scale with synthetic solutions and pre-industrial scale to treat a ground water contaminated by nitrates showed high efficiency of the hybrid process. The nitrate concentration of the treated water remained below the acceptable value (50 mg/l(-1)) and even below the recommended value (25 mg/l(-1)) for drinking water. Moreover, the ED treatment induced a softening of the treated water. The biological denitrification allowed the almost total removal of nitrates (99%) with kinetics close to 0.3kgN-NO3/kg MVS/d and a limited sludge production of 0.5gV.S.S/gN-NO3.