Pollution of ground and surface water by nitrate is a universal problem. Deleterious effects of nitrate on health are well known. An electrodialysis operation using a pilot plant having a capacity of 24 m(3)/d has been conducted to optimise industrial conditions to remove nitrate from groundwater in the centre of Morocco. Influence of the nature and of the concentration of the electrolyte in rinse and in concentrate streams were studied. The optimisation was carried out on the basis of the following parameters: correlation between demineralisation-denitrification, recovery rates, power and reagent consumptions. Electrodialysis operation was conducted continuously during an extended period, under optimised conditions to control the constancy of the electrodialysis plant performances. The treatment of the brine stream (1 g/l) by a bioreactor, previously optimised, allowed reduction of nitrate content in raw water content (100 ppm). Optimised technical and economical conditions were proposed to reduce nitrate for the groundwater studied.