A novel ion-exchange membrane bioreactor, able to prevent secondary pollution of biologically treated drinking water, was developed and specifically tested for water denitrification. This system combines ion- selective membrane dialysis and biological conversion. The ion-]selective membrane facilitates the extraction of the pollutant from the water to the biological compartment, hinders the transfer of organic and inorganic nutrients, and confines the microbial culture involved in the conversion process within the bioreactor. In the study hereby presented the system was used to investigate the removal of nitrate from a synthetic groundwater containing 50 mg-N L-1 of nitrate. The treated water obtained was free of inorganic nutrients and ethanol, the carbon source was selected for the biological process, and the surface denitrification rate achieved was 7 g-N m-2 day-1. This system proved to be effective in producing a treated water effluent that does not require the extensive posttreatment associated with conventional biological treatment.