This invention provides a method of reducing the concentration of at least one electrically charged compound present in aqueous feedstock (5), such as water and waste water. The feedstock to be treated is continuously supplied to one surface of a selectively permeable, electrically charged membrane (2) contained within a module (1). Simultaneously an aqueous reaction medium, containing an active microbial culture, contacts with the opposite face of the membrane (2). The microbial culture, fed with selected nutrients and salts, is capable to convert at least one electrically charged compound, after it permeates through the wall of the membrane (2) to by-products. The membrane is substantially impermeable to electrically uncharged compounds. The ion-exchange properties of the membrane allow avoiding secondary pollution of the treated water by metabolic by-products and residual nutrients from the reaction medium. The membrane separates physically the polluted water stream (5) from the reaction medium, which assures that the treated water (6) does never directly contact with the microbial culture.