adjacent cathodic compartment 3 between the two compartments there is provided a membrane 6 typically a nafion membrane the wall opposite to the membrane 6 of the anodic compartment 2 is given by the anode plate 4 and the wall opposite to the membrane of the cathodic compartment is given by the cathode plate 5 correspondingly the electrical current flows as indicated by the arrow 12 the anodic compartment 2 on its bottom comprises an inlet 7 for the anolyte and on its top it comprises an outlet 8 for the anolyte analogously the cathodic compartment 3 on its bottom comprises an inlet 9 for catholyte and on its top end an outlet 10 for catholyte close to the outlet as well as to the inlet the compartment 3 is provided with grids namely with an upper grid 11 u2032 and a lower grid 11 u2033 since the flow of the liquid is substantially vertical these grids are preferably located substantially horizontally they however need not be flat they may also have a three dimensional structure like for example a zigzag or wavy shape the bed of granules 14 is maintained in the reactor by such non conductive grids set on the top and the bottom of the cell and is in contact with a conductive plate 5 that brings the electrical current into the granular material 14 in the cathodic compartment 3 there is provided a bed in the form of a multitude of granules these granules however not filling up the space between the two grids 11 u2032 11 u2033 completely the situation indicated in fig1 a displays the conditions if the flow of the liquid.