NOVELTY - A biofuel cell comprises: (a) cathodic and anodic compartments containing the conductive medium in the interior of the cell; (b) an anode in the anodic compartment and cathode in the cathodic compartment; (c) an ion exchange membrane interposed between the compartments to divide the compartments.    USE - For purifying starch wastewater, a livestock and/or wastewater from septic tank (claimed).    ADVANTAGE - In the biofuel cell, a portion of reducing power generated when the electrochemically active microorganisms contained in the sludge are subjected to the energy metabolism with the substrate present in the water, is utilized for the production of biomass. At the same time the remaining portion of the reducing power is utilized to produce electric current for purifying the wastewater. The biofuel cell produces electrical energy up to 0.22 mA and purifies the wastewater by lowering the chemical oxygen demand (COD) from 1900 - 55 parts per million (ppm). The efficiency of the biofuel cell can be varied depending on the kind and concentration of the wastewater.    DETAILED DESCRIPTION - The anodic or cathodic compartment contains wastewater and active sludge and is maintained in an anaerobic condition during the operation of the cell.    An INDEPENDENT CLAIM is also included for treating waste water while producing electric power using the biofuel cell involves:    (i) introducing the waste water and the active sludge into the anodic compartment;    (ii) introducing nitrogen into the anodic compartment to remove dissolved oxygen from the anodic compartment and maintain the compartment in an anaerobic condition;    (iii) continuously introducing air into the cathodic compartment to maintain the compartment saturated with oxygen; and    (iv) densely culturing electrochemically active microorganisms present in the wastewater and the active sludge, such that the cultured active bacteria serve as a microorganism catalyst while organic substances present in the wastewater serve as a fuel.    DESCRIPTION OF DRAWING(S) - The Figure is a schematic view showing a biofuel cell of the present invention, which shows:    cathodic compartment (12)    anodic compartment (14)    cathode (22)    anode (24)