The object of the present invention is a method and an apparatus for underwater decomposition of organic content of aqueous waste solutions, involving the measurement and, if necessary, adjustment of the pH and electric conductivity of the solution, maintaining optimum pH and/or electric conductivity during the process, and further involving the partial or total decomposition of organic materials contained by the solution. The apparatus comprises a feed tank, at least one decomposition loop, and a storage tank. The method according to the invention is characterized by submerging electrodes in the solution, producing and maintaining an electric arc between the electrodes and the electrically conductive the solution, where the arc is produced by an electric current of at least 0.5 A/cm2 current density at a voltage of at least 70 V and by a symmetrical alternating current having preferably a frequency of at least 10 Hz; and by decomposing the organic content of the solution into water, carbon dioxide, and nitrogen.