NOVELTY - The method comprises: providing first and second solid polymer electrolyte electrolytic cell stacks (210, 211); supplying a flow of wastewater comprising a pollutant to the anode of each of the stacks at a flow rate and flow pressure; providing a voltage of less than 3 volts across each of the cells in the stacks; operating each of the cells in the stacks at an operating temperature and a current density of less than 10 mA/cm2 to degrade the pollutant and generate hydrogen gas at the cathode; and exhausting the generated hydrogen gas from the cathode.    USE - The method is useful for energy efficient treatment of polluted wastewater (claimed) such as industrial and municipal wastewaters.    ADVANTAGE - The method is capable of treating wastewater with lower current density and without unwanted side reactions such as oxygen evolution, while removing all pollutants in an energy efficiency manner.    DETAILED DESCRIPTION - The method comprises: providing first and second solid polymer electrolyte electrolytic cell stack (210, 211), where the cells in the stacks comprise: an anode including an anode catalyst layer; a liquid- electrolyte free cathode including a cathode catalyst layer; and a solid polymer membrane electrolyte separating the anode and the cathode; supplying a flow of wastewater comprising a pollutant to the anode of each of the stacks at a flow rate and flow pressure; providing a voltage of less than 3 volts across each of the cells in the stacks, where the anode is positive with respect to the cathode; operating each of the cells in the stacks at an operating temperature and a current density of less than 10 mA/cm2 to degrade the pollutant and generate hydrogen gas at the cathode; and exhausting the generated hydrogen gas from the cathode. One of components in the first stack and an operating condition of the first stack is different from the stack component in the second stack and the operating condition of the second stack. The different stack component includes an anode fluid delivery layer, the anode catalyst layer, an anode flow field plate, an anode filter layer, the solid polymer electrolyte membrane, and the number of cells in the stack. One of the catalyst loading and catalyst active area in the anode catalyst layer of the first stack differs by more than 10% from that of the catalyst loading and catalyst active area in the anode catalyst layer of the second stack. The different operating condition comprises flow rate of the wastewater, flow pressure of the wastewater, voltage, operating temperature, and current density. The different operating condition of the first stack differs by more than 10% from that of the second stack. The supplying step is carried out without an added supporting electrolyte. The first stack is connected upstream and in series flow with the second stack, and the anode outlet from the first stack is connected to the anode inlet of the second stack. The stacks comprise common end plates. The method further comprises incorporating a treatment unit (218) such as filter, degas unit, and pH controller, in the flow of wastewater. The treatment unit is incorporated upstream or downstream of either the stacks. An INDEPENDENT CLAIM is included for a system for treating wastewater.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross sectional view of an electrochemical cell stack.    Electrolytic cell stacks (210, 211, 212)    Valve (214)    Treatment unit. (218)