TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Electrochemical Cell: The cell includes gas diffusion electrodes that are spiral-wound. The chemicals are disinfection agents including chlorine-hypochlorite, mixed oxidants, and/or hydrogen peroxide. The water is directly treated by being passed through, or circulated within, the cell. The gas diffusion electrodes are permeable to a gas but impermeable to water. A gas channel is provided along which gases may permeate through the gas diffusion electrodes, where a reactant gas is brought into the cell via the gas channel and introduced into a water stream through the gas diffusion electrodes during water treatment. A water-borne chemical species in a water stream undergoes an electrochemical transformation at the gas diffusion electrodes info a gaseous product, where the gaseous product exits the water stream through the gas diffusion electrodes. The gas diffusion electrodes-includes a cathode that in operation produces a cathode product, where the cathode product provides a treatment action upon water passing through the cell. The gas diffusion electrodes include an anode that in operation produces an anode product, where the anode product provides a treatment action upon water passing through the cell. The chemical is or includes hydrogen gas and the generated hydrogen gas is used as an agent for catalytically reducing an unwanted species. The chemicals are or include oxygen gas and the generated oxygen gas is used as an agent to aerate bacteria that lower the biological oxygen demand (BOD) of wastewater. One of the gas diffusion electrodes is flexible and comprises: a gas permeable material that is non-conductive; and a porous conductive material attached to the gas permeable material. The gas permeable material is impermeable to water, and the porous conductive material is permeable to water, where the porous conductive material is provided adjacent to the gas permeable material. The gas diffusion electrodes includes an electrode pair of a cathode and an anode wound about a central longitudinal axis of the spiral-wound cell, where the anode is gas permeable and water impermeable; and/or the cathode is gas permeable and water impermeable.