Abstract: Apparatus for the treatment of an aqueous liquid includes a flat, cylindrical treatment chamber made of electrically insulating material. Two flat electrode plates are disposed parallel to and spaced from each other in opposite side walls of the treatment chamber. Each electrode plate is insulated from the liquid by a thin layer of electrical insulation. One terminal of a d.c. high voltage source is connected to the electrode plates so that the same voltage is applied to both plates. The opposite terminal of the voltage source is connected to electrically conductive means in direct contact with the liquid so that an electrostatic field is established between the two electrode plates and the liquid itself.