A water treatment device using a known electrolytic principle includes a pair of inner and outer concentric truncated cones serving as electrodes and providing an annular flow space for generally axial flow of liquid to be decomposed or dissociated. The flow in a preferred embodiment is from the smaller-diameter end of the cone-shaped flow space to the larger-diameter end, thus decreasing pressure; however, in certain electrolytic reactions increasing pressure is an advantage, thus the device has bidirectional flow capability. The shape of all interior components along the flow path is such as to provide smooth transitions to avoid turbulence or cavitation in the liquid flow. The two cones can be shifted relatively along an axis line so as to change the width of the annular flow space. The two electrode-cones are advantageously formed of coated titanium plates. Important features of the device include the manner of mounting the inner cone/electrode within the outer cone, which provides for sliding, relative axial movement of the two cones to adjust the size of the flow path, and this may be dynamically, during use. Features may be included to maximize the surface area of the electrodes exposed to the liquid.