An electrocoagulation treatment device includes a plurality of spaced reaction plates disposed within a reaction chamber. A voltage is applied to selected reaction plates to create an electrical field within the electrocoagulation chamber. The plates are arranged vertically which induces a vertical flow of liquid through a device. The device includes various embodiments adapted for use as a large industrial unit, a portable unit or for use within the home. The voltage and amperage of the electrical field chamber may be adjusted by placing selected reaction plates in electrical contact with the voltage source. One embodiment is especially adapted for treatment of oil slop to remove water from the oil, and for treatment of seawater to desalinate the same.