Contaminants are removed from raw water or discharge water from plants, such as sewerage and industrial plants, by applying direct current through an array of spaced, alternately charged electrodes to eliminate or minimize clogging of the electrodes with precipitated contaminants. Polarity may be switched periodically to assist in eliminating or minimizing clogging. In illustrated embodiments, electrode arrays are contained in housings of dielectric material to form modules, To increase processing capacity, the modules are arranged in parallel arrays. Alternatively, a single module is scaled up for large or industrial applications or scaled down for personal use. Instead of housing the electrode arrays in modules through which liquid passes, the electrode arrays for some batch applications are dipped in the water or aqueous solutions.