The disclosed technology includes systems and methods for clustering particles, especially water contaminants, by liquid-phase electric plasma discharge. According to certain implementations, the discharge may be applied continuously, and may be of relatively low power, e.g., a voltage between 300 V and 5000 V and an amperage between 0.1 A and 1 A. The electrodes introducing the electric discharge may be in a point-to-point configuration and with at least one electrode having a large aspect ratio. In some implementations, one or more of the voltage and amperage of the electric discharge, electrode configuration and aspect ratio, and the conductivity of the medium may enable the generation of plasma containing hydroxyl and other radicals. The radicals may catalyze polymerization of the particles, resulting in an increase in particle size and, thus, improved filterability. The polymerization process may continue after the electric discharge is discontinued or removed from influence of the medium.