A pressure vessel (28) accumulates an aqueous stream at elevated pressure and feeds it through a pressure retaining array of passages (18) in the bottom wall of a modular reaction chamber (14) that operates at atmospheric pressure. Spaced electrodes (16) treat the stream during upward flow to the open top of the chamber, where the treated stream overflows the chamber and falls into an inter-wall volume between the chamber and an outside housing (12), washing foam from the housing and chamber as it exits. A housing cover (54) establishes headspace over the chamber to accommodate the overflow. The entire chamber (14) is removable from the housing (12) by loosening fasteners (39) in the bottom wall (20) and lifting it free, with no impediment due to clogging or corrosion outside the chamber.