A reactor apparatus and method for removing chemical and biological contaminants from a contaminated fluid while minimizing disinfection by-products, sludges, and harmful residue. The reactor has a sequential, multi-stage, reaction vessel with an upper end region and a lower end region. The vessel has a liquid inlet port into which the contaminated fluid may pass. Treated liquid may exit the reactor through a liquid outlet port. A gas inlet port and a gas outlet port are provided. A plurality of stacked reaction chambers have a sieve tray and flange assembly, the sieve tray having holes up through which a gaseous phase may pass. A gas phase electrode, an electrical power supply capable of producing pulsed electrical discharges connected thereto, and a liquid phase electrode are provided. The electrodes create therebetween a pulsed corona discharge for generating reactive species in situ and destroying bacterial contaminants in the liquid. A weir-downcomer tube has an upper portion situated above a level of liquid in the associated chamber. The lower portion is situated below a level of liquid in a reaction chamber next below. The weir-downcomer tube minimizes or prevents back flow of liquid from a reaction chamber to an upstream reaction chamber.