Abstract: An apparatus for treating, such as by neutralization, oxidation, or flocculation, waste liquid streams. Waste liquid and a treating substance, such as an acid or a base, are introduced through a nozzle into a chamber and then through a second nozzle into a second chamber. The effluent from the second nozzle is directed at a sensor for an appropriate property such as pH. The rate of addition of treating substance is controlled by deviations from a set point. The effluent from the second chamber is directed to a third chamber. A portion of the contents of the third chamber is removed as product and the remainder is allowed to return to the first chamber.