Abstract: A variable residence time treatment system for liquid has a reactor vessel with variable effective holding volume. Liquid is admitted with at least one treatment material to adjust a property of the liquid, such as its pH, a discharge outlet is open full time during operation of the system to pass discharge flow from the reactor tank at its full holding volume. Treated liquid can be selectively discharged when the reactor vessel is less than full. Thus, the effective resident time of liquid in the reactor vessel is controlled by controlling the effective holding volume of the reactor tank in response to a holding volume control signal. A sensor monitors a property of the liquid and generates a corresponding input signal. A controller generates the control signal in response to the input signal.