A chlorinator for wastewater treatment systems having a circulation pump, circulation tank, and return flow line, which includes a reservoir of disinfectant fluid and a dosage container communicating with the reservoir and the circulation tank to dispense a determined quantity of disinfectant during operation of the circulation pump. The dosage container is vented to atmosphere to prevent vacuum lock in operation and communicates with the reservoir at a flow rate substantially smaller than it communicates with the circulation tank. Operation of the circulation pump causes flow through the return line, inducing distribution of the quantity of disinfectant fluid into the circulation tank. As the dosage container outgoing flow rate far exceeds the dosage container incoming flow rate, once the disinfectant level drops below the point of communication between the dosage container and the circulation tank, no her disinfectant is drawn into the circulation tank. A near-uniform volume of disinfectant fluid is thereby supplied per pump cycle.