A pressurized wastewater effluent chlorination system including a treatment tank defining a contact chamber and an aeration chamber. The treatment tank receives wastewater effluent. The system includes a pressure vessel within a liquid chlorine storage container positioned in or near the treatment tank. Preferably, the pressure vessel is a floating vessel having a check valve at its bottom, enabling the vessel to fill with a predetermined amount or measured charge of chlorine depending on the position of a float on the vessel. In response to a timer control or float switch, a valve operates to communicate air pressure from a compressor to the vessel to displace the measured charge of liquid chlorine into the wastewater effluent in the contact chamber. Effluent flow and air discharge into the contact chamber enhances mixing of the chlorine and the effluent.