Abstract: A plastic housing has an inlet for the entrance of liquid containing contaminants and an outlet for the exit of aerated liquid and an air inlet located therebetween. An inlet nozzle is located in the housing between the liquid inlet and the air inlet. The nozzle has a bore with a flared inlet and a substantially cylindrical exit portion which discharges at an exit face. A discharge nozzle is located in the housing between the air inlet and the liquid outlet, and has a bore which is flared towards the inlet nozzle and which has a substantially cylindrical exit portion of a diameter greater than that of the inlet bore exit portion. Contaminated water from a wastewater source such as a swimming pool, hot tub, or swine manure pond may be passed under pressure through the aerator, mixed with air in an expansion chamber formed between the inlet nozzle and discharge nozzle and the housing, and may take place at relatively low flow rates and pressures.