Ultrasonic standing waves are generated to trap and separate oil droplets, cellular material and/or gases from a fluid. The methods and apparatuses operate at ultrasonic resonance and are low power, e.g., in the range of 1-5 W. One or more acoustic transducers operating in the 100 kHz to 5 MHz range may be used. The methods and apparatuses may be implemented using relatively large flow chamber and flow rates, e.g., in a range of from about 200 mL/min to greater than about 15 L/min.