Ultrasound was used to improve a forward osmosis (FO) sludge dewatering process for the control of fouling by deposited sludge flocs. FO was able to concentrate activated sludge from a real-scale wastewater reclamation plant. However, the flux decline indicated a severe fouling phenomenon. To mitigate this FO fouling, ultrasound radiation using a novel cell configuration was applied, However, the application of continuous radiation unexpectedly resulted in more severe fouling. Fluorescence excitation-emission matrix (FEEM) spectroscopy showed that longer ultrasound radiation applications caused high organic release from sludge flocs. Confocal scanning laser microscopy (CLSM) clearly identified a thicker organic fouling layer on the FO membrane surface. Ultrasound cleaning was optimized for radiation length and improved by the integration of flushing. Specifically, the combination of ultrasound and flushing caused a flux loss recovery of 70% or more. This work demonstrated the possibility of ultrasound cleaning as a fouling control method for FO sludge dewatering applications. (C) 2017 Elsevier B.V. All rights reserved.