During the rejection of trace antibiotics from wastewater using forward osmosis (FO), spacer was added over the membrane to regulate the operation conditions. Interesting results were obtained, which demonstrated that when the spacer was added over the FO membrane, rejections of trace antibiotics increased by 23-2.8% in the active layer facing feed solution (AL-FS) orientation. Similarly, in the active layer facing draw solution (AL-DS) orientation, rejections of antibiotics increased by 5.3-8.5% after the spacer being added. To explore the mechanism of the enhancement for antibiotic rejections, the flow field in the FO cell was investigated using particle imaging velocimetry (PIV) technology. It was confirmed that turbulence was produced in the FO cell due to the added spacer. And it was proposed that the turbulence could disturb the boundary layer at the membrane surface and urge the antibiotics to diffuse from the layer back into the bulk solution, which reduced the diffusive driving force of antibiotics through the membrane into the draw solution, hence leading to an increase in rejections. (C) 2015 Elsevier B.V. All rights reserved.