Zinc oxide (ZnO) nanoparticles were produced via a simple and green precipitation method under stirring conditions (ZnO-St) and under ultrasonic radiation (ZnO-Us). The nanoparticles properties were characterised with X-ray fluorescence (XRF), X-ray diffractometry (XRD) and transmission electron microscopy (TEM); and compared with commercial ZnO and no ZnO. Their influence in the nanofiltration (NF) of hazardous Congo red dyes was studied in order to provide the fundamental understanding in the interacting effect of ZnO and NF membranes. The membrane performances were significantly improved after the addition of ZnO in the dye solution in descending order as follows: ZnO-Us > ZnO-St > commercial ZnO > no ZnO. It is believed that the preparation method, agglomerations and the morphology of nanoparticles influence their interaction with dye molecules and membrane surfaces. Membrane characterisations using contact angle, X-ray fluorescence (XRF), Field Emission Scanning electron microscopy (FESEM) and Energy Dispersive X-ray Analysis (EDX) confirmed that ZnO nanoparticles have great potential for fouling mitigation in industrial NF application. (C) 2014 Elsevier B.V. All rights reserved.