Green synthesis provides new avenues of nanoparticle synthesis using plants. In the present study, we attempted to synthesise ZnO nanoparticles via precipitation using different ratios of ZnO solution to leaf extract of Cymbopogon citratus (3:1, 5:1, and 9:1). The modified ZnO was characterised using Fourier transform infrared spectroscopy, transmission electron microscopy, and X-ray diffraction. The influences of the modified ZnO-C. citratus (ZnO-CC) nanoparticles on the removal of colour from and decline in the flux of palm oil mill secondary effluent (POMSE) in a membrane photocatalytic reactor (MPR) were studied. The performance of ZnO-CC 3:1 in terms of colour (99.84 %), COD (98.97 %), BOD (96.24 %) and turbidity removal (99.89 %) of the treated POMSE was significant. The improvement of the performance efficiency for POMSE treatment using MPR correlate with the reduction in the size of ZnO-CC 3:1 (6.6-42.9 nm) which promote higher degradation activity and minimize membrane flux decline during the process. Thus, it can be deduced that the coupled MPR system has significant potential for application in the palm oil mill effluent treatment industry as it mitigates membrane fouling and yields treated effluent of good quality. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.