Membrane photocatalytic reactor (MPR) has become one of the promising methods for palm oil mill secondary effluent (POMSE) polishing process. However, the utilization of green synthesis zinc oxide-Cymbopogon citratus (ZnO-CC) nanoparticles to improve MPR performance for POMSE treatment has been rather limited to date. In this study, the effects of different operating conditions (pH, initial concentrations, and catalyst loadings) and membrane cleaning using distilled water for treating POMSE via MPR assisted ZnO-CC nanoparticles were elucidated. The behaviour of the membrane fouling was evaluated through normalized flux, colour rejection, water quality and membrane surface characteristics. The results revealed that colour rejection reached 99.9%, normalized flux up to 0.33 and the water quality parameters attained are within the acceptable discharge limit of wastewater, thus verifying the effectiveness of POMSE treatment via MPR assisted ZnO-CC nanoparticles. In addition, the best performance in terms of flux, colour, chemical oxygen demand (COD), and turbidity removal efficiency was achieved after six cycles of POMSE treatment via MPR. Hence, cleaning of the membrane was able to recover the quality of permeate to a certain extent and this results in reusability of the membrane, which can serve as an alternative to improving severe membrane fouling. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.