The incorporation of green synthesis nanoparticles has a high photocatalytic degradation capability for treating anaerobic treated palm oil mill effluent (AnT-POME). Nevertheless, studies using zinc oxidelemongrass nanoparticles (ZnO-L NPs) in photocatalytic processes for the treatment of AnT-POME are still in the early phases. Therefore, the goal of this study was to assess the photocatalytic degradation of AnT-POME in terms of colour, turbidity and chemical oxygen demand (COD) removal upon introducing modified ZnO-L NPs. The outcomes of this study demonstrate that the ideal conditions for photocatalytic degradation of AnT-POME have been established in the presence of ZnO-L NPs loading 0.1 g/L and pH 8, which results in a higher percentage of removal for colour (50.88%), turbidity (85.62%) and COD (97.02%). The photocatalytic process in the presence of ZnO-L NPs proved an improvement in colour, turbidity and COD removal when compared to without the addition of ZnO-L NPs. According to the experimental results, ZnO-L 1:3 NPs exhibit superior photodegradation of AnT-POME via pseudo first-order kinetics because of higher R2 (0.9169) and K value (0.0167). Thus, the current study will gain a lot of attention because of the advantages of ZnO-L NPs’ environmental performance in photocatalytic systems.