•Rice husk ash is a potential natural coagulant for POME treatment.•CF process for POME pretreatment by rice husk ash and alum are optimized by RSM.•Coagulants and flocs were characterized using FTIR. A Coagulation-Flocculation (CF) process was used as a pretreatment for Palm Oil Mill Effluent (POME) prior to the subsequent biological processes. In this study, Rice Husk Ash (RHA) was used to explore its potential as a coagulant, in comparison with aluminium sulphate (alum), Al2(SO4)3, for the removal of Chemical Oxygen Demand (COD) and Total Solid (TS) from POME. Important process parameters including coagulant dosage, initial pH and settling time were optimized using Box–Behnken design in Response Surface Methodology (RSM). All the coagulants and coagulated flocs were characterized by using Fourier Transform Infrared spectroscopy (FTIR). POME treated with RHA yielded COD and TS removals up to 52.38% and 83.88%, respectively, under the following optimum conditions: 6.0 g of RHA, initial pH of 3.6 and settling time of 57.00 min. Meanwhile, alum under optimum conditions (6.5 g alum, initial pH of 6.0 and settling time of 41.69 min) produce slightly higher removals of chemical oxygen demand and total solids at 52.36% and 84.94% respectively. The results have shown that RHA is capable of becoming an alternative to alum and efficiently treated POME at its natural pH at relatively lower levels compared to treatment using alum. The present study shows that RHA can be utilized as a new cost-effective coagulant for palm oil mill effluent treatment.