Palm oil mill effluent (POME) from crude palm oil extraction process imposed a serious environmental threat to the nearby water resources due to high solid and organic concentration. Constructed wetland treatment systems (CWTS) has been identified as an ideal alternative solution to the POME treatment in order to comply with the regulation concern by the Department of Environment, Malaysia. CWTS offers sustainable and environmentally friendly approach with great potential for pollutant removal. In this study, three hydraulic retention time (HRT) was used in horizontal sub-surface flow CTWS with 3, 6 and 12 days with recirculation flow method to polish POME within 30 and 36 days duration. The gravel-sand bed was used as a medium for Cyperus alternifolius plant growth in the CWTS. The unplanted systems were served as control systems. The performance of horizontal sub-surface flow CTWS was measured with regards to suspended solids (SS), chemical oxygen demand (COD), ammoniacal nitrogen (NH3-N), total nitrogen (TN) and total phosphorus (TP) removal percentage and plant growth. The performance of COD and NH3-N was found to be highest in 12 days HRT with 95% removal while for SS achieved 93% removal in 6 days HRT. The growth rate of Cyperus alternifolius plant shows that the plant height increased 6.9%, 4.4%, and 7.1% respectively, for 3, 6 and 12 days HRT. In conclusion, the horizontal sub-surface flow CTWS successfully treat POME with great pollutant removal with higher HRT.
