Spatial dynamics of methane emissions and organic load reduction in a pond-based palm oil mill effluent treatment system.

Palm oil mill effluent (POME) treatment in open pond systems is a significant source of methane (CH4) emissions, yet the spatial dynamics of greenhouse gas fluxes and organic matter degradation remain poorly quantified. This study investigated CH4, carbon dioxide (CO2), and nitrous oxide (N2O) emissions alongside chemical oxygen demand (COD) reductions across a full-scale, pond system consisting of 10 ponds connected in series (Pond 1 to 10) in Sarawak, Malaysia. Using a novel, low-cost, remotely operated floating chamber, we conducted high-resolution spatial sampling of gas fluxes and water quality. The results revealed extreme heterogeneity in COD removal and CH4 fluxes, with a critical emission hotspot identified in Pond 4, where average CH4 flux exceeded 900 μmol m-2 s-1 and COD decreased by over 80 %. Stoichiometric analysis confirmed a strong correlation between removed COD and CH4 production. The cumulative climate impact of the system was estimated at 278 kg CO2-equiv. m-3 of treated effluent, with over 85 % of emissions attributed to CH4. These findings highlight the need for targeted emission mitigation strategies, such as biogas capture from high-emission ponds, and provide robust emission factors to inform sustainable palm oil wastewater management and climate policy frameworks.