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 (CH 4 ) emissions, yet the spatial dynamics of greenhouse gas fluxes and organic matter degradation remain poorly quantified. This study investigated CH 4 , carbon dioxide (CO 2 ), and nitrous oxide (N 2 O) 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 CH 4 fluxes, with a critical emission hotspot identified in Pond 4, where average CH 4 flux exceeded 900 μmol m −2 s −1 and COD decreased by over 80 %. Stoichiometric analysis confirmed a strong correlation between removed COD and CH 4 production. The cumulative climate impact of the system was estimated at 278 kg CO 2 -equiv. m −3 of treated effluent, with over 85 % of emissions attributed to CH 4 . 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. • Greenhouse gases were measured using automatically controlled floating chambers. • CH 4 and CO 2 fluxes changed during the pond processes. • Global warming potential emissions were much higher for CH 4 than CO 2 and N 2 O. • CH 4 production was similar to that estimated from chemical oxygen demand removal. • Processing ponds are hotspots for organic matter removal and CH 4 production at 60 d.