Petroleum refineries produce significant biological sludge, and the CO₂ generated during its aerobic digestion is mostly unutilized. This study examines the efficiency of CO2 production from refinery sludge under various biomass loading conditions. Sludge from PETRONAS Chemical Olefins and Glycols wastewater treatment plant was collected to operate twelve bench-scale reactors for 10 days. Water displacement and CO2 analyzer were used to measure CO2 every 24 hours. Four reactors (R1, R4, R5, and R8) were considered analytically unreliable because three showed insufficient mixed liquor volatile suspended solids (MLVSS) reduction (ΔMLVSS < 100 mg/L), while R5 exhibited anomalous gas readings. However, MLVSS continuously declined in all reactors, confirming microbial activity. Specific CO2 yields ranged from 605.98 mg CO₂/g MLVSS to 1,791.39 mg CO₂/g MLVSS throughout the eight reliable reactors. Reactor R10 (1,000 mL sludge; 50% volumetric fraction) provided the highest specific CO2 yield among the tested conditions, obtaining approximately 91.9% of the theoretical maximum carbon conversion. These findings imply that there is an ideal sludge load where microbial efficiency hits its maximum before performance is negatively impacted by overcrowding. This study provides the groundwork for improving aerobic digestion systems to enhance CO2 extraction from refinery sludge on an industrial scale.