MEA is an alkanolamine used in petroleum refineries for the removal of SOUR gases from liquid hydrocarbons. However, plants treating petroleum refineries wastewater found that MEA inhibits the removals of organic compounds in the treatment system, which causes effluent chemical oxygen demand (COD) concentrations to increase. Hence, the study on the impact of MEA on the removal of organic compounds in the activated sludge system was pursued. The objectives include, to determine the impact of MEA on the removal of organic compounds, in the removal of contaminants, the rate of COD removals of organic matter in an activated sludge system and the loading of MEA which is inhibitory to the microbes in the removal of organic compounds, by varying the addition of MEA concentrations into the raw influent. Bench scale studies were conducted, where two trains comprise of reactors with an aeration tank and a clarifier, provided with sludge recycle system and were set up. Train 1 operated at both conventional aeration-activated sludge mode and extended aeration-activated sludge mode, while Train 2 operated at only extended aeration-activated sludge mode. Both influents and effluents were analyzed according to APHA methods. All sampling parameters were conducted three times a week and were measured in triplicates. Addition of MEA at various concentration of 50, 100, 500 and 1000 mg/L was made to the raw influent once the wastewater has acclimatized. The results show that the addition of 1000 mg/L MEA increased influent COD concentrations to an average of 1774 mg/L. Average COD removals in reactor A1T1 decreased with the addition of MEA, to 88.7. However, the average COD removed/mg mlvss.day stabilized even after the addition of 100 mg/L MEA, at an average of 35.22 mg COD removed/mg mlvss.day, and 14.62 mg COD removed/mg mlvss.day, for reactor A1T1 and A1T2, respectively. Overall, effluent COD concentrations met the standard discharged limits of 120 mg/L. Both MLSS and MLVSS concentrations in reactor A1T1, A2T1 and A1T2 were stabilized throughout the study period, with MLSS/MLVSS ratio of 0.78, 0.83 and 0.84, respectively. The addition of 1000 mg/L MEA into the influent causes spikes in Sludge Volume Index (SVI) for reactors A1T1, A2T1 and A1T2 at 91 mL/g, 90 mL/g and 95 mL/g, respectively. The sludge age in reactor A1T1, A2T1 and A1T2 was maintained throughout the study, at an average of 5.4 days, 30.0 days and 30.2 days, respectively. The specific oxygen uptake rate (SOUR) declined with the addition of MEA, at an average of 5.73 mg O2/gVSS.h., 1.77 mg O2/gVSS.h and 1.85 mg O2/gVSS.h, respectively, for reactor A1T1, A2T1 and A1T2, throughout the study. © 2023, The Author(s), under exclusive license to Springer Nature Switzerland AG.