Activated sludge process (ASP) is gaining recognition as a process technique for the control of biological oxygen demand (BOD) in palm oil mill effluent (POME). Surface aerators or diffusing plates are often used in aeration tanks serving as core of the ASP. For consistent improvement in water quality within the aeration tank utilising the ASP and in particular, mitigating its BOD effluent stream, this study replaced the surface aerator with submerged mechanical aerator/agitator incorporating separate “agitation function” and “agitation diffusing function” intended for use in aeration tank of polishing plant that contains surface aerators. In order to confirm the activated state of the sludge in the aeration tanks, sludge was observed by microscopy (magnification 600 ´ or lower). The water analysis, POME, BOD, ammonium, and total nitrogen were analsyed. As a result of the study, improvement in water quality criteria including the agitation state in the aeration tank, mixed liquid dissolved oxygen, and BOD were observed. The BOD has improved from 34.7% to 93.1% at a maximum removal rate.