This paper falls into the studying of an aerated grit chamber (AGCs) within the Al-Rustamyah sewage treatment plant-third extension (50 km south of Baghdad). This treatment plant is suffering some troubles associated with inefficient grit chambers due to lack of continuous maintenance resulting in abnormal grit deposits in the subsequent units of the treatment plant, especially digesters tanks. Field tests on these AGCs were conducted for evaluating their operating performance and their efficiency of removing TSS and oil and grease content during 30 days. Samples were taken from the inlet and outlet of each AGCs every 48 hour and were analyzed. The performance of these AGCs was also re-evaluated again by plotting the velocity distribution across a chosen chamber, at four sections perpendicular to the flow in the AGC. Certain operating problems were observed in the existing AGCs as a result of the experiments.