Decreasing Settlement in the Aerator in ASP

We have a problem with settlement in an aerator in a vegetable oil manufacturing company. We have a conventional activated sludge process (ASP) treatment plant with 3 surface aerators of different capacity. The ASP is designed so that the water from the primary clarifier goes to aerator 1, water from aerator 1 to aerator 2, and from aerator 2 to aerator 3. The problem is that the settleability test shows the settlement of: 180 mg/l for aerator 1, 220 mg/l for aerator 2, but the settlement for aerator 3 is increased and is 350 mg/l after 30 mins of the settleability test. How to reduce the settlement in aerator 3?