Describes an investigation into the mechanisms responsible for sludge carry-over during decant from intermittent aeration tanks. Three possible modes of failure were identified: 1. early carry-over which may occur during the transient phase at the start of decant; 2. a failure selectively to withdraw from the supernatant alone; 3. scour failure of the sludge blanket when the interfacial shear stress exceeds some critical value. This mode of failure is analagous to the onset of erosion in cohesive sediments. A variety of experiments were conducted to develop design criteria to ensure against sludge carry-over. The experiments indicated that modes one and three were the most significant, and that the critical parameter was the velocity of the supernatant as it approached the weir. It is therefore concluded that a maximum upstream velocity is the most appropriate design criterion for a decant mechanism. Appreciation of the failure mechanisms has indicated certain design improvements which might be made to existing aeration tanks, in particular to the rate of change of weir loading, and the design of scum barriers.