Describes an investigation into the mechanisms responsible for sludge carry-over during decant from intermittent aeration tanks. Several mechanisms were identified which may cause sludge carry-over. An "early failure" mode may occur during the transient phase at the start of decant. This can be avoided by gradually increasing the rate of decant to a final steady value. The time required for flow establishment is in the order of five minutes in tanks of current size. Following the transient phase, failure can occur when interfacial shear stresses between the supernatant and settled sludge exceed some critical value. This is considered to be a "scour failure" analagous to the onset of erosion of a cohesive sediment. Experiments conducted in a variety of different types of extended aeration tanks indicated that the critical parameters governing failure are the upstream velocity of the supernatant, the settlement time, and the Stirred Sludge Volume Index, SSVI. Appreciation of the failure mechanisms has indicated certain design improvements which could be made to existing aeration tanks, in particular to the rate of changee of weir loading and the design of scum barriers. The significance of tank depth is also discussed.