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Abstract
This report describes an investigation into the mechanisms responsible for sludge carry-over
during decant from an intermittent aeration tank. 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 failure mode 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 failure mode is
considered to be a 'scour failure' analogous 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 supernatant, the settlement time,
and the Stirred Sludge Volume Index (SSVI).
Figure S.l below, shows the relationship between these three parameters for the range of tested
values.
Appreciation of the failure mechanisms has indicated certain design improvements which could be
made to existing aeration tanks, in particular to the rate of change of weir loading and the design
of scum barriers. The significance of tank depth is also discussed.