Hydraulic design of IEA wastewater treatment plant decant systems

摘要

This report describes an investigation into the mechanisms responsible for sludge carry-overduring decant from an intermittent aeration tank. Several mechanisms were identified which maycause sludge carry-over. An 'early failure' mode may occur during the transient phase at the startof decant. This failure mode can be avoided by gradually increasing the rate of decant to a finalsteady value. The time required for flow establishment is in the order of five minutes in tanks ofcurrent size. Following the transient phase, failure can occur when interfacial shear stressesbetween the supernatant and settled sludge exceed some critical value. This failure mode isconsidered 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 thecritical 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 testedvalues.Appreciation of the failure mechanisms has indicated certain design improvements which could bemade to existing aeration tanks, in particular to the rate of change of weir loading and the designof scum barriers. The significance of tank depth is also discussed.