Effect of Dewatering Methods on E. coli Reactivation and Re-growth in Digested Sludge Cake

Sylvester,J.1, Baddeley, B.2 and Burston, V.1, 1Sydney Water Corporation, Australia, 2The University of Liverpool, UK


Digested Sludge is disposed of through agricultural recycling. As such the E.coli count needs
to meet the requirements stipulated in the Safe Sludge Matrix (SSM). The digestion process
ensures that the required log reduction is achieved. However, the sludge undergoes further
processing involving dewatering prior to agricultural recycling. In this study the effect of these
processes on E.coli count was studied. Belt press dewatering and centrifuge dewatering were
run in parallel to determine dewatering efficiency and what effect the dewatering method had
on E.coli count.

Feed sludge was fed in parallel to both centrifuge and belt press dewatering processes.
Mechanically, belt press operation used significantly less power and polymer providing an
operational cost benefit. There was a consistent 1-2% dry solids benefit from using the
centrifuge, and frequently benefit was higher. The centrifuge dewatering was also a more
compact process, able to dewater a much larger volume of sludge than the similarly sized belt
press unit.

It was found that the Belt Press dewatered cake E.coli counts, on average, were 202cfu/gds.
Meanwhile centrifuge dewatered cake had an average E.coli count of 99 721cfu/gds.
Interestingly the belt press sludge cake E.coli count was always lower than its corresponding
sludge feed count, whereas the centrifuge dewatered cake counts were always higher. This
indicated a regrowth effect being present in the high shear environment of the centrifuge and
an E.coli suppression effect in the belt press.

Sludge Dewatering, E.coli reduction, E.coli re-growth, Agricultural recycling

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