Investigation of E.Coli Concentration Increases in Digested Sludge across Centrifuge Dewatering

McDonald, A., Southern Water


Southern Water (SW) has 19 Sludge Treatment Centres (STCs) 13 of which produce
“Conventionally Treated” cake for application to land using mesophilic anaerobic digestion
(MAD) followed by cake storage. In order to obtain conventionally treated compliant sludge
straight from treatment thereby removing the final process stage of cake storage, Southern
Water selected the Monsal Enzymic Hydrolysis (EH) pre-treatment process to be trialled at
Goddard’s Green STC. The EH plant was commissioned in February 2007.

Initial investigations showed that although the digested liquid has low E.Coli concentrations
(typically <10 MPN/g), there were significant E.Coli concentration increases of 2 to 4 log10
across centrifuge dewatering rendering the cake non-compliant.

The increase in E.Coli observed across dewatering is not an unknown problem having been
widely reported in the literature and attributed with various different explanatory theories.
These include contamination, E.Coli reactivation due to the presence of viable but nonculturable (VBNC) bacteria in digested liquid and shear forces.

As one of the main reasons for commissioning the EH pre-treatment plant was to be able to
remove cake straight from site thereby alleviating storage constraints and to provide
increased contingency storage for a nearby dryer site, possible reasons for this increase were

Biosolids, E.coli, Enzymic Hydrolysis, qPCR, Reactivation, VBNC

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