Since 2003 a number of WWTPs have observed elevation of E Coli densities following dewatering of digested sludge by high solids centrifuges, mainly in the US. A recent study by WERF shows that this applies to 4 out of 7 facilities studied in detail. A review of all the literature shows that the phenomenon occurs in 31 out of 33 facilities and includes those with thermophilic digestion or pre-pasteurisation. The VBNC hypothesis for faecal reactivation is
explained. The US regulations apply to the use of product at point of use or disposal and the regrowth of E Coli makes testing and mitigation of non-conforming sludges necessary. The phenomenon occurs also in UK – but there is no collected data or research into the matter. There is a need to take reactivation into account when considering HACCP compliance.