Effect of hygienization treatment on the recovery and/or regrowth of microbial indicators in sewage sludge

Aims Escherichia coli (EC) is the primary indicator micro-organism in regulations for sewage sludge reuse. The aim of this work was to assess the ability of EC to enter and recover from a viable-but-not-culturable state (VBNC) after sludge hygienization treatments. Methods and Results The persistence of EC, somatic coliphages (SOMCPH), spores of sulphite-reducing clostridia (SRC) and Salmonella spp. was assessed in digested sludge after different pasteurization treatments and storage conditions. Pasteurization at 55°C produced EC-injured cells that were resuscitated during the first 24 h. Different sludge treatments altered the inactivation kinetics of EC, while SOMCPH and SRC did not resuscitate and showed lower die-off than EC. No regrowth was observed in stored sludge for up to 60 days. Conclusions EC monitoring by culturable methods is not by itself a suitable method for assessing the hygienization achieved in sludge as EC can enter into VBNC from which it can recover during the first hours of storage. Significance and Impact of the Study The regulations should indicate the time when monitoring of EC should be performed to avoid the period when EC can resuscitate from VBNC or add alternative microbial indicators, such as SOMCPH, which do not have a VBNC state.