Survival of Escherichia coli and Salmonella spp. after application of sewage sludge to a Pinus radiata forest

Aims: This study investigated the survival of Escherichia coli and Salmonella spp. in sewage sludge applied to young and old Pinus radiata forest in Spring and Autumn/Winter. Methods and Results: Large numbers of E. coli were present in sludge applied to the forest blocks but Salmonella spp. numbers were low or nondetectable. In the mature stand in Spring, numbers of E. coli returned to back-ground after 3 weeks and die-off was significantly correlated with per cent solids of sludge. E. coli survived longer in mature and young stands in Autumn/Winter where numbers did not significantly decrease until weeks 5 and 13, respectively. Salmonella spp. was detectable in the mature stand until week 4 and in the young stand until week 11 in Autumn/Winter. Conclusions: Microbial die-off was related to desiccation of the sewage sludge, and was faster in warmer, drier conditions. Significance and Impact of the Study: In many countries, environmental and health risks associated with the application of sewage sludge to land are minimized by 'best management practice' guidelines, where risks are managed by restriction of public access to these sites. This study provides supporting evidence that withholding periods of greater than 6 months are sufficient to reduce microbial contaminants to background levels.