Regrowth of faecal coliforms and salmonellae in stored biosolids and soil amended with biosolids

Abstract

Stabilised wastewater sludge (biosolids) has beneficial re-use properties but these are limited by the presence of human pathogens. In this study soil amendment with biosolids and storage of biosolids prior to re-use were examined as disposal and treatment options. In a soil amendment trial biosolids were mixed with sandy soil and monitored for 37 weeks. In two storage trials biosolids were stored in piles 1m high and monitored for <60 weeks. Included in the monitoring programme were tests to determine the concentrations of faecal coliforms, faecal streptococci and salmonellae. In both the soil amendment trials and biosolids storage trials, concentrations of indicator organisms and salmonellae decreased through an extended hot, dry summer period. Although these organisms were not detected in the majority of samples taken during the summer, repopulation of faecal coliforms and salmonellae occurred in the trials following rainfall at the beginning of the winter. In the case of one of the storage trials repopulation occurred following a period of 50 weeks when salmonellae and faecal coliforms were not detected. When repopulation occurred, faecal coliform concentrations increased to higher than those at the beginning of the trials. These results suggest that faecal coliforms and salmonellae were at undetectable concentrations through the summer period but were able to grow when provided with favourable conditions. From this limited trial it was concluded that soil amended with biosolids could not be considered free from pathogens for at least one year following amendment.