Fate of pathogenic microorganisms during lagooning sludge composting and exploration of bacteriophages as indicator of hygienization

Composting is a suitable solution to reduce the high quantity of sewage sludge produced every year. Microbiological quality of compost should be evaluated before its use. In this circumstance, researching the faecal bacteria indicators becomes more interesting to confirm the compost safety in terms of pathogenic microorganisms, which can cause health risk for the soil–plant–human​ system. This work monitored the evolution of pathogenic bacteria (faecal coliforms, faecal streptococci, Shigella spp and Salmonella spp.) during windrows co-composting of three mixtures with different proportion of lagooning sludge and green waste under semi-arid climate. It also investigated the evolution of two bacteriophages (Coliphages and Salmonella-phages) as an indicator of long-term compost hygienization. The final composts showed a high reduction that reached 99% for faecal coliforms, faecal streptococci; and 100% for Shigella spp. and Salmonella spp. These reductions revealed the efficiency of composting on bacteria inactivation. Comparison between the survivals of the two bacteriophages showed that Salmonella-phages have less tolerance to inactivation than coliphages, it was totally eliminated after 45 days of composting, while coliphages were completely inactivated after 200 days. The complete destruction of the bacteriophages showed the safety and the good microbial quality of the three co-composting trials. These findings revealed that in the case of composting on windrows, a maximum temperature about 50 °C and time which, exceeds 5 months are essential to decrease pathogenic bacteria and virus to a safe level. We conclude that co-composting conditions of mixtures containing half and one-third of the sludge were more adequate for a rapid destruction of pathogens, compared to mixture containing two-third of the sludge. Based on this study, we anticipate that the proposed analysis will yield a rapid indicator of co-composting hygienization design with less dependency simple and fast technique.