Fate of faecal pathogen indicators during faecal sludge composting with different bulking agents in tropical climate

In recent years, composting has increasingly been promoted as a reliable method for sanitizing Faecal Sludge (FS) from onsite sanitation systems, particularly where there are opportunities to use the recovered nutrients in agriculture. However, there remain gaps in our understanding of the fate of infectious faecal pathogens during composting, particularly in tropical climates. This study investigated the influence of different locally available bulking agents on the inactivation efficiency of composting by tracking the fate of four key indicator organisms (E. coli, Salmonella spp., Enterococci spp., and viable helminth eggs). Dewatered FS was mixed with different bulking agents - i.e. Sawdust (SD), Coffee husks (CH) and Brewery waste (BW). Compost piles of FS:SD, FS:CH, and FS:BW in a volumetric ratio of 1:2 were set-up in duplicate (3 m3 each), composted on a pilot scale and monitored weekly for the survival of pathogen indicators for a period of 15 weeks. The study findings suggest that the different bulking agents have a statistically significant (p <0.05) effect on the temperature evolution and survival of pathogen indicators in compost. CH was the most suitable bulking agent for composting with FS as piles containing CH exhibited higher pathogen inactivation efficiency and shorter inactivation periods of 6 weeks compared to 8 weeks for SD and BW piles. Time-temperature was the most important factor responsible for pathogen inactivation. However, other mechanisms such as indigenous microbial and toxic by-products such as NH4+-N also played an important role in the inactivation of pathogens. The results suggest that co-composting of FS with a sawdust, coffee husk or brewery waste for 8 weeks with thermophilic temperatures of about 48–60 ◦C sustained in the composting piles for more than 38 days, using 7 days turning frequency, is sufficient to ensure complete sanitization of FS before reuse in agriculture.