An Enhanced Direct Process Temperature Validation Framework in Composting: Case Study of a Full-Scale Covered Aerated Static Pile

ABSTRACT

To ensure the safety of compost products, the Canadian Council of Ministers of the Environment's compost guidelines specify upper limits for certain pathogenic and indicator microbes, which are presumably achieved by exposing every particle of compost to temperatures ≥55°C for at least three consecutive days. A rugged temperature probe that behaves like a random compost particle was used to investigate whether every compost particle meets the time temperature criterion and to measure sanitation efficacy. An inoculum consisting of Salmonella enterica var. Meleagridis, Escherichia coli K12, and phi-S1 bacteriophage (all at levels of ∼1 × 10⁶ CFU/PFU mL⁻¹) was added into 17 probes. The probes were randomly introduced into a covered, aerated static pile along with 17 probes that only monitored the temperature. After 56 days of composting, with one pile turn the probes were recovered. Organism levels were determined via culture-based methods. Before turning, 80% of the randomly introduced probes satisfied the time-temperature criterion. After turning, this number increased to 87%, demonstrating that turning is somewhat useful for sanitation. The cool zones largely remained mesophilic with the pile turning having minimal impact, which could potentially be an indication that the pile was not turned thoroughly. One of the 17 probes with cryovials reached only 40.2°C, and survival of S. meleagridis (2.5 × 10⁶ CFU ml⁻¹) was observed. The remaining probes with cryovials exceeded 55°C and were pathogen free. It appears that the specified time-temperature conditions are likely adequate. However, more observations are needed before a firm conclusion can be made.