Sanitary Assurance at Biosolids Composting Facilities: Development of a Temperature Contact Time Test Protocol

Pathogen inactivation in aerated static piles is dependent upon assuring that every particle of compost is exposed to a temperature of 55°C or higher for at least 3 consecutive days. Regardless of maintaining high temperatures, there may still be pathogenic microorganisms that survive composting. Hypothetically, this could be because: (1) temperature test methods may not account for spatial and temporal temperature variation in large composting piles, giving the false impression the temperature-contact time condition has been met; and/or (2) the existing temperature contact time criterion may be inadequate, which could lead to pathogen survival via entrance into a viable but not culturable state (VBNC). The objectives of the current study were: 1) to demonstrate how the two hypotheses mentioned above can be tested with minimal bias; and 2) to analyze whether gradual increases in temperature, as occurs in compost piles, triggers the selected E. coli and Salmonella strains to enter VBNC. A covered aerated static pile (CASP) was used in the study. 22 temperature probes which behave like random particles were seeded with E. coli and Salmonella and introduced into the CASP. The biosolids feedstock was composted for 56 days. It was found that 92% of compost pile experienced temperatures >55°C for at least three consecutive days during the first stage of composting. This number rose to 93% after pile turning. At the end of composting the biosolids matrix contained fecal coliforms at a level of <103 MPN g−1 dry solids and Salmonella at a level of <3 MPN 4g−1 dry solids. There were no culturable Salmonella or E. coli in the samples retrieved from temperature probes. The molecular method, however, indicated that there still were live cells which had apparently entered a VBNC state.