Assessing the temperature - contact time criteria and turning effect in compost sanitation

For a finished compost to be safe for market, it should have reached the necessary sanitation conditions. This means thatthe number of known pathogenic microorganisms in compost should comply with the levels specified in the nationaland provincial guidelines. North American guidelines, such as those published by the USEPA in the United States andthe CCME in Canada, specify the levels of some pathogenic and indicator microorganisms as <3 MPN·4 g-1 dry solidsfor Salmonella, <1000 MPN·g-1 dry solids for fecal coliforms and < 1 PFU·4 g-1 dry solids for enteric viruses. Further Author: K. Wichuk - Civil & Environmental Engineering

by Pulat Isobaev, Dr. Daryl McCartney, Norman Neumann

These levels are generally assumed to be achieved by operating the composting process such that every particle is exposed to atemperature of 55°C or higher for at least 3 consecutive days. For in-vessel and static pile systems, this means that thewhole pile should be maintained at or above 55°C for at least 3 days, while in turned windrows the high-temperatureperiod should last for at least 15 days with the pile being turned at least 5 times during this period. A review of theliterature illustrated that, regardless of maintaining high temperatures and frequently turning piles, there may still bepathogenic microorganisms, which survive composting. This survival may be linked to spatial and temporaltemperature variation in large composting piles, or it could be due to the fact that recommended temperature-contacttime conditions are not adequate (the contact time is shorter or the target temperature lower than it actually needs to befor industrial scale operations).The overall objective of the current study was to assess the efficacy of existing sanitation requirements imposed byFederal and Provincial regulators. Specific objectives were to investigate: 1) the likelihood that a random compostparticle would actually meet the required temperature (≥55°C) for three consecutive days in a full-scale operation; and2) how effective the time-temperature requirement actually is in reducing the number of pathogenic microorganisms tocompliance levels. To the knowledge of the authors, no similar study aimed at validating the time-temperature criteriafrom a random particle perspective has been undertaken to-date.To satisfy the objectives of this study, a temperature probe was developed consisting of temperature recording circuitand a compartment for a cryovial (to hold microorganisms) enclosed in a cylindrical casing made of anodized 6061grade aluminium. Two field trials confirmed that the temperature probe behaved like a random particle in compost.Twenty-four temperature probes were used to assess the compliance of an aerated static pile with national regulationsand to correlate observed temperature patterns with inactivation of pathogens. Seventeen temperature probes containinga mixture of Salmonella meleagridis, Escherichia coli K12, and phi-S1 bacteriophage (all at levels of ~1×106 CFU (orPFU)·ml-1) were randomly introduced into a covered aerated static pile, along with seven probes monitoringtemperature only. After 8 weeks of composting, with one pile turning, they were recovered. Organism levels weredetermined via culturing methods, and half of the organism mixture from each probe was stored for future molecularanalysis.Before pile turning, 80% of probes satisfied the time-temperature criteria. After turning, this number increased to 87%,demonstrating that turning was useful for sanitation. One of the probes with cryovials reached only 42°C, and survivalof S. meleagridis (2.5×106 CFU·ml-1) was observed. The remaining probes with cryovials exceeded 55°C and werepathogen free. It appears that the specified time-temperature conditions are likely adequate. However, more observations are needed before a firm conclusion can be made.

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published: European Compost Network ECN e.V., Orbit 2012 (June 2012), 6|2012
Keywords: Material Recovery, Policy Tax Instruments, Biomass, Canada, Mexico, United States of America