Consistent potential pathogen kill during composting – how do we achieve this?

Composting regulations in most countries include minimum temperatures required to be achieved for a certain time period. The reason for this is that we know from extensive research that most potentially pathogenic microbes are not able to withstand temperatures of greater than 55 C for very long. The goal of the regulations are that all of the material being composted reaches these temperatures.

“By mixing the composting material, turned windrow systems insure that all portions of the pile are exposed to interior conditions sufficient to kill pathogens.  In-vessel and static pile systems are often insulated to insure that exterior temperatures are sufficiently elevated.” (CIWMB 2000)

“Unless the entire composting mass is subject to the pathogen reduction temperatures, organisms may survive and repopulate the mass once the piles or windrows are cooled. Therefore it is crucial that temperatures be attained throughout the entire pile. For aerated static piles or invessel systems using static procedures such as tunnels or silos, temperature monitoring should represent points throughout the pile, including areas which typically are the coolest.” (EPA 2003)

“Aerated static pile – Aerated static piles should be covered with an insulation layer of sufficient thickness to ensure that temperatures throughout the pile, including the pile surface, reach 55° C. It is recommended that the insulation layer be at least 1 foot thick.” (EPA 2003)

German Composting Regulation for windrow composting requires: 14 days at 55 °C; 7 days at 65 ° C – 3 representative zones: edge, core and base, and for aerated static pile and in-vessel: 7 days at 60 ° C – diagrams demonstrating process requirements include insulated layer (Gilbert, undated)

There has been some excellent research on temperatures throughout the composting material by the University of Alberta Civil and Environmental Engineering Department on covered aerated composting windrows in 2014 (Isobaev 2014)

  1. “During composting in CASP [covered aerated static pile] with single turning, the likelihood of every particle’s compliance to TTC [time temperature criteria] typically achieves 76 to 93% compliance.
  2. Pile turning is a significant step towards assuring the particle’s compliance with TTC.
  3. The CASP surface, if not insulated, remains in the mesophilic temperature range (≤45°C) up to 50 cm depth and provides a good environment for microbial proliferation. After a single turning with the front-end loader, 40% of the particles in the cool zones still remain mesophilic. It is recommended that: 1) the insulating layer up to 50 cm in thickness be used to cover the pile and ensure that the traditionally cool zones reach thermophilic temperatures; and 2) the cool zones be adequately handled during the mixing step to ensure that what was in the cool zone gets into the pile core after mixing.”

The recommendations for composting practitioners and regulatory authorities in this thesis included:

“several pile turnings don’t ensure 100% compliance with TTC. Each additional turning increases the chances of every particle to meet the required TTC. However, after 6th turn the probability of compliance increases to 98%; thereafter the impact of turning becomes negligible and practically unattractive. Furthermore, the effect of turning is significantly profound during active composting stage. Since, on average, the CASP requires more than three days for its temperature to reach 55°C we recommend not to turn the pile until seven days have passed from its construction, and every three days thereafter.”

We can conclude from evidence and research that piles must be insulated, or turned in order for all the particles to reach the composting temperatures of 55 C or higher. As we investigate further, we learn that the microbiology during composting and curing is complex. How the material is managed during the curing process has a significant impact in ensuring that the final composted product contains no potential pathogenic organisms.

References

California Integrated Waste Management Board (CIWMB) 2000. Composting Reduces Growers’ Concerns About Pathogens. Publication #442-00-014.

Gilbert, undated. Monitoring sanitisation in practice at composting sites. The Composting Association. http://ec.europa.eu/environment/waste/compost/presentations/gilbert.pdf

Isobaev P. 2014. Developing and Testing a Framework to Measure the Sanitation Efficacy on a Random Particle Level in the Composting Industry. PhD Thesis. Department of Civil and Environmental Engineering, University of Alberta.

US EPA. 1992. Environmental Regulations and Technology Control of Pathogens and Vector Attraction in Sewage Sludge (Including Domestic Septage) Under 40 CFR Part 503  EPA/625/R-92/013 Revised 2003.

 

 

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