Pathogen kill – more than just high temperatures?

Temperatures > 55 C are required for at least a few days to  achieve pathogen kill during composting. We would then naturally agree that if the high temperature requirement was not met, we would have a higher risk of potential pathogens in our compost. Unfortunately, experience does not always support this conclusion. We cannot assume that just because we met the > 55 C requirement, our compost is pathogen free! It is clear that there is more to potential pathogen kill than simply high temperatures!

In a study of 19 small scale composters where the temperature rarely exceeds 55 C, 60% of samples had fecal coliform counts < 1000 MPN (Class A compost) (Cornell Composting Institute (2004). In contrast, with large scale composting facilities (no biosolids), 40% had fecal coliform counts < 1000 MPN (Brinton et al. 2009). This is surprising in that one would expect that > 95% of the compost from large scale facilities would meet Class A requirements, and that > 95% of the compost from small scale composters to fail! Christensen (2002) also observed high E. coli counts in the compost from large scale thermophilic compost facilities, and suggested that a longer time is required for pathogen kill.

While some have suggested that fecal coliform counts are not representative of potential pathogenic organisms, there does not appear to be evidence to support this. Brinton et al. (2009) measured a positive relationship between fecal coliform and E. coli. The Cornell Composting Institute (2004) observed that small scale composters that did not include meat waste actually had higher E.coli counts than those that did. In our own research with small scale composting of food waste, fecal coliform and E. coli counts were the same, suggesting that most of the fecal coliform were E.coli. When we measured discharges from food waste compost facilities, elevated concentrations of E. coli as well as fecal coliform were observed.

Research and experience suggests that microbial diversity is required to kill potential pathogens, and that this microbial diversity is as important or even more important than the high temperatures! In our own small scale work, we were intrigued about why fecal coliform and E.coli counts were < 3 MPN/g after 3 months of “curing” at 12-15 C, following two weeks of composting at 40-50 C.  It was a small scale batch of 25% foodwaste plus 10% poultry litter, which was expected to have a significant fecal coliform and E.coli count!

Others have observed fecal coliform and E.coli destruction at lower temperatures. Henault-Ethier (2007) observed reduction of fecal coliform and E.coli to <1000 MPN/g after 16 days of vermicomposting at 25 C. They attributed the reduction primarily to the microbial diversity in the composting environment.  Henault-Ethier et al. (2016) concluded that E. coli destruction during low temperature composting was primarily due to antagonistic activity of the indigenous microbial population.  Kim and Jiang (2010) reported that E. coli and Salmonella inoculated in autoclaved compost survived for a much longer time than in composts that did not have the microbes killed, suggesting that the microbial diversity in compost was very important to kill potential pathogens that survived the high temperature phase, or were introduced during curing. Paniel et al. (2010) added a number of different pathogenic bacteria to composting green waste, biowaste, sewage sludge and municipal solid waste, and concluded that the indigenous microbial community was critical for the destruction of pathogens during a 25 C curing process. Kim et al. (2011) concluded that the survival of E. coli O157:H7 in compost was negatively correlated with the population of indigenous microbes, particularly the actinomycetes and fungi.  Doffner and Brinton (1995) observed the survival of potential pathogenic organisms at temperatures > 55 C in the composting process and concluded:

“These results suggest that the mechanism for removal of these microorganisms during aerobic composting is complex and not simply the result of a thermal physical environment”

The importance of the microbial community in the destruction of potentially pathogenic bacteria was already reported almost 60 years ago!

“Pathogen destruction during the composting process may occur primarily as a result of two actions: a) thermal kill by sufficiently high time-temperature, and b) kill by antibiotic action or by the decomposing organisms or their products. In light of recent findings, the latter may be equally important as the former” (Wiley, 1962)

The microbiology of composting is even more amazing than I had first considered, where the the microbiology of pathogen kill is more than high temperatures and competition for carbon. In the next posts, we will review aeration requirements and their relevance to odour control and microbial diversity and pathogen kill, and how temperatures > 60 C in the composting process may decrease microbial diversity, which may delay the destruction of potentially pathogenic organisms.

References

Brinton, W.F. Jr., P. Storms and T.C. Blewett. 2009. Occurrence and Levels of Fecal Indicators and Pathogenic Bacteria in Market-Ready Recycled Organic Matter Composts. Journal of Food Protection 72: 332–339.

Cornell Composting Institute. 2004. Hygienic implications of small scale composting in New York State. Final Report of the Cold Compost Project.

Christensen, K.K., M. Carlsbaek, E. Norgaard, K.H. Warberg, O. Venelampi, and M. Brogger. (2002) Supervision of the sanitary quality of composting in the Nordic countries: evaluation of 16 full-scale facilities. Nordic Council of Ministers, Environment TemaNord 2002: 567.

Droffner, M.L. and W.F. Brinton. (1995) Survival of E. coli and Salmonella populations in aerobic thermophilic composts as measured with DNA gene probes. Zbl. Hyg. 197, 387-397.

Henault-Ethier, L. 2007. Vermicomposting: from microbial and earthworm induced effects in bacterial sanitation to the chemistry of biodegradation under batch or continuous operation. M.Sc. Thesis, Concordia University, Montreal, Quebec.

Henault-Ethier, L, V.J.J. Martin and Y. Gelinas. 2016. Persistence of Escherichia coli in batch and continuous vermicomposting systems. Waste Management 56: 88-99.

Kim, J. and X. Jiang. 2010. The growth potential for Escherichia coli O157:H7, Salmonella spp. and Listeria monocytogenes in dairy manure based compost in a greenhouse setting under different seasons. J. Applied Microbiology DOI: 10.1111/j.1365-2672.2010.04841.x

Kim, J., C.M. Miller, M.W. Shepherd Jr., X Liu and X. Jiang. 2011. Impact of indigenous microorganisms on Esherichia coli O157:H7 growth in cured compost. Bioresource Technology 102: 9619-9625.

Paniel, N., S. Rousseaux, P. Gourland, M. Poitrenaud and J. Guzzo. 2010. Assessment of survival of Listeria monocytogenes, Salmonella Infantis and Enterococcus faecalis artificially inoculated into experimental waste or compost. J. Applied Microbiology 108: 1797-1809.

Wiley, J.S. 1962. Pathogen survival in composting municipal wastes. J. Water Pollution Control Federation 34: 80-90

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