Are we feeding potentially pathogenic microorganisms during curing?

Inactivating potential pathogens during composting is a two step process. The first step is the thermophilic or temperature kill process that kills most potentially pathogenic microorganisms. The second step is removal of readily available energy compounds for potentially pathogenic organisms. We are well aware that some potential pathogens survive the thermophilic stage either because they are in cooler zones in the composting material or they are able to enter a VBNC state (viable but not culturable). We normally assume that during the curing process, aerobic microbes that decompose the more resistant carbon compounds flourish and outcompete the potentially pathogenic microbes that thrive on readily available energy compounds.

Is it possible that in some of our curing processes, we may actually be feeding and culturing potentially pathogenic microbes? Can thermotolerant anaerobic microbes such Clostridia decompose cellulose and other more resistant carbon compounds and produce readily available carbon compounds to feed potential pathogenic bacteria?

Presence of Clostridia in Finished Composts

Anaerobic microorganisms such as Clostridia survive the composting process (Bohnel and Lube 2000, Jones and Martin 2003, Cornell Waste Management Institute 2004). While testing 94 marketable composts in the US, 70% of the composts contained measurable Clostridia and 20% contained > 1000 CFU/g. (Brinton et al. 2009).

If oxygen limiting conditions persist during composting and curing, Clostridia produce odorous substances that persist (acetic, butyric and valeric acids) (Partanen et al. 2010, Sundberg et al. 2013, Yu 2014).

Volatile Fatty Acid Production in Composts

Clostridia are anaerobic bacteria that decompose cellulose and other organic materials, producing short chain organic acids such as acetic and butyric acids. Butyric acid production is well known with animal feed storage (silage), where Clostridia flourish if the grass silage is not properly processed.

In a study of over 700 finished composts from throughout the US, Brinton (1998) measured > 500 ppm volatile fatty acids in 95% of the composts. Volatile fatty acids including acetic, propionic and butyric acids are excellent and easily degradable carbon sources for aerobic bacteria during the composting process.

Presence of E. coli in Finished Composts

In the study of 94 marketable composts in the US, Brinton et al. (2009) found that more than 50% of composts contained quantifiable fecal coliform bacteria, and that the fecal coliform counts were closely related to the measured E. coli. Many other research reports have discussed potential pathogen regrowth or its persistence during the composting  and curing process despite high temperatures.

Many of the potentially pathogenic organisms are known as facultative anaerobic bacteria. This means that they would prefer to use oxygen as an electron acceptor, but  are able to use other electron acceptors if oxygen is not available.

Are the Anaerobic Bacteria Feeding the Fecal coliform and E. Coli?

If our curing process occurs on large piles, where little to no oxygen is present, could Clostridia and other anaerobic bacteria continue to produce readily available carbon in the form of volatile fatty acids in anaerobic microsites, which then allow potential pathogenic microorganisms to thrive by consuming these volatile fatty acids in the presence of oxygen?

It may help understand some of the “frustration” over the “persistence” of fecal coliform during the compost process, leading some of us to suggest “false positives”.

Perhaps we need to focus on maintaining an aerobic environment to limit the ability of Clostridia and other obligate anaerobic bacteria to thrive, rather than simply focusing on keeping the temperatures high, particularly given that both Clostridia and many of the potential pathogenic organisms are thermotolerant?

In the next blog, we will address potential pathogen kill at lower temperatures during compost curing.

References

Bohnel, H., and K. Lube 2000. Clostridium botulinum and bio-compost. A contribution to the analysis of potential health hazards caused by bio-waste recycling. J. Vet. Med. B Infect. Dis. Vet. Public Health: 47: 785-795

Brinton, W. F. 1998. Volatile organic acids in compost: Production and Odorant Aspects. Compost Science and Utilization 6: 75-82.

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

Jones, P., and M. Martin. 2003. A review of the literature on the occurrence and survival of pathogens of animals and humans in green compost. WRAP (Waste and Resources Action Programme, UK.

Partanen, P., J. Hultman, L. Paulin, P Auvinend and M. Romantschuk. 2010. Bacterial diversity at different stages of the composting process. BMC Microbiology 2010 10: 94 doi.org/10.1186/1471-2180-10-94.

Sundberg, C., D. Yu, I. Franke-Whittle, S. Kauppi, S. Smars, H. Insam, M. Romantshcuk and H. Jonsson. 2013. Effects of pH and microbial composition on odour in food waste composting. Waste Management 33: 204-211.

Yu, D. 2014. Microbial community profiling of biodegradable municipal solid waste treatments – aerobic composting and anaerobic digestion. PhD thesis. Faculty of Biological and Environmental Sciences, University of Helsinki, Finland.

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Managing Potential Pathogenic Bacteria – MPN or CFU – are they the same?

Fecal coliform and E. coli in compost or leachate is usually reported in MPN per g compost or MPN per 100 mL water (or leachate). Sometimes we see results in CFU/g, or per 100 mL. Is there a difference?

CFU refers to “colony forming units”, whereas MPN refers to “most probable number”.  The difference is that CFU/100ml is the actual count from the surface of a plate, and MPN/100ml is a statistical probability of the number of organisms (American Public Health 2012). The US EPA appears to prefer MPN rather than CFU “because a colony in a CFU test might have originated from a clump of bacteria instead of an individual, the count is not necessarily a count of separate individuals.” (US EPA 2003).

It is important to note that some test methodology for specific organisms report the results in CFU, whereas for fecal coliform and E.coli, MPN is most often used.

Although we would intuitively think that CFU and MPN should be equivalent, and we normally assume that they are, research suggests that this is not always the case. One research report indicated that “especially in fall, E. coli concentrations in MPN are one order of magnitude greater than that in CFU” (Cho et al. 2010).

The Organic Matter Recycling Regulation, the Approved Water Quality Guidelines for British Columbia (BC MOE 2001), the CCME Compost Quality Guidelines (CCME 2005) and the US EPA (US EPA 2003), and the UK Compost Regulation (BSI 2011) report the fecal coliform requirements for compost in MPN per g solids.

References

BC Ministry of Environment. 2001. Approved Water Quality Guidelines Microbiological Indicators 2001. (http://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/wqgs-wqos/approved-wqgs/miceoindicators-or.pdf)

BSI. 2011. PAS 100:2011. Specification for Composted Materials. http://www.wrap.org.uk/sites/files/wrap/PAS%20100_2011.pdf

CCME 2005. Guidelines for Compost Quality. PN 1340 (http://www.ccme.ca/files/Resources/waste/compost_quality/compostgdlns_1340_e.pd)

Cho, K.H., D. Han, Y. Park, S.W. Lee, S.M. Cha, J.H. Kang and J.H. Kim. 2010. Evaluation of the relationship between two different methods for enumeration fecal indicator bacteria: colony-forming unit and most probable number. J. Environ Sci (China) 22: 846-50.

American Public Health Association, American Water Works Association, Water Environment Federation. 2012. Standard Methods for the Examination of Water and Waste Water.

US EPA 2003. 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(https://www.epa.gov/sites/production/files/2015-04/documents/control_of_pathogens_and_vector_attraction_in_sewage_sludge_july_2003.pdf)

 

 

 

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Potential Regrowth and the Role of Viable but Nonculturable Bacteria

As practitioners in the compost industry, we assume that when the temperature of our compost has reached 55 C, we have killed all potential pathogens. There are many studies that demonstrate the effect of temperature on the viability of potential pathogens. When we find elevated fecal coliform or E. coli after the high temperature phase, we assume that its either regrowth, or false positives due to “other” bacteria. However, the elevated fecal coliform following high temperature composting may be due to Viable but Nonculturable Bacteria (VBNC). Sunar et al. (2009) observed that high temperatures during composting killed E. coli and Salmonella as measured using traditional plating methods, but found that using polymerase chain reaction (PCR) methodology indicated that the E. coli had survived.

There are some excellent review articles on VBNC (Fakruddin et al. 2013, Li et al. 2014, Ramamurthy et al. 2014, Pinto et. al. 2015, Zhao et al. 2017). The concept was first reported in 1982 (Xu et al. 1982). Li et al. (2014) described as bacteria reducing their function as a survival mechanism allowing them to wait for suitable conditions to revive. and also described that bacteria are not able to survive the VBNC state for extended periods of time. Ramamurthy et al. (2014) described how the resuscitation process of VBNC bacteria required favorable growth conditions with a source of energy, which we observe during composting when the temperature decreases following the thermophilic phase.

While we would like to think that its only the non-pathogenic bacteria that can survive high temperatures, its not true.  Li et al. (2014) reported that 51 potential human pathogens are able to enter the VBNC state including those commonly found in our composting processes.

Some excellent work was done on the topic of potential pathogen kill and viable but nonculturable bacteria during composting by the Engineering group at the University of Alberta (Isobaev 2014). They concluded that:

“Gradual exposure to TTC [time temperature criteria] induces a VBNC state in E. coli and Salmonella. The VBNC state helps both E. coli and Salmonella survive at appreciable concentration throughout the 56 days long composting cycle. With certain constraints the VBNC at the early state in E. coli and Salmonella can be reverted when optimum growth conditions are supplied”.

“it is not recommended to view the temperature as an effective stand-alone sanitation factor. According to the collected evidences, pathogens like E. coli and Salmonella can survive thermophilic conditions, similar to those in the composting pile. The cells, when exposed to 55°C for more than 3 consecutive days can induce stress-response mechanism and subsequently transit into VBNC state. During direct process validation the organisms in VBNC successfully skip culture-based detection methods and pose the risk to regrow during storage and transportation. The stakeholders should always keep that in mind when distributing the product. At least the existing direct process validation methods should be amended to incorporate the pathogens in VBNC.”

The humbling news is that potentially pathogenic organisms may survive the high temperature phase of composting.

The good news is that potentially pathogenic bacteria that enter the VBNC state are only able to remain in that state for a limited length of time. Other studies have confirmed that when the compost matures, there is no readily available carbon left for the potentially pathogenic organisms, and they are no longer able to resuscitate. This confirms that when we follow adequate composting and curing procedures, we are destroying potential pathogens.

References

Fakruddin, M., K.S.B. Mannan and S. Andrews. 2013. Viable but nonculturable bacteria: food safety and public health perspective. ISRN Microbiology. http://dx.doi.org/10.1155/2013/703813.

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.

Li, L., N. Mendis, H. Trigui, J.D. Oliver and S.P. Faucher. 2014. The importance of the viable but non-culturable state in human bacterial pathogens. Frontiers in Microbiology. Doi 10.3389/fmicb.2014.00258

Pinto, D. V., M.A. Santos and I. Chambel.  2015. Thirty years of viable but nonculturable state research: unsolved molecular mechanisms. Critical Reviews of Microbiology 41: 61-76. review

Ramamurthy, T., A. Ghosh, G.P. Pazhani and S. Shinoda. 2014. Current perspectives on viable but non-culturable (VBNC) pathogenic bacteria. Frontiers in Public Health. Doi: 10.3389/pubh.2014.00103. review

Sunar, N.M., D.I. Stewart, E.I Stentiford, and L.A. Fletcher. 2009. A rapid molecular approach to determine the occurrence of pathogen indicators in compost. Proceedings of the Twelfth International Waste Management and Landfill. Sardinia 2009 Symposium.

Xu, H.S., N. Roberts, F.I Singleton, R.W. Attwell, D.J. Grimes and R.R. Colwell. 1982. Survival and viability of nonculturable Escherichia coli and Vibrio cholerae in the estuarine and marine environment. Microbial Ecology 8: 313-323.

Zhao, X. J. Zhong, C Wei, C-W Lin and T. Ding. 2017. Current perspectives on viable but non-culturable state in food-borne pathogens. Frontiers in Microbiology doi: 10.3389/fmicb.2017.00580

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Compost Awareness Week – Four Great Things to be Aware of

During this Compost Awareness Week, there are four great things that we should be aware of with composting our food waste.

If food waste is well aerated during composting, odour is eliminated within two weeks, and the moisture content reduced from 65% to 50% or below.
Achieving this requires a well designed process and a well designed recipe, allowing air to move freely through composting mixtures that include up to 50% food waste. Its easy to find news stories of odorous composting processes. The key is good management, including compost system design and compost recipe preparation. If we don’t get it right early on by providing enough air, the odour may linger in the compost for a long time, possibly many months.

This plate of compost was two weeks old. It was 50% food catering waste and 50% ground yardwaste. It was composted in a small insulated bin at our home. There was no odor during the process.

This plate of compost was two weeks old. It was 50% food catering waste and 50% ground yardwaste. It was composted in a small insulated bin at our home. There was no odor during the process.

Encouraging fungal growth is important to kill potential pathogenic bacteria such as E. coli. We like to see temperatures of > 55 C for at least three days in all of the food waste being composted. Controlling moisture, temperature and the oxygen after this initial composting phase is important to encourage fungal growth.

This compost still contained fecal coliform and E. coli after the initial high temperature composting, but was completely eliminated by encouraging fungal growth.

This compost still contained fecal coliform and E. coli after the initial high temperature composting, but was completely eliminated by encouraging fungal growth.

Compost produced from food waste is rich in nutrients, so its more like an organic fertilizer than a “soil”. We can’t use it directly to grow plants in, but is a great product to add to existing soil to provide nutrients and healthy microbes. Food waste compost is an excellent product to improve the health of our soil, and should be returned back to the land in order to grow food or beautify our environment.

This compost was applied to the grass in a layer about ¼” thick. The beneficial microorganisms encouraged decomposition of any remaining thatch from the winter. It also provided fertilizer for the grass to grow.

This compost was applied to the grass in a layer about ¼” thick. The beneficial microorganisms encouraged decomposition of any remaining thatch from the winter. It also provided fertilizer for the grass to grow.

Successful composting can be done at home or on a small scale. It is important to contain the process, protect it from rats and other vectors, and ideally insulate it to allow the higher temperatures to be achieved. The composter has be be designed to allow hot moist air to escape and allow cool air to be drawn through the bottom.

This small insulated composter is able to achieve temperatures of up to 70 C, with no external heat or power requirement. It was able to produce the compost shown the photographs above.

This small insulated composter is able to achieve temperatures of up to 70 C, with no external heat or power requirement. It was able to produce the compost shown the photographs above.

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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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Indicator Organisms for Potential Pathogens: Fecal coliform or E. coli?

How do we know that our compost is free of potential pathogenic organisms? The test that we use in our British Columbia regulation (OMRR) is fecal coliform bacteria, as an indicator organism. This is in part based on the US EPA Biosolids Rule 503, which suggests that fecal coliform is a good indicator organism:

“Fecal coliforms are enteric bacteria that are used as indicators of the likelihood of the presence of bacterial pathogens. Although fecal coliforms themselves are usually not harmful to humans, their presence indicates the presence of fecal waste which may contain pathogens. These bacteria are commonly used as indicators of the potential presence of pathogens in sewage sludges. They are abundant in human feces and therefore are always present in untreated sewage sludges. They are easily and inexpensively measured, and their densities decline in about the same proportion as enteric bacterial pathogens when exposed to the adverse conditions of sludge processing.” (EPA 2003).

More recent Compost Regulations use E.coli as the indicator organism rather than fecal coliform, because of concerns with bacteria of non-fecal origin are included in the fecal coliform test.

“The microbiology literature is replete with reports of studies that correlate results of fecal coliform levels with the presence of E. coli including several recent examples that advocate the fecal coliform test as an acceptable indicator in manure composts and foods. However, the value of the fecal coliform assay as an indicator of fecal contamination is negated when bacteria of nonfecal origin are the principal microbes detected by the assay.” (Doyle and Erickson 2006).

Brinton et al. (2009) observed an excellent correlation between E.coli and fecal coliform in a composts produced in Oregon and California. More recent regulations (Ontario Ministry of the Environment 2012) and the UK PAS 100:2011 (BSI 2011) use E.coli as the indicator organism for fecal contamination. Gilbert (undated) reported that measuring E.coli “is useful to demonstrate a stable product without feacal bacteria” and measuring feacal coliforms “may be problematic due to variable species composition”.

In Scandinavia, recommendations for testing included both fecal coliform and E. coli:

“Most of the experts suggested that thermotolerant coliform bacteria [fecal coliform] or E. coli should be included in both direct process evaluation and the end-product analysis. These organisms were proposed because they have a heat resistance very similar to Salmonella and because of the increasing interest for E. coli as a pathogen in the environment.” (Nordic Council of Ministers 2000).

The discussion with indicator organisms has been occurring with water for a longer time. The World Health Organization (2001) has an excellent review of the indicators of water quality, and suggested that E.coli is a more specific indicator organism:

“Many members of the total coliform group and some so-called faecal coliforms (e.g. species of Klebsiella and Enterobactero) are not specific to faeces, and even E.coli has been shown to grow in some natural environments. Hence, the primary targets representing faecal contamination in temperate waters are now considered to be E. coli and enterococci.”

The BC Approved Water Quality Guidelines also discusses the limitations of measuring only fecal coliform:

“Fecal coliforms have historically been the indicator of choice, but their presence does not always correlate well with the incidence of disease. Coliforms are therefore now being supplanted by more specific indicators. These include Escherichia coli and enterococci which are good indicators of gastrointestinal disease, and Pseudomonas aeruginosa which correlates well with ear and skin infections. Criteria are set in this report for these three other indicators as well as for fecal coliforms. Although the fecal coliform criteria are the only ones that apply now, they will be phased out in the future as the change to other organisms occurs.”

We suggest that in BC, we measure fecal coliform as required by the current regulation, but also measure E.coli as a more specific indicator of potential pathogenic organisms.

References

BC Ministry of Environment. 2001. Approved Water Quality Guidelines Microbiological Indicators 2001 (http://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/wqgs-wqos/approved-wqgs/miceoindicators-or.pdf)

BC Ministry of Environment. 2002. Organic Matter Recycling Regulation.

Brinton JR., W.F., 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.

BSI. 2011. PAS 100:2011. Specification for Composted Materials. http://www.wrap.org.uk/sites/files/wrap/PAS%20100_2011.pdf

Doyle, M.P. and M.C. Erickson. 2006. The fecal coliform assay, the results of which have led to numerous misinterpretations over the years, and may have outlived its usefulness. Microbe April 2006 (https://woodsend.org/pdf-files/MicrobeNews.pdf)

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

Nordic Council of Ministers. 2000. Sanitary Aspects of Composting Biodegradable Waste. Towards a Nordic Evaluation Model. Copenhagen 2000

Ontario Ministry of Environment. 2012. Compost Quality Standards.  http://www.ewswa.org/wp-content/uploads/2011/06/Ontario-Compost-Standards.pdf

US EPA. Control of Pathogens and Vector Attraction Reduction in Sewage Sludge. Environmental Regulations and Technology. EPA/625/R-92/013 . Revised July 2003. (https://www.epa.gov/sites/production/files/2015-04/documents/control_of_pathogens_and_vector_attraction_in_sewage_sludge_july_2003.pdf)

WHO. 2001. Indicators of Water Quality. Chapter 13. In L. Fewtrell and J. Bartram, eds. Water Quality: Guidelines, Standards and Health. IWA Publishing, London, UK.

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