High Airflow Rates Reduces Odor and Kills Pathogens Faster

Supplying adequate air to composting material, particularly in the first week, dramatically reduces the potential for odour emission later. It appears that adequate aeration early in the process also encourages a microbial community that is antagonistic to potential pathogenic organisms. While traditionally we have thought that the greatest requirement for aeration is heat removal, it appears that the greatest requirement for air in the first few days or week of composting is to provide adequate oxygen.

How much air is enough? Recommendations from the literature suggest from 5.6 m3 air per tonne per hour (Smet and Langenhove 1998),  6-10 m3 air per tonne per hour (Shen et al. 2011), to more than 30 m3 air per tonne per hour (Arslan et al. 2011). The volume of air also depends primarily on the readily available carbon in the composting material. The size of the pile, the moisture content and bulk density affects the ability of the air enter and be distributed in the compost pile.

In our small scale experiments, we observed natural convective aeration rates up to 100 m3/air/dry tonne with 25-35% food waste and 65-75% yard waste.

Temperature of a food waste yard waste blend and corresponding airflow into the bin

Temperature of a food waste yard waste blend and corresponding airflow into the bin

Oxygen concentrations remained above 18%. We observed that when we restricted airflow, oxygen concentrations could drop to 0-1% within 24 hours during the first few days of composting.

We blended the composting material after one week. At that time there was already negligible odor. After the end of the second week, the distinct odor associated with Actinobacteria (geosmin, or an earthy smell) was obvious. We further noticed that after three months of further curing, the fecal coliform and E. coli was not detectable.

These observations have also been made by others in large scale compost facilities. In some excellent research in Scandinavia, Sunberg et al. (2013) observed that aeration rates of 25 m3/h/dry tonne during composting of foodwaste/yardwaste blends dramatically reduced odour and increased populations of Actinobacter and Bacillus, compared with aeration rates of 1.5-3 m3/h/dry tonne, which resulted in dramatically increased odor throughout the composting process, as well as a proliferation of anaerobic bacteria. They concluded:

“An important strategy for reducing odour from food waste composting is to rapidly overcome the initial low pH phase. This can be obtained by a combination of high aeration rates that provide oxygen and cooling, and additives such as recycled compost.”

There are a number of practical recommendations resulting from the requirement for high aeration rates early in the composting process:

  1. The compost facility must be designed to provide and allow high enough aeration rates that encourage the beneficial microbes that eliminate odour and pathogens.
  2. The compost facility must be designed with adequate odour control to manage the high rates of air emission during the first few days of composting. This is the period where potential odour compounds already present in the composting material may be released into the air.
  3. Because the compost dries quickly at these high aeration rates, the compost system must allow turning or mixing the material after 1-2 weeks.
  4. After 1-2 weeks, odour control requirements may be minimal if the compost continues to be managed properly.

In conclusion, our nose is an incredible tool. If we smell compounds characteristic of anaerobic activity, we are likely to have sustained odour and possibly sustained potential pathogens in our compost. On the other hand, when the compost smells like earth after a week or two, we may be more likely to have successful pathogen kill.

The fascinating facts here is that our nose is very sensitive to both the unpleasant smells associated with anaerobic activity such as butyric acid, as well as the positive smells such as geosmin associated with Actinobacter. Our noses can detect both butyric acid (unpleasant smell), and geosmin (earthy smell) at concentrations of parts per trillion.  Its an amazing world!

References

Arslan, I, U. Ayhan, and M. Topal. 2011. Determination of the effect of aeration rate on composting of vegetable and fruit waste.  Clean Soil Air Water DOI 10.1002/clen.201000537

Shen YJ, Ren LM, Li GX, Chen TB, Guo R. 2011. Influence of aeration on CH4, N2O and NH3 emissions during aerobic composting of a chicken manure and high C/N waste mixture. Waste Management 31(1): 33-38. DOI: 10.1016/j.wasman.2010.08.019.

Smet E, and HV Langenhove. 1998. Abatement of volatile organic sulfur compounds in odorous emissions from the bio-industry. Biodegradation 9(3): 273-284. DOI: 10.1023/a:1008281609966.

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.

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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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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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