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.
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:
- The compost facility must be designed to provide and allow high enough aeration rates that encourage the beneficial microbes that eliminate odour and pathogens.
- 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.
- Because the compost dries quickly at these high aeration rates, the compost system must allow turning or mixing the material after 1-2 weeks.
- 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!
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.