I was not aware that composting odors are 10-1000 times stronger when the composting material pH values is below 6, rather than above 6.6! This was the finding of Sundberg et al. (2009) during studies of food waste composting in Norway. They also further reported that these high odor concentrations at low pH can linger for more than 90 days after the start of composting (Sundberg et al. 2013).
This was a surprise as many of us in the composting world have concluded that pH sorts itself out during composting – and not many have reported that food waste having a low pH at the start of the process may have a long and lingering impact on the potential for odor emission.
If we consider the Compost Facility Requirements Guideline in BC (Forgie et al. 2004), we see that pH is not a direct factor that affects odor from compost facilities. The following are:
- Feedstock degradability – the less degraded the material, the greater the odor potential
- Aeration (particle/pile size) – not enough porosity or large piles increase odor potential
- Temperature – higher temperatures increase odor potential because of more microbial activity and greater oxygen demand
- Moisture content – higher moisture contents will decrease oxygen exchange and increase risk of odor production
San Diego University (2007) summarized the factors that affect odor production from compost facilities for the California Integrated Waste Management Board as follows:
- Feedstocks – Increasing sulphur content and increasing degradability increases the risk of odor production.
- Nutrient balances – low carbon to nitrogen and carbon to sulphur ratios increase potential for odor production.
- Lignin content – the low degradability of lignin artificially changes the C:N and C:S ratios.
- Oxygen – adequate oxygen throughout the composting material decreases risk of odor production.
- Aeration – Aeration increases oxygen content but also can transport odorous compounds out of the composting mass.
- Turning – Turning is important for allowing aeration that reduces odor production, breaking preferential air pathways to allow more thorough aeration in forced aerated systems, but can also release odors created inside the material.
- Time – odor production potential decreases with time – most odors are released during the first 14 days of composting.
- Moisture – moisture contents higher than 60% reduce oxygen exchange and may result in higher risk of odor production.
- Bulk density and porosity – lower bulk density and higher air-filled porosity decreases the risk of odor production.
- Temperature – increased temperature increases microbial activity and oxygen demand, increasing the risk of odor production – temperature is also influenced by aeration rate
- pH – ammonia more likely to be emitted at high pH, hydrogen sulphide and volatile fatty acid production at low pH – combination of low pH and anaerobic conditions increases production of intermediate compounds that may produce odor.
Cornell Composting suggests that aerating or mixing composting material with a low pH will reduce the acidity (increase the pH), implying that low pH may be temporary (http://compost.css.cornell.edu/monitor/monitorph.html).
The research by Sundburg et al. (2009) found that the microbes typically found in food waste include Pseudomonas and enterobacteria (E. coli, Klebsiella, Enterobacter), and Lactic acid bacteria. These microbes can produce organic acids that lower pH. During the composting process, Bacillus and Actinobacteria, already present in the waste, flourish under aerobic conditions, decompose organic acids, and maintain a higher pH that reduces the risk of odor production.
Sundberg et al. (2013) found that very high rates of aeration initially during the composting process are required to overcome the continued organic acid production and switch the microbial population to one dominated by Bacillus and Actinobacteria. It appears that the microbes that produce organic acids can function very well, even at high temperatures. In most recommendations for composting, a fast temperature rise is indicative of a good composting process. Normally the air requirement for cooling is greater than the air requirement for maintaining adequate oxygen, therefore controlling aeration to a specific temperature will ensure that the pile is adequately oxygenated. It appears from this research that initially, the aeration rate has to be high enough to limit the temperature increase that allows the microbial population to switch to one dominated by Bacillus and Actinobacteria.
In many of our food waste composting programs, we combine yard waste and food waste during collection. The amount of food waste in the blend is usually in the order of 10%. This is the case with the residential organics program at the District of Mission. In this case, we have an aerated windrow system, where the temperatures rapidly increase to 75 C or higher. There is very little concern with odor from this process, even in the outdoor aerated windrow covered with breathable fabric.
When we have food waste collection programs that do not include yard waste, the proportion of food waste entering the composting process is typically higher, which triggers this phenomenon of reduced pH impacting the composting process. Washington State University (2011) has completed a very good literature review, mostly citing the Norwegian research on the impact of food waste on pH, microbial community and odor during the composting process.
There are several strategies to overcome the low pH and dominant organic acid producing microbial colonies:
1. add high pH materials such as ash or lime to increase pH (not allowed in British Columbia)
2. add recycled and finished compost (if it has a higher pH) to provide inoculation of other microbes.
3. vigorously aerate to oxidize the organic acids and change the microbial colonies to one dominated by Bacillus and Actinobacteria.
It seems like a pH monitor should become one of the “tools of the trade” for composters, especially those contemplating increased amounts of food waste.
Forgie, D.J.L., L.W. Sasser and M.K. Neger. 2004. Compost Facility Requirements Guideline: How to Comply with Part 5 of the Organic Matter Recycling Regulation. http://www.env.gov.bc.ca/epd/codes/omr/pdf/compost.pdf
San Diego State University. 2007. Comprehensive Compost Odor Response Project. California Integrated Waste Management Board. http://www.calrecycle.ca.gov/publications/Documents/Organics%5C44207001.pdf
Sundberg, C., H. Jonsson, M. Romantschuk and S. Smars. 2009. Minimization of odour from composting of food waste through process optimization. Nordic Council of Ministers, Copenhagen.
Sundberg, C., D. Yu, I. Franke-Whittle, S. Kauppi, S. Smars, H. Insam, M. Romantschuk and H. Jonsson. 2013. Effects of pH and microbial composition on odour in food waste composting. Waste Management 33: 204-211.
US EPA. 1994. Composting of Yard Trimmings and Municipal Solid Waste. EPA530-R-94-003.
Washington State University. 2011. WSU Compost Odor Project: Literature Review. http://toxipedia.org/download/attachments/10191820/WSU%20Compost%20odor%20lit%20review%20June%202011%20final.pdf?version=1&modificationDate=1346805250000&api=v2