As composting educators, we have been teaching residents and community project leaders in the Northeast that small-scale composting piles will routinely freeze solid during the winter months, only to thaw and become active again the following spring. This concept is based upon the conventional wisdom regarding the inverse relationship between surface area and volume, in that most compost bins tend to be small-scale (usually less than one cubic yard in volume) and do not contain sufficient mass to support and sustain microbial metabolic activity. The surface area to volume relationship was tested in a study conducted by Woods End Laboratories by measuring the gaseous emissions from a series of composting bins ranging from 8 to 50 cubic feet (ft3) in volume (Brinton, 2010). Each of the bins tested contained the same optimized compost recipe. The findings suggested that in small-scale, well-balanced systems, surface area to volume ratios directly influence the composting process. Optimal oxygen concentrations of 5 to 10 percent were noted in bin sizes ranging from 20 to 30 ft3, or roughly one cubic yard (cy) in volume.
Additionally, microbial activity, measured by carbon dioxide (CO2) evolution, was also noted to be robust at similar bin size ranges. Smaller bin volumes of less than 20 ft3 did not appear to contain sufficient mass to adequately support the self-heating process, whereas when pile volumes exceeded 30 ft3, decreased porosity inhibited ‘core’ microbial activity, due to decreased oxygen permeability (Brinton, 2010). Other studies have focused on the influence of ambient temperature on the composting process (T. Luangwilai et al., 2010; Huang et al., 2005; and Fery, 2007). In all instances, porosity was determined to exert a significant influence on the composting process by regulating the ease and depth of airflow throughout the pile, moisture content and microbial activity. These studies suggest that the ideal small-scale composting bin should hold approximately the volume of 1 cy, or 27 ft3, of compostable material.
The following research project attempted to determine if ambient temperature fluctuations influence the activity of a series of optimally sized small-scale compost bins (containing approximately 1 cy of compost mix) during the winter season, when cooler temperatures may inhibit the compost process activities.

Materials And Methods

Between December 1, 2011 and March 21, 2012, a series of replicated composting trials were conducted at the University of New England, Marine Animal Rehabilitation Center (UNE MARC) Compost Facility located in Biddeford, Maine. A total of four cylindrical bins measuring 4-feet high by 4-feet in diameter were constructed from marine grade lobster trap wire (0.5-inch diameter mesh). Each composting bin held approximately 1 cy of compost recipe. Three test bins (Bins #1, #2 and #4) held a mixture of food scraps from the UNE cafeteria layered with leaves collected from campus clean-up activities during the previous year, while a separate control bin (Bin #3) was filled with only leaves. The leaves were a mixture of maple and oak with some pine needles mixed in.
On November 17, 2011, each bin was loaded in a series of alternating layers of leaves and food, except for Bin #3 which served as the control bin and contained only leaves. The initial layer of leaves was approximately 18 inches thick to provide adequate leachate retention, followed by a 6-inch layer of food, and then a 10-inch layer of leaves, alternating until the bin was full. Bins #1 and #2 contained approximately 1.75 barrels of food scraps (approximately 400 pounds), whereas Bin #4 contained 2.5 barrels of food scraps (approximately 550 pounds) or 38 percent more food. It is well known that layering of ingredients allows for the absorption of liquid generated from the breakdown of putrescible materials, such as food scraps, while simultaneously enhancing materials contact (carbonaceous and nitrogenous) and airflow throughout the bin. Periodic ‘physical’ turning further enhances composting processes by homogenizing ingredients ensuring better contact and redistributing airflow throughout the bin (Rynk, 1992). For the purposes of this study, bins were not disturbed for the entire study period; bins were not mixed nor was additional material added.
Bin temperatures were continuously monitored at hourly intervals, using HOBO® data loggers (Model #U-12). Additionally, a 3-foot long Reotemp® analog thermometer (300 series) was placed in the center of each bin to serve as a data collection back-up should the HOBO® data loggers fail, and to facilitate observations of bin activity. Bins were checked daily to record temperatures and to check for nuisance odors, leachate generation and vector activity.

Results And Discussion

Figure 1 displays the distribution of temperatures for each of the compost bins along with ambient temperatures recorded during the 18-week study period. Each of the bins performed as expected throughout the course of our study. Bin #3 (control) failed to gain thermophilic composting temperatures and, after two weeks, dropped down to ambient temperature levels. This expected drop was attributed to a combination of factors, including a relatively low bulk density, excessive porosity, high carbon to nitrogen (C:N) ratio and resultant lack of microbial activity.

In contrast, Bin #1 and Bin #2 were constructed using an optimal compost recipe (C:N=25:1 to 35:1), moisture content (55-60%), and adequate porosity. Both bins were able to gain thermophilic composting temperatures early into the process (by week two), and sustain optimal composting temperatures independent of ambient temperatures throughout the study. The mixture in Bin #4, which contained approximately 38 percent more food residuals than Bins #1 and #2, was very dense and saturated at the outset, and daily observations revealed strong odorous emissions (likely volatile organic acids) emanating from this bin. The odors, coupled with an inability to reach thermophilic temperatures, suggest that this bin had poor porosity and was operating anaerobically.
This was further evidenced by bin excavations at the end of the study (March 22, 2012). Excavations revealed advanced decomposition in Bins #1 and #2, whereas Bin #4 contained a dense, saturated core, which still emitted strong odors. However, it is important to note that even though this bin mixture was not optimal, it still was able to maintain temperatures in excess of 70°F for the entire study period. Bin #3 revealed a mixture of leaves that were still intact with very little decomposition noted.
Based on the results of this investigation, it appears that composting bins measuring one cubic yard in volume have the ability to sustain active composting temperatures independent of ambient temperatures, provided that the initial recipe favors an optimal C:N ratio (25:1 to 35:1), moisture content (55%-60%) and adequate porosity. Above all, daily temperature monitoring provides an opportunity to inspect the bins to ensure that they are working efficiently (without producing odors or leachate), while allowing opportunities to correct any issues as they arise.
M. A. King, G. MacDonald and M. S. Clark are with the Maine Department of Environmental Protection in Augusta, Maine. K. A. Matassa, A. Cariddi, M. Huitt and R. Burgess are with the University of New England in Biddeford, Maine.

References

Brinton, W.; Compost Emissions; Small and Large Scale CO2, CH4,NH3; Presentation BioCycle West Coast Conference, April 13-14, 2010.
Fery, M.; Keep the Compost Cooking This Winter. Oregon State University Extension Service Publications, Winter 2007, Volume 1, Number 1.
Huang QF, Chet TB, Gao D, Huang ZC.; Ambient air temperature effects on the temperature of sewage sludge composting process. Journal Environmental Science (China), 2005; 17(6): 1004-7.
Luangwilai, T., Sidhu, H.S., Nelson, M.I., Chen, X.D. (2010), Modelling air flow and ambient temperature effects on the biological self-heating of compost piles. Asia-Pacific Journal of Chemical Engineering, 5:609-618. doi:10.1002/apj.438
Rynk, R., ed. 1992. On-Farm Composting Handbook. NRAES-54. Cooperative Extension, Northeast Regional Agricultural Engineering Service, Ithaca, New York.