Top: Compost feedstocks were a heterogeneous mixture of slaughterhouse waste materials including offal, hooves, bones, and hides, and paper pulp and wood chips. Photos by Jeremy Ferrell

Seth Wells and Jeremy Ferrell

Of the more than 1,500 slaughterhouses regulated by the U.S. Department of Agriculture, 56% are considered “very small” — defined as facilities with under 10 employees or annual sales under $2.5 million (USDA, 2025). Out of these small-scale slaughterhouses, it is estimated that between 44% to 70% of their offal and non-marketable byproducts are landfilled (Rime et al., 2004; Stacey et al., 2021). If 500 of these facilities operate with a 50% landfill disposal rate, approximately 30,000 tons/year of offal will be wasted, generating approximately 42,000 tons/year of carbon dioxide equivalent (CO₂e) in methane using the global warming potential (GWP) 100-year horizon multiplier. For small slaughterhouses, it is simply cheaper to landfill than to pay for a renderer.

This problem statement, coupled with an interest to develop a small-scale “kill in chill” meat processing facility in rural Watauga County, North Carolina (where Appalachian State University is located), was the impetus for this study to explore composting as an organic waste management solution and evaluate the effects of co-composting with biochar. Adding biochar in small amounts (~5% by volume or 1-2% by weight) has been shown to promote air and water retention and add structure during composting, creating favorable environments for microorganisms (Halder, et al, 2022). The porosity of biochar promotes bacterial habitation in compost, while providing additional surface area for air and water to be used by the bacterial colonies.

These factors are reported to increase the population of bacteria within compost, which in turn raises the temperature as bacterial activity is increased (Mao et al, 2018). By adding structure to the compost and creating a favorable environment for microorganisms, biochar can extend the duration of the thermophilic phase, ultimately increasing organic matter degradation and potential material throughput, according to the literature (McIntosh et al., 2025; Cao et al., 2024; Irvan et al., 2018).

Researchers from Appalachian State University’s NEXUS Project partnered with Wilkes County Abattoir, a vertically integrated business that raises and processes cattle and retails the products. The company also operates a commercial composting facility to recycle its non-marketable waste materials for application to pasture. Wilkes County Abattoir was initially interested in the partnership to learn if biochar could help optimize its composting process, and assess the effects of using biochar with its recipe.

Experiment Design

The experiment was designed to examine temperature differences throughout the active composting phase by introducing biochar to the mix at a predetermined concentration. There was a control sample with no biochar and a sample with biochar. The sample batches were monitored for temperature throughout the process. Our team performed two trials in the summer and fall of 2025 to evaluate the effects of biochar on compost conditions, specifically temperature and carbon dioxide flux, a proxy measurement for microbial activity.

The experimental apparatus consisted of two aerated static pile (ASP) compost bins located at the Appalachian State NEXUS Facility where the experiment took place. The compost feedstocks were a heterogeneous mixture of slaughterhouse waste materials including offal, hooves, bones, and hides. The composition was approximately 50:50 carbon to nitrogen by weight. One bin was filled with compost feedstock mixed with 5% biochar by volume (approximately 1% by weight), and the other was filled with only compost feedstock as a control. The biochar was made from mixed hardwood feedstock in a flame cap Oregon kiln. It was dried and screened to a half-inch particle size before being added to the compost mixture.

Two aerated static pile composting bins (left), with capacity of about 1 cubic yard each, were used for the trial.

The total volume of each ASP was approximately one cubic yard. The ASPs were aerated using an electric Black and Decker leaf blower attached to perforated 3-inch PVC piping located behind the bins. The PVC had one-quarter-inch holes drilled every 9 inches at 5 and 7 o’clock orientation inside the ASP for aeration. The orientation was chosen to reduce air channeling during aeration and eliminate the possibility of leachate or fine material leaking into the piping. The leaf blower was wired with a timer to blow air into the bins at intervals of five seconds every 15 minutes. The bins were lined with 2-inch XPS foam board insulation on all sides except the bottom. A four-inch layer of finished compost was added to the bottom of the ASPs and atop the composting materials to act as a biofilter. Ten gallons of finished compost were mixed evenly into each sample to inoculate the piles.

About 1,480 ± 20 pounds of offal mixed with wood chips and paper pulp were split between the two ASPs. The nitrogen source in the recipe is composed of various offal materials, and the carbon source is a mixture of paper pulp and wood chips. Seven pounds of biochar were evenly mixed in with the experimental ASP to achieve 5% biochar by volume. Ten gallons of water were added to each ASP to increase the starting moisture content and prevent drying out due to the active aeration.

The experiment consisted of one trial that lasted for approximately one month. It assessed the pile temperatures and CO₂ levels during the active phase of composting. The compost temperatures were monitored with data collected in 30-minute intervals by six total Onset HOBO temperature loggers positioned at the top, center, and bottom of the compost pile in each ASP. A LI-COR CO₂ gas flux analyzer was used to characterize the CO₂ flux from each ASP. The ASPs were not turned during the process and were located in a shady area under the cover of trees.

Results and Analysis

Temperature

Temperature was monitored from September 12 to October 10, 2025. Using the 30-minute temperature data collected from the thermocouple in the core of each ASP, daily average temperatures were calculated. The average daily temperature of the control ASP was 141.7°F, and the biochar ASP was 149.3°F. The result is a difference of 7.6°F, indicating that the biochar treatment had an average daily temperature that was approximately 5.4% higher than the control during the measured period.

Average daily temperature in each bin remained similar while the temperatures climbed initially, but the temperature in the biochar ASP continued rising when the control began to plateau (Figure 1). Additionally, temperatures in the control ASP began dropping at a faster rate than the treatment as thermophilic activity started slowing down. The maximum average daily temperature reached in the control was 156.6°F, while reaching 163.6°F in the biochar ASP.

According to the 30-minute average temperature data taken over the measurement period, the control spent 24.2 days over 131°F and the biochar ASP spent 25.5 days over 131°F. According to these recordings, the biochar ASP spent approximately 5.4% more time over 131°F compared to the control.

CO₂ Flux

CO₂ flux measurements were taken eight times during the experiment, starting on September 16 and ending on October 10. The average flux was calculated for each ASP based in units of µmol*m-2*s-1. Average flux of the control was calculated to be 246.7, and 327.1 in the biochar ASP, resulting in a 32.6% increase in the biochar ASP compared to the control. This difference indicates a higher average of microbial respiration in the biochar ASP compared to the control, suggesting a higher microbial activity in the biochar ASP.

As shown in Figure 2, CO₂ flux in both ASPs started out high, reaching their maximum with the first measurement. The maximum recorded CO₂ flux was 461.0 for the control and 453.9 for the biochar ASP. Flux quickly drops as temperature increases in each pile. A possible explanation for this behavior is that the pile may have experienced some settling and compaction, which lowered the effectiveness of aeration and reduced overall airflow.

The minimum CO₂ flux measured in each ASP was 150.5 for the control in roughly the middle of the measurement period, and 145.4 for the biochar ASP at the tail end of the measurement period.

Discussion and Future Research

According to the data analysis, the biochar appears to have a positive correlation with CO₂ flux, maximum temperature, duration over 131°F, and average temperature during thermophilic composting conditions. These results corroborate results we have observed from previous co-composting experiments — the initial offal ASP trial as well as food waste trials in 60-gallon compost tumblers, that used 5% biochar addition by volume. The authors plan to replicate this experiment with 5% biochar added to slaughterhouse waste as the composition of both the offal and carbon materials are variable and subject to change.

Additional studies may look at increasing the concentration of biochar added to the feedstock recipe and the nutrient retention in the finished compost over time. Although higher volumes of biochar have been shown to exhibit diminishing returns during composting, slaughterhouse waste can be particularly dense with a high moisture content and may benefit from higher concentrations of biochar. Additionally, the effects of mixing the pile could be studied to potentially increase the duration of thermophilic conditions and allow the materials on the periphery to be mixed into the core. Finally, it could be beneficial to include more temperature sensors than what was present in this experiment to more accurately determine the average temperature of the whole pile, as opposed to only the temperature in the core.

Seth Wells, a Graduate Research Assistant, and Jeremy Ferrell, Assistant Department Chair & Associate Professor, are with the Appalachian State University Department of Sustainable Technology and the Built Environment.