Exploring the Potential of Biochar for Improving Anaerobic Digestion

Introduction

You may have heard about biochar as a soil amendment for farming, and many of you are probably familiar with the concept of anaerobic digestion to produce biogas on farms. Is there a potential connection between these two opportunities?  This article gives an overview of a recent test by Penn State Extension to investigate that possibility. 

1. What is Biochar?

Biochar is a versatile material created from organic matter such as wood chips, crop residues, or other plant materials through a process called pyrolysis. During pyrolysis, these materials are heated in a low-oxygen environment, turning them into a stable form of carbon. The result resembles charcoal and has a porous, spongy texture.

This porous structure gives biochar its unique properties. It's especially valuable for carbon sequestration, which involves capturing and storing carbon to mitigate climate change. By converting organic waste into biochar, we not only recycle nutrients but also lock carbon into a stable form that can stay in the soil for centuries. This helps reduce carbon dioxide levels in the atmosphere, mitigating global warming. Additionally, biochar improves soil health by enhancing its structure, increasing water retention, and providing essential nutrients. This boosts crop yields and reduces the need for synthetic fertilizers.

Beyond its role in carbon sequestration, biochar has significant applications in environmental cleanup. Its porous nature makes it effective at adsorbing pollutants from water and soil. For instance, in urban areas, biochar is being tested for stormwater management, where it helps filter and clean runoff before it reaches water bodies. Learn more about its impact on riparian buffers, its broader properties and potential, and its use for water quality in the following Penn State Extension articles:

• Biochar in Riparian Buffers
• Biochar Properties and Potential
• Using Biochar for Water Quality

While the usefulness of biochar has been touted for many applications, its practical and economic feasibility is not yet fully established. Thus, there is a need to test and evaluate its usefulness in different settings and locations. 

2. Using Biochar to Enhance Anaerobic Digestion and Biogas Production

Biochar's benefits extend beyond its environmental and agricultural applications into more specialized areas, including its use in anaerobic digesters. To understand how biochar enhances this process, it's important to first know what anaerobic digestion is.
Anaerobic digestion is a biological process that breaks down organic material-like manure, food waste, and plant residues without the presence of oxygen. This process occurs in a sealed tank called a digester, where microorganisms decompose the organic matter, producing biogas as a byproduct. Biogas is primarily composed of methane and carbon dioxide, and it can be used as a renewable energy source. The remaining material after digestion, known as digestate, can be used as a nutrient-rich fertilizer. For more background information on anaerobic digestion see the Penn State Extension article on Basics of Anaerobic Digestion.

Biochar plays a significant role in improving this process. It is a porous material created from organic matter, like wood chips or crop residues, through a process called pyrolysis. This process transforms the material into a stable form of carbon, giving biochar its spongy texture and large surface area.

In anaerobic digesters, biochar offers several advantages:

1. Microbial Habitat: The porous structure of biochar provides a habitat for beneficial microbes that are essential for the digestion process. These microbes help break down organic material more efficiently. The increased surface area of biochar allows for more microbes to attach and flourish, which enhances the overall efficiency of the digestion process.
2. Enhanced Biochemical Reactions: Biochar supports biochemical reactions by improving electron transfer conditions within the digester. This boosts the breakdown of organic matter, leading to increased production of biogas. Higher-quality methane is produced as a result, which is a valuable and energy-rich component of the gas.
3. Hydrogen Sulfide Reduction: Biochar also helps reduce hydrogen sulfide (H2S) levels in the biogas. Hydrogen sulfide can be corrosive and undesirable, so reducing its concentration improves the quality of the methane and the overall efficiency of the biogas system. Biochar adsorbs sulfur compounds during its time in the digester, preventing them from exiting as hydrogen sulfide gas and therefore, leading to cleaner, higher-quality biogas.
4. Long-Term Impact: Once biochar exits the digester, as part of the digestate, it remains in a stable form and can be applied to fields, where it continues to offer benefits for soil health and carbon sequestration.

Much of the research on biochar's benefits for anaerobic digestion has been conducted in laboratory settings. Our recent field trials with biochar in farm anaerobic digesters are focused on translating these findings into practical applications. By evaluating biochar's performance in real-world farm environments, we aim to assess how it can enhance energy production, improve digester efficiency, and contribute to overall farm sustainability.

These trials are important for understanding biochar's effectiveness outside of controlled lab conditions. We are investigating its potential to boost and improve the quality of the biogas output. This work aims to provide actionable insights that will help integrate biochar into commercial farming practices and support more sustainable agricultural operations.

3. Farm Trials: Testing Biochar in Farm Anaerobic Digesters

To translate the promising laboratory results of biochar into real-world applications, we partnered with three local farms. Each farm agreed to add biochar to their anaerobic digesters and monitor the resulting biogas production, while sharing about their experiences during the trial. This biochar is made from hardwood chips, providing a rich source of carbon. Delivered in large bags, the biochar was added to the digesters daily.

Figure 1. Biochar used in the anaerobic digester test (Photo: K Lopez, Penn State).

The biochar used was sourced from Metzler Forest Products of Reedsville, PA, and consisted of mixed hardwood chips that were pyrolized to a carbon content of 90%. The biochar was delivered to the farms in "super sacks", and manually added to the digester infeed tanks on a daily basis.

Weekly measurements were taken of biogas Hydrogen Sulfide level and power production from the farm's generator. Samples of manure feedstock, digestate, and separated solids were also collected for further analysis. Biochar concentration in the digester was calculated based on the mass added, assuming fully mixed conditions in the digester tank.

The dosage of biochar (0.05-0.15 g/litre) was relatively small when compared to some laboratory research (often closer to 1 g/litre). The purpose of the lower dosage was to minimize the likelihood of operational problems, to confirm the safety of the biochar for the digester equipment and microbial community, and to assess the practicality of adding biochar, even at relatively low doses. The farmers were interviewed after the tests were completed, and their impressions were recorded in terms of the digester's operation during the test and the feasibility of utilizing higher doses of biochar.

We introduced a controlled amount of biochar to each digester to avoid any potential operational issues. The goal was to observe how varying quantities of biochar could impact the performance of the digesters, without overwhelming the systems. Regular monitoring was used to evaluate the effectiveness of biochar. We tracked several key metrics, including biogas production, energy output, and the condition of the digestate. These observations helped us understand the immediate and longer-term impacts of biochar on digester performance.

Farm Trial Results

Overall, it can be said that the biochar additions caused no detectable problems with the digesters, with smooth operation observed at all of the farms. This suggests that biochar is compatible with current digester systems, in terms of the physical operation of the equipment, piping, etc.  Specific findings from each of the farms are as follows: 

Farm #1: At this long-standing dairy farm, biochar was added daily to the infeed pit, where it was mixed with manure before being pumped into the digester.

Results: The daily addition of biochar corresponded to a slight increase in energy production. The digester operated smoothly with no major issues reported. The farmer suggested that automating the addition process could improve efficiency.

Farm #2: This farm uses manure and trap grease as feedstock for its digester. The biogas flare operates frequently, indicating excess biogas relative to the generator's capacity. For this trial, two 5-gallon buckets of biochar were added daily to the infeed pit over a three-month period.

Results: The introduction of biochar in larger quantities led to variable energy production and fluctuations in gas quality. The lower concentration of biochar compared to Farm #1 may have influenced these results, highlighting the need for optimizing biochar levels.

Farm #3: The third farm uses manure along with food waste as feedstock for its digester. Biochar was introduced at a gradually increasing rate, allowing us to observe the effects of rising biochar concentrations over time.

Results: The gradual increase in biochar provided detailed insights into how varying concentrations affect biogas production and quality. Notable changes were observed in the digestate's moisture and nutrient content, offering valuable information on the broader effects of biochar.

4. What It Means: Insights and Implications

Overall Trends

The trials revealed several insights into the performance of biochar across different farm settings. Across all three farms, biochar demonstrated the potential to enhance biogas production, although the effects varied depending on the farm's setup and the amount of biochar used. Farm #1 showed a slight increase in energy production, suggesting that daily additions of biochar can positively impact power generation. However, the results at Farm #2 were more variable, with fluctuations in gas quality indicating that the optimal concentration and application method of biochar are crucial for consistent performance. Farm #3 provided detailed data on how gradually increasing biochar affected both biogas production and digestate quality, highlighting the need for tailored approaches based on specific farm conditions.

Practical Impact

The practical implications of using biochar in anaerobic digesters are significant. On the positive side, biochar's ability to improve microbial health and support biochemical reactions can lead to increased biogas production and better-quality methane. This can enhance energy output and provide a more stable and efficient operation of the digesters. Additionally, biochar's role in reducing hydrogen sulfide levels improves the quality of the biogas, making it more suitable for energy generation and reducing potential maintenance issues related to corrosive gases.

However, there are challenges to consider. The variability in results suggests that biochar's effectiveness is influenced by factors such as the amount used, the type of digester, and the specific conditions of each farm. Ensuring consistent application and determining the optimal biochar concentration for different setups are key challenges that need to be addressed. Farmers may need to experiment with different approaches to find the most effective way to integrate biochar into their digesters.

What Farmers Think

Feedback from farmers has been instrumental in understanding the real-world implications of using biochar. Farm #1's experience highlighted the potential for biochar to be a valuable addition to digesters, with the suggestion to automate the process indicating a desire for more efficient implementation. On the other hand, the varying results at Farm #2 and Farm #3 underscore the importance of practical adjustments and ongoing evaluation. Farmers appreciated the potential benefits of biochar but also noted the need for further optimization and clear guidance on its use.

5. Next Steps for Biochar: Advancing Farm Efficiency and Sustainability

Biochar involves an initial investment for both its purchase and integration into anaerobic digesters. This cost includes not only buying the biochar itself but also adapting farm operations to incorporate it into existing systems. This might involve logistical adjustments and potential modifications to the digesters to accommodate the new material.

Despite these upfront costs, biochar potentially offers significant long-term advantages that can make it a worthwhile investment. Firstly, biochar enhances the efficiency of energy production in anaerobic digesters. By improving the microbial environment and boosting biogas production, biochar helps in generating more energy from farm waste. This can lead to increased energy savings and a more stable energy supply for the farm.

Additionally, biochar improves the quality of the methane produced in these digesters. Higher-quality methane is more energy-dense and can be used more effectively as a fuel source, which adds to the overall energy efficiency of the farm.

Beyond the digesters, biochar contributes to better soil health. Its application to fields helps in improving soil structure, increasing water retention, and providing essential nutrients. This enhancement of soil health can lead to better crop yields and a reduction in the need for synthetic fertilizers, which not only lowers input costs but also benefits the environment.

In summary, while the initial costs of biochar can be significant, its benefits in terms of increased energy efficiency, improved methane quality, and enhanced soil health often outweigh these expenses. The long-term gains include higher farm productivity, reduced reliance on synthetic fertilizers, and a positive impact on environmental sustainability. Thus, biochar may be a valuable investment for farms aiming to improve their sustainability and operational efficiency

Support and Contributions

This research was made possible through the support of the US Forest Service via their Wood Innovation Grant program and the USDA-funded C-Change Grass-To-Gas project. These programs provided the necessary funding and resources for investigating the practical applications of biochar in farm anaerobic digesters. The collaboration with our partner farms facilitated the implementation of this study, contributing valuable data and insights into the real-world impact of biochar on farm operations.