Rewired Anaerobic Digestion: Developing More Sustainable Chemical Factories

By Jorge L. Rico, Ph.D. Candidate in the Department of Civil and Environmental Engineering and Trainee in the CSU InTERFEWS Program
According to the World Bank, humans produce about 2.01 billion tons of municipal solid waste each year. (Photo: | ©likstudio)

The growing demand for food, energy and water seems unstoppable on a crowded planet. Each year, considerable amounts of waste are added to landfills and open dumps globally.

According to the World Bank, humans produce around 2.01 billion tons of municipal solid waste every year (World-Bank, 2019). Most of the waste generated is “organic”; in other words, it is biodegradable and comes from either plants or animals (primarily associated to our food systems). At least 33% of this waste is not safely managed and ends up in open dumps all over the world. One might think about this problem from different perspectives.

On one side, our food production systems are inefficient and poorly designed to satisfy our societal needs. In developing countries, like United States, we are producing more food than what we need. On the other hand, food supply chains lack of optimal food preservation practices that result in large amounts of wasted food, which is a more common issue in developing countries. However, in both developing and developed countries, current practices for waste disposal lack economic incentives to encourage organic waste utilization technologies. These practices could avoid accumulation of food residues in landfills and associated issues like habitat disruption, methane emissions, and production hazardous leachate.

We can obtain energy in the form of methane biogas through anaerobic digestion, a process where microbes transform food and animal residues into methane. We have used this technology for centuries, however, nowadays turning these residues into methane biogas is not economically attractive due to the recent drop in natural gas prices (Han, He, Shao, & Lü, 2019).

The need for more profitable products from food and animal residues has inspired humans to develop new waste valorization technologies. One of these technologies relies on “rewiring” anaerobic digestion to produce more valuable chemicals, such as fatty acids, instead of methane.

Fatty acids are intermediate industrial chemicals with growing markets in several industries, including pharmaceuticals, food preservatives, polymers, and fuels. The petrochemical industry dominates current fatty acid markets; therefore, developing an alternative production system is an opportunity to mitigate environmental and social issues associated with current petrochemical systems. On one side, food and animal residues are renewable carbon materials. They are not as limited as petroleum, and plants can recover them “more sustainably” through photosynthesis regenerating plant carbon materials that animals or humans can use as food. On the other hand, several petrochemical plants and oil & gas extraction wells are close to vulnerable communities. These communities have been either unjustly ousted from their land or impacted by emissions of toxic pollutants. Transitioning from these petrochemical plants to rewired anaerobic digestion facilities that add value to their waste and do not pollute their environment is also an opportunity to empower and strengthen their economies.

Rewired anaerobic digestion is a complex biochemical process. Achieving high yields of specific fatty acids is challenging and unpredictable. We know that thousands of microbial species can interact with organic waste molecules through multiple pathways. However, we do not fully understand these relationships and the role of several microbes is unknown. This lack of understanding has resulted in a technical limitation for process scale-up.

The De Long lab researchers are studying anaerobic digestion using microbes from wastewater sludge and the stomachs of bison and cattle, known as the rumen. (Photo: | ©lightpoet)

In the De Long lab at Colorado State University, we have been inspired to address this challenge. We aim to develop a better understanding of the process of rewired anaerobic digestion. We have used microbes from anaerobic wastewater sludge and the stomachs from bison and cattle (known as the rumen). Our approach involves designing and assembling bioreactors to test the fatty acid production potential of these communities and then using DNA sequencing to figure out which microorganisms contribute to the production of desired fatty acids.

The microbial communities we use have piqued our interest because of their potential for fermenting organic residues. Microbes living in the rumen of cattle and bison play an essential role in the breakdown of difficult to digest feeds, like the ones we observed in food and animal residues. They can transform these feeds into short chain fatty acids for energy metabolism in their host animal. Although they have been characterized in animal nutrition investigations, a limited number of studies have used them to produce fatty acids from organic residues.

In a recent study published in Bioresource Technology, we used bison rumen in bioreactors fermenting cellulose for the first time and evaluated their potential for rewired anaerobic digestion (Rico, Reardon, & Susan, 2021). Interestingly, microbes from these communities were different from those present in cattle and sludge and produced the highest rates of butyric acid, the most economically valuable short-chain fatty acid. We found that microbes belonging to the genus Alistipes were potential contributors to production of this fatty acid. This type of microbes has been associated with butyric acid and carbohydrate decomposition in the human and termite gut but not in anaerobic digesters (Poulsen et al., 2014; Qing et al., 2019). These results are fascinating since isolating and growing these microbes in bioreactors could promote strategies to achieve more profitable butyric acid yields from organic residues.

Similarly, cattle rumen produced more propionic acid, which was linked to a potential symbiosis between species of Fibrobacter succinogenes and Prevotella ruminicola. These microbes are one of the most active bacteria in the rumen. Certain P. ruminicola species cannot use cellulose, but they can produce propionic acid (Strobel, 1992). On the other hand, F. succinogenes can breakdown cellulose to release sugars promoting the growth of other microbes (Burnet et al., 2015; Suen et al., 2011). These capabilities suggest that increased production of propionic acid in cattle rumen bioreactors could have been driven by the activity of F. succinogenes promoting propionic acid production by P. ruminicola. Propionic acid is a three-carbon short chain fatty acid with an increasing market in the food preservatives industry. Engineering microbial communities with these bacteria could enable the development of technologies that utilize food or animal residues instead of petroleum-based feeds to produce propionic acid.

Our study provided new insights into the ability of microbes to convert cellulose to different fatty acids. We found that we can get more or less of the fatty acids we want to produce depending on what set of microbes we use. However, rewired anaerobic digestion is still a complex and not-fully understood biochemical system. Our ongoing work uses advanced molecular biology tools to characterize genomes and fatty acid production pathways. This knowledge will be crucial in developing more sustainable chemical factories to integrate into current waste management systems. Microbial-based processes, like rewired anaerobic digestion, can utilize less energy than traditional petrochemical processes. They operate at lower temperatures and pressures and do not demand toxic solvents (like the ones used in petrochemical manufacturing). Using food and animal residues instead of petroleum feedstocks is a win-win opportunity to reduce the tremendous amount of landfilled waste and its associated carbon emissions. Furthermore, it is also an opportunity to rethink the establishment of alternative chemical production systems that are more just and sustainable to vulnerable communities currently affected by petrochemical systems worldwide.

About the Author:

Jorge is a PhD candidate in the CSU Department of Civil and Environmental Engineering, which he began in fall 2019. Jorge also became part of the first cohort of the InTERFEWS program in Fall 2019. He has a B.S. in Chemical Engineering from the Industrial University of Santander (Colombia), and an M.S. in Civil and Environmental Engineering from Colorado State University. His areas of focus are environmental biotechnology, organic waste valorization, and food-energy-water systems. His research interests include anaerobic digestion, composting, microbiome engineering, systems analysis, social movements, and environmental justice.


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Han, W., He, P., Shao, L., & Lü, F. (2019). Road to full bioconversion of biowaste to biochemicals centering on chain elongation: A mini review. Journal of Environmental Sciences, 86, 50–64.

Poulsen, M., Hu, H., Li, C., Chen, Z., Xu, L., Otani, S., . . . Schindler, P. M. (2014). Complementary symbiont contributions to plant decomposition in a fungus-farming termite. Proceedings of the National Academy of Sciences, 111(40), 14500–14505.

Qing, Y., Xie, H., Su, C., Wang, Y., Yu, Q., Pang, Q., & Cui, F. (2019). Gut microbiome, short-chain fatty acids, and mucosa injury in young adults with human immunodeficiency virus infection. Digestive diseases and sciences, 64(7), 1830–1843.

Rico, J. L., Reardon, K. F., & Susan, K. (2021). Inoculum microbiome composition impacts fatty acid product profile from cellulosic feedstock. Bioresource technology, 323, 124532.

Strobel, H. J. (1992). Vitamin B12-dependent propionate production by the ruminal bacterium Prevotella ruminicola 23Appl. Environ. Microbiol., 58(7), 2331–2333.

Suen, G., Weimer, P. J., Stevenson, D. M., Aylward, F. O., Boyum, J., Deneke, J., . . . Chertkov, O. (2011). The complete genome sequence of Fibrobacter succinogenes S85 reveals a cellulolytic and metabolic specialist. PLoS One, 6(4).

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