Powering Planes through Arrested Anaerobic Digestion

Pumpkin rotting on the ground, orange color, background or backdrop
At face value, organic waste is an unsightly problem. Within it, however, are stores of energy and nutrients that researchers are learning how to effectively access. Photo: Adobe Stock under education license.

Waste is not glamorous. Just look at the moldy pumpkin leftovers from Halloween and Thanksgiving (yes, there are still quite a few around my neighborhood!) and you know why so many of us prefer to not spend our time thinking about wastes. From an energy standpoint, however, waste contains a largely untapped reserve of resources that can be recycled into the products we utilize daily as consumers. When we recover these materials, we have fewer materials to deal with as waste – and also reduce our consumption of raw materials. So why is waste recovery not a typical component of our infrastructure?

Picture that moldy pumpkin again (you’re welcome). Whether a whole pumpkin or just scraps, it is full of chemical elements including nutrients like nitrogen, potassium, and phosphorous taken from the soil as a loss from the natural environment to feed (or entertain) you. These chemical components still exist in the waste, but they must be broken back down into a usable form and incorporated into the soil if they are to be useful again for food production. The problems in converting the pumpkin back into usable components? If I think about that pumpkin on a personal level, I’m thinking

  • It’s gross
    • If new materials are available, why would I want to deal with the mess that requires more tools and sanitation?
  • It’s time consuming
  • New materials – in this case fertilizer – can sometimes be equally inexpensive, or sometimes even cheaper, than a renewable alternative – in this case compost.

Similar considerations are relevant at an industrial scale. Historically, there has been readily available infrastructure to support the consumption of new materials and limited incentive for change. This has meant that waste recovery technology has been slower to develop. Increasing pressure from climate change, however, has set the scene for increased understanding and implementation of waste recovery and the promotion of a circular economy is growing significantly. One of the biggest strides forward is the use of anaerobic digestion to create fuel from organic resources (an ever-renewing waste stream) rather than fossil fuels. Anaerobic digestion is a platform that can be used to generate a range of fuels, including electricity, renewable natural gas, hydrogen, or even potentially other fuels.

Volatile Fatty Acids Play a Role

Making strides towards that big picture goal, however, requires a significant amount of work on nailing all the details of the processing. With anaerobic digestion, those details are often smaller than microscopic. Volatile fatty acids (VFAs) -like acetic acid, propionic acid, butyric acid, valeric acid, and caproic acid- are produced as intermediates during anaerobic digestion. VFAs are energy-rich compounds and as such, are extremely valuable chemical building blocks to a number of industrial products, from plastics to pharmaceuticals to jet fuels and have an estimated demand of 18.5 million tons per year worldwide (Bhatia & Yang 2017). VFAs produced from anaerobic digestion have a lower carbon footprint than traditional petrochemical VFA production.

The typical process for anaerobic digestion breaks down the stable carbon bonds in organic wastes, first into volatile fatty acids, then further into biogas and digestate that can be applied as a soil amendment. If you can stop the digestion process after VFA production, you can recover VFAs as a primary product.  Inputs that contain a lot of carbon, such as those with high levels of lignin and cellulose, are prime candidates for producing VFAs. There are two primary challenges in VFA production from anaerobic digestion:

  1. Anaerobic digestion processes VFAs as an intermediary step, forming biogas as an end product rather than the more valuable fatty acids.
  2. With these higher carbon content feedstocks, a pretreatment step is usually required to break the lignin and cellulosic bonds, which creates an additional barrier to the feasibility of anaerobic digestion as a tool in waste recovery.

The Difference in Arrested Anaerobic Digestion

Through work supported by WSU and WSDA Applied Bioenergy Research Program, WSU researchers Anthony Giduthuri and Dr. Birgitte Ahring are investigating methods of arresting anaerobic digestion at the VFA production stage, rather than allowing the process to continue through biogas production. Their review encompasses the relative success of chemical pathways to arrest anaerobic digestion, as well as the potential for those VFA products to be transformed into high-value material like jet fuel.

There are two promising pathways to arrested anaerobic digestion: inhibiting methanogenesis or increasing acidogenesis (more on those processes in this kangaroo blog). In both of these pathways, the microbial communities involved in anaerobic digestion are targeted in order to alter the pathway and produce intended end products. Pretreatment of the waste material is of importance no matter if you produce methane or stop the process at VFA to increase the carbon conversion efficiency of the process. However, the VFA allow for production of more diverse types of energy and chemical products. Pretreatment becomes more economically feasible with higher value volatile fatty acids compared to biogas and digestate, which creates a better economic basis for the waste recovery to occur.

Jet Fuel?

The increasing feasibility of capturing VFAs generated from arrested anaerobic digestion is driving greater interest for the conversion of VFAs into renewable aviation fuels to curb a portion of the transportation sector’s emissions. This fuel source could not only divert potential emissions from organic wastes (like those from food waste) and utilize a resource that would otherwise be a societal “problem”, but also reduce the emissions generated by traditional petrochemical production pathways.

So how feasible is it to create jet fuel from arrested anaerobic digestion and put all those decomposing pumpkins to work? The Giduthuri and Ahring study shows that arrested anaerobic digestion holds a lot of promise for sustainable aviation fuel generation. The resulting VFAs can be chemically converted to ketones and ultimately create the base for fuel production. The big question that remains is how to efficiently convert a mixed composition of VFAs like that from arrested anaerobic digestion into ketones without separating each individual acid (e.g. isolating acetic acid from the mix and converting acetic acid directly). Either efficient separation methods or broad chemical conversion would help move arrested anaerobic digestion into the spotlight for VFA and sustainable aviation fuel generation. This research brings us closer to the ideal of a circular economy- and makes good use of all the products we hope to never see again!

References:

Bhatia, S. K., & Yang, Y.-H. (2017). Microbial production of volatile fatty acids: current status and future perspectives. Reviews in Environmental Science and Biotechnology, 16(2), 327–345.

Giduthuri, A.T.; Ahring, B.K. Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids. Fermentation 2023, 9, 13.

This is part of a series of posts highlighting work by Washington State University researchers through the Applied Bioenergy Research Program of the Agricultural Research Center at Washington State University, College of Agricultural, Human, and Natural Resource Sciences.