Anaerobic digestion (AD) is a natural process that sees microorganisms break down organic materials. Its potential is tremendous, especially in the face of a growing and increasingly urgent need to decarbonise our societies. AD can divert organic waste from landfills, create biogas to power engines, make our soils healthier and reduce fugitive methane emissions which currently account for 10 % of total greenhouse gas emissions.
So, what are we waiting for, you might ask? Well, there are two major hurdles to AD that researchers have yet to fully overcome. “The first problem is that digestion is sensitive to many factors, making it unstable and inefficient if those factors are not kept under check,” says Lin Richen, Marie Skłodowska-Curie research fellow at University College Cork, Ireland. “Then, the second problem is that the digestate produced after AD still holds a significant amount of energy and nutrient load. Absorbing this nutrient load requires a large land area and, if not managed properly, it could lead to eutrophication of waterways.”
Richen has been working on a solution with Jerry Murphy, director of the Science Foundation Ireland-funded MaREI centre, thanks to EU funding under the DIET project. The objective is to make biogas production through AD much more efficient than it is. To do so, they have been investigating a process called Direct Interspecies Electron Transfer (DIET), which enables the reduction of the digester’s size while producing the same amount of biogas.
“Through both of these outcomes, we can reduce the cost of sustainable renewable gas while reaching the International Energy Agency (IEA)’s ambition to increase biogas sector output 20-fold in order to create a decarbonised world,” Richen explains.
Conductive materials to the rescue
What DIET essentially does is add an electrically conductive material such as graphene to a digester. The material acts as an electron highway between bacteria (which produce volatile fatty acids) and archaea (which produce biogas), thereby reducing hydrogen partial pressure and improving the overall biogas production process. As Richen points out: “Reactions between bacteria and archaea during the breakdown of wet organic material into biomethane can be inefficient due to the accumulation of hydrogen. DIET helps prevent this from happening.”
From a mere hypothesis at project launch, the DIET process was turned into a verified solution over the past 2 years. Solutions with and without the conductive material were compared using a thermodynamic comparison between systems as well as laboratory experiments examining biomethane production for a variety of feedstocks in both scenarios.
“We can now suggest a graphene-based direct electron transfer model using a variety of substrates (including ethanol and glycine) under different digestion temperatures. This lays out a theoretical foundation to understand influential factors for Direct Interspecies Electron Transfer. The theory was supported by laboratory experimentation, which achieved higher methane production rates and generated an overall increase in methane yield,” Richen explains.
In the case of glycine being used as a substrate, the addition of 1 g/L of graphene increased the peak biomethane production rate by 28 %. The project successfully opened a pathway to more effective AD, and further efforts are now ongoing to apply the same principle to advanced feedstocks such as seaweed using pyrochar. “We already demonstrated that pyrochar is almost as effective as graphene in its role in DIET, while typically costing 200 times less,” Richen concludes.