3D-Printed, Solid-State Bioreactor Converts Methane Waste To Useful Chemicals

When landfills and wastewater treatment plants create methane as a byproduct, much of it is simply burned and its value is lost. But the gas is an energy-rich resource, and it could be recycled as a fuel, chemical feedstock or bioproduct. Capturing and converting waste methane offers an opportunity to recover energy and create useful products from an otherwise wasted gas stream.

To harness that resource, researchers at Lawrence Livermore National Laboratory (LLNL) have developed a solid-state bioreactor that can convert methane into succinate, a valuable chemical used to make polymers, stabilize drugs, enhance food flavor and beyond. The small and efficient bioreactor performs over 10 times better than conventional liquid-state systems — and it consumes less power.

“Bioproduction using poorly soluble gases has long been limited by slow mass transfer and low efficiency because conventional liquid-phase bioreactors are not well suited for gas fermentation,” said LLNL scientist and corresponding author Fang Qian. “With growing interest in recovering energy and valuable products from waste gas streams, new bioreactor technologies designed specifically for gas fermentation are greatly needed.”

The new method, published in Scientific Reports, exploits the unique metabolisms of methanotrophs, bacteria that naturally consume and process methane.

“These microbes are naturally designed to do the conversion. They work without added heat, without added pressure, without all these things that a chemical process normally requires,” said LLNL scientist and author Samantha Ruelas. “That’s the beautiful thing about this biology. Because it’s natural, you don’t need those things, and without them, it can be more cost effective.”

Usually, bioprocesses take place in a large vat. The gas is dissolved in water, the mixture is vigorously stirred, and the bacteria work their magic. But for poorly soluble gases like methane, it is a slow process that requires significant energy and operating costs. As a result, it has been an economic challenge to perform methane bioconversion at scale.

“Rather than have this big tank of a ‘broth’ that you're putting gas into, we produced these thin structures that we call scaffolds,” said LLNL scientist and author Nathan Ellebracht. “They're 3D printed. They have a lattice structure, and we put the microorganisms into a hydrogel that fills these really thin walls.”

The hydrogel walls are mechanically robust and resistant to degradation. The scaffolds supporting them can accommodate many times more bacteria than a vat, and because the walls are like a thin, porous sponge with high surface area and excellent permeability, the methane reaches the microorganisms quickly.

The gas flows directly through and around the scaffold, eliminating the need to dissolve it in liquid and stir the tank and allowing efficient interactions with the bacteria.

“LLNL’s advanced additive manufacturing capabilities enabled us to integrate the selection of scaffold materials printing methods, geometric design and performance modeling into a single development workflow,” said LLNL engineer and author Hawi Gemeda. “Once the optimal design parameters were identified, 3D printing allowed rapid fabrication of prototypes across different sizes and scales, accelerating the transition from reactor design to testing and scale-up.”

This multidisciplinary project brought together LLNL’s broad expertise, and it integrated advances in biomaterials, additive manufacturing and bioreactor modeling.

“One of the challenges was integrating the engineering design and the limitations of our bacteria,” said LLNL scientist and author Natalie Hwee. “It was a combination of figuring out how we could engineer our scaffold to maximize our cell density and gas-cell interaction, but also how we keep the cells alive for more than a few days.”

The team started off with a two-milliliter reactor and increased it to one liter. While the device requires further scale-up for real-world deployment, its small size could eventually be an advantage.

“Economically, chemical plants only work at large scales. At smaller landfill or wastewater treatment facilities, there aren't really good, existing solutions that scale down,” said Ellebracht. “Our approach could be applied at a small plant and allow them to get value out.”

The novel scaffold structure could also be useful for other types of cells and biochemical conversions.

“Beyond gas fermentation, most industrial bioproduction relies on aerobic sugar fermentation, where oxygen mass transfer remains a major bottleneck. In addition, the immobilization of the biocatalyst would eliminate the downstream process of separating it from the reaction products,” said Qian. “We are now actively extending the solid-state bioreactor concept to other bioprocesses.”

This work was done with collaborators at the National Laboratory of the Rockies, the Quasar Energy Group and the University of North Texas. It was supported by a Laboratory Directed Research and Development project, LLNL’s Innovation and Partnerships Office and a DOE Technology Commercialization Fund.