Microbes Enrich Biogas And Synthesize Valuable Chemical

Biogas is a renewable energy source that can be used to heat homes or power vehicles. But in its raw state, the gas mixture contains both methane and carbon dioxide, so biogas plants must remove and release the carbon dioxide to purify the methane. Now, researchers report an approach to enrich the methane in biogas using bacteria that convert the carbon dioxide into a valuable chemical instead of emitting the greenhouse gas (Environ. Sci. Technol. 2014, DOI: 10.1021/es504000h).

Anaerobic microbes produce biogas by digesting manure, plant matter, and other organic materials that may otherwise end up as waste. This biogas contains between 50 and 75% methane, which, if purified, can be pumped directly into the natural gas grid for use in homes or compressed into a fuel. Biogas producers remove the carbon dioxide using specialized filters and expensive processes, says Ingólfur B. Gunnarsson, a graduate student in the laboratory of Irini Angelidaki at the Technical University of Denmark.

Angelidaki’s team wanted to develop a simple biological method that could transform this carbon dioxide into something useful instead of releasing it into the atmosphere. To do so, the researchers enlisted other anaerobic bacteria for help. Several species of these microbes convert carbon dioxide into succinic acid, a four-carbon molecule with multiple industrial applications, including as a precursor for polymers, a food additive, and a replacement for petroleum-based chemicals. Using these bacteria, the scientists could turn raw biogas into pure methane and succinic acid.

For a proof-of-principle experiment, the researchers selected Actinobacillus succinogenes, a carbon-fixing bacterium that tolerates high concentrations of sugar and succinic acid. They constructed a closed fermentation vessel that allowed them to pump a simulated biogas consisting of 60% methane and 40% carbon dioxide over a culture of A. succinogenes.

The team increased pressure in the vessel to 140 kPa to increase the solubility of the carbon dioxide in the culture. At this pressure, a 24-hour fermentation produced 14.39 g of succinic acid per liter of culture and a biogas composed of 95.4% methane, which is good enough for most applications.

The purity achieved is exciting, says Caixia (Ellen) Wan of the University of Missouri, Columbia. However, she says, real biogas may require additional purification steps because of impurities, such as hydrogen sulfide, that could potentially harm bacterial cultures. Also, given the amount of carbon dioxide present in the simulated biogas, the team’s yield of succinic acid is low, Wan says. Gunnarsson thinks they should be able to increase yields by optimizing the bacterial strain and fermentation conditions.