Digester gas is a fuel produced when microorganisms break down organic material in the absence of oxygen. It’s composed primarily of methane (50 to 75 percent) and carbon dioxide, making it a renewable alternative to natural gas. You’ll find it generated at wastewater treatment plants, farms, and food processing facilities, where it can be burned on-site for heat and electricity or upgraded to pipeline-quality fuel.
What’s in Digester Gas
Raw digester gas is mostly methane, the same combustible molecule found in natural gas. The EPA puts the methane content between 50 and 75 percent, with carbon dioxide making up most of the remainder. Smaller amounts of hydrogen sulfide, water vapor, and trace gases round out the mix.
That composition matters because it determines how much energy the gas carries. Pipeline-quality natural gas has a heating value of roughly 1,030 BTU per cubic foot. Raw digester gas, with its lower methane concentration and diluting carbon dioxide, lands closer to 500 to 600 BTU per cubic foot. It’s usable fuel, but it packs about half the punch of conventional natural gas per unit volume.
How Microorganisms Make the Gas
Digester gas is the end product of anaerobic digestion, a four-stage biological process that happens inside sealed tanks called digesters. Each stage involves a different community of microorganisms passing material down the chain.
- Hydrolysis: Bacteria release enzymes that break down complex organic matter (carbohydrates, fats, and proteins) into simpler molecules like sugars, fatty acids, and amino acids.
- Acidogenesis: A second group of microbes absorbs those simpler molecules and converts them into short-chain fatty acids and other intermediate compounds.
- Acetogenesis: Those intermediates get further converted into acetate and hydrogen gas.
- Methanogenesis: In the final stage, a specialized group called methanogens consumes the acetate and hydrogen to produce methane, the target fuel.
The whole process depends on these microbial communities working in sequence. If conditions shift too far (temperature swings, a sudden change in what’s being fed into the digester, or a pH drop from acid buildup), any stage can bottleneck and reduce gas output.
What Goes Into a Digester
Almost any organic material can serve as feedstock, but gas yields vary dramatically depending on what you’re digesting. In small-scale trials, leftover cooked food produced up to 261 liters of biogas per kilogram of solid material. Fish waste came in close behind at around 249 liters per kilogram, while potato waste yielded about 137 liters per kilogram.
The pattern is straightforward: materials rich in fats and easily digestible carbohydrates produce more gas. Food waste can generate roughly three times the methane of sewage sludge per unit of material. That’s why many wastewater treatment plants now co-digest food waste alongside sewage, boosting gas production without building new infrastructure.
Common feedstocks in practice include municipal sewage sludge, dairy and livestock manure, food processing waste, crop residues, and fats, oils, and grease from restaurants. Large dairy farms often run dedicated manure digesters, while municipal plants typically process a blend of sewage sludge and trucked-in organic waste.
How Digester Gas Gets Used
The most common use is generating electricity and heat on-site through combined heat and power (CHP) systems. These units burn the gas to spin a generator while capturing the waste heat for building heating or industrial processes. Total system efficiencies typically run between 65 and 80 percent, meaning most of the energy in the gas gets put to work rather than lost up an exhaust stack.
For wastewater treatment plants, this is especially valuable. These facilities are energy-intensive operations, and burning their own digester gas can offset a significant portion of their electricity bill. According to Argonne National Laboratory, plants that recover energy from biogas generally use it to generate electricity for on-site needs.
The other major pathway is upgrading digester gas to renewable natural gas (RNG). This involves stripping out carbon dioxide, hydrogen sulfide, and moisture until the methane concentration reaches pipeline standards (typically above 95 percent). The cleaned gas can then be injected directly into the natural gas grid or compressed for use as vehicle fuel.
Contaminants That Cause Problems
Raw digester gas isn’t clean enough to use in most equipment without some treatment. The most damaging contaminants are hydrogen sulfide and siloxanes.
Hydrogen sulfide is corrosive and, when burned, produces sulfur dioxide, which damages engine components and catalytic systems. Siloxanes are silicon-containing compounds that enter digesters through personal care products in wastewater (shampoos, lotions, deodorants). When siloxanes combust, they form hard deposits of silica, essentially tiny glass particles. In engines, these deposits force frequent oil changes and rebuilds. In boilers, they coat heat transfer surfaces and reduce efficiency. In fuel cells, they deactivate catalysts.
Gas cleanup systems range from simple (iron sponge filters for hydrogen sulfide) to sophisticated (activated carbon beds or chilled condensation for siloxanes). The level of cleanup depends on the end use. A basic boiler can tolerate more impurities than a fuel cell or a pipeline injection point.
Why It Matters for Emissions
Digester gas sits at an interesting point in the climate equation. Methane is a potent greenhouse gas, with a global warming potential 81 to 83 times greater than carbon dioxide over a 20-year window (27 to 30 times over 100 years). When organic waste decomposes in open lagoons or landfills, that methane escapes directly into the atmosphere.
Capturing it in a sealed digester and burning it converts the methane into carbon dioxide and water. Since carbon dioxide is far less potent as a greenhouse gas, burning digester gas for energy is a net climate benefit compared to letting the same organic material decompose in the open. The energy generated also displaces fossil fuel that would otherwise be burned, adding a second layer of emissions reduction. This is why digester gas projects often qualify for renewable energy credits and carbon offset programs.