Natural gas is primarily methane, and there are several ways it’s produced: nature makes it over millions of years underground, bacteria make it in weeks by digesting organic waste, and industrial processes can synthesize it from coal, biomass, or even carbon dioxide and hydrogen. The method that’s most accessible to individuals is anaerobic digestion, where microorganisms break down organic material in an oxygen-free environment to produce biogas that’s roughly 50 to 74 percent methane.
How Nature Makes It Underground
The natural gas you buy from a utility company formed deep underground over millions of years. It starts with organic material, mostly ancient marine organisms, buried under layers of sediment. As that sediment piles up, the increasing heat and pressure slowly transform the soft material into rock, and the trapped organic matter begins to change chemically.
When buried rock temperatures exceed about 120°C (250°F), the organic remains start to “cook,” releasing both oil and natural gas. Above 150°C (300°F), any remaining oil breaks down further into natural gas alone. This entire process takes millions of years just to reach the right burial depth, then millions more to generate enough gas to fill the porous rock formations where it accumulates. The gas migrates through cracks and pores in surrounding rock until it gets trapped beneath an impermeable cap layer, forming the reservoirs that drilling operations tap into.
Making Biogas With Anaerobic Digestion
The most practical way to produce methane at a smaller scale is anaerobic digestion. You feed organic waste into a sealed, oxygen-free container called a digester, and naturally occurring bacteria break it down in stages, ultimately producing biogas. This is the same process that happens in landfills, swamps, and animal guts, just controlled and contained.
The basic setup requires a sealed tank (the digester), an inlet for feeding organic material, an outlet for the leftover digestate (which works well as fertilizer), and a gas collection system. The bacteria work best at consistent temperatures, typically between 35°C and 55°C depending on the type of digestion. Heated digesters should have their internal temperature measured continuously, and operators should monitor flow rates and loading rates daily to keep the biology stable.
What you feed the digester matters enormously for how much methane you get. Fish waste produces biogas with the highest methane concentration, around 74%. Leftover cooked food comes in at about 59% methane, and potato waste around 56%. In terms of total volume, food waste produces the highest cumulative methane yield at roughly 173 liters per kilogram of material, followed by leftover cooked food at about 126 liters per kilogram. Animal manure is another common feedstock, and combining it with food scraps can improve both yield and process stability.
Cost and Payback
A commercially manufactured home biogas digester using flexible plastic (HDPE) costs around $1,600 to install, while a more permanent brick-built fixed-dome digester runs between $3,200 and $5,000. That upfront cost is significant, especially for small farms, but economic analyses consistently show positive returns. Studies have found payback periods as short as 1.3 years when the leftover digestate is used as fertilizer alongside the gas, and within four years even for more conservative setups. The financial case improves when you’re offsetting the cost of both cooking fuel and commercial fertilizer.
Capturing Methane From Landfills
Landfills are essentially massive, uncontrolled anaerobic digesters. As buried waste decomposes, it produces landfill gas that’s roughly 50% methane and 50% carbon dioxide, with trace amounts of other compounds. Rather than letting this escape into the atmosphere (where methane is a potent greenhouse gas), collection systems can capture it.
The most common approach involves drilling vertical wells into the waste mass and connecting them with lateral piping to a central collection header. A blower or vacuum system pulls the gas through. Horizontal trench collectors work better in areas where waste is still actively being added, and many landfills use a combination of both. At the collection point, the raw gas passes through a knockout drum to remove moisture, then through filters and blowers before reaching either a flare (for destruction) or an energy recovery system.
Upgrading Raw Biogas to Pipeline Quality
Whether it comes from a digester or a landfill, raw biogas isn’t the same as the natural gas in your stove. It contains too much carbon dioxide, moisture, and contaminants like hydrogen sulfide, a toxic, corrosive gas that smells like rotten eggs. Turning raw biogas into renewable natural gas (RNG) that can be injected into a pipeline requires several treatment stages.
Primary treatment removes moisture using a knockout pot, filter, and blower. Secondary treatment handles additional moisture removal along with hydrogen sulfide and siloxane removal, plus compression. Advanced treatment strips out carbon dioxide, nitrogen, oxygen, and volatile organic compounds, compressing what remains into high-energy gas suitable for vehicle fuel or pipeline injection.
For hydrogen sulfide specifically, the most common removal methods include water scrubbing, chemical absorption, biological desulfurization, membrane separation, and adsorption using materials like activated carbon, metal oxides, or zeolites. Activated carbon is one of the most effective options. Research has found that carbon treated with potassium hydroxide performs significantly better than other preparations, maintaining removal efficiency for over twice as long.
Synthetic Natural Gas From Industrial Processes
There are two main industrial routes to making methane from scratch. The first is coal gasification, where coal reacts with steam and oxygen at high temperatures (around 700 to 750°C) and pressures of about 35 atmospheres in the presence of a catalyst. This produces a mix of gases that can be further processed into methane. Various catalyst types have been tested, including single-component, composite, and disposable formulations, each with tradeoffs in cost, performance, and lifespan.
The second route is the Sabatier reaction, which combines hydrogen with carbon dioxide over a catalyst to produce methane and water. This approach is especially interesting for renewable energy because the hydrogen can come from water electrolysis powered by wind or solar electricity, and the carbon dioxide can come from biogas, industrial exhaust, or even direct air capture. A prototype reactor achieved 91% conversion of carbon dioxide into methane with 100% selectivity, meaning no unwanted byproducts, and ran stably for over 100 hours without any external heating after the initial startup. With proper energy integration, the overall efficiency from electricity to methane reaches about 59%. When powered by renewable electricity, the resulting gas qualifies as renewable natural gas.
Safety Considerations for Methane Production
Methane is flammable in air at concentrations between about 5% and 15% by volume. Below 5% there isn’t enough fuel to ignite, and above 15% there isn’t enough oxygen. That narrow window might sound reassuring, but it means any leak in a confined space can quickly create an explosive atmosphere. Safety guidelines recommend that methane concentrations should never exceed 5% by volume in any occupied space.
For anyone operating a digester or handling biogas, this means all piping and connections need routine leak testing. Valves should be exercised regularly so they don’t seize, and pressure relief valves need periodic checks to confirm they aren’t stuck. Biogas also contains hydrogen sulfide, which is toxic at relatively low concentrations and, dangerously, deadens your sense of smell at higher levels so you stop noticing it. Proper ventilation, gas detection equipment, and sealed gas handling systems aren’t optional.
Carbon Footprint of Renewable Natural Gas
Renewable natural gas produced from waste sources has a substantially lower carbon footprint than fossil natural gas, partly because capturing methane that would otherwise escape into the atmosphere provides a double benefit. The carbon intensity of RNG depends not just on how it’s produced but how it’s transported. Pipeline transmission is the most efficient delivery method over medium to long distances. For shorter hauls under 250 miles, compressed gas trailers have the lowest carbon intensity at about 3.2 grams of CO2 equivalent per megajoule. Over longer distances above 650 miles, liquefied gas trailers become more efficient due to their larger payload capacity, despite higher baseline emissions from the liquefaction process.