What the Digester Actually Produces

When a renewable natural gas project is announced, it comes with a number. The press release says the facility will produce so many million British thermal units of pipeline gas per year, enough to heat some round figure of homes, displacing some headline volume of fossil fuel. That number is a forecast. It is built before a single cow has contributed to the digester, before the first winter cold snap slows the microbes, and before the herd that justified the project gets sold or thinned.

The number that matters comes later, and it is almost never the one on the press release. It is the volume that actually moves through the gas meter after the digester has been running for a few years. Across the livestock digester fleet, that metered number tends to run below the forecast. Sometimes well below. The gap is not a scandal. It is engineering. But it has a consequence that rarely gets discussed: the climate claim was sized to the forecast, and the atmosphere only responds to the meter.

Nameplate Is a Promise, Not a Measurement

Nameplate capacity describes what a system can do under ideal conditions. For an anaerobic digester, ideal means a steady supply of fresh, high-solids manure, a herd that stays at full size, a digester held at its target temperature year round, and equipment that runs without interruption. Real dairies do not operate under ideal conditions. They operate under weather, cull cycles, milk prices, and breakdowns.

The EPA's AgSTAR program, which has tracked livestock anaerobic digesters for more than two decades, maintains a database of operating projects alongside a separate list of projects that were built and then shut down. The shut-down list is not short. Projects fail for reasons that have nothing to do with the chemistry of methane: a digester operator leaves, a maintenance bill arrives that the farm cannot cover, a feedstock partner walks away, or the gas offtake contract expires and is not renewed. Each one started life with a confident production forecast.

Why Real Output Runs Low

Biogas production is biological, and biology is variable. Methane yield from manure depends on what the animals eat, how fresh the manure is when it reaches the digester, and how warm the digester stays. In northern states, winter pulls digester temperatures down and slows the microbial activity that makes the gas. A system that hits its design output in July can fall short in January.

Herd size is the other moving part. A digester is sized to the number of animals on the farm the year it is financed. Dairies consolidate, downsize, or exit. When the herd shrinks, the manure shrinks with it, and the digester runs below the volume that justified its capital cost. The forecast assumed a static farm. The farm was never static.

Then there is uptime. Digesters need pumps, mixers, gas cleaning equipment, and flare or engine backup, and all of it requires maintenance. Time spent down for repairs is time the system produces nothing. Industry performance reviews of agricultural digesters have repeatedly found capacity factors below design, with real output landing at a fraction of nameplate once downtime, temperature, and feedstock variability are counted.

The Climate Claim Inherits the Gap

Here is where the production gap stops being an operational footnote and becomes a methane problem. The environmental value of an RNG project is the methane it keeps out of the atmosphere. That value is usually estimated up front, from the projected gas volume, and credited through fuel standards and voluntary markets on that basis.

But credited destruction and actual destruction are not the same thing. If a project was sized to capture the methane from 2,000 cows and the herd is now 1,400, the missing methane is not being upgraded into pipeline gas. It is doing what manure methane does when nothing captures it. It is venting from the lagoon.

Layer on the slip that every RNG pathway carries. Methane escapes at the digester, at the gas cleaning skid, at the compressor, and along the pipeline. The reduction that gets counted is the design figure minus an assumed slip rate. The reduction that actually occurs is the metered throughput minus the real slip, and the real slip is measured far less often than it is assumed. Two gaps, the production shortfall and the unmeasured leakage, both push the same direction. The atmosphere gets less than the ledger says.

A Flare Destroys What Is Actually There

An enclosed flare does not have a nameplate problem in the same way, because it is not trying to manufacture a marketable product to a forecast. It is sized to destroy the gas the farm produces, and it destroys whatever shows up. When biogas flow rises, the flare burns more. When flow falls, it burns less. There is no offtake contract that has to stay signed, no pipeline that has to accept the gas, and no upgrading equipment whose downtime idles the whole system.

This matters because methane destruction should not be conditional. A flare keeps working when the herd shrinks, when the credit market softens, and when the nearest pipeline says no. Its job is narrow: convert methane to carbon dioxide and water on site, continuously, at a capital cost in the few hundred thousand dollars rather than the tens of millions a pipeline-grade RNG project requires. The narrowness is the point. There are fewer ways for it to fall short of its number, because its number is just the gas in front of it.

Measure the Meter, Not the Forecast

None of this means RNG never works. Where a large herd sits close to a pipeline with a stable offtake agreement, a well-run digester can deliver real fuel and real reductions. The problem is the accounting, not the technology.

Public methane policy should credit the methane that was actually destroyed, verified at the meter and on a schedule, not the methane a financing model projected on the day the ribbon was cut. That single change would do two things. It would reward projects that perform and stop rewarding projects that underperform. And it would let the simplest, most reliable form of destruction compete honestly, because a flare that destroys metered methane every day of the year has nothing to hide behind a forecast.

The question for a methane program is not how much gas a project was supposed to make. It is how much methane is no longer reaching the air, this year, with a meter to prove it.