Cow manure, cleaned up: Washington's small but real renewable gas

How a dairy digester makes a pipeline fuel

An anaerobic digester is a sealed tank or covered lagoon where bacteria break down manure without oxygen (that is what anaerobic means) and give off biogas that is roughly half methane. On its own that biogas is wet, dirty, and good mostly for burning on site. To get renewable natural gas, meaning gas cleaned up until it is chemically the same as the fossil gas already in the pipeline, the raw biogas runs through scrubbers that strip out carbon dioxide, water, and sulfur.

The cleaned gas then has to meet the pipeline operator's spec and get an interconnection, which is often the slowest and priciest part of any project. Washington's Department of Ecology permits the manure side under its dairy nutrient rules, and the state Utilities and Transportation Commission reviews the tariffs that let Puget Sound Energy and Cascade Natural Gas buy the gas and pass the cost to you. Neither step is quick, and both leave a public paper trail you can read.

The honest math on cows and volume

Here is the part the brochures skip. A single dairy cow produces only so much manure, and manure makes only so much gas, so a project needs scale: on the order of a few thousand cows to justify the cleanup and interconnection equipment. Washington has roughly a quarter million dairy cows at last public count, concentrated enough in the Yakima Valley and the lower Columbia Basin that a cluster of neighboring farms can sometimes feed one digester.

Even so, the total gas is modest. At last public report the state had on the order of a dozen or fewer manure digesters running, and the renewable gas they make is close to a rounding error against all the gas Washington burns in a cold week. That does not make it worthless. It makes it a niche fuel that does real work for specific farms and for the climate in specific places, not a replacement for the gas system you heat with.

Where the climate math holds, and where it leans

What makes the strongest climate case for dairy gas is not the gas itself. It is the methane you stop from leaking. Under Washington's Clean Fuel Standard, the rule run by Ecology that makes fuel suppliers cut the carbon in what they sell or buy credits, dairy gas can earn a very low or even negative carbon intensity score, meaning a measure of greenhouse gas per unit of energy, because the accounting credits all that avoided methane from open manure lagoons.

That credit is where honest people argue. Count the avoided methane and dairy gas looks almost miraculous; stop counting it, and it looks like ordinary gas with extra steps. The avoided methane is real, but it rests on a baseline assumption that the manure would otherwise have sat venting in an open lagoon, and it rewards the dairy for a problem the dairy created. Worth knowing when an ad calls the gas carbon negative: that label is doing a lot of accounting work, and the program rules behind it can change.

Worth watching this month

1. Watch the Utilities and Transportation Commission docket pages for any new Puget Sound Energy or Cascade Natural Gas renewable gas tariff or supply filing, since those set what you actually pay.

2. Ecology's Clean Fuel Standard posts periodic credit and carbon intensity data, routine but worth a look to see which direction dairy gas scores are trending.

3. The EPA's livestock digester database updates from time to time and is the cleanest public count of which Washington dairies actually have a running digester, not just a proposed one.

4. Any state budget or legislative line on manure-to-energy grants, which matters because most of these projects need public help to pencil out.

5. Local Ecology permit notices for new or expanded dairy digesters in Yakima and Benton counties, usually routine but often the first public sign a project is real.