Microsoft’s history of dominating the market for carbon removal continued when the tech giant announced late last week it was buying 4.9 million tons of removal credits from Vaulted Deep, a startup that buries organic waste underground. Here’s what prospective buyers and other carbon credit players need to know about the deal.

It’s not all about human waste

The Wall Street Journal described the deal thus: “Microsoft wants your poop to lower its emissions.” 

That’s not quite right, according to Vaulted co-founder and CEO Julia Reichelstein. The startup takes multiple types of organic waste, including manure and sludge from paper mills, and injects it hundreds or thousands of feet below the ground. This “bioslurry” contains carbon that was removed from the atmosphere by plants before being eaten by animals or used in paper processing, making the process carbon negative.

But, yes, excrement is involved. Vaulted’s bioslurry injection technology was originally developed as means of disposing of waste from a water treatment plant in Los Angeles, and human fecal matter will be an important input going forward.

Will the credits deliver real climate value?

One of the biggest problems in carbon markets is proving “additionality”— knowing that credit revenue is essential to making a project work. Some forest conservation schemes, for example, have been criticized for selling carbon credits to protect forests that were really not at risk. 

Vaulted’s process is clearly additional, said Reichelstein, because the vast majority of the organic waste it’s targeting in the U.S. is spread on land, incinerated or sent to landfill, releasing carbon dioxide and methane in the process. Without a commercial incentive or regulatory requirement to do otherwise, credit revenues are needed to fund the removal.

Buyers will want that and other claims — including guarantees that the carbon will not seep back into the atmosphere — to be verified by an independent third party. At present, no carbon credit rating agencies have assessed Vaulted’s projects. But the startup does have important proof points. It follows a methodology developed by Isometric, a credit registry with a reputation for thoroughness. Microsoft is also known for doing extensive due diligence on prospective sellers, as is Frontier, a coalition of removal buyers that purchased a total of slightly more than 150,000 credits from Vaulted in 2023 and 2024.

“Having them do months and months and months of diligence on us and deciding to purchase from us is good industry validation,” argued Reichelstein.

What you can expect to pay for a Vaulted credit

The cost of the Microsoft deal was not disclosed, but Frontier paid $58 million for its credits, putting the per-ton price just over $380. For comparison, other recent deals involvin