For centuries, dead bodies have been wrapped in shrouds, placed inside coffins, buried below ground, or cremated.

Soon, residents of Washington state will have another option: To turn a loved ones' remains into composted soil for use in the garden.

The new rule, which allows for "the contained, accelerated conversion of human remains into soil" will go into effect in May 2020, the first of its kind in the US.

The news is exciting for a startup called Recompose, which aims to provide consumers with a body-composing service as an alternative to cremation (by far the most popular death ritual practiced in Washington today). In order to accelerate the natural decomposition process, Recompose plans to put corpses into steel vessels, where they'll stay for a month. During that time, microbes from the body naturally heat up to between 120 and 150 degrees Fahrenheit.

Katrina Spade, founder and CEO of Recompose, said she's aiming to open the first "natural organic reduction" shop of its kind in Seattle next year.

Turning bodies into compost is better for the Earth than cremation: Recompose estimates that one metric ton of CO2 is saved for every person who opts to compost a body instead of burning it. (That's roughly equivalent to taking a gas-powered car off the road for about three months.) Washington Gov. Jay Inslee, who's running for president with a progressive environmental platform, signed the bill into law on Tuesday.

"I would prefer to have the last gesture I make on this planet be something that was gentler and, perhaps, benefited the planet," Spade told Business Insider.

Human bodies decompose in a hurry

Spade said she hopes to "host" around 750 bodies annually at Recompose's next-generation funeral home , starting with 20 or 25 bodies at a time.

An artist's vision of the future recompose facility in Seattle, complete with composting vessels on the wall. MOLT Studios

Human composting is remarkably straightforward. After a person dies, our immune system stops working, and decay begins near-immediately.

"You become food for your own bacteria first, and then also bacteria from the soil," microbiologist Maria Dominguez-Bello recently told Business Insider.

Read More: Why you never really die: A microbiologist explains all the ways the body lives on, and why we don't decay until we're dead

When people get buried in the ground, bodies can take months or even years to fully decompose, depending on the environment. So Recompose's technology is designed to speed up this process.

Katrina Spade Craig Willse

"The body goes in [to a vessel] on wood chips, alfalfa and straw," Spade said. "The microbes are naturally occurring — they're on you and me as we speak. As long as we're providing the right mix of carbon and nitrogen via those materials and providing enough oxygen to the vessel, the microbes start breaking down the body right away."

The company's method has already been tested in trials at Washington State University, where six human bodies were recently composed.

"We do rotate our vessels periodically during the month-long process to ensure a good mixture and again, ensure aeration," Spade said. "But it's really quite amazing what those microbes can do."

The only people who are ineligible for this human composting are those with rare neurodegenerative prion diseases (Creutzfeldt-Jakob disease is one example), and those who die of highly contagious illnesses like Ebola. Recompose says any artificial limbs, hips, pacemakers and other implants are all tossed out before the composting or, ideally, recycled.

"We know a lot about the process, because we're not so different from livestock really, but it had never been proven with humans," Spade said.

A more Earth-friendly way to go

Recompose is still raising funds for its first composting site. Spade said turning one corpse into soil will cost around $5,500, and that price would include some funeral-like services on site "to actually have what, hopefully, is a meaningful ritual for people."

The finished product from Recompose looks like this. WSU Communications

Spade is part of a growing group of people advocating for greener ways to deal with dead bodies. Washington also just legalized a method called alkaline hydrolysis, a water-intensive process that turns corpses into bone fragments (it is not the first state to allow this, though.)

"I realized that cremation was becoming the default choice," Spade said. "As people decide they don't want to be buried, we go for cremation. It's just kind of what we have."

In Washington State, nearly 80% of corpses are cremated, a process that requires "about two SUV tanks worth of fuel," according to the Atlantic. Some people opt to turn those human ashes into blue diamonds.

Another more eco-friendly burial method involves cloaking a body in a biodegradable mushroom coat. Actor Luke Perry's daughter said her father is buried in one of these "infinity burial suits," made by a startup called Coeio.

The suits cost $1,500 and are made of a biodegradable mix of mushroom myclium and other microorganisms that help bodies decompose, flush out toxins, and "transfer nutrients to plant life," the company's website says. Artist Jae Rhim Lee wore an earlier version of the suit on stage for a TED talk eight years ago, but the design has changed in part "due to the high sensitivity of spore germination," as the company explains it.

Artist Jae Rhim Lee designed the mushroom burial suit. TED

Likewise, Spade said Recompose, which is set up as a for-profit B Corp., plans to continue refining its compost "recipe" in the coming years and tweaking the design of its vessels to help break down bodies as fast as possible.

"The finished product smells a lot like top soil you would buy at the nursery," she said. "Really beautiful, rich soil."