bbach@mercurynews.com

The most radical “green” features of the City of San Jose’s new Environmental Innovation Center are concealed behind two doors marked “Women” and “Men.”

There, plopped between the other conventional stalls, are two “composting toilets,” the first ever installed in a California office building.

“You assume you get an outhouse on a trail,” said Nora Cibrian, an environmental services specialist with the city. “But you just don’t think you can do it indoors too.”

These two toilets resemble every other commode, but at a total cost of approximately $20,000, they aren’t your great-grandmother’s latrines. They skip the sewage system entirely, funneling waste into a tank in the basement the size of a commercial trash bin.

Unlike an old-fashioned outhouse, where the waste goes straight into a hole in the ground, the composting toilets use the magic of chemistry to convert it to fertilizer. Just as in a backyard compost pile full of leaves and fruit rinds, the nutrient-rich waste mixes with carbon from wood shavings in the tank. Hungry bacteria then break down the material into tiny pieces, generating heat and carbon dioxide. A fan circulates air, reducing odor and ensuring the bacteria have plenty of oxygen. It takes some additional work to complete the yearlong composting process. You have to toss in some earthworms, sift occasionally, and add about a gallon of water per day.

When it’s all done, the city will have access to mature compost, according to toilet manufacturer Don Mills, a sales director for Clivus Multrum, a Massachusetts company founded in the 1970s by environmentalist Rockefeller scion, Abby Rockefeller.

“It’s not a highly complex device,” Mills said. “It’s a consciousness-raiser as well as an actual technology.”

The design team saw the toilets as a natural fit for the city’s new Environmental Innovation Center, a $31-million facility on Las Plumas Avenue that includes a variety of Earth-friendly innovations, said city spokeswoman Cheryl Wessling. The toilets use just ounces of water per flush — just enough to moisten a biodegradable foam solution that coats the tanks before and after each use, and much less than the one to five gallons guzzled by average toilets.

“This is definitely a learning process,” Cibrian said. “We’ll see what works and what doesn’t work and whether it can be applied elsewhere.”

The Bay Area is home to two other composting toilet installations, both outdoors. One is at the Presidio’s El Polin Spring in San Francisco and the other at Crystal Springs Golf Course in Burlingame.

The Presidio Trust installed the composting toilets near the spring in 2011 to preserve a unique watershed with a rich archeological history, said Allison Stone, the organization’s director of trails and philanthropy. She admitted they were a bit nervous at first, but are now quite happy with their off-the-grid choice.

“It’s been a great experience,” Stone said. “We have not had any major problems.”

But despite the water-saving benefits, the composting toilets have one big drawback apart from the steep upfront cost that has limited their use: governments still see the resulting compost as sewage, sharply limiting disposal options.

Though San Jose expects to approve permits soon for the unconventional toilets at the Environmental Innovation Center, city officials haven’t decided how they will dispose of the resulting compost. Wessling said it might be used on-site to fertilize the landscaping, or the city could have Clivus haul it away for $350 a month.

The Innovation Center’s new tenants are expected to pay the maintenance costs. They include: ReStore, Habitat for Humanity’s building materials store and donation center; a household hazardous waste drop-off facility; and Prospect Silicon Valley, which will run a program to help develop clean technology.

The Presidio Trust hired a contractor to regularly sift and care for the toilets and that contractor removed the waste, Stone said.

Vermont Law School installed composting toilets indoors in 1998 and gets numerous requests to see them, said Lori Campbell, the school’s facilities manager.

“People are fascinated,” she said.

But the compost disposal poses a big drawback that advocates face coast to coast.

“It is presently the case in nearly every state that you must regard this material as sewage,” Mills said.

Vermont state law still precludes the school from using the compost on its grounds, Campbell said, calling the restriction “unfortunate.”

“It’s beautiful compost,” she said. “It’s so rich, it’s unbelievable.”

Peter Scott, an emeritus professor of physics at UC Santa Cruz and California’s foremost expert on the commodes, believes the steep upfront cost chiefly dissuades many potential buyers. He first learned about the toilets while visiting the Vermont Law School in the early 2000s with his wife, Celia, a former Santa Cruz mayor, and he has been advocating their use ever since.

“It’s an interesting problem and an interesting puzzle as to why there aren’t more of them,” Scott said. “It seems like we have a lot of opportunities.”

Contact Becky Bach at 408-920-5862. Follow Becky Bach at Twitter.com/troutbach.