Adithya Sambamurthy/The Bay Citizen

According to the Clean City Data Report, Dodge Street, shown Aug. 31, 2011, had 123 cleanups of human waste in 2010 — the most in the Tenderloin

An innovative plan would replace some street parking spaces with environmentally friendly public restrooms — facilities that do not flush or connect to the sewer system. Instead, human waste would be collected and someday possibly composted to add back into the ecosystem as plant fertilizer.

The idea is in early development, but an initial restroom could be on the street by spring. Partly inspired by the city’s “parklets” program, which has transformed parking spaces into miniature parks, the proposed curbside restrooms have been nicknamed “pooplets.”

Beyond its green ambitions, the idea — officially called “the ecological toilet project” — could help solve a problem plaguing the city: a lack of public restrooms.

It has been a health concern for decades, tied to the rise of homelessness in the city, which has increased the need for restrooms while causing businesses and government offices to restrict access because of feculence left by transients.

This led to a spike in the use of sidewalks as toilets. In 1985, the columnist Herb Caen, after a malodorous stroll through the downtrodden Tenderloin neighborhood, is said to have quipped that the city could soon be called Latrine-by-the-Bay.

Today, however, it is no joke. There were nearly 10,000 documented “incidents of human waste” cleaned up last year in the Tenderloin alone, according to Dina Hilliard, executive director of the North of Market-Tenderloin Community Benefit District, a neighborhood improvement effort financed by local property owners that pays for sidewalk cleaning.

“This is out of control,” Hilliard said of the waste.

Ideas for solving the problem — like portable toilets or renting out a storefront with facilities — were determined too unsightly, problematic or expensive.

The eco-restrooms in parking spaces would offer a different approach.

The restrooms are the vision of Hyphae Design Laboratory of Oakland and its founder, Brent Bucknum, who helped create the celebrated living roof at the California Academy of Sciences in Golden Gate Park. The science behind them is already used in developing countries: solid and liquid wastes are separated, reducing smells and the risk of disease. No toxic chemicals are used, making it possible to eventually transform waste into plant nutrients (although initially, in San Francisco, it would be trucked to waste treatment facilities).

“We have to deal with this lack of public facilities, and the lack of public decency,” Bucknum said.

Bucknum said the public’s suggestions would go into the final design, but several features are part of the preliminary concept, including ultraviolet lights to destroy germs and to act as “lanterns” to attract patrons.

To thwart vandalism and misuse, like drug use or prostitution (problems that have besieged 25 green JCDecaux kiosk restrooms since they were installed in the city beginning in 1996), the washing facilities would be on the outside of the structure.

And the walls could be translucent, making abstract silhouettes of people using the restroom visible from outside, allowing the police to note any illegal activity.

Courtesy of Hyphae Design Laboratory

Differences between the proposed restrooms and existing public toilets. Click to view larger.

The cost would be $40,000 to $50,000 each. Hyphae, together with the community benefit district, is financing the prototype restroom. If successful, Bucknum said he hoped the city would pay for others, perhaps to provide relief to the crowds expected for the 2013 America’s Cup yacht races. The restrooms are intended to be free.

It’s not just the homeless who feel the dearth of public restrooms. Facilities at BART and Muni stations, with thousands of transit passengers daily, have been closed for years because of what officials say are security and maintenance concerns.

The 2010 California Plumbing Code indicates that transit stations, and businesses that serve the public, must provide customer restrooms, according to Lynne Simnick of the International Association of Plumbing and Mechanical Officials, a professional group that helps carry out construction standards.

“They have to have them — there is no choice,” she said.

But Simnick said the rules were difficult to enforce and might not cover older buildings. Regardless, she said, “a business is supposed to be serving their patrons.”

Not everyone sees it that way. Last week on Haight Street, a destination for tourists and transients alike, a female friend’s request to use the restroom was denied at 7 out of 10 randomly selected shops. One sympathetic clerk said she had been told to pretend that the restroom was broken whenever customers asked to use it.

It’s a situation that most seem to accept. City building and health inspectors said they received few complaints about customers being denied restrooms.

However, for the homeless, who have little choice but to use the streets, the need for a solution is urgent. Perhaps soon a curbside “lantern” illuminating a silhouette could show the way.

Courtesy of Hyphae Design Laboratory

This map shows the areas in the Tenderloin that had the highest numbers of cleanups of human waste last year. In total, the neighborhood had nearly 10,000 such incidents in 2010. Click to view larger.

This article also appears in the Bay Area edition of The New York Times.