SEATTLE — When an office building here that bills itself as the world’s greenest officially opens later this month, it will present itself as a “living building zoo,” with docents leading tours and smartphone-wielding tourists able to scan bar codes to learn about the artfully exposed mechanical and electrical systems.

Tenants have already begun moving into the six-story Bullitt Center, in advance of its grand opening on Earth Day, April 22. With the final touches nearly complete on the 50,000-square-foot office building at 1501 East Madison Street, at the edge of the city’s Capitol Hill neighborhood, its occupants are about to embark upon an unparalleled — and very public — experiment in sustainability.

Once settled in, they will be guinea pigs in a $30 million living laboratory distinguished by its composting toilets, strict energy and water budgets and a conspicuous lack of on-site parking. To earn its environmental bragging rights, the Bullitt Center must complete a rigorous one-year certification process called the Living Building Challenge, which requires both water and energy self-sufficiency, among a list of 20 demands.

Provided that the building clears a few remaining regulatory hurdles, all its water will be supplied by rainwater collected in a 56,000-gallon cistern before being filtered and disinfected. A rooftop array of photovoltaic panels, extending beyond the building like the brim of a graduation mortarboard, will produce an estimated 230,000 kilowatt-hours a year, hopefully just enough to break even for a building that is 83 percent more efficient than the city’s typical commercial site.