Since opening in 2013, Seattle's Bullitt Center has been claimed to be the world's greenest commercial building. Now, it's been awarded Living Building certification, a mark that's regarded as the highest standard of building sustainability.

The Living Building Challenge considers projects based on the seven categories (or "petals") of place, water, energy, health and happiness, materials, equity and beauty. The categories are split into 20 imperatives, all of which must all be achieved in order to receive certification.

According to the Bullitt Foundation, which owns the Bullitt Center, the building succeeded in certification of all the imperatives. It was assessed over a 12-month period, as all submitted buildings must be, starting from January 1, 2014.

The Bullitt Center was built on a previously-developed site, specifically chosen to be accessible by foot, bike and public transportation. Safe storage for up to 29 bikes takes the place of a car park, with on-site showers and a repair station to further encourage cycling instead of car use. Space around the Bullitt Center has been reserved for wildlife and agriculture.

The building has been shown to use zero net water. Once approved by regulators, it will capture rainwater for all uses, including drinking. The systems were modeled on 500 gal (2,273 L) of water usage per day, but in practice only 300 gal (1,364 L) or less is actually being used.

The Foundation is still working with regulators to show that the building can deliver safe drinking water (it is currently using City of Seattle water), but it was able to be assessed based on testing of the systems it has installed and the amount of water being used.

Grey water from sinks, showers and dishwashers is captured and filtered in a constructed wetland before being infiltrated back into the ground to recharge the aquifer. Black water from toilets, meanwhile, is used in the building's composting system. The building's compost and leachate goes through a secondary treatment before being used as fertilizer.

The Bullitt Center also had to show that it used no more energy that it produced. Over the course of the assessment period, the building used 152,877 kWh and produced 243,671 kWh, by way of the 575-panel solar array installed on its roof. Surplus electricity is fed into the grid in the summer months when production is high, with electricity taken from the grid in the winter months when production is low.

The building is heated using a geothermal system, with tubing containing a water and glycol mixture running down into 26 closed-loop wells. In winter, the mixture is heated to a constant 53 ºF (12 ºC) and pumped up into the floors of the building. In summer, the system can be reversed, with warmth removed from the building and piped back into the ground.

To help ensure the health of building users, all occupiable spaces within the building have operable windows that provide access to fresh air and daylight. Systems to promote good indoor air quality, such as ventilation, are also employed.

Biophilic (natural or living) design elements are incorporated to satisfy the human desire for exposure to natural systems and processes. These include ensuring that all desks are located around the perimeter for direct access to fresh air, natural daylight and views of nature, along with the use of exposed wood throughout the building.

A total of 362 "Red List" hazardous materials or chemicals were excluded for use in the building's construction. Materials used were shown to have been responsibly and locally sourced, where possible, and measures were taken to minimize waste production. An offset was also purchased against the amount of carbon produced during construction.

The aim of contributing to a just, equitable world is met in a few ways. The building is designed to promote culture and interaction among its occupants. It is built to be accessible to all and so as not to block access to natural elements, such as light or air.

Finally, elements intended solely for human delight are incorporated into the design, and educational materials about the building's performance and operation are made available to the public. The main one of these elements is positioning of the stairwell, which was designed to deliver the "best views in the building" and accounts for 85 percent of trips between floors as a result.

The Bullitt Foundation says its certification will be announced by the International Living Future Institute today, with an awards ceremony taking place this evening.

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