Mosholu Golf Course in the Bronx is one of a dozen run by the city’s Department of Parks and Recreation. Its compact layout is typical of New York’s urban courses—nine holes, tree-lined fairways, the odd sand bunker—save for one highly unusual obstacle: the $2.1 billion drinking water treatment facility under construction on what used to be the driving range.

When this heavily secured compound is completed in 2012, it’s due to be topped by far more than just new turf. Grimshaw and landscape architect Ken Smith have designed one of the largest and most intensive green roofs to date, which is also a fully functioning driving range. And an irrigation system for the golf course. And an integrated security program for the facility below. Think Pebble Beach meets the Biosphere meets Rikers.

“The distinction here is it’s not just a green roof, but a performative green roof that needs to provide all these functions,” Smith said in an interview. “I think we’re pushing both the design of the green roof and the design of the golf course in new directions. We’re working to see how far we can push the diversity of the ecology and still adhere to the constraints of the golf course.”

This quietly radical project is the result of more than a decade of debate over whether or not water from the Croton Reservoir, the smallest of the city’s three, needed treatment after more than a century of going without. That was followed by battles with Bronx residents over which and even whether the borough’s parks would be torn up to make way for the new plant. The city finally broke ground on the facility in 2004, and the driving range has moved to a temporary site while the complex roofscape takes shape.

The engineering challenges are formidable. At nine acres, the $95 million driving range is the largest contiguous green roof in the country. So when it rains at the range, it pours, which creates a paradoxical hazard for the plant below. “It’s of paramount importance to the City of New York that this building stay dry, despite being full of water,” said David Burke, the project architect at Grimshaw. So to handle the millions of gallons that can accumulate on the green roof during a storm, the design team has devised a natural filtration system to collect, process, and store the runoff.

The range’s unique topography not only provides green-like targets for golfers, who tee off from the perimeter of the circular structure, but helps channel rainwater into the collection basins, where it meets groundwater pumped in from the plant’s four sump pumps. The water then travels through a series of ten cells that ring the range, each one modeled on a different native ecosystem to serve different filtration purposes. It takes up to eight days for water to travel through the cells, at which point it’s collected and used to irrigate the golf course.

“We’re not just dumping it in the sewer,” said Mark Laska, president of Great Ecology & Environments, one of two ecological designers on the project. “It’s a true display of sustainable green design in an urban environment.”

The design team wanted to convey such sustainable lessons to the public, especially the kids enrolled in the First Tee outreach program at Mosholu, and so the cells were left in plain view. Furthermore, because they are sunk ten feet below grade, they serve as a moat of sorts that helps protect the city’s water supply, which is seen as a potential target for terrorists.

To that end, Grimshaw has also designed the guardhouse and screening buildings that security constraints required, in addition to the new clubhouse and tee boxes on the range. (Grimshaw is not designing the plant, however, which is the work of a specialized engineering firm.)

It’s an unlikely commission, to be sure, but one the architects embraced. “It’s very fitting for Grimshaw,” as Burke put it. “We tend to gravitate toward these oddball projects.”