MILWAUKEE - With a grinding roar, an excavator takes a huge bite from a heap of debris. Cardboard, branches, ribbons of metal strapping, slices of vinyl siding, chunks of wood and rocks tumble from the machine's jaws onto a conveyor belt.

Seventy tons of debris an hour ride up two stories and over a sorter whose 18-inch metal prongs let dirt and rocks fall to a conveyor belt below. A magnet sweeps over the belt and pulls away metals. The river of stuff turns a corner and is seized upon by City Wide Recycling's line workers.

Two guys, one on each side of the line, grab mashed cardboard and toss it into chutes that empty into bins a floor below. The next two guys pull wads of plastic that once shrink-wrapped materials. More guys, more stuff pulled and tossed: Sawed-off hunks of wood, non-ferrous metals - even a rusty wheelbarrow.

Not much makes it to the end of the 10-second ride down the belt. Some soggy insulation, plaster-crusted buckets, painted wood, treated wood and the occasional half-eaten sandwich are all that end up in the landfill-bound bin.

Three years ago, nearly all the leftovers from Milwaukee building sites ended up in landfills. There was nowhere else to put the stuff.

But John Hansen, who started and sold a waste disposal firm in the late 1990s, realized two years ago that the time was ripe for wholesale recycling of construction debris. Now, this plant on the north side of Milwaukee processes an ever-deepening avalanche of construction leftovers, and Hansen plans a new, multimillion-dollar facility on the city's south side.

Skimming recyclables is one of the easiest ways to green up a building project, says Jenna Kunde, executive director of WasteCap Wisconsin, a 10-year-old Milwaukee nonprofit consultant to state businesses.

A third of the stuff filling Wisconsin's municipal landfills is construction waste, she says. The typical 2,000-square-foot new house throws off a literal ton of waste - half of it, by weight, is wood and cardboard.

WasteCap has been gradually making inroads with homebuilders, Kunde says. To date, the bigger local builders have the volume to adopt wholesale recycling programs. Kunde says sending construction debris to a recycler can save money, especially considering the rising value of scrap materials.

Hansen, though, says he can't count on downstream revenue from scrap haulers to keep City Wide operating.

He's pitching contractors on the convenience of sending their containers of mixed debris to City Wide - they don't have to sort anything at the job site - and the fact that it costs the same - about $35 to $50 a ton - to tip the stuff into City Wide's cavernous plant as it does to dump it in a landfill.

"It's definitely market-driven," Hansen says. "The top people in the industry are doing it."

Recycling debris is one of the hallmarks of one of the best-known, most authoritative standards for green building, the Leadership in Energy and Environmental Design, or LEED, Green Building Rating System sponsored by the U.S. Green Building Council.

Commercial builders have been quick to jump on the recycling bandwagon; residential builders, not as fast - and typically only when customers so dictate in their contracts, Hansen says.

About 75 percent of new-construction waste can be recycled. But the throwaways from remodeling often include painted or treated wood, old insulation and wiring, and other hard-to-recycle materials, yielding only 35 percent to the recycle bins. Old painted cabinets, for instance, might be impregnated with lead and can't be recycled.

Once builders and clients start recycling, they see their construction sites differently, even finding ways to reuse castoff materials later in the project, Kunde says.

"We worked with a couple of home builders who ground wood and used it for landscape mulch, but now they've gone to having their wood picked up and processed offsite," she says. "Gravel's an easy one to recycle onsite."