AUGUSTA – Gov. John Baldacci on Tuesday signed into law new restrictions on the burning of construction and demolition debris, which is imported into Maine by the ton and is used as a cheap biomass fuel.

The legislation, which took effect immediately upon Baldacci’s signature, limits the volume of wood from construction and demolition debris that may be substituted for conventional fuel in boilers to 50 percent per year.

The new law also establishes for the first time a standard to make sure the debris is as clean as technically possible.

“The bill adopts a standard we think is one of the most rigorous in the country,” said Rep. Ted Koffman, D-Bar Harbor, a leading supporter of the bill and co-chairman of the Natural Resources Committee.

Some 600 million pounds of construction and demolition debris were burned in biomass boilers in Maine in 2004. The state Department of Environmental Protection says more than 80 percent of the total came from out-of-state sources.

Imports of the waste exceed the volumes sent out of Maine, and “there’s a concern this volume will grow,” Koffman said. And if the CDD isn’t consumed in boilers, there is an even greater concern it could be destined for Maine’s landfills, he said.

Simply blocking waste from being imported into Maine isn’t an option because such restrictions have been deemed unconstitutional.

“It’s important that we take some action to assure that the fuel is as clean as it can technically be made now,” said Koffman.

The new 50 percent limit on burning of construction and demolition debris could doom a proposal by GenPower LLC of Massachusetts to build a plant in the western Maine town of Athens.

GenPower developed a gas-fired power plant in Westbrook, Maine. Its Athens proposal would have roughly doubled the volume of such debris that’s burned in the state today, driving up the cost for debris for other Maine plants.

Lawmakers who supported the bill had also argued that the rules were needed to help the state find a new buyer for the Georgia-Pacific Corp. mill in Old Town, which uses a biomass boiler to reduce energy costs. DEP officials have cautioned against connecting the rules with the Old Town mill, however, saying that G-P was not burning construction and demolition debris because the boiler could not meet existing air pollution standards.

Improper or inefficient incineration of CDD can result in higher levels of dioxin, arsenic and other harmful chemicals being released into the air.