Maryland Gov. Martin O’Malley, who touts his environmental bona fides, has enthusiastically championed wind power, solar power and electric cars, all in recent months. Now he is facing a much tougher call on an issue that boils down to this:

Just how green is garbage?

Environmentalists in Maryland are urging O’Malley (D) to veto legislation that would put incineration of garbage on par with solar and wind power as a source of renewable energy.

Debate over the bill puts O’Malley in a difficult spot. Many of the groups opposed to the bill stood side by side with the governor as he pressed unsuccessfully for legislation to spur the development of offshore wind power and limit construction of septic systems.

But Maryland faces a deadline to increase its use of renewable energy. Its energy administration backed the incineration bill, and so did the government in Montgomery County — the only local jurisdiction in Maryland that controls a waste-to-energy plant that is certified as a renewable source. The facility, in Dickerson, generates enough electricity to power about 50,000 households and earns millions in annual revenue.

“This bill does not take away from any other renewable project,” said Robin Davidov, executive director of the Northeast Maryland Waste Disposal Authority, a regional governmental organization that develops, finances and manages waste-to-energy projects. “The technology has evolved to the point that it’s so clean and so reliable that it’s the best option for managing solid waste.”

Despite support from county officials, the bill was opposed by seven of Montgomery’s eight state senators, who view the measure as a boon for the waste companies that operate facilities in Maryland and that lobbied lawmakers to introduce and pass the legislation.

“The question is, should we put as high a priority on waste incineration as we do on solar and wind energy? The answer is not just no, it’s hell, no,” said Sen. Brian E. Frosh (D-Montgomery), who has urged the administration to reject the bill. “This will result in a windfall for existing waste-to-energy plants. It’s unjustified, and it’s a lousy policy.”

Maryland, 28 other states and the District have renewable energy standards; all include landfill gas collection as a renewable source, but only 19 include trash incineration, according to a 2009 federal study.

Over the next decade, Maryland law requires the state’s power companies to get an increasing percentage of their energy from renewable resources. Companies that sell electricity must show they are meeting the standard by purchasing credits, essentially claim checks for renewable power.

Under current law, electricity from waste incineration is classified as a second-tier credit that will phase out of the market after 2018. The District, Pennsylvania and New Jersey, for instance, classify waste-to-energy as a second-tier resource.

The bill on O’Malley’s desk would elevate its status, making credits from waste-to-energy plants more valuable, removing the 2018 expiration date and making new plants eligible for the credits. Plans are underway for facilities in Frederick County and Baltimore.

In Montgomery, electricity from the plant in Dickerson generated $20.5 million in fiscal 2010, including $304,000 from the sale of renewable credits. If the bill becomes law, the county would earn about $250,000 more a year.

About 20 miles northwest of Rockville, the county’s waste-to-energy operation burns about 1,800 tons of trash each day.

The plant was designed, built and has been operated since 1995 by New Jersey-based Covanta Energy, the largest operator of waste-to-energy plants in North America, with 41 facilities, including sites in Alexandria and Lorton.

A smelly mix is tipped into a 90-foot-deep trash pit and then dropped into the mouth of boilers lined with water-filled tubes. Heated to temperatures of up to 2,100 degrees, steam powers turbines that are hooked into a generator connected to the grid.

The plant is outfitted with state-of-the-art scrubbers and filters to capture toxic pollutants such as mercury, and emissions from the 275-foot-tall stack are tracked and posted on the county’s Web site.

Paul Gilman, who is responsible for Covanta’s environmental compliance program, pointed to the 2009 study, which found that waste-to-energy plants produce electricity more cleanly and efficiently than landfill gas-to-energy, which is classified as a top-tier renewable credit in Maryland.

Sen. Thomas M. Middleton (D-Charles) said he agreed to sponsor the bill this year after meeting with Covanta representatives, who emphasized the alternative: Households and companies will continue to generate trash that would otherwise be diverted to landfills.

“You have to be a realist. Would you rather landfill waste or use it as a renewable energy?” said Middleton, who chairs the committee that oversees the state’s energy policies. “Looking at the new technology that’s out there, these plants aren’t the big polluters that we experienced years ago.”

Covanta has been a political player in Annapolis, contributing $4,000 to O’Malley since 2009, according to campaign finance records, and $3,500 to Senate President Thomas V. Mike Miller Jr. (D-Calvert) since 2007. The legislation passed the Senate on a vote of 24 to 20 in the final hours of this year’s General Assembly session.

In their letter to O’Malley this week, environmental groups argued that enhancing incentives for garbage incineration undercuts recycling because the facilities rely on trash for fuel. Providing more valuable credits to the facilities, they said, weakens the state’s clean-energy goals by subsidizing existing facilities instead of encouraging new technologies, such as wind power.

“We should meet these goals squarely and fairly . . . not by creating even greater incentives for incineration or other destructive, polluting industries,” said the letter signed by such groups as the Chesapeake Bay Foundation and the Maryland League of Conservation Voters.

The governor, who has until the end of May to act on the bill, has expressed support for converting waste to energy. He visited Middleton in his office in the final weeks of the session, the senator said, and asked whether Middleton needed help shepherding the legislation.

O’Malley spokesman Rick Abbruzzese said Tuesday that the governor is reviewing the bill.

On the final day of the session this month, O’Malley told reporters that the country needs to move toward “zero waste,” according to the Associated Press. “And part of that zero waste will, by necessity, involve converting our waste to energy, albeit with a very robust recycling component on the front end.”

Staff writer John Wagner contributed to this report.