Getting rid of Maine’s garbage was easy in 1970, when the first Earth Day was celebrated. Trash was shoved into piles at the town dump and set afire.

The pollution caused by open dumps was one of the driving forces behind Earth Day, and the landmark clean air and clean water laws that followed.

Forty-two years later, roughly 35 percent of Maine’s municipal solid waste is recycled. One-quarter goes to modern landfills designed to protect groundwater.

Most of the rest is still burned, but in four plants that control air emissions, reduce the volume by 90 percent and generate electricity.

Maine is a national leader in turning waste into energy. Behind Connecticut, Maine ranks second in the country in the percentage of trash handled by these plants.

But waste-to-energy has a public relations problem in southern Maine. Longstanding complaints about truck traffic, noise and odor at Maine Energy Recovery Co. in Biddeford have the city and the owner talking — once again — about closing the plant and sending trash elsewhere.

The friction involving MERC obscures the vital role waste-to-energy plays in preserving Maine’s limited landfill space, as well as trends to expand and build new plants in the United States and overseas.

The propects for growth will be discussed by experts starting Monday, when the 20th annual North American Waste-To-Energy Conference opens at the Holiday Inn by the Bay in Portland. More than 300 people are expected to attend.

The three-day event is taking place as both the industry and its expansion plans face some obstacles.

Many waste-to-energy plants were built in the 1980s and 1990s. They were able to negotiate lucrative, long-term power contracts, earning revenue that kept disposal fees down.

Those windfalls are ending, in an era when low natural gas prices are cutting the wholesale cost of making electricity. So waste-to-energy plants are under pressure to control their tipping fees, to compete with lower-cost landfills.

“It’s really a matter of economics,” said Ted Michaels, president of the Energy Recovery Council, a trade group that’s helping organize the conference. “It’s not an easy path to a resurgence, but there are opportunities.”

After years of limited development, the industry is starting to grow again. Five waste-to-energy plants are expanding in the United States and four new ones are being built, according to Michaels.

A recent report by Pike Research, a clean-tech market forecaster, is projecting multibillion-dollar growth in Asia, where population and urbanization are taxing waste disposal. There’s also a move to refine technologies that go beyond combustion, such as pyrolysis and gasification, to create oils and transportation fuels from trash.

These topics will be explored at the conference. Presentations include news on a 3,000-ton-per-day waste-to-energy plant in Palm Beach County, Fla.; comparing the environmental performance of gasification plants versus mass-burn units, which combust at high temperatures; the rapid growth of waste-to-energy in China, and the impact of removing food waste on heating values.

Of particular local interest is a presentation on how to restructure plant operations and contracts at an older plant that burns refuse-derived fuel, which is made by separating metals and other non-combustibles.

MERC fits that description. The 25-year-old plant currently is the subject of talks between Biddeford and Casella Waste Systems, the owner. But rather than improving MERC’s performance, the city wants to close the plant, which burned 284,718 tons in 2010. One option being discussed involves taking some of the waste to the state-owned Juniper Ridge landfill in Old Town, 136 miles north.

This idea was embodied in a controversial, last-minute bill in the Maine Legislature to allow Casella to sell the plant and buy Juniper Ridge. The proposal was tabled earlier this month, for lack of time to consider it.

But it immediately highlighted potential conflicts, including whether the move would violate Maine’s 23-year-old waste management hierarchy. The policy, enacted by the Legislature, sets a priority order: Reduce, reuse, recycle, compost, waste-to-energy and landfill.

Maine had more than 450 local dumps during the first Earth Day. Typically unlined, they often were near wetlands or streams.

Today, Maine has eight municipal waste landfills, one commercial landfill and state-owned Juniper Ridge, which is operated by Casella. Any policy changes that could modify the waste hierarchy and steer more trash to landfills are on hold until after the fall elections.

“I think that discussion will be held next year,” said George MacDonald, who heads Maine’s waste management and recycling program.

Maine’s other three waste-to-energy plants are in Portland, Auburn and Orrington. If MERC closed, it would be technically possible for the remaining waste-to-energy plants to handle the in-state trash that now goes to Biddeford, MacDonald said. But costs would rise if waste is trucked from southern Maine farther north, at a time of high diesel prices.

“It looks good on paper,” he said. “The issue then becomes the transportation of that waste.”

Waste-to-energy plants must take in consistent volumes of trash as fuel to meet their electricity contracts — not a problem in growing regions. But Maine’s stagnant year-round population can result in trash shortages during the winter. That’s one reason the plants, to varying degrees, accept trash from out of state. Roughly 65 percent of MERC’s waste is trucked in, mostly from Massachusetts and New Hampshire.

“Our waste stream isn’t growing, as it was in the 1980s and 1990s,” MacDonald said.

Recycling and composting also reduce the waste stream. The statewide recycling rate, which has averaged 38 percent or so in recent years, is likely to increase as more communities roll out “single-sort” recycling programs, MacDonald said.

But recycling also has economic benefits. The materials have value, and the reduction in waste cuts disposal costs for communities.

The state’s largest recycling facility is at a waste-to-energy plant — ecomaine, which serves Greater Portland. It recycled more than 35,000 tons last year. The nonprofit facility, owned by 21 communities, is considering a plan to pull out food waste and compost it.

“You can’t burn lettuce,” said Kevin Roache, ecomaine’s executive director.

Roache, who will make a keynote presentation at the conference, said he expects Maine’s four waste-to-energy plants to remain in operation because they dispose of waste closer to where it’s generated.

“Storing waste in landfills will become more and more of an issue,” he said. “How long can we send our waste to faraway places?”

But for now, a more pressing issue for Maine’s waste-to-energy plants is the power-purchase contracts. The deal at the Penobscot Energy Recovery Co. in Orrington, for instance, expires in 2018. Lower power revenue in the future could increase disposal costs for the 178 communities that rely on the plant.

At ecomaine, low natural gas prices have cut power revenue from nearly $7 million to $3.1 million over the last fiscal year. But ecomaine’s members are fortunate, Roache said, at least for now. The plant’s outstanding debt will be paid off this summer, and revenue from recycling is higher than expected.

“That has allowed us to insulate the downturn in the electric market from our member communities,” he said.

Staff Writer Tux Turkel can be contacted at 791-6462 or at:

tturkel@pressherald.com