BILLINGS (AP) — For 40 years, the Billings Regional Landfill has been accepting truckloads of garbage. Last year, about 250,000 tons of garbage was deposited in the landfill just south of Billings. Now the trash is about to start giving back.

The bugs and microbes that break down the garbage over time give off methane, or natural gas. With the rush to find alternative energy sources and the need to control methane emissions from the landfill, Billings has teamed up with Montana-Dakota Utilities to trap the gas before it escapes into the air above or the water below.

MDU has drilled three test wells at the landfill and is monitoring the mix of gases brought to the surface. If the test wells confirm that methane extraction is a good idea — and most involved are convinced that it is — about 40 more wells will be drilled.

MDU can process and sell the gas to its customers and the city can turn a potentially expensive problem into a steady source of new income.

Other cities, faced with government requirements to prevent methane emissions at landfills, often burn off the gas or use it to generate electricity. Officials in Billings foresaw the problem and had set aside $725,000 to deal with the methane. Now they can now use that money elsewhere.

The contract between Billings and MDU is for 40 years, with automatic renewal every 10 years unless either side wants changes. The city will receive 15 percent of the net gas sales, and unless gas prices collapse unexpectedly, MDU estimates Billings will see an average return of at least $500,000 a year. That means a windfall of at least $20 million over 40 years for something that could have cost the city.

And for an upfront investment estimated at $8 million and annual operating costs of $600,000, MDU could easily earn more than $130 million over 40 years.

Information from: Billings Gazette, http://www.billingsgazette.com