With the current landfill filling up, Renton and King County are looking for solutions for what to do with King County’s trash in the future.

The county conducted a study looking at long-term disposal options to prepare for the eventual closure of Cedar Hills Regional Landfill. The landfill, which is the only active landfill in King County, is set to close in about 2040, King County Strategic Planning Manager Brian Halverson said: “That’s when it will reach capacity.”

The study examined five disposal options: waste export by rail, mass burn, pyrolysis, gasification and refuse-derived fuel. The study also examined the economic, environmental and social impacts, as well as capacity, logistics and operating history for each disposal method.

Halverson said waste export by rail and mass burn are the two methods being considered for long-term disposal.

• Export by rail: Halverson said many of Renton’s neighbors use rail to export waste, including Seattle, Snohomish County and Kitsap County, shipping the garbage to available landfills in Eastern Washington. Mass burning, where they incinerate trash to create energy, would require siting and building of a new facility and the disposal of ash, he said. The process leaves about 25 percent of the total weight of garbage as ash that will need to be exported by rail.

Waste export by rail is less expensive with capital costs of $3.3 million and operational costs of $72 million a year, for a cost per ton of $108 in 2040 dollars. Halverson said this method could utilize existing infrastructure and would not need to site or build a new facility and capacity in eastern Washington wasn’t a concern.

“The report says there’s 300 years worth of landfill life and there wouldn’t be a significant impact by adding King County’s tonnage to the rail through the study period,” said Halverson, noting that one downside to the rail option is that shorter rail contracts could result in changes in future price.

• Pyrolysis and gasification: Usually referred to as thermal treatment, pyrolysis and gasification involve the superheating the garbage to create in a limited oxygen environment oils and synthetic gasses that can be sold or refined to make energy. Halverson said the largest facility they looked at only could handle a tenth of the capacity needed for their lowest tonnage scenario in the future and have struggled to show they can handle mixed municipal solid waste.

• Refuse-derived fuel: This is waste ground up, dried out and turned into either pellets or fluff that is then used in boilers. Halverson said refuse-derived fuel would be capable of processing the tonnage, but comes with other challenges such as salt in food waste creating impure fuel and the challenge of selling the fuel once made. “We’ve basically made our landfill more expensive and still have to landfill it or burn it,” Halverson said.

• Mass burning: This method would be the more expensive method at costs for the capital totaling an estimated $1.2 billion, and operational costs of $54 million plus $23 million in ash disposal per year. Halverson said that would result in an estimated cost of $231 per ton in 2040 dollars. The siting, permit and design of a mass burn facility can take more than 10 years. He said another drawback is the method disincentivizes lowering the amount of garbage produced.

“Because the capital cost is so high, there is a financial incentive to push through as many tons as possible,” Halverson said.

Energy produced by a potential facility would be classified as “non-renewable” by the Clean Energy and Transformation Act and can’t be sold within the state, he said.

Halverson said there will be many opportunities for cities and other interested parties to engage and provide feedback in the decision. A final environmental impact statement will be finished by the end of 2026. The county will seek advisory committee recommendations in early 2027 before an executive office decision in mid-2027.

Check it out

King County’s Solid Waste Division will host an in-person meeting at 6:30 p.m. Wednesday, May 28, at Maywood Middle School, 14490 168th Ave. SE, Renton. The meeting will give people the opportunity to learn and ask questions about the Cedar Hills Regional Landfill, including current and planned construction projects, environmental monitoring activities and operational activities.