A sprinkler sprays a smoldering pile of unprocessed yard debris at the Knott Landfill Recycling & Transfer Facility on Thursday. 

Beneath the surface of a large heap of unprocessed yard debris at Knott Landfill Recycling & Transfer Facility, a fire started to smolder.

But nobody noticed until Wednesday night when it erupted in flames at the Deschutes County landfill on SE 27th Street.

The spot fires began at 11:45 p.m. Wednesday and never threatened anyone but burned some of the pile of yard debris that was roughly 20 feet tall, 200 feet long and 100 feet wide. The flames grew as it spread to a nearby pile of wood.

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Two engines were needed to contain the fire. Dan Derlacki, deputy fire marshal of Bend Fire & Rescue, said firefighters stayed at the scene for 10 to 12 hours. Once visible flames were extinguished, county crews provided water trucks to monitor the site. Despite the flames being contained, smoke was still wafting in the air at 2 p.m. Thursday.

The debris becomes compost after being ground and treated. Compost is used in soil to generate heat, help seeds sprout and retain moisture. It also provides beneficial organisms to reduce the use of pesticides. However, if unattended and in large quantities, the weight of it can start a fire.

“That pressure on it keeps it heating up, and eventually, if you don’t break it up, it will get to a point where it’ll ignite and then start smoldering down there,” Derlacki said. “That smoldering will eventually make it out to where it gets oxygen and a breeze and now you get that actual open flame.”

Processed compost piles at the landfill are regularly monitored for this reason, but unprocessed piles don’t receive the same attention. Bend fire officials are now working with the landfill to come up with a way to mitigate the risk of spontaneous fires.

Tim Brownell, director of the landfill, described the fire as “impressive” and smelling like any other wildfire.

It might take days to fully extinguish the fire and could take weeks to move the unburned debris to the other side of the site.

Even though the debris was burned, it can still be processed to create compost. The burned wood will not be usable. For now the recycling center will still receive yard debris. However, it will not be able to process it until the smoldering and smoke ceases.