What Causes Compost Fires: Knowing the Conditions

Fires, especially small fires—and more so in mulch facilities than commercial compost facilities— are not uncommon. The trick is to manage those fires before they become larger. A better strategy yet is to prevent them in the first place, which begins with knowing the conditions that can lead to these events.

Arlene Karidis, Freelance writer

June 10, 2024

5 Min Read
Norma Jean Gargasz / Alamy Stock Photo

If you Google "fires in compost" or “fires in mulch" you will get plenty of returns, especially as municipalities divert more and more green waste.  Fires, especially small fires—and more so in mulch facilities than commercial compost facilities— are not uncommon. The trick is to manage those fires before they become larger.

A better strategy yet is to prevent them in the first place, which begins with knowing the conditions that can lead to these events.

Kurt Schoppe, director of operations for Viably, a waste and recycling equipment and system supplier, calls out some top conditions that can lead to and or fuel fire:

  • Compost piles that are too large. At over 12 feet, it becomes difficult for heat to escape.

  • Poorly maintained or damaged equipment.  Oil leaks or damaged hoses can spark a fire.

  • Piles left for long periods of time without being monitored. 

  • Compost that has not been turned or had sufficient water added.   Unturned piles become anaerobic and dense, resulting in heat buildup. 

  • Backlogs of feedstock that become excessively dry.

The good news for compost operators is that sound composting practices also serve as sound fire prevention practices, says Matt Cotton, owner of Integrated Waste Management Consulting.

“Key is managing inventory via first-in, first-out processing; adequate moisture content of materials; monitoring temperatures; regular turning and dewatering of piles,” he says.

Staying on top of fire risk takes hypervigilance in this industry. Compost and mulch operators have to deal with inconsistent feedstock deliveries, the challenges of equipment reliability, the vagaries of weather (especially wind and rain), and seasonal markets.

Still there is a lot that good operators can do to minimize risk.  Above all, site management is in their control. Some facilities go so far as training on what to do when a fire occurs and rehearsing the protocol of managing a site with a fire.  Facilities may invite the local fire crew out to educate them about the site and its practices.

“But keeping material moving, from incoming, to processing, to load out may be the most important thing,” Cotton says.

The phenomenon of compost piles igniting is called spontaneous combustion. This is the most common, yet least-understood cause of these facility fires.

While it’s known that large piles of organics can self-generate heat that if left unchecked can lead to spontaneous combustion, this type of fire can seem mysterious. It may be triggered by a number of causes.  Not all of those triggers are quickly identified, potentially leaving the real cause unattended to.

An unseasonable wind can push a spontaneous combustion fire over the brink; though, without wind, most of these events would be well managed quickly, industry experts have found.

Firefighters refer to spontaneous combustion fires as low-density fires, which can be slow to control and extinguish. It is not uncommon for a compost or mulch fire to burn for several days. In extreme cases some may burn for weeks, depending on the size of the pile.

And know that although some of the conditions that can lead to fires might be more common in drier, desert regions, they can happen anywhere.  Even during the winter, Schoppe says.

Early detection is critical to controlling and extinguishing fires.

There have been some new approaches to early detection that use thermal imaging and video monitoring. 

Then there are the long-established protocols, mainly to know what to do as soon as a problem is spotted.

“In a basic compost or mulch fire that you suspect was caused by spontaneous combustion, the best approach is to carefully remove cooler outside material first (with fire resources present); remove burned or burning material (with caution and with fire resources; and communication in place); spread out the burning or burned material, and douse with water until extinguished,” Cotton says.

He cautions that smothering the fire with dirt is ineffective at extinguishing the fire for the long term but can buy time.  And he suggests waiting until the fire department is on scene before going this route.

Of note is that simply dousing water on the pile is not effective in fighting a compost fire.  Compost and large brush/feedstock piles tend to shed water, making it difficult to extinguish, Schoppe says, adding that

firefighting requires heavy equipment to open compost and feedstock piles to allow the water to extinguish the active or smoldering fire.

Compost operators should know that fire departments unfamiliar with these types of fires may use large amounts of water. But compost experts say that reducing the size of the pile (with water and other firefighting resources nearby) using loaders or dozers will likely be more effective. 

It is very important to make sure that all firefighting equipment is inspected on a weekly basis, and to make sure it is in working order, says Eddie E. Reyes, general manager at SA Recycling’s Coachella & Thousand Palms facility.

SA does twice-monthly trainings, conducted as fire drills, in its compost area. Training consists of knowing the firefight tools and making sure they are in working order.

In these sessions fire hoses are pulled and heavy equipment is leveraged such as front loaders to separate the hot spot or fire from the wind rolls in order to gain control. And temperature of wind rolls is measured with large thermometers.

“If we find a pile that is too hot, we will add water and flip the pile with a large wind roll turner to control the temperature of the pile,” Reyes says.

The most common mistakes he’s seen are failure to continually and properly track temperatures and/or failure to properly flip piles to allow oxygen in. The other slip-up is not cutting or breaking down piles fast enough.

Reyes recently presented on compost facility fires at the US Composting Council Conference in Daytona. His main takeaways?

“Drill. Drill. Drill.  Most folks don’t take fire prevention and firefighting training seriously.  But practice makes permanent and is important because prevention is key. Develop a prevention and firefighting plan. You can’t get ready [once there’s a fire.] You have to be ready.”

About the Author(s)

Arlene Karidis

Freelance writer, Waste360

Arlene Karidis has 30 years’ cumulative experience reporting on health and environmental topics for B2B and consumer publications of a global, national and/or regional reach, including Waste360, Washington Post, The Atlantic, Huffington Post, Baltimore Sun and lifestyle and parenting magazines. In between her assignments, Arlene does yoga, Pilates, takes long walks, and works her body in other ways that won’t bang up her somewhat challenged knees; drinks wine;  hangs with her family and other good friends and on really slow weekends, entertains herself watching her cat get happy on catnip and play with new toys.

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