Fire in Solid Waste

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California Integrated Waste Management Board

1. Fire Conditions and Requirements

There are several times throughout the year when ambient conditions can promote fires at sites where compostable materials are accumulated. After record high temperatures as recently experienced in many parts of the state, compostable materials site operators may need to apply increased diligence to prevent fires. 14 CCR 17867(a)(8) requires operators to monitor temperatures as part of a fire prevention program.

Section 17867. General Operating Standards.
(a) All compostable materials handling operations and facilities shall meet the following requirements:

(8) The operator shall provide fire prevention, protection and control measures, including, but not limited to, temperature monitoring of windrows and piles, adequate water supply for fire suppression, and the isolation of potential ignition sources from combustible materials. Firelanes shall be provided to allow fire control equipment access to all operation areas.

2. Look Out for Problematic Conditions

Fire prevention is key! Quoting from Rynk (2000): “Key conditions that lead to spontaneous combustion are biological activity, relatively dry materials or dry pockets, large well-insulated piles or vessels, limited air flow, and time for temperature to build up. In addition, there may be other contributing factors such as short circuiting of air flow, a non-uniform mix of materials, poor moisture distribution, difficulty in knowing temperatures throughout a pile, and sometimes a lapse or oversight in monitoring. These key conditions are usually more prevalent within large undisturbed piles containing raw feedstocks, curing compost and finished compost than in the active composting system.” Temperature monitoring and compostable materials management to address “wet/dry interface” concerns are two of the most important aspects of fire prevention for most operators. Temperature monitoring can pose problems, though, as it is difficult to "know" temperatures throughout the pile unless the pile is very small and/or highly homogeneous. Beyond temperature monitoring, it is difficult “know” the pile’s composition, level of aeration, and moisture content. Since biological decay produces hydrocarbons and other intermediates that may ignite and/or burn at lower temperatures, fires may start deep within the pile and be difficult to detect.

Rules of Thumb

More homogeneous, freshly-ground material is less likely to combust. For instance, a well-ground, uniformly wet, 20-foot-high pile that peaked at 195 plus degrees Fahrenheit just after grinding, and trailed off in a two week period to a very stable mid-140's degrees Fahrenheit, did not combust. Conversely, a marginally ground pile of mixed grass and wood waste when pushed together by an operator climbed in temperature overnight and caught fire.

Pile size dictates heat retention. When some very old, relatively hard-packed material in large piles had pile heights reduced to 6 feet or less, internal temperatures were directly lowered to less than 20 degrees above ambient. A pile over 12 feet high with sub-surface temperatures above 160 degrees Fahrenheit would in most cases be cause for extreme alarm (see below).

Wet/dry interface starts most compostable materials fires -- in other words, a pile that has dry spots which come in contact with a portion of the pile that is wet and composting robustly has a good chance of catching on fire. The robustly composting material reaches temperatures that support chemical oxidation, and the fuel from the dry portion continues the chemical oxidation and eventually combusts. Combustion occurs between 205 to 400 degrees Fahrenheit. Two possible scenarios illustrate this: 1) some very old, dry material comes in contact with new wet, unground material after a late rain, and a hot north wind blows at the end of a long, hot day to start a fire; 2) in late summer, after several unusually hot days in a row, some dust suppression water gets on the edge of a drying pile, the wet area heats up and starts a fire -- the water encourages vigorous biological activity, raises the temperature, and the dry portion of the pile ignites. Quoting again from Rynk (2000), “Moisture is a crucial requirement for composting and spontaneous combustion. In both cases, proper moisture is a matter of balance. The critical moisture range that supports spontaneous combustion is roughly 20 to 45 percent. Above 45 percent, there is enough moisture available for evaporation to hold down temperatures. Below 20 percent, there isn’t enough moisture to sustain the biological activity that initiates the temperature rise.”

There are other sources of ignition to keep in mind. Equipment sparks and off-site fires are common sources of ignition. The location of piles of combustible compostable materials needs to be critically observed. Could the piles ignite because of nearby grass fires? Are the piles near structures, equipment or other components that could also catch fire? Can all sides of the pile be accessed in case fire suppression is needed?

4. Follow-ups and Information Gathering

If you find that you do have piles of stored material, please let CIWMB staff know where they are and any other information about those sites.

The Board continues to work with the EAC and CCDEH on a project involving fires in stored piles that will lead to workshops with LEAs, Board staff and Fire Authorities on the issues common to the fires that have occurred. Please contact Dennis Corcoran for information on those cases or if you have any fire incidents that occurred in your jurisdiction. Dennis can be reached at dcorcora@ciwmb.ca.gov or 916-341-6395.

Fire in Solid Waste

1. Fire Conditions and Requirements

2. Look Out for Problematic Conditions

3. Help?

4. Follow-ups and Information Gathering

References