Biosolids TOPICs: Interesting Ideas Connected to the World
When Biosolids is Hot Stuff
On nearly every Thanksgiving Day over the past 30 years, a moment inevitably comes to me when I recall, with a slight shudder, the great Thanksgiving Day conflagration of biosolids compost that threatened to shut down Philadelphia’s international airport. A huge pile of Philadelphia’s unscreened compost had caught fire spontaneously on that windy holiday morning and was billowing acrid smoke across the runway approach path. The FAA got involved, asking should it halt flights into the airport. It did not; the compost was spread out and sprayed with water, and the fire and smoke were finally extinguished.
Spontaneous combustion. This is a serious issue with any operation in which biomass is stored in big piles for extensive periods. That is just as true for biosolids products as it is for other organic materials. The National Fire Protection Association (NFPA) reports over 14,000 fire department responses annually to fires believe to have started spontaneously (Investigating Claims Involving Spontaneous Combustion). A quick Google Search of news articles on “spontaneous combustion” reveals an amazing array of such fires. Tempura flakes spontaneously combusted in Wisconsin in 2019 , which recalled spontaneous nacho chip fires (Flakes Aflame, Spontaneously combusting tempura flakes spark blazes, recalling 2018 nacho incident”). MBM, or meat and bone meal, is vulnerable, as described in the Japanese Failure Knowledge Database (”Spontaneous ignition of meat and bone meal under storage caused due to fermentation with rain water”), and by the French (Self-ignition of animal meal). Soy meal is similarly at risk: Spontaneous combustion sparks fire in previously flooded mountain of soya beans. Manure is a major culprit, as explained by state extension agencies ( e.g., Delaware ). International news services have covered fires in Spain (Manure pile spontaneously combusts to spark 13,000-acre wildfire, Spanish authorities say), as well as the U.S. (Spontaneous combustion concerns lead to warnings over height of chicken manure piles and Crap's Spontaneously Combusting in Upstate New York.) And hay is, too, at major risk for spontaneous combustion (Firefighters battle combine blaze to save crops ); the U.K. reports that damages due to haybale fires rank above other causes for insurance payouts (Fires cost farming industry more than rural crime).
Closer to home, several items in the home are culprits. Oily fabric rags are worst, as in the news story from November 2019, Improperly stored rags cause house fire in New York State and ‘Spontaneous combustion’ causes North Hollywood auto shop fire in California. And in today’s high tech world, an emerging source of spontaneous combustion is lithium batteries (Lithium Batteries Rechargeable batteries in your favorite devices can ignite and burn down your house). The solid waste industry is close to this issue, recommending management of transfer station pits (see A combinational approach). A less appreciated spontaneous combustion risk is laundry. Recently washed, dried and folded cotton towels seem to be a culprit (Could your laundry burn your house down? Fire started when washing spontaneously combusted after being folded and put away). This is true, too, for commercial laundering facilities, leading one manufacturer to provide this to brochure to customers: Preventing Laundromat Fires - Spontaneous Combustion in Dryers.
Spontaneous combustion of wood piles is a major issue, and many kinds are vulnerable, including wood chips, bark, yard trimmings and wood biofuel pellets. A review article from nearly 20 years ago, Self-Heating In Yard Trimmings: Conditions Leading To Spontaneous Combustion, concluded “the bigger the pile, the lower the ambient temperature at which SC may occur.” It usefully pointed to earlier research by the coal industry (SPONTANEOUS COMBUSTION OF COAL: “There are many factors which may contribute to spontaneous combustion including: pile size, atmospheric moisture content, wind, coal type, internal heat and oxygen transfer.” The emergence of a vigorous marketplace for wood pellet biofuel has focused research into methods of avoiding pile fires: Recent Health and Safety Incident Trends Related to the Storage of Woody Biomass: A Need for Improved Monitoring Strategies. This arises from a few incidents that have caught the industry’s attention, especially a sequence of fires at a Georgia manufacturer (Wood pellets 'spontaneously combust' at Logistec). Such landmark events have pointed to the importance of industry guidance documents, such as Self-heating Hazards of Biomass Materials, describing equipment operational risks, and Early Detection of Spontaneous Combustion in Pellet Mills, describing product storage risks, for which remote monitoring for pile temperatures and carbon monoxide are recommended.
What does all of this have to do with biosolids? If you are truly a hands-on, field-tested biosolids managers you know the answer. Many biosolids products are subject to spontaneous combustion. Air-dried granules, heat-dried pellets and biosolids compost are all prone to spontaneous combustion; it’s not a matter of if, but of when.
Conference papers, journal articles and technical manuals on the mechanisms of biosolids fires and the best practices for preventing and managing fires are rare. To its credit, the US EPA covered the risk of self-heating and spontaneous combustion in the 2000 Guide to Field Storage of Biosolids, but in cursory fashion. In 2006, at the 11th, European biosolids conference, Steven Manchester, owner of a specialty consultancy BRE Global Ltd., discussed how to characterize the propensity of different biosolids products to self-heat (“Application of Self-Heating Test Data to the Safe Handling and Storage of Dried Sewage Sludge”), and in 2007 at the 12th annual conference Manchester discussed the risk of thermal dryers (“Fire Risk Assessment of Sewage Sludge Dryers”), but these papers appear no longer accessible. Recent journal articles are not terrifically helpful, with the article Flammability properties of thermally dried sewage sludge confirming dried sludge will burn, and Self-heating of dried wastewater sludge telling us dried biosolids can self-heat.
Research on non-biosolids residuals has been more robust than biosolids and provides some information on the mechanisms that may be involved with biosolids self-combustion. One study of dried sludges from industrial treatment explored the complex interplay of moisture content, air flow/permeability and particle size. The report, Self-heating of dried industrial wastewater sludge: Lab-scale investigation of supporting conditions, explained ”[t]he higher the O2 concentration, the higher the solids heating rate. More added water prolongs the exothermic phase… The sludge particles size strongly determines the strength and extent of the heat release, indicating that surface reactions are taking place. In pelletized particles, limitations to water and air permeability mitigates the reaction course.” Another study, also on industrial sludges, showed an influence of iron on self-heating. Iron is used for phosphorus and odor control in some wastewater plants, and the biosolids in these plants run high in iron concentrations. Research suggest that iron reactions may accelerate self-heating to the point of combustion: Self-heating of dried industrial tannery wastewater sludge induced by pyrophoric iron sulfides formation. Research on wood pellets (Self-Heating and Spontaneous Combustion of Wood Pellets during Storage) shows that slow temperature rise from oxidation to the boiling point of water can initiate a pyrolytic (absent of oxygen) reaction that causes rapid self-heating: “the heat generated from the pyrolysis reaction is too large to be dissipated, leading to the accumulation of heat in the silo center,” a condition it terms “thermal runaway.”
How ought we manage risks of spontaneous combustion? We need to turn to our network of colleagues who are “in the trenches.” For operations, Brandon Gott offers the GHD Fire and Explosion Hazard Assessment and Training and lectures on “Dust Hazard analysis & Thermal Dryer Design Safety Considerations.” Robert Pepperman, Synagro’s director of product sales (AllGro biosolids compost and Granulite biosolids pellets), advises his large, nationwide customer base on management of bulk products distributed to agricultural and horticultural users, including storage challenges. Other fire-tested colleagues of ours include Chicago’s Dan Collins, compost consultant Craig Coker (see his January 2019 BioCycle article Managing Compost And Mulch Fires), and Inland Empire’s (California) Jeff Ziegenbein.
A first step is to know your product and the vulnerable aspects of your production process. Laboratory characterization of the biosolids product for aspects of flammability can inform storage monitoring. Synagro has developed for its own company’s laboratory protocol for judging the tendency of biosolids pellets to heat during storage. Pellets with high iron and oil contents are flagged for higher risk. Outdoor storage presents more risks of spontaneous combustion than covered storage, but storage facilities need to be built with fire risks in mind. Inland Empire’s indoor composting has smoke monitors that compel immediate actions to dismantle and cool piles when smoke is detected, and happily such events have been rare because Inland Empire aggressively moves compost through the entire plant in 60 days. Inland Empire also has written procedures kept close at hand for fire emergencies, and these plans have been reviewed with local fire fighters. Compost and pellet customers are advised regularly by both Synagro and Inland Empire to avoid long storage and to use products promptly, and they are instructed to carefully break into hot spots and douse them with water.
Here is additional advice culled from written work and interviews applicable to mitigating spontaneous combustion of biosolids compost and pellets: 1. Keep pile size small, (e.g., under 8 feet tall) which allows for heat dissipation. 2. Do not let piles become too dry, as moisture will slow heat build-up, and not too moist, as moisture spurs heat-generating microbial activity. 3. Do not allow piles to age too long, as old piles tend to trap heat and decompose. 4. Constantly monitor piles for hot spots, using automatic equipment (such as thermocouples and infrared detectors), as appropriate, and, also, visual inspection for smoldering areas, and once heat is detected, take action, as in pile breakdown. 5. Anticipate adverse weather factors that increase risks, such as rain, strong winds and warm air temperatures and increase inspections accordingly during such weather.
Even so, unexpected events of spontaneous combustion can occur. One customer of Philadelphia’s compost asked for reimbursement for a wooden fence the customer needed to replace. What I observed when I visited the site was an event of spontaneous combustion in which a 2-inch layer of biosolids compost had been overlain by an inch of triple screened hardwood mulch, effectively sealing the biosolids from heat dissipation after a rainfall had stimulated microbial heating. Smoldering biosolids compost led to a burning fence posts, which led ultimately to a burning fence. Had I not seen this for my own eyes, I would not have guessed this phenomenon possible. In a similar way, had not a security camera at a sushi restaurant in Wisconsin recorded ignition of tempura flakes, the cause of the restaurant fire would have remained unknown. Biosolids, like sushi, is a complex material, and no less so when the Biosolids is Hot Stuff. |
