December 1, 2005 -- A dust explosion occurred in a large storage silo holding dried sewage sludge. The sludge was dried in a gas-fired, air-convection dryer and stored in the silo until discharged into trucks and hauled away. The silo was approximately 35% full at the time of the incident. Immediately preceding the explosion, a powered bin vent and air intake fans, had been started. The dryer was not in operation at the time of the event and had last been in operation 3 days prior to the event. The plant was cold and being started up following a long weekend.

The investigation concluded that the explosion started because a smoldering nest in the silo was disturbed by the increased air flow when the fans and vent were started. The nest quickly transited from a smolder to a very vigorous combustion zone. The dust was disturbed into a dust cloud that reached explosive concentrations very rapidly.

Smoldering nests of this type are often caused by a chemical or biological reaction, in the case of sewage sludge, which is an organic material, this is an aerobic reaction. The reaction is exothermic and the heat so generated can build up to levels not only higher than the surrounding area, but to a point where there is spontaneous ignition of the material itself. The resulting smolder can take from several hours to several days to develop, and there are cases where these nests continued to smolder for weeks.

Generally such smoldering nests form deep inside a critical mass of the material and they smolder rather than burn more vigorously due to a lack of sufficient oxygen. The unburned dust actually acts as both an insulation blanket, and an oxygen starving mechanism. If the mass is disturbed, however, so that the burning part is exposed to more air, or even worse, suspended into a dust cloud, the result is an accelerated burning rate and either a fire or an explosion.

The type of aerobic respiration that occurs in sewage sludge is facilitated if there is moisture present in excess of approximately 10%. A review of the logs for the plant indicated that the material coming out of the dryer and into the silo immediately prior to shutdown, was running at over 15% moisture. Samples sent to an explosion lab confirmed both the high moisture content and the explosive behavior of the dust.

The combination of the critical mass of material, the sufficiently high moisture content, and a 3 day holiday weekend that left this material undisturbed for a lengthy period of time, combined to set the stage for the explosion.

The damage was limited by the bin vent, which sacrificed itself, but saved the silo from what could have been much more significant damage. There were no injuries. Steps were taken to ensure that in the future the material exiting the dryer would be below 10% moisture. Proper explosion venting was recommended for the silo. Procedures were instituted to prevent dried sludge from resting inside the silo undisturbed for several days. These steps have resulted in no re-occurrence of this unfortunate event.

There are many materials that have been identified as being prone to self-heating. While the mechanism for the exothermic reaction can vary from the aerobic one reported here, the result of spontaneous ignition is much the same.