Spontaneous Combustion in a Municipal Sewage Treatment Plant - A
Dust Explosion Case Study
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
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
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
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.