Despite the fact that sawdust piles are fire hazards, they are very unlikely to spontaneously combust as a result of heat build-up as the composting process of pure sawdust is too slow. This argument was accepted by the judge in a court case in which a sawmiller was sued by a neighbouring landowner who claimed that a sawdust pile spontaneously combusted, causing a fire that damaged his plantation. DAVE DOBSON reports …
This case study deals with allegations of spontaneous combustion in sawdust heaps on the Defendant’s property that resulted in a fire that devastated a neighbouring commercial pine plantation.
The client was the Defendant in this case; the owner of a property on which eucalypts were grown to supply a sawmill that processed the timber to produce pallets.
The sawdust arising from the milling operation was dumped at various localities on the property and not incinerated on account of the danger associated with this operation.
This case deals specifically with the claim by the Plaintiff that as a result of the manner in which the Defendant managed the sawdust, spontaneous combustion occurred. This lead to the wild fire that burnt the neighbouring commercial pine plantation belonging to the Plaintiff.
A number of challenges arose in the case but the most important one related specifically to the sawdust. In the summons it was claimed that the sawdust piles constituted a fire hazard in that:
• The Defendant had not intermittently layered the sawdust with soil.
• The Defendant had not restricted the height of the sawdust heaps so as to avert or minimise the risk of spontaneous combustion occurring in the sawdust pile.
The Plaintiff claimed that these two omissions were largely the reason for the spontaneous combustion occurring.
A third claim was that the Defendant failed to maintain an effective firebreak around the perimeter of the sawdust piles. Such a firebreak - if implemented - would have contained the fire to the sawdust pile.
While sawdust fires are recognised as being a potential fire risk, SAFIRE Insurance Company Ltd. had at the time that this court action commenced (2012) never received a claim emanating from smouldering sawdust piles.
However there was a single incident reported to SAFIRE of a fire in a sawdust pile, but this was not the result of spontaneous combustion of the pile. This was a fire in sawdust at a sawmill on the farm Etterby in the Richmond district. This fire was extinguished by digging out the smouldering sawdust and dousing the area with water. The fire had entered the sawdust while the landowner was burning a firebreak around the sawdust pile.
Spontaneous combustion does at times occur at composting facilities when the compost heaps self-heat to temperatures high enough to ignite. In these instances no external heat source is required. In order for composting organic material to ignite very specific conditions are required. These are:
• A C:N (Carbon:Nitrogen) ratio of 20:1 to 35:1 is required. Sawdust has a C:N ratio of between 300:1 to 400:1. The result of this is that the composting process for pure sawdust will be extremely slow. (Nitrogen is required to feed the micro-organisms that are responsible for the composting process. There is simply not enough of this nutrient for them to thrive!)
• The moisture content of the compost heap must be greater than 50%. Below this percentage the composting process slows down.
• Aeration is required for rapid, efficient composting. Allowing the organic material to become anaerobic (compacted) will slow the composting process.
In the composting process temperatures in the compost stack rise and can reach 70OC to 80OC as a result of the activity of the micro-organisms breaking down the organic material. Above 80OC micro-organisms die and chemical reactions take over. This chemical heating can continue to raise the temperature of the organic material until it reaches about 150OC at which point ignition can occur. It is important to note that both the biological and chemical oxidation processes require oxygen to proliferate. Progress is extremely slow under low oxygen (anaerobic) conditions.
Back to the spontaneous combustion sawdust pile court case. In this case the Expert for the Plaintiff used the example of silage production, likening the heat build-up in silage to spontaneous combustion. From the previous discussion a number of points arise. The first is that maize used for silage is green and thus contains a high proportion of nitrogen that is required by the micro-organisms to break down this organic material. The organic material is also moist which fulfils the moisture requirement. However, silage is compacted thus resulting in anaerobic conditions. The silage will simply not burst into flame!
Further issues that mitigated against spontaneous combustion in the sawdust on the Defendant’s property were that the sawdust was spread and compacted i.e. this would have limited the oxygen available to the micro-organisms responsible for composting this organic material. The sawdust pile in question was merely one meter deep and any heat build-up would have been rapidly dissipated. Finally, had there been any combustion in the sawdust pile a source of fine ash would have marked the site of ignition. No such evidence was found.
Sawdust waste – like this – is highly unlikely to spontaneously burst into flames due to heat build-up as the rate of decomposition is too slow!
Spontaneous combustion will not occur in sawdust. The primary reason being the C:N ration of this organic material. Sawdust fires will invariably be the result of fire from the outside entering the sawdust pile - as was reported in the Richmond case.
The result of the trial was a finding in favour of the Defendant. Spontaneous combustion was ruled out as the origin of the fire, and honey hunting was identified as the source. This activity had set a stump alight which later - under extreme weather conditions - ignited grass on the edge of the sawdust pile. Despite attempts by the Defendant’s team to put the fire out, it swept across the sawdust pile as well as the firebreak around the sawdust pile into a gum compartment and on into the Plaintiff’s property.
*Related article: How to calculate plantation fire damage