Speaking at the Waste Management Association of Australia’s 2017 Australian Landfill and Transfer Station conference at the end of March, Mockinya Consulting director Paul Lightbody told delegates the project started as a means to find a use for trommel fines produced at a refuse-derived fuel facility in Adelaide.
The plant, which was established in 2006, processed some 150,000 tonnes of construction and demolition and commercial and industrial waste a year to produce an engineered fuel.
The feedstock it receives is screened, goes through a size reduction process, and is then separated to produce process engineered fuel, aggregates, fines, as well as a small amount of residual waste that is landfilled.
“Two streams come out of this process – one’s a coarser stream and the other’s a finer stream,” Lightbody said.
“Parts of the equation that framed this project was to look at what these materials were, how they could be used, which characteristics might be important, and whether these materials and residuals could become a resource.”
At the moment, there is simply no local market for the oversized fines (12-40mm) and heavy fines (12mm minus).
Typically, these fines, which Lightbody said are “studied extensively”, are composed of inert mineralogical materials including plaster/gyprock (85.6%), organic inclusions (12.13%), glass (1.54%), hard plastic (0.17%), light plastic (0.02%), other miscellaneous materials (0.42%) and an almost negligible percentage of metal and foam.
Because of the fines’ composition, there were characteristics that Lightbody needed to evaluate.
“The issues for the use of these materials on landfill as cover were potential flammability particularly because there were visible inclusions of plastics. There was also potential for it to generate odour and further contribute to methane emissions in the landfill because of the organic fraction. It also needed to be suitable from a chemical composition perspective and we already were pretty satisfied about that in the quality of the material,” Lightbody explained.
Additionally, under the SA EPA licence requirements, landfill cover must be a “soil” and not a waste material although in SA, there is an exception for waste fill. Or, Lightbody said, the fines could be approved as an alternative cover under approved specifications. Thus, discussions with the EPA began around the parameters required for the fines to be used as an alternative cover.
Testing the fines
Lightbody noted that in Australia, there were few common methods have been used to test the flammability, odour and methane emissions, and physical performance of trommel fines as an alternative landfill cover, though these tests certainly exist.
He turned to the ASTM D4982 “flammability potential screening – waste” to test the material’s flammability and a static method based on the ASTM D5975 to test respiration activity in order to determine the odour and methane emissions.
“For the physical performance of this material, there hadn’t been a lot of characterisation of particle size distribution (PSD) so we looked at that to see how closely the methods were replicated for the fuel,” Lightbody said.
Mockinya performed initial tests on a stockpile of material in 2013 over 10 consecutive days, followed by a five-day test in 2015.
“The ASTM method for flammability tested whether the material would be ignited easily and sustain a flame. This testing was undertaken by an independent third party laboratory,” Lightbody said.
The result was that no, the material did not ignite easily nor did it sustain a flame. All samples were later classified as not potentially flammable.
However, there was a small amount of flammable material present in the fines, specifically:
- plastic and paper, which would ignite and burn when contacted directly with the flame. It would then smoulder for up to 15 seconds; and
- larger pieces of wood, paper and plastic that would ignite and burn when contacted directly with the flame and remain burning for up to 15 seconds.
Extinguishment followed after, Lightbody said.
Turning to the respiration activity, Lightbody relied on a modified version of the ASTM method developed by the laboratory that used proprietary and independent techniques around the containment of the gas within the chamber. Table 1 details the differences in the respiration activity tests; the middle column is the test that was undertaken.
Table 1: Testing respiration activity. (Source: Mockinya Consulting)
The test revealed that the rate of oxygen consumption had decreased over time with the highest four-day rate at less than 1mg of oxygen per gram.
“We were looking for variability in the factors that might affect the results but the results were consistent. We started to look at shorter timeframes because we were seeing a plateau in the respiration rates and we did, over time, develop confidence that we were seeing good and consistent data and we weren’t getting significant outliers in the data,” Lightbody said.
“One of the benchmarks we developed as we went through this program was based on Irish EPA guidance around the use of residual material for landfill cover application, which established the criteria of between four and 10mg O2/g and we were seeing results that were well inside what was thought to be a reasonable target. Thus, the conclusion was the organic matter was biologically stable.”
Finally, when it came to particle size distribution, Lightbody told delegates that this was “relatively consistent across the material” over the two testing periods. There was quite a good gradient of material and a reasonable amount of fines content – coarser content that provided the material some strength that would make it suitable to be used as a cover material.
“Other discussion points were the presence of glass and the perception that there is a fire risk due to the glass in the material,” Lightbody added.
“This is a myth in Australia that glass on the side of the road causes bush fires and our own fire authorities do teach their firefighters that it is a myth.
“The volatile solids are also within the range of carbonaceous soils, so that wasn’t a concern. We looked at all the impacts, even the stability of the material and it benchmarked reasonably well against the current soils that are used for cover.
“In summary, it forms a dense cover, it’s well graded, and it physically limits direct access for vermin – it’s not a food source for vermin. It is also not generating odour.”
The waste inclusions were also not significant to function as a cover though they are within approved specification limits.
In terms performance, Lightbody noted that the material has been used as an interim cover for three years now and in that time, it has remained stable despite some very heavy rainfalls.
“It has been accepted now by the EPA,” Lightbody said.