Packed with treasure from a more wasteful time, it seems inevitable that historic landfill sites will one day be mined for energy and materials. In Spain Ferrovial Servicios has been running a project to determine just how soon that day may be.
As resources become increasingly scarce, it seems inevitable that the day will come when recovering the huge quantities of valuable materials deposited in landfill sites since the 1950s will make economic sense. Back in 2014 Ferrovial Servicios began a project to determine just how soon that day might be, and what the optimal recovery strategy should be.
The project was carried out at a non-hazardous waste treatment facility in Jijona, Alicante. The site consists of a Municipal Solid Waste (MSW) mechanical biological treatment plant, a composting facility and a landfill.
Gonzalo Magdaleno, chief of innovation and studies at Ferrovial Services’ Centre of Excellence for Environment, explains that the company decided to conduct the project at a landfill site that was in operation before the Landfill Directive came into force, as much less of the recyclable content would have been diverted.
“If you run this kind of project at a landfill that operated in the last decade of the 20th century, it’s very likely that you will find waste that is of a similar composition to the waste that is treated in current facilities,” he tells WMW.
The Jijona site was also ideal as it had a waste sorting and processing facility onsite, which meant the excavated waste would not need to be transported for treatment.
The project was split into four stages: waste extraction, processing, refused derived fuel production and organic fraction refining. For each stage, characterisations and physico-chemical analyses were conducted to determine the process mass balance as well as the potential recovery of waste streams.
Excavation
The waste was extracted from the site using conventional excavation methods. Particular attention was paid to discarding the top layer of soil from the landfill covering so as not to contaminate the waste beneath. The extracted material was then moved to the unloading beach of the site’s biological mechanical treatment plant, where testing and further characterisation were carried out.
During the course of the project the site was excavated to a depth of around 6 metres and over 2640 tonnes of wastes were extracted.
As they were dealing with an older site, Magdaleno says that the excavation was also carried out with considerable care in case they found anything unexpected which should not have been there. One of the main goals of the project was to understand how to go about the excavation process without risking any environmental contamination, as well as to define safe working procedures.
Processing
Once excavated, the landfill waste was moved to the unloading beach of the mechanical biological treatment plant and categorised by age into two groups for testing. The aim was to assess if there were behaviour differences during processing.
The sorting operation was conducted using the current equipment configuration of the MBT plant, and was focused on the recovery of Tetra Pack type packaging, aluminium, ferrous metals, HDPE and PET. The process yielded the following fractions:
Bulky waste, mainly stones
Flat fraction from the ballistic separator
Reject fraction from the byproducts line which was used for refuse-derived fuel production
Organic matter from <70 mm trommel-screened fraction and <70 mm ballistic separator fraction
Trommel reject, which was returned to landfill.
Magdaleno explains that the lightweight material from the ballistic separator and the reject from the packaging sorting line, both of which had a high plastics content, were used for RDF production. The organic matter, meanwhile, was refined using a 12 mm trommel. A sample of the resultant material was extracted for agrochemical analysis.
“Regarding the quality of the recyclable material, after being sorted we found that packaging waste changed colour and had a certain degree of deformation, but we think that it has potential to be recovered as a valuable material,” says Magdaleno. “We didn’t put into practice any additional more cleaning processes than we do with fresh waste.”
“As well as a high moisture content of around 10%, one of the main difficulties we found was its high density and high compaction causing clogging problems,” he adds.
Results
According to Magdaleno after completing his analysis of the excavated waste, over 16% was potentially recoverable.
That included 1.42% of packaging waste, which in spite of its years in the ground was suitable for material recovery. A further 1.73% was the refined material, potentially useful as landfill engineering product or for soil restoration. The remaining 13% of recoverable material was appropriate for the production of RDF.
Of the waste unsuitable for recovery, 57% was refining reject, which Magdaleno says could still be useful as temporary cover for landfill or as regulation layer prior to final closure layers being applied. Just 17% of the extracted waste was returned to landfill.
Conclusions
In addition to the recovery of materials, other benefits of landfill mining include an increase of useful volume in landfill cells, a reduction in potential emissions from methane and leachates, and reduced costs associated with post-closure supervision of landfill facilities.
For those contemplating a similar project, Magdaleno summarises by offering two critical insights into the economic viability of conducting such a landfill mining project.
“The first conclusion is that this project could only be profitable because we had the treatment plant there to process the waste at the site. If we had had to make an investment to set up new treatment facilities, or transport the waste any distance, we couldn’t have done it,” he says.
“The second conclusion from this project is that if you compare this waste with fresh waste, the composition is different. The fresh waste contains only those materials that it is not possible to recover with mechanical treatment,” he explains.
However, perhaps the most important achievement was in laying the foundations of how such projects can be safely executed. “With the limited information we have after only one project, we are currently developing a procedure to conduct landfill mining in a safe way,” concludes Magdaleno.
Unfortunately most of the world’s landfill sites are not conveniently located alongside the mechanical treatment facilities needed to recover their hidden treasure. For those sites it seems that the day when it makes economic sense to mine them is not yet upon us. But the work done here certainly brings it closer.
