Recent major infrastructure projects in Melbourne, such as the West Gate tunnel project, have highlighted the challenges and the subsequent cost and time blow-outs that disposing soil contaminated with per-and poly-fluoroalkyl (PFAS) into landfill can cause. But what if there was a better solution?
For researchers at the Institute for Frontier Materials, a better, holistic solution lies in taking a circular economy approach.
Associate Professor Will Gates, Dr Andras Fehervari, Mr Chathuranga Gallage, Associate Professor Damien Callahan, Dr Alastair MacLeod and Professor Frank Collins have shown how to reuse heat-treated soil, free of PFAS, as fine aggregate for concrete and mortar. The aim is to help make the entire remediation process more efficient, less costly compared to excavation and chemical extraction and to remove waste soils from landfill.
The IFM project is one of 13 projects nationwide funded through the Australian Research Council (ARC)’s PFAS Remediation Research Program that is run through ARC’s Special Research Initiatives scheme, which has been funded by various Federal Government stakeholders.
PFAS are manufactured chemicals found in materials such as aqueous film-forming foams (AFFF – used in firefighting) and carpet and upholstery waterproofing sprays, and have been detected in soil, air, surface water and groundwater. PFAS has not been shown to cause disease in humans however, because these chemicals can remain in humans and the environment for many years, it is recommended that as a precaution exposure to PFAS be minimised where possible.
Associate Professor Will Gates says of all the ARC Special Research projects nationwide, IFM’s project is the only one that takes a holistic approach to remediation targeting the waste-stream itself to produce useful end products.
‘Clear air, water and soil are necessary to support growing populations,’ Assoc. Prof. Gates says.
‘As we expand our cities and industrial centres it is therefore imperative that we use our resources as fully as possible.
‘This project brings together widely disconnected but well-established industries in soil and groundwater remediation, PFAS destruction and cement and concrete engineering. It underpins a circular economy approach that is required for sustainable PFAS – actually any – remediation industry.’
Assoc. Prof. Gates says a crucial part of the project has been IFM’s partnership with Renex Group. Renex Group’s Pyrolysis Rotary Kiln Technology at its plant in Dandenong South can completely destroy PFAS in soil – and unlock new purposes for the treated material that is left behind.
‘Our industry partner– Renex Group– showed that it was possible to inject concentrate into the pyrolysis chamber and still also heat-treat soils so that a dual activity was achievable, thereby reducing costs,’ he says.
‘The Renex process creates heat-treated soil, but it destroys PFAS. So, you do not have to worry now about PFAS getting back into the environment; it’s completely destroyed.
‘The process is designed to remove the fluorine that comes out of the stacks so they can make calcium fluoride. They reflux as much of the outgas back into the plant as possible, so that the final emissions from the plant are basically CO2, heat and water vapor, and harmful fluorine emissions that can occur in other processes are avoided. It’s quite thorough in that regard.’
For the West Gate Tunnel project, the 3 million tonnes of PFAS contaminated soil to be excavated will be dumped in landfill, but Prof Gates argues why not give that soil a new purpose?
‘There’s really two options after you have treated the soil – use it as a fill or dispose of it as landfill, with the later costing money,’ he says.
‘For myself, and many other people around the world, we firmly believe that landfill is not the place for treated soil because it takes up a lot of space and there are many other things that should be going into landfill.
‘So, a third option is to utilise the heat-treated soil as a resource. That’s what this project is about – to find a use for this soil.’
Assoc. Prof. Gates is quick to admit that a two-phase pyrolysis plant is expensive to build and run, however, he says, the benefits outweigh costs.
Currently, the cost of processing contaminated soil can be anywhere between $100 and $250 per tonne exclusive of transport costs, and disposal in landfill exceeds $180 a tonne. However, disposal and some transportation costs can be recovered – with remediated soil having the potential to be sold to the concrete industry for perhaps $50 a tonne.
‘By not putting contaminated soil – or even heat-treated soil – into the landfill, you’re now saving landfill space that would otherwise be utilised by this space filling material,’ Assoc. Prof. Gates says.
‘The concrete industry would be reducing their carbon footprint. They also won’t be using virgin materials from quarries or river sands. So, they are reducing environmental harm and saving on the existing resources that can go to more important things.
‘It’s kind of hard to put price tags on it all, but they all add up to some pretty common-sense things that would point towards significant improvements economically.’
The team has conducted a field trial where the remediated fine aggregate has been used in reinforced concrete slabs for a fully instrumented entrance to a parking lot.
‘The concrete that you can make is strong,’ Assoc. Prof. Gates says. ‘We’ve proven in the labs that we can make greater than 40MPa concrete, which means it’s high strength.
‘At our field trial we took cores to test the seven-day strength and achieved 26MPa strengths, at 28-days better than 41MPa strengths and after 56 days the cores had 45MPa compressive strengths. This means that the heat-treated soil can be used as a fine aggregate replacement in many concrete applications.
‘We’re matching the field trials to our laboratory trials to prove we have good strength concrete. And we’re taking the steps to get this to be taken up by industry.’
The final goal in the soil remediation research would be to take the remediation technology to site.
‘Renex can process eight to 10 tonnes an hour, but the problem is, it’s a big, fixed plant in Dandenong South and bringing the soil to the plant does increase the cost,’ Assoc. Prof. Will Gates says.
‘Ultimately what is best is to basically try to convert it into a field portable system.
‘The problem is taking that extreme efficiency to a field; it is going to be a major industrial and engineering challenge.
‘But if you can make this all efficient, then more and more companies will be able to get involved in the remediation and the remediation will proceed faster and more efficiently, and we can eventually get PFAS out of the environment as much as possible.’
One other aspect of their research that was severely hampered by the COVID-19 pandemic, but equally as important, was addressing how organic concentrates from groundwater contamination can be destroyed using this same circular economy approach.
He says this area of research asks: can the efficiency of the heat treatment process be further improved by using alternative calorie enriched materials (calorifics); can the residuals of these calorifics be used as adsorbents for use in groundwater remediation; and can activated carbon products be rejuvenated using this same process?
‘With industry partner The Remediation Group we have shown that a variety of agricultural wastes – almond, walnut hulls, apricot, nectarine and olive pips, saw dust and even lees from breweries – contain substantial calories,’ he says.
‘And result in material with a sufficient absorptive capacity for targeted contaminants – so far chemicals such as PFHexS and PFOA – to be useful in groundwater remediation.
‘All these organic components or agricultural waste can be used as fuel. They can be combined and by thermally treating them under controlled conditions, the same kind of conditions that you treat the soil in, you can basically pyrolyse these materials.
‘And so that pyrolysis process activates the carbon or activates the surfaces of these carbon materials. And so, you get what you call an activated carbon type material. And these are really useful for absorbing organic contaminants in remediation.’
The traditional groundwater remediation method pumps the contaminated water through filters which contain granular activated carbon, which strips out the organics from the water.
‘The granular activated carbon that’s used in industry is really highly refined, very expensive on the order of $3,000 to $5,000 a tonne,” Assoc. Prof. Gates says.
‘The material we’re able to make isn’t near as high quality as the granular activated material, but again, can be made at a fraction of the cost.
‘What we want to find out is, how efficient can we make it? And can we use it to actually replace some of the granular activated carbon?’
Assoc. Prof. Gates is hopeful that this novel remediation approach will change the status quo of industrial waste processing.
‘This can apply to any kind of remediation technology that requires heat treatment,’ he says.
‘There are many, many different types of organic contaminants, and the only effective way to treat them on a large scale is by heat treatment. In the eyes of the EPA, this then renders the soils as an industrial waste rather than as something that can still be used.
‘The only way you can get the Environmental Protection Agency to accept these things as anything more than industrial waste is to find a use for them that can be prescribed.
‘And that’s what this project is designed to do – help advance that hierarchy of dealing with materials. And by being able to prescribe it as having a use, then it can have a value and it can be sold. It doesn’t have to go to landfill. In fact, it shouldn’t go to landfill.’