Zero-waste cities: #2 Remediation

When Fox River on New Zealand’s South Island flooded in 2019, it caused the sudden erosion of historic landfill situated on its banks. Hundreds of tonnes of rubbish, much of it plastic, was washed into the river and distributed along its banks and on beaches downstream. Even after a clean-up operation costing NZ$3m, many tonnes of toxic waste and plastic were lost to the ocean.

This is far from an isolated incident, or an unusual situation. In Europe alone, there are estimated to be up to 350,000-500,000 of these “legacy” landfills, 90% of them pre-dating modern legislation and a significant number already at risk from flooding. This is just one example where waste has been inadequately dealt with in the past, and it all adds up to a colossal challenge for us today. Historic mining areas throughout the world, for example, are often disfigured by huge deposits of tailings, the potentially toxic residue that remains once the sought-after substance has been extracted. Heaps of coal ash the size of small hills are common wherever coal has been used to generate power. Land around oil and gas extraction sites is often blighted by hydrocarbon contamination that leaks into rivers and seas.

“As well as the waste we generate today, we also have a considerable problem with the waste we have already created and failed to deal with properly,” says Carole Smith, technical director, environment with WSP in New Zealand. “As in many countries, legacy landfills in Aotearoa New Zealand were located on poorly considered sites around the coast in flood plains or in gullies. The sites were not engineered to today’s standards, often because the risks associated with these sites were not well understood. They were not always lined, so leachate discharge was poorly contained. Unlike modern landfills, they were not capped, and there was limited or no landfill gas control systems to capture methane.” These sites, like Fox River, are now in danger of being washed out to sea — a risk exacerbated by rising sea levels, flood events and more frequent extreme weather events caused by climate change. 

But Smith does see some hope — and even opportunity. “First, our knowledge of landfill-related risks and design solutions is much improved,” she says. “Leachate is collected to prevent it polluting land and water, and improved capping solutions mean that methane can be safely flared or used to produce energy.” The CO₂ produced during this process is a much less potent greenhouse gas than methane.

For the oldest, most problematic sites, the only option may be to excavate the landfill and reposition it in a safer location — an expensive process, but one that does open up the possibility of reclaiming some of the materials. “We recently looked at removing metal from a legacy landfill with a large number of scrap cars in it,” says Smith. “Sadly, the economics didn’t quite stack up — partly because the metal was usually attached to other materials. But as reprocessing technology improves, and perhaps as the cost of landfill rises, this sort of approach should prove more feasible.”

Proponents of landfill mining point out that dumps are full of potentially recoverable materials such as steel and aluminium, along with plastic and organic matter that could be burnt in waste-to-energy plants. Research by Cranfield University analysed waste at four UK landfill sites and estimated they contained US$400m of copper and aluminium, along with US$141m of palladium and US$9m of neodymium.

The other valuable asset that we can recover from landfill is space. Where legacy fill sites can be safely re-engineered — removing the voids, improving overall stability and enhancing protective capping layers — they become valuable community spaces. These can be used for active or passive recreation, community events and even provide important biodiversity havens.

Smith gives one more example of how landfill might be rich in future opportunities: “We’ve made cocktails of waste in landfill and, while that is concerning, it is possible that they contain new and useful microbes.” Leachates within these sites are made up of different chemicals, she explains, which can be highly toxic if released into the environment. “But the microbiome present in this poisonous cocktail does play a role in slowly breaking down the constituents into less harmful substances. If we could harness the power of these microbes, we may be able to use them in different applications — for example, to enhance remediation or improve energy efficiency.”

In the 1990s, anammox bacteria, responsible for oxidizing ammonia, were discovered in wastewater plants, and early indications show this group of bacteria may be helpful in accelerating the nitrogen cycle and treating ammonium. They are also present in some landfill leachates. “If we knew more about what goes on in landfill, we might be able to manage it better or extract more value,” says Smith. “There may be compounds and microorganisms we can use.”

Even the most unpromising waste streams can have value, says geochemist Shameer Hareeparsad, an associate with WSP in South Africa, where the mining and energy sectors are major producers of waste materials, including rock, tailings and fly ash from coal combustion. “These waste materials are often disposed of in surface dams and dumps, which can have significant environmental and social impacts if not managed properly,” he says. “It is encouraging to see efforts to extract value from them, rather than simply treating them as liabilities that need to be disposed of.” Mining companies are extracting more gold from historic tailings, for example, while fly ash is used as a cement substitute in concrete and for brick making. “This not only reduces the amount of waste that needs to be managed but also creates economic opportunities and helps to address environmental and social challenges.”

Other waste — such as toxic brines from waste-water treatment processes— seem valueless, but they do have potential when you combine them or add to them, says Hareeparsad. Mixing harmful brines with ash to create a solid backfill material is a way to deal with both waste streams. “By stabilizing the mix with lime or cement, harmful elements can be fixed and trapped, preventing them from leaching and contaminating the environment.” This approach is particularly useful for addressing the problem of acid mine drainage (AMD), where certain sulfide-bearing rocks left exposed in old mine workings can react with air or water. “There are thousands of disused and abandoned mines in South Africa where AMD is an issue, with the acid getting into land and water courses. So if you backfill with naturally alkaline fly ash, stabilized with brine and lime, you cut off the air and safely neutralize the acid as well as removing brine and ash from the surface.”

Using materials that would otherwise be discarded to treat other environmental issues is an idea whose time has come, according to Jeff Paul, technical fellow with WSP in Florida, and a below-ground remediation specialist. “We’ve actually been upcycling materials that would have been waste in this way for decades. It is classic circular-economy thinking. But it’s only recently that people have really started to take notice and ask about the possibilities,” he says.

Paul helped to pioneer a technique that uses waste alcohol or ethanol to remove nitrites and ammonium from contaminated soil and groundwater. It was first used in the UK in the 1970s, and deployed again in 2005 on a much larger scale on a project in Dalton, Georgia in the US. “We don’t necessarily know why the, in that case, alcohol is being got rid of. It might be out of spec or not pure enough for its original purpose. What matters is that we get it cheap or free on the second-hand market, analyse to check fitness for use, and the donor benefits because they don’t have to pay to have it disposed.”

This has proved quicker and cheaper than traditional methods of dealing with contamination, he adds, so it’s always been a win-win. “The difference now is that there is less suspicion and much more interest. Donors are particularly keen to see their discards reused in a useful way as it helps with their ESG [environmental, social and governance] credentials.”

Paul’s ethanol-ammonia technique is an example of dealing with contamination in situ — almost always cheaper than excavating or removing it. As global environmental standards tighten, and waste management authorities review their legacy liabilities, it is likely to become the focus of more interest and research.

“Dealing with low levels of contamination where they are present is usually preferable,” agrees Smith. “Digging up slightly contaminated soil and trucking it miles away to landfill is often not really worth it — both because it takes up valuable void space in our landfills and because of the carbon burden generated through excavation and transport. Quite often it is better to retain it on site where people and the environment can be isolated from the contamination. Existing demolition dumps that may have an associated asbestos risk might be better left alone.”

This does depend on whether the waste in question is sufficiently safe and stable — not at risk of erosion, and not containing substances that can leach out and contaminate water. But it is possible to stabilize waste too. “This is often fairly basic chemistry,” says Paul. “Though you can’t change one element into another, you can often modify, fix or neutralize harmful substances.”

And, as in the case of anammox bacteria, if chemistry won’t fix the problem, then biochemistry might: “Take cobalt,” says Paul. “This is a common contaminant arising from the aluminium, paint and other industries. It is classed as toxic if it gets into water. But we can deal with it using bacteria.”

By optimizing conditions for naturally present microbes — providing them with food and oxygen — cobalt is taken up into the bacterial biomass. “When the microbes die, the biomass hold on to the cobalt, so it is dealt with permanently. A modified method works for molybdenum contamination too.”

It is an exciting field, with new remedial techniques coming to market with increasing regularity. “The rate of change is incredible,” says Paul. “The number of people around the world researching this kind of thing is enormous. As an industry, we are getting better at remediating pretty much everything — hydrocarbons, chlorinated solvents, paraffins, pesticides, herbicides, every organic you can think of — and we are doing it faster and cheaper than before.”

The fact that many of these advances use material that would otherwise become waste as part of the solution is more than just satisfying, says Paul: “It points to a much-needed change in mindset. Taking a more creative approach can transform a waste stream into a significant source of value, and I think people are finally beginning to understand that it’s not waste leaving their plant, it’s dollars.”

This article is the second in a four-part series exploring the transition to a circular economy. To read the rest, click the links below, or download the whole series as a PDF.

#1

The circular economy

By 2050, global cities could produce 3.88 billion tonnes of waste every single year — unless we do something about it. Can the circular economy save us from drowning in our own wastefulness?
Read the story

#3

Plastic

Humankind’s most useful invention has become its biggest problem, and one that just won’t go away. What can we do about plastic?

Read the story

#4

Equity

Waste has always had a disproportionate impact on low-income communities, but social and environmental goals don’t always align neatly. How can we clean up past mistakes, and create a fairer future?
Read the story