A bold reuse initiative by Icon Water has transformed clay spoil and water treatment solids into a valuable topsoil product, the project standing as a leading example of collaboration for circular economy outcomes and long-term sustainability.
During Ozwater’25, the Spoil to Topsoil project won the national R+D Excellence award (sponsored by Water Research Australia) at the Australian Water Awards. The R+D Excellence award recognises projects that challenge current practice and address the need for a sustainable future.
Delivered by Icon Water, Yass Earth Movers and ACT NoWaste Transport Canberra and City Services, the project reinvented hard-to-manage waste streams from water treatment and network maintenance operations to generate a valuable product for rehabilitation across the ACT, and potentially Australia.
Know of an outstanding water project? Nominations for state and territory awards are now open.
Icon Water's James Gibson said the research behind the project is extensive and started about six years ago, with Yass Earth Movers being a key partner as Icon Water’s excavation company, regularly handling burst water mains and dig-ups.
"We used to dump the excavated soil in a paddock, clean it as best we could, and reuse it back into the system. But, in Canberra, our soil is very clay-based. Even when cleaned, it looked fine but would reform into chunky clay lumps again,” Gibson said.
“My team fixes burst water mains, repairs sewer mains – we dig every day. We wanted something more consistent and easier to work with.
“I didn’t know much about water treatment solids, but I started asking around – could we blend something to improve the soil properties? That’s how we landed on using water treatment solids.”
Icon Water's Nupur Khanna said the project brought various areas of the business together to work towards reducing multiple waste streams.
“My team had been working on finding a reuse for the water treatment solids – a project going on for about ten years. Water treatment solids are one of our largest waste streams. We generate about 2,000 tonnes a year – and all of it was going to landfill,” she said.
"Leading the circular economy plan at Icon Water, my focus was on finding sustainable uses for our waste. When James and I were talking, we realised we had two problems and wondered if we could combine them in the solution.
“It started as just an idea. We weren’t sure it would work, which is why collaboration was so important. We’d already worked with ACT NoWaste on earlier trials – they hadn’t been successful, but we still had strong relationships. That helped us kick this off.
Khanna said research around water treatment solids has been unfolding over the past decade, and this project leaned into the work of other utilities’ reports and research.
“We had reports from other utilities and universities going back ten years, looking at national applications for water treatment solids. Each utility waste has similarities, but differences, too, depending on the influent,” she said.
"The early research led to a more localised project here in the ACT. One of the findings was that water treatment solids could be useful in compost, though we didn’t yet know what it needed to be mixed with.
“There were trials – like making mud bricks, which technically worked, but had no real market. Another trial involved mixing FOGO (food and garden organics) with water treatment solids.
“That compost product came out well and even got some regulatory approval, but due to uncertain FOGO supply, the project didn’t proceed. When I joined Icon Water, we decided to take those learnings and try something new – mixing water treatment solids with soil to create topsoil.”
Gibson said the testing of the product involved many different blends and, in the end, a 50-50 mix was found to achieve the best results.
“We tested 15 different blends in a paddock over three months. A third party monitored it. The best result was a 50-50 mix – water treatment solids and clay spoil,” he said.
“Since February, we’ve made three large batches – over 1,000 cubic metres reused in the network. Every day, we’re backfilling holes with it. The challenge now is making it consistently. We’ve had teething problems, but it's going well now.
“We’ve designated an area on our land as a “no-waste facility” – basically our resource recovery centre. All our excavation waste goes there to be sorted. We're almost putting everything back in the ground now.”
Not to be confused with wastewater treatment biosolids, water treatment solids have great moisture-holding capacity, Khanna said. While this makes the material clumpy and slow to dry, mixing it with clay produced a fantastic result.
"The result is really great looking, fluffy topsoil. Plus, it has good organic matter, which is great for plant growth – it retains nitrogen and phosphorus, making it excellent for use in nature strip restoration,” she said.
Gibson said there used to be complaints about subsidence with the reused, unmixed clay, but not anymore: “Now we fill entire holes with this product – no need for a second visit. We’ve had no complaints since we started using the new mix”.
Khanna said making a difference with reused materials is all about scalability: "If this can be replicated across Australia – or even globally – it’s a win for the environment and for the industry”.
“We made our decision on the 50-50 mix based on visual monitoring (weeds, weather resilience) and soil testing (organic matter levels). That combination of evidence gave us confidence it could be scaled,” she said.
And when it comes to being recognised for the achievement at a national level? Gibson said the experience of winning at the Australian Water Awards was amazing.
"I was proud just to be nominated for the ACT awards. Winning the national award was next level, especially considering how massive that category was,” he said.
“But the thing that hit home was the criteria – R&D with future implications. We’ve genuinely turned a waste stream into our new normal. For the people who come after me and Nupur, this process will just exist. That’s something to be proud of.”
Know of any amazing water research and development work occurring in your state or territory? Nominations for awards are now open.