In current wastewater treatment processes, a seemingly beneficial material is produced – the nutrient-rich biosolids. A mixture of water and organic matter, this by-product can be harnessed as a type of fertiliser for food crops, or used for regenerative purposes – helping to stimulate revegetation in areas impacted by industries such as mining and construction, or in aiding forest productivity. With such potential applications, it seems on the surface to be a wonder-product that works in service of the circular economy. However, questions have been raised over the potentially harmful long-term impacts of these materials given the presence of heavy metals within them (such as arsenic, mercury, nickel and selenium). While they may seem environmentally beneficial, it seems they may hold longer-term dangers that warrant closer examination.
Now, researchers from RMIT University in Melbourne, Australia, have developed a novel technology to eliminate pollutants from biosolids, using high temperatures to convert them into biochar – a stable, carbon-rich charcoal-like product that is used to increase soil fertility, improve water holding capacity and crop productivity. So what does this process involve, and what role could it play in the circular economy? We take a closer look.
Biosolids: why should we care?
In Australia, biosolids have been used in the agricultural industry since the 1990s, with their application as fertilisers seen as a simple and effective solution to eliminating waste products from water treatment processes. In 2019 Australia was found to produce approximately 371,000 dry tonnes of biosolids per year, with around 70% of this applied to agricultural land, 24% for landscaping or land rehabilitation. The remaining 6% was stockpiled, landfilled or discarded into the ocean.
Speaking with RMIT Associate Professor and research lead Kalpit Shah, he says biosolids can be incredibly useful in soil and plant regeneration schemes.
“Biosolids can be used as a fertiliser and soil conditioner to improve and maintain productive soils and stimulate plant growth,” he says. “They are also used to fertilise gardens and parks and reclaim mining sites. In Australia, biosolids have been primarily used for land application in agriculture and forestry, land rehabilitation and composting.”
However, not all biosolids are appropriate for this kind of application, and environmental issues arise when considering what happens to those that are too low-quality for use.
“There are variable biosolids quality across Australia,” Shah adds. “Only the highest grade of biosolids can be used to grow crops for human consumption. Lower qualities are typically used for road base and mine site rehabilitation. The bulky nature of biosolids means they are not amenable to be used as a mainstream agricultural product, and currently around 30% of the world’s biosolids resource is stockpiled – creating an environmental challenge.”
In addition, recent investigation into the potentially harmful long-term impacts of this material on surrounding ecosystems has given rise to calls for a safer, toxin-free alternative. This is where the RMIT team comes in.
The team at RMIT have created a novel pyrolysis (PYROCO) technology, which uses high temperatures to eliminate pathogens, as well as any microplastics that may be contained within biosolids.
Using this process, biosolids are transformed into biochar – an organic material that not only improves soil quality, but also increases soil carbon content, reducing overall greenhouse gas emissions as a result.
“The biosolids produced from conventional wastewater treatment processes (after dewatering process) are converted into biochar in a hyper-efficient new type of reactor developed and patented by RMIT University,” says Shah. “This reactor radically optimises heat and mass transfer while shrinking the technology to make it highly mobile. The technology is based on slow pyrolysis – a process for splitting up organic materials into their chemical components by heating them in the absence of oxygen, so they do not combust.”
The team’s technology can enable a more than 30% reduction in CO2 emissions as compared to existing waste management options, in addition to a reduction of more than 70% of waste volume, transport cost and associated emissions. The new approach would also help divert potentially toxic waste from landfill.
Producing biochar and applying it to soils could also reportedly create carbon offsets under the Carbon Farming Initiative.
The technology is currently being trialled by South East Water, Intelligent Water Networks and Greater Western Water, which Shah says will be investigating the technology’s efficacy. The trials will be utilising RMIT’s patented reactor, which itself also has potential applications in the biomass and plastic sector.
“The trials will confirm the improvement in the heat and mass transfer around the reactor and overall process which can have a significant impact on the capital and operating costs,” Shah says. “This will be measured by logging temperature and pressure profiles across the reactor throughout the trial. The trials will continue to produce biochar which will be comprehensively characterised for soil properties, contaminants and carbon.”
“The next stage of the demonstration trial will involve scaling up the technology in collaboration with a manufacturing partner,” he adds. “This dedicated unit will then be based at a recycling plant over a longer period of time, validating the target markets for biochar. This is a crucial step before the technology can be commercialised.”
The team is currently looking at funding options and potential partners to take part in the next step of the trial. With the project now looking at building a full-scale demonstration plant, taking the tech to the next level seems well within reach, and biochar may well be set to play a crucial role in future land rehabilitation projects.