Earlier this year, the European Union launched POST-PURPLE, a Horizon Europe project pushing the boundaries of zero-waste urban biorefineries with purple phototrophic bacteria. These microscopic powerhouses will transform wastewater and biowaste – the organic fraction of solid municipal waste – into nutrients and microbial biomass, which will then be converted to high-value biological products.
These bioproducts include carotenoids, omega-3 fatty acids, and coenzyme Q-10, all widely used in dietary supplements, with nutrients also being used to enrich irrigation water. Critically, by combining this resource recovery with emission reduction, the project is set to reshape how cities manage waste, helping them become more sustainable and move closer to true circularity.
“For the first time we are tackling solid waste, wastewater and gas emissions together, rather than as separate problems,” highlights project leader, Ass. Prof. Daniel Puyol, Universidad Rey Juan Carlos (URJC). “The project will promote technologies based on innovative photobioreactors that contain purple bacteria to prevent, reduce and convert gaseous, solid and liquid waste streams from wastewater treatment and municipal solid waste treatment plants.”
Critically, by combining this resource recovery with emission reduction, the project is set to reshape how cities manage waste, helping them become more sustainable and move closer to true circularity
POST-PURPLE comes at a time when Europe’s sprawling wastewater and municipal waste systems face persistent pressures from high volumes of waste that, if left untreated, can harm ecosystems and human health. More than 30,000 urban wastewater treatment plants serve hundreds of millions of people across Europe, treating sewage that contains pathogens, chemicals, and excess nutrients. The continent also generates around 229 million tonnes of solid municipal waste each year, of which some 229,000 tonnes of organic waste is treated.
The levels of greenhouse gas emissions are significant – wastewater treatment accounts for about 0.85% of emissions, while the entire solid waste sector accounts for around 3% of EU greenhouse gas emissions, largely driven by methane from the decomposition of the biowaste in landfills. But by tapping the nutrients and organic matter in these waste streams, POST-PURPLE aims to convert what is now a major environmental challenge into sustainable raw materials and valuable microbial bioproducts.
Real innovation
POST-PURPLE follows the EU-funded project – DEEP PURPLE – that also aimed to recover energy and nutrients from urban waste-streams, using photobiorefineries with purple phototropic bacteria. Two demonstration wastewater treatment plants and a biogas facility, fed by municipal solid waste, were set up around Spain, which successfully extracted cellulose and other resources from the domestic wastewater and solid waste. Each plant operated independently, and as Puyol puts it: “Putting our lab-scale technologies into these larger, demo-scale plants was fantastic, but now we'll explore the technologies, in more detail, at a smaller scale.”
“With POST-PURPLE, we're working with the organic fraction of municipal solid waste and domestic wastewater – two connected but very different lines of waste with very different problems,” he adds. “This integrated biorefinery approach is where some real innovation lies.”
With funds of nearly €4 million, the three-year project aims to convert nutrients from wastewater and municipal solid waste treatment plants into high value products and reduce the associated emissions via integrated photo-biorefineries – in line with the European Union’s Zero Waste and Zero Pollution objectives. Numerous novel technologies, pioneered by key sustainability players from around Europe, are involved.
Puyol highlights how the project will use purple phototrophic bacteria in large, low-emission photobioreactors, developed by the URJC, water management firm, Aqualia, and the AIMPLAS Technological Institute of Plastics – all based in Spain. These reactors are designed to metabolize nitrogen, phosphorus and carbon from the waste streams, generating microbial biomass for high-value products. Additionally, by capturing nitrous oxide – a potent greenhouse gas typically released by wastewater treatment plants – the project will reduce emissions, with recovered nitrogen being re-used in fertilizers. These fertilizers will be tested by the Portuguese research centre, and project parnter, Food4Sustainability.
With funds of nearly €4 million, the three-year project aims to convert nutrients from wastewater and municipal solid waste treatment plants into high value products
Meanwhile, Spain-based waste treatment firm, Econward, will use its advanced thermal hydrolysis system to breakdown organic municipal solid waste. The set-up generates odorous gases that, while potentially harmful, contain nitrogen and ammonia that can be converted to the high-value compounds used in dietary supplements – also known as nutraceuticals. On top of this, municipal waste from Western Macedonia will be transformed into high-value microbial protein – essentially protein-rich biomass of bacteria or other microbes. Greek company DIADYMA, alongside the University of Western Macedonia and the non-profit Cluster of Bioeconomy and Environment of Western Macedonia (CLuBE), will process municipal solid waste into 'syngas.' This gas is then used by the Austrian research institute BEST to cultivate microbial protein.
POST-PURPLE is also building on technology from DEEP Purple that used bacteria,as part of a hypersaline methane oxidation process, to break down methane from decomposing waste. In POST-PURPLE, project partners, including researchers from the Institute of Sustainable Processes, University of Valladolid, will recover carbon dioxide and nutrients from hypersaline methane oxidation. The carbon dioxide will then be fed into open ponds to grow Dunaliella salina. These microalgae will convert the captured carbon into valuable biomass, which can then be processed into bioproducts such as carotenoids, lipids, and proteins. In a final step, researchers from the Faculty of Technology Novi Sad, in Serbia, will analyse these extracts, along with the high value-added produced from purple bacteria, to determine their quality for industrial use. Along the way, researchers from Imperial College London, UK, will assess the sustainability of POST-PURPLE developments.
As Puyol highlights, by combining wastewater, solid municipal waste and syngas into a single biorefinery that ultimately results in valuable resources and reduced emissions, POST-PURPLE is set to demonstrate a circular economy in action. “In terms of metabolism, purple bacteria are amongst the most versatile organisms on Earth, and using these bacteria [in all of these different ways] is new,” he asserts. “We've seen small-scale studies using microalgae, but this has never been tried with purple bacteria.”
Next steps
Clearly the project is breaking new ground. As Puyol puts it: “From a metabolical point of view everything here is feasible… but this is novel and the processes have not been explored before – so this is our big challenge for now.”
“We have a number of smaller, dynamic companies working with us that have a particular expertise in a specific field,” he adds. “This project covers fundamental [topics] and these companies can concentrate on the detail and will bring good ideas.”
Tech development aside, Belgian non-profits, STRANE Innovation and Water Europe, will also be helping to exploit and publicise project results. “We've got this really heterogeneous consortium, and this is going to help spread the word – and our results – all over Europe,” concludes Puyol.
