Sewage could underpin industrial collaborations and green hubs that aim to keep resources in circulation as long as possible - and could help meet Europe's challenging climate change mitigation targets.

That is according to scientists at SINTEF, a research organisation based in Norway.

“Entire regions and their industries must start adopting circularity by exchanging their materials and energy,” says researcher Richard Heyn at SINTEF. “If we expand this form of collaboration to incorporate urban areas and regional businesses, we can establish what is termed an industrial-urban symbiosis."

An industrial symbiosis is established when different industries operating within a region exchange energy, materials or waste with the aim of making optimal use of resources. Reused resources can include anything from waste heat and coolant water to critical raw materials and sewage.

Not only does sewage include plentiful supplies of water that can be treated and reused, it is also a potential source of energy in the form of biogas and biofuels, heavy metals, and fertiliser nutrients, including nitrogen, phosphorus, potassium and sulphur.

The aim of the collaborative approach is to circulate the resources for as long as possible, reducing consumption of non-renewable energy sources, reducing water abstractions and sewage emissions to the environment, and mitigating CO2 emissions. Well-known symbioses of this type include district heating systems, which exploit waste heat obtained from industrial plants for domestic use.

“Two further examples are the use of ash from combustion plants as a replacement for cement in concrete, or the use of sewage derived from a medium-sized town, either as the basis for generating household electricity, or as an industrial raw material,” says Heyn.

The aim of green hubs is to gather industrial-urban symbioses on individual sites, and at scales large enough to make it possible for hundreds of tonnes of materials to flow between participating industries and other facilities. This can achieve significant reductions in both resource consumption and carbon footprint.

One place where this is already happening is in the city of Antwerp in Belgium, which is closing the loop on wastewater with a €100 million project that will repurpose 20 billion litres of water every year. From 2025 this treated effluent will undergo further treatment to produce huge quantities of cooling water for industries based in the city's port.

The EU is aiming to increase the number of green hubs in Europe, which is the main focus of the EU-funded project Hubs4Circularity.

“The project intends to establish a network for the sharing of knowledge and resources, enabling participating actors to design their processes and arrive in a position where they can implement new technologies as a basis for creating new, circular value chains,” says Heyn.

He says the solution that works best for a given hub will vary from community to community and each region will needs its own unique system for exchanging energy and material flows. Hubs4Circularity will help participants learn from other regions as a basis for identifying the solutions that suit them best.

“Regardless of the solutions that individual regions arrive at, we want activities to revolve around the process industries. In this way, the waste or secondary materials that they generate can be 'decontaminated’, enabling them to be reused in the manufacture of new products by other industries,” he explains.

By making knowledge, experience and best practice accessible via the digital platform, the project researchers hope that as many industries and regions as possible will be in a position to participate in green hubs and embrace circularity.

“The knowledge and experience sharing aspects of the project are being designed as we speak, so we’re encouraging all industries, clusters, municipal and county administrations to register on the project’s website," says Heyn.