## 3 Resource Potential--Wastewater  
Recovering resources throughout the water cycle is crucial for sustainability, reducing costs, energy consumption, and emissions. Technological advances make it possible to recover nutrients, such as nitrogen and phosphorus, and to produce biogas from by-products of wastewater treatment and organic solid waste, with examples at the local level [54,55] and in centralised systems [15,55]. Figure 2 synthesis this transition, demonstrating how wastewater evolves from being a simple waste or cost to becoming a valuable resource, highlighting the increasing potential for recovery across the reuse value chain. The advantages include energy recovery since wastewater contains chemical, thermal, and hydraulic energy. The chemical energy in domestic wastewater, which comes mainly from organic matter, is estimated at 1.5 kWh/m\({}^{3}\), with anaerobic digestion being the most widely used technology for converting this organic matter into biogas. On the other hand, thermal energy, although less exploited, is significant, with the potential to extract approximately 5.8 kWh/m\({}^{3}\) by reducing the temperature of wastewater by 5 \({}^{\circ}\)C [56]. Another advantage of utilising wastewater in the urban water cycle is the recovery of organic matter and nutrients such as nitrogen. The direct use of NH\({}_{4}\)+ from wastewater in the production of protein for animal feed can have a significant impact on reducing carbon emissions since producing one kilogram of nitrogen as fertiliser requires two litres of fossil fuel [56]. Projects such as \"Power to Protein\" ([[https://www.powertorprotein.eu/about/](https://www.powertorprotein.eu/about/)]([https://www.powertorprotein.eu/about/](https://www.powertorprotein.eu/about/)) (accessed on 11 April 2024)) are committed to developing processes to produce protein from the ammonia that comes from anaerobic digesters, forming part of the circular economy and focusing on recovering and valorising ammonium from the wastewater cycle. Phosphorus, although not a renewable resource, can be recovered from wastewater and sludge, making it a viable alternative for fertilisation. It is estimated that 22% of global phosphorus needs can be met by recycling domestic wastewater [56; 57]. Urine, which accounts for approximately half of the phosphorus load in domestic wastewater, represents a highly concentrated nutrient stream that can be efficiently recovered through source separation technologies [55]. With the average adult excreting approximately 1 g of phosphorus per day through urine [57], these systems offer a sustainable and locally adaptable solution for nutrient recovery in urban environments. Recovered phosphorus can be used in urban agriculture, fertilising public gardens and green roofs. This practice not only contributes to food security by providing an essential fertiliser but also reinforces the fundamental link of the water-nutrient nexus, essential for a sustainable circular economy and the future of cities. Ensuring adequate supplies of food, energy, and water faces significant challenges globally. Instead, local, adapted, and context-specific approaches are needed. Integrating these processes is key to promoting environmental and economic sustainability, as well as improving the quality of urban life. The relationship between water, energy, and nutrients plays an essential role in the sustainable development of the built environment. In urban environments, this integration should be seen as local resource management. Urban systems, including infrastructures and buildings, must adopt models that facilitate integrated management of energy and locally produced or recovered resources. Figure 2: Wastewater as a resource in the urban water cycle (source: authors). Progress in the potential to recover value from wastewater and biosolids in the urban water cycle.