Urban wastewater treatment plants as resource hubs: evaluating circularity and sustainability of nutrient recovery and water reuse.

Wastewater treatment plants (WWTPs) are evolving from pollution control facilities into resource recovery hubs, aligning with circular economy principles. However, assessing their transition requires robust methodologies that integrate environmental, economic, and circularity dimensions. This study aims to evaluate the sustainability and circularity performance of a WWTP in Cyprus upgraded with additional treatment stages to recover nutrient-rich sludge for fertilizer and treated water for irrigation. To achieve this, a combined methodological framework was applied, incorporating standardized Circularity Assessment (based on ISO 59,004), Life Cycle Assessment (LCA), and Life Cycle Costing (LCC). Circularity was assessed using a dual-indicator approach: "resource flow indicators" (covering material, energy, and economic flows) and "circular actions indicators" (capturing process optimization, repurposing, cascading, and regeneration). Environmental and economic impacts were quantified using LCA and LCC, respectively, while sensitivity analyses explored the effects of renewable energy integration and process simplification. Results show that while the upgraded WWTP enhances circularity through increased resource recovery and high-value product generation, it also incurs higher environmental impacts and operational costs due to energy-intensive technologies (distillation, reverse osmosis, and nanofiltration), increasing the impacts in values that can achieve +60 % in some impact categories. Sensitivity scenarios demonstrated that reducing energy demand and increasing renewable energy use can improve sustainability outcomes, with the higher average impact reduction achieving -20 %, though often at the expense of circularity performance. The study highlights the trade-offs between maximizing resource recovery and minimizing environmental and economic burdens, emphasizing the need for optimized process design and supportive policy frameworks to enable WWTPs to function as effective circular economy enablers. The findings show that advanced treatment technologies enhance circularity through increased resource recovery, but also entail elevated energy demands and limited economic viability. While frameworks like ISO 59,004 support multidimensional assessment, further methodological development is needed to guide sustainable decision-making.