Water circularity index: A novel approach for authorities and operators

The "transition to a circular economy" is frequently articulated in conventional frameworks regarding water use efficiency, predominantly emphasizing quantitative dimensions. However, water is ubiquitous, and its various applications are often directly or indirectly interlinked. Examining water quantity and quality within the water usage cycle is imperative to ensure security and optimise added value throughout the process while promoting the natural values of the surrounding environment. The present study introduces an innovative metric known as the Water Circularity Index (I) that encompasses the complex balance between water quantity and quality throughout the water cycle at urban and industrial installation levels. The methodology for developing the I is supported by a Multi-Criteria Decision Analysis, where key factors are divided into sub-factors categorised and weighted. The I comprises eleven key factors, including freshwater consumption, wastewater discharge, water reuse, best management practices and technologies, hazardous substances defined under the Water Framework Directive, microplastics and emerging contaminants, biodiversity, nutrient recovery, internal industrial symbiosis, sludge management, and voluntary or incentive-based instruments. For each key factor, inputs are classified as negative, neutral, or positive, with final results categorised into five levels: negative circularity, no circularity, and low, medium, or high circularity. The index was applied to facilities across seven countries, encompassing fourteen installations across various industrial sectors and urban wastewater treatment plants, showing the versatility of the index in promoting best practices in multiple processes. In one case study involving a pulp mill, the I was computed before and after the revision of its environmental permit. This assessment facilitated an evaluation of the measures implemented during the transition from a discharge permit aligned exclusively with the Industrial Emissions Directive principles to an integrated water management approach that integrated both directives' frameworks. The findings revealed a substantial improvement in the plant's performance, progressing from negative to medium circularity.