Industrial Symbiosis and Waste Management Under a Circular Economy Management Standard in the Water Sector

Industrial symbiosis is one of the main routes for turning waste streams into useful materials and energy within a circular economy. Little empirical evidence exists, however, on how symbiosis projects are managed within a circular economy project management standard. This exploratory single-case study examines the motivations, implementation, and outcomes of symbiosis initiatives in the water sector, developed by an organisation certified under the XP X30-901 standard and covering treated water, biomethane, thermal energy, and fertiliser recovery. An externally assured indicator series for 2016 to 2025 compares the years before the portfolio was formalised in 2019 with those after certification in 2021. Sludge sent to landfill fell from 16.4% of the total managed in 2016 to zero from 2022, the share sent to agriculture and fertiliser production rose from 46.0% to 64.1% between 2019 and 2023, and the average transfer cost fell from EUR 134.34 to EUR 107.15 per tonne. The standard supported the identification, prioritisation, and monitoring of circular projects, although these outcomes cannot be attributed to it alone. Indicators exposed to hydrological, market, and regulatory conditions proved less stable than those that the organisation controls directly, suggesting that standardisation consolidates controllable outcomes rather than improving overall performance.