Quantitative correlation between DNSH Criteria and LCA Indicators: methodological proposal and case studies validation in the wastewater sector

Technical Summary: Quantitative Correlation between DNSH Criteria and LCA Indicators in the Wastewater Sector

Problem Statement
Wastewater treatment plants (WWTPs) are essential for public health and environmental quality but are significant sources of greenhouse gas emissions and other environmental impacts. While the European Union Taxonomy requires activities to comply with six "Do No Significant Harm" (DNSH) criteria, there is currently no normalized quantitative approach to evaluate environmental impacts across all these criteria simultaneously. Existing assessments often focus narrowly on carbon footprints, failing to capture the full spectrum of sustainability requirements mandated by the EU Taxonomy and the Corporate Sustainability Reporting Directive (CSRD). Furthermore, the new Urban Wastewater Treatment Directive (EU) 2024/3019 emphasizes energy neutrality and circular resource use, necessitating a robust framework to evaluate how specific technological interventions affect compliance with DNSH principles.

Methodology
The study employs a Life Cycle Assessment (LCA) methodology following ISO 14044 standards to evaluate three WWTPs in Northern Italy, serving populations between 35,000 and 50,000 population equivalents (PE). The functional unit is defined as 1 m³ of treated wastewater. The analysis covers two distinct phases:

1. Realization Phase: Assessing the environmental impacts of the construction and implementation of interventions (cradle-to-gate for materials).
2. Operational Phase: Comparing pre- and post-intervention performance (cradle-to-grave).

The study utilizes the ReCiPe 2008 and ReCiPe 2016 (Hierarchical perspective) methodologies to quantify 16 environmental impact categories. These results are normalized to person-equivalents (PE) per year and correlated with the six DNSH criteria to create a quantitative mapping between LCA indicators and regulatory requirements.

Three specific intervention strategies were analyzed:

• WWTP1 (Centralization): Decommissioning of two smaller plants and redirection of influent flows to a single upgraded facility via new pipelines.
• WWTP2 (Energy Optimization): Replacement of electromechanical equipment with high-efficiency alternatives and the introduction of an incineration system with energy recovery.
• WWTP3 (Sludge Management): Implementation of a bio-drying system for sludge processing.

For WWTP3, a specific greenhouse gas sampling campaign was conducted using an Infrared Photoacoustic Detector (IPD) to measure CO₂, N₂O, and CH₄ emissions at the bio-dryer, scrubber, and biofilter stages to derive real emission factors. Additionally, an Environmental Payback Period (EPP) analysis was conducted for WWTP1 and WWTP2 to determine the time required for operational savings to offset the environmental burdens of the realization phase.

Key Results

• DNSH Correlation and Impact Reduction:

  • WWTP1 (Centralization): Demonstrated the most consistent environmental benefits. Climate Change (DNSH 1) impacts decreased by 43%, Fossil Depletion (DNSH 4) by 36%, and Pollution Prevention (DNSH 5) by 24%. The centralization strategy effectively leveraged economies of scale, reducing energy demand, chemical usage, and sludge transport.
  • WWTP2 (Energy Optimization): Showed moderate improvements. Climate Change impacts decreased by 18%, and Fossil Depletion by 11%. However, impacts on toxicity categories were negligible or slightly increased due to the new incineration process.
  • WWTP3 (Bio-drying): Exhibited a heterogeneous and largely negative environmental response. Climate Change impacts increased by 50%, and Fossil Depletion increased by 58%. While sludge transport was reduced, the energy demand and GHG emissions from the bio-drying process (particularly N₂O) created significant new burdens, negatively affecting DNSH 1 and DNSH 4.

• Realization Phase Impacts:

  • For WWTP1, the realization phase was dominated by Fossil Depletion (1.30E-03 PE m⁻³), driven primarily by fossil-based materials for pipeline installation (asphalt, backfilling, piping).
  • For WWTP2, the realization phase was also dominated by Fossil Depletion (5.83E-04 PE m⁻³), but driven by electrical infrastructure, specifically cables (6.28E-04 PE m⁻³) and transformers.

• Environmental Payback Period (EPP):

  • WWTP1: The climate change indicator was recovered in 1.23 years. Fossil depletion required 3.43 years. The longest payback periods were for Human Toxicity (11.72 years) and Metal Depletion (10.47 years). All indicators recovered within the 50-year service life of the infrastructure.
  • WWTP2: Climate change was recovered rapidly (0.37 years). Fossil depletion required 13.92 years. Human Toxicity and Freshwater Ecotoxicity did not show improvement due to increased sludge transport associated with incineration. Most indicators recovered within the 20-year service life of electrical devices.

Significance and Claims
The paper claims to provide a methodological framework for quantitatively linking LCA indicators to the EU Taxonomy's DNSH criteria, enabling a more comprehensive assessment of sustainability in the wastewater sector. The study highlights that intervention typology is the primary determinant of environmental outcomes and DNSH alignment.

• Flow Centralization is identified as the most effective strategy for reducing overall environmental impacts and improving compliance with multiple DNSH criteria.
• Energy Optimization offers measurable benefits, particularly for DNSH 4, without introducing significant new burdens, though its impact is less pronounced than centralization.
• Sludge Bio-drying is presented as a solution requiring careful context-specific assessment, as it can shift impacts from one category to another (e.g., from eutrophication to stratospheric ozone depletion) and may increase overall emissions despite operational efficiencies.

The authors conclude that integrating LCA with DNSH criteria provides a robust validation framework for developing taxonomy-aligned sustainability assessments, moving beyond simple carbon footprinting to address the full spectrum of environmental regulatory requirements.