Carbon footprint analysis and energy efficiency of urban wastewater treatment plants in the context of a low-carbon city: A case study of Kish Island

Municipal wastewater treatment plants (WWTPs), while essential for safeguarding public health and water resources, are among the most energy-intensive urban infrastructures, particularly due to the activated sludge process. This study analyzes the energy efficiency and carbon footprint of the Kish Island WWTP to evaluate its alignment with the objectives of a low-carbon city and the three dimensions of sustainable development. Employing a mixed-methods approach and the DPSIR (Driving forces–Pressures–State–Impact–Response) analytical framework, one year of operational data (2025–2026) and in-depth interviews with 12 experts were systematically analyzed.Findings reveal that the plant’s specific energy consumption (SEC) stands at 0/737 kWh/m³, significantly exceeding the global benchmark of 0/3–0/6 kWh/m³, thereby classifying its performance as “requiring urgent improvement.” Aeration in the activated sludge system accounts for 51/7% of total energy use, confirming its status as the primary energy consumer. The annual carbon footprint is estimated between 1,461 and 1,848 metric tons of CO₂-equivalent, with the higher value derived from Iran’s national emission factor (0/687 kg CO₂/kWh)—26% above the IPCC global average (0/543 kg CO₂/kWh).A three-dimensional sustainability assessment further indicates a critical environmental condition (1/5), economic weakness (2/8), and moderate social performance (3/2). Qualitative analysis identifies key systemic barriers, including the agricultural electricity tariff (which undermines economic incentives for conservation), institutional fragmentation, absence of a dedicated energy management unit, and lack of legal mandates for transparent carbon reporting.In response, a four-dimensional management model—integrating technological (e.g., DO-based aeration control, VFDs, biogas recovery, and photovoltaics), institutional (e.g., ISO 50001 implementation), policy (e.g., tariff reform and carbon disclosure mandates), and behavioral (e.g., energy-awareness campaigns) components—is proposed within the DPSIR framework. Cost–benefit analysis demonstrates that implementing this model could yield annual net savings and revenue of 253/9 million Iranian Rials, with an internal rate of return (IRR) of 28/7%.The study concludes that the sustainable transformation of WWTPs cannot be achieved through technical interventions alone; rather, it requires a systemic, multi-dimensional, and simultaneous strategy across technical, institutional, policy, and social domains. The proposed model is not only context-sensitive to Kish Island’s unique challenges—including sanctions, aging infrastructure, and seasonal tourism—but also offers a transferable framework for other coastal and island cities in Iran and similar developing contexts pursuing low-carbon urban development.