Water-energy-carbon nexus and de-carbonation pathways in integrated urban water system for a megacity study

The holistic urban water cycle represents a highly energy-intensive system, posing a significant challenge to the achievement of the United Nations Sustainable Development Goals. Despite its critical importance for urban sustainability and carbon neutrality, a comprehensive analysis of greenhouse gas (GHG) emissions, energy flows, socio-economic drivers, and viable pathways to de-carbonization remains lacking for the integrated urban water system. This study addresses these research gaps by developing a holistic model to analyze the current performance and project future de-carbonization pathways for the megacity of Shenzhen. The model encompasses the entire urban water cycle, including raw water abstraction, reservoir allocation, water treatment and distribution, wastewater collection and treatment, sludge processing, and final effluent discharge. Our findings indicate that in the base year 2021, the system consumed 2.09 × 10⁹ kW·h of energy and generated 1.88 × 10⁹ kg CO₂-eq GHG emissions. The freshwater subsystem was the dominant contributor, accounting for 62.4-68.1 % of total energy use and 51.7-59.9 % of total GHG emissions. A stochastic analysis model was used to identify the key socioeconomic drivers of GHG emissions. Using this model, four distinct de-carbonization pathways were proposed. Among these, an enhanced low-carbon scenario emerged as the most favorable, offering optimal environmental and economic benefits. This study provides actionable insights to support policymakers in designing more sustainable and climate-resilient urban water systems for Shenzhen and other global megacities.