Wastewater treatment is a substantial and growing source of global greenhouse gas emissions as urban infrastructure expands to meet rising demand and stricter standards. In rapidly urbanising nations such as China, where treatment capacity has more than doubled in the past decade, operational inefficiencies caused by mismatched influent flows and fixed plant capacity represent a major but underexploited decarbonisation opportunity. Conventional load rate metrics systematically overestimate true alignment between wastewater generation and treatment capacity, masking the spatiotemporal mismatches created by intense human mobility. Here we demonstrate that human mobility acts as a powerful yet previously overlooked lever for decarbonising wastewater treatment plants. Using more than 170 million high-resolution population heatmap records from Shenzhen, a megacity in southern China with a highly mobile population exceeding 17 million, we show that Spring Festival outflows reduce citywide population presence by 64%, lowering the treatment and inflow dynamic equilibrium (TIDE) index by 23% and raising greenhouse gas emission intensity by 28%, whereas intercity commuting improves alignment and cuts emission intensity by 27%. Two mobility-informed operational strategies—gradient shutdown during holidays and flow equalisation under routine conditions—reduce plant-level emissions by up to 18% and offer an average 22% improvement in TIDE across Chinese cities. These findings establish human mobility as a scalable, low-capital lever for aligning wastewater infrastructure with real behavioural dynamics and advancing climate mitigation goals.

Legacy phosphorus, accumulated from past anthropogenic activities, poses persistent and complex threats to global water quality. Despite extensive efforts to control phosphorus inputs, legacy phosphorus can persist for decades and undermine restoration goals. Emerging evidence suggests that shifts in regional development patterns profoundly reshape the dynamics and environmental risks of legacy phosphorus accumulation and mobilization. However, the mechanisms by which development pattern shifts reshape legacy phosphorus trajectories remain poorly understood. Here we show the complex pathways linking development-driven land-use changes, biogeochemical buffering capacities, and legacy phosphorus mobilization through an integrative modeling framework that couples developmental shift coefficients, anthropogenic phosphorus inventories, and riverine time-lag modeling to diagnose and predict long-term legacy phosphorus risks. Using the Songhua River as a case study, our results reveal that shifts from industrial to agricultural dominance significantly amplify legacy phosphorus accumulation by 86 times. Consequently, legacy phosphorus accounts for 65.4%–69.9%, surpassing current-year inputs and becoming the primary driver of riverine pollution. Furthermore, we demonstrate that development shifts systematically alter the dominant controlling factors, from fossil fuel emissions and drainage infrastructure to soil retention characteristics and agricultural practices, reshaping mitigation priorities. Our framework provides a generalizable methodology for quantifying legacy phosphorus risks under dynamic development patterns, offering immediate applications for water quality management. More broadly, this framework offers critical insights that can guide sustainable management strategies for linking evolving regional development patterns with long-term ecological restoration.