Human mobility acts as a lever for decarbonising wastewater treatment plants.

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.