Spatio-temporal distribution and comprehensive risk assessment of heavy metals in sewage sludge from wastewater treatment plants in the Dianchi Lake basin

Heavy metals (HMs) in sewage sludge from municipal wastewater treatment plants (WWTPs) pose a latent threat to lake ecosystems. Accurately characterizing their pollution profiles and associated risks is crucial for ecological conservation in cities within globally sensitive lake basins. Focusing on 27 WWTPs across the Dianchi Lake Basin, this study collected 335 sludge samples over 12 consecutive quarters. Leveraging multi-dimensional comprehensive assessment approaches, we analyzed the pollution characteristics, risk levels, and sources of HMs to provide scientific evidence and a scalable framework for safe sewage sludge management in plateau lake basins. Key findings: ① Arsenic (As) was the dominant pollutant with a high detection rate, with 20 exceedance events in 8 WWTPs; Cadmium (Cd) concentrations ranged from 19 to 259 mg/kg, showing higher pollution risk in the dry season and a spatial pattern of “higher in the northeast and lower in the southwest”. ② Cd and mercury (Hg) accounted for 55.4% and 34.6% of the total ecological risk, respectively, being the core risk-driven pollutants. ③ Metal processing and smelting industries were the primary source (41.8% of total load). ④ Membrane bioreactor (MBR) and intermittent cyclic extended aeration system (ICEAS) led to the highest HMs accumulation, with comprehensive pollution indices of 0.280 and 0.250. These findings clarify the key HM pollutants and their risk drivers in the basin, providing targeted support for optimizing sludge management strategies in plateau lake ecosystems.