Waste sorting policy impact on mercury emissions from municipal solid waste incineration: Evidence from China

Source separation of municipal solid waste (MSW) is advocated as a sustainable strategy to mitigate pollution from waste-to-energy facilities, yet quantitative, field-based evidence of its impact on mercury (Hg) emissions remains scarce. Here, we investigate the effects of Beijing's city-wide mandatory waste sorting policy, implemented in 2020, by conducting a comparative study at a representative incinerator before (2019) and after (2021) its enactment. The policy drove a profound 67.7% reduction in the Hg input load to the incinerator, primarily by diverting high-Hg components from the waste stream. This source-level change cascaded through the system, resulting in a dramatic 82% decrease in total Hg emissions from the stack (from 1.35 ± 0.6 to 0.24 ± 0.05 μg/m³). Counterintuitively, the relative abundance of oxidized mercury (Hg² ⁺) in the final flue gas dropped from 45.5% to 28%. We attribute this to a powerful synergistic mechanism: increased chlorine from a higher proportion of plastics in the sorted waste enhanced the in-furnace and catalytic oxidation of elemental mercury (Hg⁰), which was then captured with exceptional efficiency by the plant's advanced air pollution control system. This work provides the first field-scale validation that front-end policy intervention is a highly effective tool for controlling industrial Hg emissions, demonstrating a direct link between public policy and the operational performance of pollution control technologies.Implications: Controlling mercury emissions from municipal solid waste incineration is a significant global environmental challenge, often reliant on expensive end-of-pipe technologies. Our study provides crucial mechanistic evidence demonstrating that waste classification policy is not merely a waste management strategy, but a direct and effective chemical pre-treatment for pollution control.We reveal that by altering waste composition at the source, specifically the chlorine content, mandatory waste sorting fundamentally shifts mercury's chemical transformation pathways during incineration. This process promotes the oxidation of difficult-to-capture elemental mercury (Hg⁰) into water-soluble divalent mercury (Hg² ⁺), which is much more easily removed by existing air pollution control systems.This finding establishes a clear synergistic link between environmental policy and engineering reality: policy can chemically enhance the effectiveness of pollution control technology. Our mercury mass balance and Sankey diagram provide a quantitative model that validates this synergy.Consequently, this research offers a compelling, scientifically-backed rationale for policymakers to implement and enforce stringent waste classification programs as a direct, proactive, and cost-effective strategy for mercury emission reduction. It bridges the gap between policy goals and chemical processes, offering a new paradigm for integrated waste management that simultaneously advances sustainability and protects public health.