Elemental sulfur–siderite composite filler empowers sustainable tertiary treatment of municipal wastewater even at an ultra-low temperature of 7.3 °C

Tertiary treatment, the ‘polisher’ for wastewater nutrients, has assumed an increasingly greater role in municipal wastewater treatment plants, particularly given the growing demands for wastewater treatment worldwide and more stringent discharge standards. However, most municipal wastewater treatment plants in service use first-generation tertiary treatment processes (for example, additional carbon source-dependent denitrification and chemical dephosphorization), raising significant sustainability concerns. For effective, yet sustainable nutrient polishing, we develop an elemental sulfur (S⁰)–siderite composite filler (S⁰SCF) using a melting–embedding strategy based on the liquid immersion granulation technique. As a prerequisite for engineering use, S⁰SCF overcomes the poor mechanical properties and safety concerns plaguing traditional S⁰-based reactive fillers. S⁰SCF inherits efficient S⁰-driven autotrophic denitrification and acquires an effective dephosphorization capability, with the dephosphorization mechanism linked to S⁰-driven autotrophic denitrification-induced Fe²⁺ leaching from siderite and subsequent Fe²⁺–PO₄³⁻ coprecipitation. During ultra-low temperature tests (7.3 ± 0.3 °C), the S⁰SCF-packed bed bioreactor demonstrated robust removal rates for NO_x⁻ (NO₃⁻ and NO₂⁻) (0.29 ± 0.02 kg N m⁻³ per day) and PO₄³⁻ (0.014 ± 0.004 kg P m⁻³ per day), with removal efficiencies reaching 91.2 ± 3.2% and 81.4 ± 7.8%, respectively. Meanwhile, the low levels of nitrous oxide emissions and free sulfide generation further highlight the sustainability implications of S⁰SCF-based nutrient polishing. This work sheds fresh light on developing low-carbon and eco-friendly tertiary treatment processes, taking a necessary step towards addressing the sustainability crisis in the wastewater treatment sector.