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 NOx− (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. Tertiary treatment in wastewater treatment plants serves as the final barrier against the discharge of nutrients into natural waters but requires large inputs of chemical agents. An elemental sulfur–siderite composite filler demonstrates efficient and sustainable denitrification and dephosphorization, even at ultra-low temperatures.