Waste Valorisation and Contribution to the Circular Economy: The Evaluation of Water Treatment Sludge as a Phosphorus Adsorption Material and its Potential for Agricultural Reuse

Aluminium-based coagulants are extensively used in water treatment, producing significant quantities of waste sludge (WTS), that poses challenges for landfill disposal. Eutrophication, mainly driven by phosphorus (P) enrichment from wastewater, remains a critical environmental concern in aquatic ecosystems. WTS contains substantial amounts of aluminium (Al), which exhibits a high affinity for phosphate. This study aimed to assess the phosphorus adsorption capacity (qmax) of WTS and its potential for agricultural use. The WTS samples were characterized using various analytical techniques. Kinetic and isothermal experiments were conducted using dried (105 °C) and calcined (650 °C) WTS samples. Characterization revealed crystallinity differences between dried WTS and calcined WTS Kinetic tests, indicating equilibrium times of 16 h for dried WTS and 1 h for calcined WTS. Isothermal tests showed maximum adsorption capacity values at pH 7 of 13.81 mgP g⁻¹ for dried WTS and 52.03 mgP g⁻¹ for calcined WTS, highlighting the enhanced phosphorus removal efficiency of calcined WTS. Phytotoxicity assessments demonstrated that dried and calcined WTS promoted enhancements in germination and root elongation of Lactuca sativa of 20 and 10%, respectively, suggesting its potential agricultural benefit. Therefore, WTS demonstrates promise for tertiary wastewater treatment, contributing to a circular economy by potentially reclaiming phosphorus-rich compounds for agricultural reuse.