Empowering the role of sewage sludge in circular economy: Struvite and biochar production

Balancing environmental concerns and resource recovery, this study explores an innovative approach to sewage sludge valorization in the context of a circular economy. Sewage sludge, a byproduct of wastewater treatment, presents significant disposal challenges due to its pollutant content. However, it also contains valuable nutrients and carbon that can be harnessed. In this work, we focus on fully recycling sewage sludge, ensuring that no residues or additional wastes are generated. Our primary objective is to maximize the recovery of phosphorus from sewage sludge and find practical uses for the remaining solid residue. During the pretreatment stage, we experimented with various acidic and alkaline leaching agents to determine the most efficient one for phosphorus removal. Among the agents tested (including sulfuric, nitric, hydrochloric, oxalic, and citric acids, as well as sodium and potassium hydroxides) sulfuric acid emerged as the most effective, achieving an impressive 89% phosphorus removal efficiency. Following this, the liquid fraction containing dissolved phosphorus was separated and processed, resulting in an 84% phosphorus recovery in the form of struvite. The solid residue from the separation process was then pyrolyzed to produce sludge-derived biochar, which showed potential for supercapacitor applications. This biochar exhibited a specific surface area of 208 m² g^-1 and a specific capacitance of 53F g^-1 in 1 M H₂SO₄. Overall, this study demonstrates a comprehensive and sustainable method for transforming sewage sludge from a waste product into valuable resources, contributing to global efforts in waste reduction and resource recovery.