Sewage sludge-derived biochar as a circular “waste-to-resource” strategy for wastewater treatment

The increasing production of wastewater and sewage sludge (SS) is a major environmental challenge of the 21^st century, while the need for sustainable waste management and resource recovery drives the development of innovative technologies for sludge utilization. The thermochemical conversion of SS through pyrolysis to biochar (BC) is a promising “waste-to-resource” strategy, as it allows both the reduction of sludge volume and the production of a functional, value-added material.

This study aims to examine sewage sludge-derived biochar (BCxSS), focusing on its characterization methods, the effect of pyrolysis conditions on its physicochemical properties, and its practical applications in water and wastewater treatment. By applying a structured PRISMA-based methodology, 170 studies concerning the production, modification, and environmental utilization of BCxSS were studied. The results showed that pyrolysis conditions, and particularly pyrolysis temperature, have a major influence on the properties of the BC, such as yield, ash content, pH, specific surface area, porous structure, and surface functional groups. Furthermore, BCxSS effectively removes heavy metals, dyes, phenolic compounds, and emerging organic micropollutants, such as pharmaceuticals and antibiotics. These removals occur through mechanisms such as physical adsorption, ion exchange, surface complexation, and π-π interactions. BCxSS is also attracting attention as a precursor for catalysts capable of degrading persistent pollutants. Despite these advances, the application of BCxSS for the adsorption and inactivation of pathogenic microorganisms and antibiotic resistance genes remains limited, revealing a critical research gap. Understanding BC-microorganism interactions is vital, given the significant public and environmental health risks posed by waterborne pathogens.

Overall, BCxSS provides a tangible example of circular economy in practice, transforming wastewater treatment byproducts into valuable resources that reduce waste and mitigate pollution.