Characterization of sewage screenings from wastewater treatment plants and upgrading via torrefaction

Sewage screenings generated during wastewater pretreatment are typically disposed of through landfilling or incineration, limiting opportunities for resource recovery. This study presents a multi-season characterization of sewage screenings collected from two wastewater treatment plants and evaluates their potential for upgrading through torrefaction. The screenings were dominated by biomass-derived fractions (>90 wt% on a dry basis), while plastics accounted for a minor but variable fraction (<10 wt% on a dry basis). Radiocarbon analysis showed that biogenic carbon exceeded 60% across all samples, indicating that the screenings were a mixed-origin material containing both biogenic and fossil-derived carbon. Seasonal variation exerted a greater influence than site differences, particularly on compositional distribution. Torrefaction at 300 °C increased carbon content, reduced O/C and H/C ratios, and improved the heating value to 25-27 MJ/kg (approximately 25-30% higher than raw screenings). Thermogravimetric analysis further indicated reduced low-temperature reactivity and a shift in decomposition behavior toward higher temperatures. These findings indicate that torrefaction enhances carbon densification, producing a material with improved properties for solid-fuel applications, highlighting the potential of sewage screenings as a recoverable resource within wastewater management systems. However, the findings represent an initial assessment under a single operating condition, and further evaluation of environmental performance, contaminant behavior, and process scalability is required.