From hazardous waste to valuable resource: Trends in sewage sludge composition and reuse challenges at Croatia’s largest wastewater treatment plant

• Examined sewage sludge (SS) meets criteria for energy recovery and agricultural reuse. • Pollutants in SS comply with international standards, showing stable or declining trends. • Metal concentrations decrease in order: Zn > Cu > Pb > Cr > Ni > Co > Mo > As > Hg > Cd. • Organic micropollutants decline in order: PCBs > PAHs > OCPs. • SS reuse is limited by regulations, land constraints, and lack of energy recovery infrastructure. Sewage sludge (SS), a by-product of wastewater treatment plant (WWTP), requires diligent management to minimize environmental risks and protect public health; however, properly treated SS becomes a renewable resource, providing: i) a high-calorific fuel for energy, ii) essential nutrients for crops, and iii) rare metal(oid)s for various industries; thus contributing to a circular economy. In this study we assessed long-term monitoring data on the physicochemical properties of anaerobically digested SS from Croatia’s largest national WWTP in Zagreb. The key characteristics (heating value, dry matter, N, P, K, C content) of the examined SS meet the criteria for energy recovery and reuse in agriculture. Moreover, the most critical parameters; organic micro-pollutants (polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and organochlorine pesticides) and heavy metal(oid)s (Cd, Cr, Cu, Hg, Ni, Pb, Zn, Mo, As, and Co) were found to be within acceptable limits and compliant with the most stringent inter/national standards for the use of SS in agriculture, composting, organic fertilizers or soil amendments. Most of these indicators demonstrated stable or decreasing trends over time. However, the practical reuse of SS for agricultural application or energy recovery remains negligible at regional/national levels. This underutilization is largely driven by stringent regulatory requirements, limited infrastructure for thermal conversion, the scarcity of suitable agricultural land near WWTPs, and the prevalence of land-use restrictions. Our findings provide critical insights into SS quality trends, supporting evidence-based, region-specific strategies for its safe and sustainable reuse aligned with circular economy objectives.