Valorization of Sewage Sludge via Co-Composting with Iron Ore Tailing Enhanced Formation of Aggregated Soil-Like Compost

Co-composting sewage sludge with industrial mineral residues offers a promising strategy for integrated waste valorization and the development of functional soil-like materials. In this study, two representative inorganic wastes─iron (Fe) ore tailings and fly ash, were individually incorporated into a sludge composting system to evaluate their effects on soilification performance and compost quality. The results showed that Fe ore tailing addition enhanced compost soilification, increasing the level of composting-induced aggregation by 8.1%. However, fly ash initially increased particle size by 10.5% before composting but ultimately hindered composting-based soilification by suppressing microbial richness and organic matter (OM) transformation. High-resolution mass-spectrometry-based molecular analysis revealed that Fe ore tailings promoted the removal of amino groups through deamination reactions, accelerating the breakdown of sludge-derived proteins. During the cooling stage, dealkylation and decarboxylation processes became dominant, generating lipid- and protein-like small molecules. These compounds subsequently formed stable mineral-binding organic matter (MOM), facilitating the formation of aggregated, soil-like structures. Microbe–dissolved organic matter (DOM) network analysis further confirmed that Fe ore tailing addition enhanced microbial-mediated conversion of key molecular components, with Pseudoxanthomonas dominating the thermophilic phase and Aminobacter enriched during the cooling stage. Finally, compost quality assessments indicated that Fe ore tailing addition enhanced the germination index (GI) and nitrogen retention capability without leading to heavy metal accumulation. This study demonstrates the feasibility of co-utilizing iron ore tailings and sewage sludge to produce soil-like bioresources, advancing sustainable waste co-treatment and land application.