Feedstock-Adaptive Valorisation of Agro-Industrial Organic Waste: Pretreatment, Biological Conversion, and Resource Recovery

Agro-industrial organic wastes differ markedly in moisture, structure, biochemical composition, nutrient value, and contaminant risk, so pretreatment should be selected from the limiting feedstock constraint rather than from a generic technology hierarchy. This critical narrative review examines feedstock-adaptive valorisation routes centred on anaerobic digestion and subsequent recovery of energy, nutrients, carbon-rich fractions, fertilisers, humic-like materials, and plant-biostimulant products. Unlike technology-centred syntheses, the review uses the dominant feedstock constraint as a common comparison axis across pretreatment, biological stability, and product qualification. Mechanical, mechanochemical, cavitation-based, thermal, chemical, and biological treatments are compared in terms of the barrier they remove, their energy and material burden, downstream biological response, and recovered-product quality. The evidence shows that particle-size reduction, solubilisation, or methane yield alone is insufficient for process selection. A minimum-sufficient-pretreatment heuristic is therefore proposed: treatment intensity is increased only until a predeclared conversion or product-quality objective is reached while stability, safety, and positive incremental net benefit are retained. Zero pretreatment remains a valid outcome when concentration, co-digestion, feeding control, biomass retention, or another intervention better addresses the dominant constraint. The framework also highlights the need for harmonised energy reporting, mass balance, continuous validation, and product-specific qualification before laboratory improvements are translated to industrial operation.