European Journal of Nutrition & Food Safety

Dairy and food processing generate large, continuous streams of residues, including wastewater treatment sludges, whey, peels, shells, bones and spent coffee grounds, whose composition differs markedly from the wood and straw feedstocks that dominate biochar research. Converting these residues into biochar or hydrochar has been proposed both for wastewater treatment and as fillers or active components in bio-based food packaging, but the two literatures have developed separately and have rarely been appraised critically. This critical narrative review evaluates whether residue-derived chars have a defensible role in either application and under what conditions the applications can be integrated. Literature published from 1 January 2010 to 27 July 2026 was identified through structured searches of Crossref, Europe PMC, PubMed, OpenAIRE, the Directory of Open Access Journals, Semantic Scholar and Google Scholar, supplemented by citation tracking and official regulatory sources, and was appraised for test realism, design and transferability. The inorganic fraction of the feedstock emerged as the principal determinant of performance and risk. Calcium- and magnesium-rich chars from dairy sludge, eggshell, bone and shell remove phosphate, fluoride and metals mainly by precipitation, and activated shell- and scale-derived carbons adsorb antibiotics strongly. However, most capacities come from batch tests in synthetic solutions at high concentrations, real-effluent and continuous-flow studies are few, and competing ions, dissolved organic matter and mineral release reduce effectiveness. Phosphorus-laden dairy-derived products often show low plant availability. In packaging, low biochar loadings can stiffen polymers, nucleate crystallisation and delay photo-oxidation, whereas antimicrobial effects usually derive from co-incorporated silver, zinc oxide or essential oils. Biochar consistently accelerates degradation of poly(lactic acid) and poly(3-hydroxybutyrate), barrier and ethylene-scavenging evidence is limited, and no study reported migration testing. A conceptual framework is proposed that routes mineral-rich residues to nutrient recovery and low-ash residues to packaging, subject to a food-grade benchmark, and permits movement from food-contact to environmental uses but not the reverse. Priorities include realistic effluent testing, food-contact safety characterisation, mechanistic studies of polymer degradation and site-scale life-cycle assessment. Residue-derived biochar is a credible but conditional technology whose benefits must be demonstrated rather than assumed.