Anaerobic Biorefineries for Slaughterhouse Waste and Wastewater: From Process Fundamentals to Resource Recovery

Slaughterhouse effluents represent one of the most concentrated and compositionally complex industrial waste streams, characterized by extreme variability in the lipids, proteins, and suspended solids. While traditionally regarded as a treatment liability, these effluents constitute a high-value feedstock for anaerobic biorefineries capable of simultaneously delivering energy, reusable water, and nutrient-rich byproducts. This review critically examines recent advances (2021–2025) in the engineering of anaerobic digestion for slaughterhouse waste valorization, with a focus on reactor design, process intensification, and integration into circular bioenergy systems. Several reactor configurations, including upflow anaerobic sludge bed reactors, anaerobic sequencing batch reactors, continuous stirred tank reactors, and anaerobic membrane bioreactors, are systematically discussed in terms of operational performance and methane productivity. The analysis further integrates energy recovery pathways, including combined heat and power generation and biomethane upgrading. Techno-economic and environmental analyses are addressed to identify scalable process configurations that maximize the methane yield and overall resource efficiency. By consolidating the fragmented literature into an integrated engineering framework, this review provides guidance for designing robust anaerobic biorefineries tailored to the slaughterhouse industry, supporting both decarbonization and circular economy objectives.