Animal manure in the context of renewable energy and value-added products: A review

Animal manure has a complex structure and can be considered a lignocellulosic material. Manure management generates greenhouse emissions, mainly N2O and CH4. Currently, efforts are being made for the sustainable use of animal manure to obtain bioenergy and value-added products. This work shows thermochemical and biochemical methods for manure valorization and proposes strategies to mitigate the inherent environmental problems, thus helping to meet the Sustainable Development Goals (SDG), particularly SDGs 7, 9, 12, and 13. The physicochemical properties and high heating values (HHV) of different types of manure are presented and discussed. The HHV follows the order of cow≈horse>sheep>poultry>pigmanure. Thermochemical processes such as pyrolysis, gasification, and combustion were studied. The operating conditions of anaerobic digestion were also studied. Physical, chemical, thermochemical, and biochemical (e.g., with microbial consortia and enzymes) pre-treatments were analyzed to improve the performance of the above processes. Limitations and obstacles in technological applications were identified, such as manure distribution, lack of government incentives, or the presence of antibiotics in manure. Animal manure has a clear potential to produce organic acids, biomaterials, biofuels, and bioenergy. So, bioprocesses or biorefineries using this biomass as a feedstock may be promising in the near future in the context of the bio-economy, helping to increase the production of renewable energy, promoting inclusive and sustainable industrialization, fostering innovation that adds value to the main livestock activities that generate manure, and guaranteeing sustainable consumption and production modalities that allow mitigating the effects caused by the current treatment of manure.