Chicken meat and bone meal valorization by hydrothermal treatment and anaerobic digestion: Biofuel production and nutrient recovery

In this work, chicken meat and bones (C-MBM) waste is treated through a sequence of stages including hydrothermal treatment (HTT), nutrient recovery and anaerobic digestion, with the aim of evaluating their potential synergy as a circular economy approach. HTT was carried out at 170, 200 and 230 °C, under non-acidic and acidic conditions using 0.5 M HCl (HTT-A). Phosphorous from process water was recovered by chemical precipitation with the addition of a Mg salt, and the liquid effluent was anaerobically treated to degrade organic matter and produce a methane-rich biogas. Hydrochar obtained under non-acidic conditions presented poor combustion characteristics, while HTT-A yielded a bio-oil with high higher heating value (≈38 MJ/kg), good combustibility performance and high reactivity. More than 95% phosphorous (as phosphate) and almost 100% nitrogen (being 30% as NH4–N) content in C-MBM were solubilized in the process water upon HTT-A, while these nutrients were mainly retained in the hydrochar in non-acidic reactions. Chemical precipitation of P and NH4–N from HTT-A process water allowed recovering a crystalline solid identified as struvite and a struvite-apatite mixture, with negligible heavy metals content. High methane production (250–300 mL CH4/g CODadded) and organic matter removal (up to 75%) were achieved in the anaerobic tests. HTT proves to be a suitable treatment for material and energetic valorization of C-MBM, within a circular economy framework, which allows to obtain high value-added products (hydrochar/bio-oil, biofertilizers and biogas).