Optimizing biomethane production and plants growth with biochar-enhanced anaerobic digestion

• Anaerobic co-digestion (AcD) was tested for energy and agronomic effects. • Mesophilic (M) and thermophilic (T) AcD were assessed with two biochar doses. • T-AcD with two biochar doses increased biogas yield by 25 % more than the control. • T digestates improved biomass accumulation by +20 % and chlorophyll content by +10 %. • M-digestates enhanced root development more than thermophilic ones. Food waste (FW) poses environmental, economic, and ethical challenges, contributing to greenhouse gas emissions. Anaerobic digestion (AD) offers the dual benefit of enhancing FW management while generating biogas as renewable energy source and producing nutrient-rich digestates. This study explored a circular and integrated model where anaerobic co-digestion (AcD) of FW and cow manure is enhanced with biochar-derived digestate applied in two doses (7, 14 g/L) under mesophilic and thermophilic conditions. Residual digestates are tested as fertilizers on Solanum Lycopersicum L. growth in greenhouse trials. Digestates, mineral fertilizer, and commercial compost were applied at three different dosages: 85, 170, 340 kgN/ha. Results showed significant improvements in biogas yield, particularly under thermophilic conditions where the AcD with both doses of biochar produced around 890 NL/kgVS of biogas, 25 % v/v more than the AcD without biochar, and 44 % more than mesophilic AcD. Greenhouse trials using digestates as fertilizers for tomato plants revealed differential agronomic impacts. Thermophilic digestates outperformed mesophilic ones as well as mineral fertilizer in promoting biomass accumulation (+20 %) and chlorophyll content (+10 %), while mesophilic digestates favored root development despite no significant differences emerging in their physic-chemical composition. Results support previous research, proving that the addition of biochar affected methane production, nutrient composition, and bioavailability of digestates, with notable benefits for plant root development and nutrient uptake. The novelty of this study is the comprehensive evaluation of both biomethane production and agronomic effects of the residual digestate, that offers a closed circular-bioeconomy model for sustainable FW management, energy production, and crop cultivation.