Physicochemical characterisation and dry anaerobic digestion of food and garden waste

Optimising the anaerobic digestion of food and garden waste is traditionally governed by the carbon-to-nitrogen ratio (C:N). However, in high-solids environments, this approach often fails to account for the physical recalcitrance and mass-transfer hurdles of lignocellulosic biomass. This study presents a detailed characterisation of food and garden waste and evaluates their specific biomethane yield through anaerobic mono-digestion (AmoD) and co-digestion (AcoD) across C:N range of 15-45 and total solids content (TS) of >15%. In AcoD, the results showed a synergistic peak at C:N 33, achieving a specific methane yield of 198 NmL/gVS_in. In comparison, AmoD of grass (C:N 26.8) yielded 149 NmL/gVS_in higher than AcoD at similar C:N. This highlights that digestion performance depends on substrate composition beyond nutrient balance, a key insight not previously reported for AcoD of food and garden wastes. Biodegradability Index proved that FG33 (FG33-stands for food and garden waste with C:N 33) reached 46% of its theoretical biomethane potential, compared to only 23% in the lignin-rich FG39. This difference highlights the lignin sheath as the main factor to sequester carbon in high-solids systems, regardless of available nitrogen. Furthermore, adding food waste and acclimatised inoculum from AmoD to AcoD eliminated the extended lag phase, demonstrating that lignocellulose-adapted microbes can reduce dependence on pretreatments to hydrolyse lignin. This has practical implications for scaling up, where pretreatment may not be feasible due to economic and practical constraints. Optimisng the substrate-to-inoculum ratio with lignocellulose-adapted microbes may further improve biomethane yield and volatile solids reduction in dry AD.