Valorization of spent mushroom substrate via anaerobic digestion: Process stability, co-digestion strategy, and energy recovery

Abstract

Spent mushroom substrate (SMS), an abundant lignocellulosic waste material from the mushroom production process, is an interesting feedstock for renewable biomethane production. This study cracks performance of anaerobic digestion (AD) of SMS under mesophilic conditions (35±1°C) during batch fermentation for 45 days. The impact of inoculum type and co-digestion with nitrogen-rich livestock manures (cow, pig, and chicken) was evaluated with regard to the optimization of the methane yield. Raw SMS had moderate methane potential because of fungal delignification; however, mono-digestion resulted in an unstable input because of a high C/N ratio and VFA accumulation. Methane yields from raw SMS only ranged between 140-180 mL CH4 g-1 VS. Co-digestion showed a significant improvement in the performance, especially for chicken manure at a ratio of 1:1 in volatile solids. 300-350 mL of methane gas per g of volatile solids was obtained, representing more than a fourfold increase in comparison to SMS mono-digestion. Optimum mixtures have had balanced C/N ratios (25-30), stable pH (7.0-7.5), low VFA build-up, and improved volatile solids reduction (up to 65%). Manure digestate had superior buffering and a rapid onset of methane compared to using sewage sludge inoculum. Energy analysis revealed 55% of the energy content of SMS could be recovered in the form of biomethane as a result of co-digestion. The digestate contains mineral nutrients that are suited for biofertilizer circular agriculture. This study results in the confirmation of the potential of SMS co-digestion in improving biomethane production and stability, showing the feasibility for biogas production while supporting biomass vaporization.