Free nitrous acid (FNA) pretreatment enhances biomethanation of lignocellulosic agro-waste (wheat straw)

Agro-residues having lignocellulosic biomass are considered the most effective source (heating value 16 MJ/kg) for energy production through anaerobic digestion (AD). However, complex lignocellulosic fraction in agro-residue obstructs its biotransformation and is a rate-limiting step of the AD process. This study presents a novel free nitrous acid (FNA) pretreatment to enhance the biomethanation of wheat straw (WS). The effect of FNA pretreatment was evaluated through anaerobic digestion test and model-based evaluation performed in two phases, i.e., mono-digestion (phase I) and co-digestion of wheat straw with cattle manure and food waste (phase II). For phase I, the first-order, transference, and dual pool first-order models showed the best model fit with R² > 0.98, but the dual pool first-order model (DP-FOM) showed the least root mean square error (RMSE) of 1.88–62.10. The DP-FOM analysis of assays from anaerobic digestion test showed the biogas yields of 576, 564, and 565 mL/g VS_added at 1.77, 3.54, and 5.31 mg HNO2–N/L FNA concentrations and 12 h reaction time, respectively. The corresponding increment in biogas yields was 10.87, 8.85 and 8.80%. In phase II, DP-FOM analysis revealed the best model fit with R² > 0.99 and the least RMSE of 8.88–13.04. The DP-FOM analysis from assays from anaerobic digestion test for 12 h FNA pretreatment showed the peak biogas yields of 715, 648, and 654mL/gVS_added at 1.77, 3.54 and 5.31 mg HNO₂–N/L FNA concentrations, respectively. The corresponding improvement in biogas yields was 32.24, 19.81, and 20.90%. The FNA pretreatment can potentially improve the biogas yield from wheat straw effectively under mono- and co-digestion conditions. Even co-digestion could achieve enhanced biogas yield over mono-digestion.