Effects of NaOH, thermal, and combined NaOH-thermal pretreatments on the biomethane yields from the anaerobic digestion of walnut shells

Anaerobic digestion (AD) of walnut shells (WS) results in only a limited biomethane yield because of their high fibre content, which ultimately represents an essentially nonbiodegradable lignocellulosic biomass. In the present study, thermal (i.e. 50-250 degrees C), alkaline (i.e. 1-5% w/w NaOH) and combined alkaline-thermal (i.e. 4% w/w NaOH + 150 degrees C thermal) pretreatment methods have been applied to increase the anaerobic biodegradation of WS. The highest biomethane yields of 159.9 +/- 6.8 mL CH4.g VS-1 and 169.8 +/- 6.8 mL CH4.g VS-1 were achieved after pretreatment at both 250 degrees C and with 4% NaOH. After combined NaOH-thermal pretreatments, the AD process showed the largest total VFA concentration (i.e. 1280.1 mg Hac L-1) but a relatively high lag phase (i.e. 3.90 days) compared to thermal and NaOH pretreatments alone, from which the highest biomethane yield (i.e. 192.4 +/- 8.2 mL CH4.g VS-1 ) was achieved at the end of the AD process. The highest biomethane yield from the combined NaOH-thermal pretreated WS was corroborated by the corresponding highest SCOD/TCOD ratio (i.e. 0.37 +/- 0.02) and the highest lignocellulosic fibre removal (i.e. 41.1 +/- 2.7% cellulose, 35.6 +/- 1.8% hemicellulose, and 58.7 +/- 3.2% lignin). The cumulative biomethane yields were further simulated via a modified Gompertz model. This study provides a promising strategy in the sense that the biomethane yield of WS containing large amounts of lignin can be significantly increased via thermal, NaOH, and combined NaOH-thermal pretreatment methods.