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.

Automated summary

The study shows that the biomethane yield of walnut shells, with high fiber content, can be significantly increased in anaerobic digestion process through thermal treatment, NaOH treatment, and combined NaOH-thermal treatment methods.