Fungal pretreatment of unsterilized yard trimmings for enhanced methane production by solid-state anaerobic digestion

• Unsterilized yard trimmings (YT) successfully pretreated by white rot fungus.
• Pre-colonized YT used as inoculum for fungal pretreatment.
• Solid-state anaerobic digestion of fungal pretreated YT for biogas production.
• Methane yield was comparable to traditional fungal pretreatment with sterilization.
• Fungal pretreatment resulted in two-fold increase in net biogas energy production.

Fungal pretreatment is an environmentally friendly process that has been widely studied to improve the digestibility of lignocellulosic biomass. However, sterilization of feedstocks, a costly process, is generally required prior to the fungal pretreatment. In this study, fungal pretreatment of unsterilized yard trimmings using yard trimmings pre-colonized with Ceriporiopsis subvermispora as an inoculum was investigated. Degradation of lignin, cellulose, hemicellulose, and dry matter in yard trimmings during 30 days of fungal pretreatment using different inoculum/substrate ratios (1:19, 1:9 and 1:4) was 14.8–20.2%, 8.1–15.4%, 20.7–27.8%, and 9.8–16.2%, respectively. Methane yields of 34.9–44.6 L/kg volatile solids were achieved during solid-state anaerobic digestion (SS-AD) of the pretreated yard trimmings, which were comparable to those obtained by using the traditional method requiring feedstock sterilization. The technology developed in this study can save about 501–789 kJ/kg of dry yard trimmings processed, which is about half of the total biogas energy produced by SS-AD.