## 1 Introduction
OFMSW is highly biodegradable and has a high-energy-yield potential of up to 200 m\({}^{3}\) of biogas per metric on of treated OFMSW [19]. Moreover, the use of digestate, in place of mineral fertilizers, provides a further environmental benefit due to reduced CO\({}_{2}\) emissions estimated at 30-40 kg per metric ton of biowaste [20]. However, OFMSW has certain characteristics that may limit its efficacy as such a resource due to issues such as high solids content, large particle size, slowly biodegradable components (lignin-rich, woody wastes), and the waste's heterogeneous nature, which makes process control challenging. These difficulties are usually overcome with biomass pretreatments and by co-digestion with different combinations of municipal, industrial, agricultural, and farming waste materials [21]. The main advantage of the co-digestion process is the improvement of biogas production (25 to 400% over the mono-digestion of the same substrate) and methane yield, as well as the economic advantages from the sharing of apparatuses and costs [22]. GW is a complex structure of cellulose, hemicellulose, and lignin, which makes it resistant to hydrolysis by microbial enzymes during AD. Consequently, biogas generation and digestion time of GW are negatively influenced, resulting in an inefficient process [23]. This explains why only a limited number of experiments have been reported for mono-digestion of GW and none for co-digestion with OFMSW [24; 25; 26], even though biogas production from GW and lignocellulosic materials in general have a remarkable potential in terms of both environmental and social sustainability [27]. The large gap between the actual and potential biogas production from such feedstocks can in fact be ascribed to the their recalcitrant structure [28]. This makes pretreatments before AD a desired step to overcome the obstacle and make the biomass accessible to microorganisms [29]. Many pretreatments, generally classified into mechanical, physical, chemical, and combined, are available for cellulosic biomass as the basis for biological biorefineries [30]. In recent years, many studies have evaluated the feasibility of these methods for accelerating the AD process and improving biogas production from lignocellulosic biomass [31]. Several attempts have been reported for delignification of GW, and herbaceous waste in general, in order to obtain fermentable sugars, fine chemicals, or renewable biofuels using mechanical, chemical, or physical pretreatments [32]. Most of the processes involve harsh conditions, require the use of chemicals, and produce waste and toxic byproducts, making such processes unsuitable from economic and environmental points of view [33]. A promising and low-cost alternative is biological pretreatment, or bio-delignification, which involves the direct action of microbial metabolism on lignocellulosic substrates and can be used to increase biogas production [34]. In nature, _B. adusta_ plays an ecologically important role in the global carbon cycle by decomposing wood and leaf litter [35], whereas its biotechnological advantages have gained interest in the last decades; it is used as a dye-decolorizing fungus and is able to degrade aromatic xenobiotics in textile wastewater [36] and transform halogenated toxic compounds in pesticides [37]. Previous experiments in lignin degradation by _B. adusta_ have been reported for pulp and paper mill wastewater [38] and for wheat straw for bioethanol production [39; 40]. The aim of this study was to evaluate the application of _B. adusta_ to bio-delignification pretreatment on GW to enhance biogas production yield from anaerobic co-digestion of OFMSW and GW and to realize a sustainable alternative for the valorization of waste biomasses, whose potential from a circular economy perspective is often underestimated. The bio-delignification process was also compared with chemical treatments of GW, based on alkali, sulfuric acid, and chlorine, in terms of yield and overall sustainability.