The Potential of Fungi to Enhance the Yields and Sustainability of Anaerobic Digestion.

Anaerobic digestion (AD) is a biological process that foresees the conversion of organic feedstocks into biogas (methane), which can be exploited to obtain heat, steam, electricity, fuel and chemical. Moreover, AD has the potential to produce various by-products and particular attention has been given to the digestate, which mainly contains water, inorganic compounds, and undigested organic matter. AD is among the most efficient technology for renewable energy production, but there is still room for some significant improvements. For instance, the increased use of dedicated crops for energy purposes has led to competition with food-feed production and land use. The enhancement of the process efficiency and/or the use of alternative and sustainable feedstocks, such as agro-industrial wastes and by-products, may contribute to improve the environmental and economic sustainability of AD. In this context, previous studies have reported the possibility of exploiting the solid fraction of digestate (SFD) as a feedstock for a further AD step, in order to recover economically attractive amounts of methane and to reduce greenhouse gas emissions during the storage. Unfortunately, the proposed use is not generally a feasible option as SFD is largely composed of recalcitrant lignocellulose. Nevertheless, many fungi are known for the ability to produce lignocellulolytic enzymes capable of effectively degrading the structural components of plant cells and they can be exploited to develop biological pretreatments aimed at increasing the anaerobic digestibility of lignocellulosic biomasses. In the present study, the fungal biodiversity was investigated to develop processes aimed at improving the biogas yields, and consequently the valorisation, of the SFD. Following a preliminary screening, 3 strains belonging to the species Coprinopsis cinerea (MUT6385), Cyclocybe aegerita (MUT5639), Cephalotrichum stemonitis (MUT6326) were selected to carry out the pretreatment in solid-state fermentation and non-sterile conditions. The main physicochemical characteristics and the content of plant cell wall polymers (cellulose, hemicellulose, lignin) of untreated and fungal-pretreated SFD samples were analysed. The effects of fungal pretreatments on subsequent AD were studied by means of biochemical methane potential (BMP) tests in batch systems. The fungi showed the ability to degrade the lignocellulosic fractions, but different degradation profiles were observed depending on the species and the pretreatment duration. All the fungal pretreatments improved the digestibility of the SFD, leading to a significant (p <0.05) increase in the daily and cumulative production of biogas and methane in the samples pretreated with fungi compared to the untreated controls (Fig. 1). In detail, the most effective pretreatment was that carried out with C. stemonitis MUT6326, which led to approximately three-fold higher biogas and methane yields (+182% and +214%, respectively) than the untreated SFD (Fig. 1). The increase in AD yields was ascribable both to the addition of fungal biomass, which acted as organic feedstock, and to the lignocellulose transformation due to fungal activity during pretreatments. In conclusion, the developed fungal pretreatments have the potential to enhance the polysaccharides accessibility of SFD and untap its biogas potential for a further digestion step, thus allowing a potential improvement in the total biogas plant yields and contributing to the development of a by-product management strategy, in accordance with a circular economy and biorefinery perspective.