Fallen Leaves as a Substrate for Biogas Production

Fallen leaves in cities are often treated as waste; therefore, they are collected, transported outside urban areas, and composted, which contributes to greenhouse gas (GHG) emissions. Instead of this conventional management approach, fallen leaves could be utilized as a feedstock in biogas production, helping to reduce GHG emissions, increase renewable energy generation, and provide fertilizer. The aim of this study was to compare the mono-digestion of fallen leaves from three tree species commonly found in parks and along streets—northern red oak (Quercus rubra L.), small-leaved lime (Tilia cordata Mill.), and Norway maple (Acer platanoides L.)—in both wet and dry anaerobic digestion (AD) systems. A biochemical methane potential (BMP) test was conducted in batch assays for each of the three substrates in both AD technologies at a temperature of 38 ± 1 °C. The highest specific methane yield (SMY) was obtained from Quercus leaves in wet AD technology, with a methane yield of 115.69 ± 4.11 NL kgVS−1. The lowest SMY (55.23 ± 3.36 NL kgVS−1) was observed during the dry AD of Tilia leaves. The type of technology had no significant impact on the SMY of Acer and Tilia leaves; however, the methane yield from Quercus leaves in wet AD was significantly higher (p < 0.05) than that from dry AD. Studies on the use of fallen leaves from Tilia cordata, Quercus rubra, and Acer platanoides as substrates in mono-digestion technology have shown their limited suitability for biogas production. Nevertheless, this feedstock may be more effectively used as a co-substrate, mainly due to the low concentrations of ammonia (NH3) and hydrogen sulfide (H2S) in the biogas produced from these leaves, both of which are considered inhibitors of the AD process.