Anaerobic digestion of bean straw applying a fungal pre-treatment and using cow manure as co-substrate

The significant amounts of agriculture residues such as bean straw (BS) in rural areas, advises its valorisation for energy recovery. The feasibility of using BS for biogas production through anaerobic digestion was assessed. Prior to this, a fungal pre-treatment to hydrolyse BS with Pleutorus ostreatus was studied at 30°C and 100 rpm in orbital incubators with 1, 10 and 30 mg fungus/g straw for 14, 21 and 28 days. Then, anaerobic digestion experiments were performed in batch with cow manure (CM) as co-substrate and pre-treated BS at ratios (g/g total solids) of 1/2, 1/3, 1/5 and 0/1. Maximum lignin (18%) and hemicellulose (44%) degradation occurred at 30 mg fungus/g straw and 28 days, along with the highest total methane yield (38 mL CH₄/g VS loaded). The total amount of methane decreased when increasing CM in the experiments (701.4–191.5 mL CH₄), suggesting inhibition owed to a component of CM. Self-sustained biogas production of BS occurred due to the presence of bacteria (i.e. Bacilli and Bacteroidia) and archea (i.e. Methanobacteria and Methanomicrobia). However, the usage of a full-active inoculum should be studied for higher biogas production rates.