Co-digestion of Pig Slurry and KOH Pre-Treated Giant Reed (arundo Donax L.) Enhances Methane Yield and Digestate Characteristics

The objectives of this study were: to evaluate the methane yield attainable by co-digestion of pig slurry with thermo-alkaline pre-treated giant reed; to assess the convenience of washing the pre-treated giant reed biomass before anaerobic digestion; to evaluate the composition of the digestate, in the prospect of its possible agronomic use. This study is the first to show a clear advantage in co-digesting potassium hydroxide pre-treated giant reed with pig slurry. In mono-digestion, the maximum methane production per reactor from pre-treated biomass was 263 mL g−1 volatile solids, which was 28% higher than untreated biomass. The co-digestion of pre-treated giant reed biomass with pig slurry increased the volatile solids content by 65% and doubled the methane production per reactor (533 mL); a synergistic effect was also observed (+10%). Washing the biomass after the pre-treatment had a detrimental effect on the overall methane production and also prolonged the anaerobic digestion time both in mono-digestion and in co-digestion. The digestate derived from the pre-treated and unwashed biomass co-digestion has a considerable content of plant nutrients (nitrogen, phosphorous, and potassium) and carbon making it promising for agronomic utilization.