Enhancing the solid-state anaerobic digestion of fallen leaves through simultaneous alkaline treatment

► Simultaneous NaOH treatment and solid-state anaerobic digestion (SS-AD) of leaves. ► Methane yield of SS-AD improved greatly by simultaneous NaOH treatment. ► Failure of SS-AD process observed at high substrate loadings. ► Buffering capacity increased with the addition of alkali by increasing alkalinity.