Biochar as an Additive in Anaerobic Digestion of Municipal Sludge: Biochar Properties and Their Effects on the Digestion Performance

Fast pyrolysis of biomass pretreated with mineral acid produces high quality bio-oil, however, biochar produced from this process has not been characterized and its performance as an additive to anaerobic digestion (AD) is unknown. This study reports the effects of physicochemical properties of two distinct biochars on AD of municipal sludge: one was produced from pyrolysis of raw corn stover (BC-1); the other was produced from sulfuric acid pretreatment of the same corn stover (BC-2). BC-1 had higher carbon content, alkalinity, and specific surface area but lower ash and sulfur than BC-2. Both biochars contained volatile fatty acids and residual sugars serving as substrates for anaerobic bacteria to enhance biogas/methane production. When the biochars were added to the AD, their effects on biogas production showed contrary trends. Addition of BC-1 resulted in higher methane yield and content, while BC-2 reduced methane yield and content. The strong buffering capacity of BC-1 was a major factor for its beneficial effect, while the high sulfur content of BC-2 was inhibitory to anaerobic bacteria. Collectively, the results indicated that the effects of biochar on AD depends on biochar properties, and selection of appropriate biochar was important in facilitating higher biogas production and maintaining a stable process.