Amending anaerobic bioreactors with pyrogenic carbonaceous materials: the influence of material properties on methane generation

Abstract

Amending anaerobic digesters with pyrogenic carbonaceous materials (PCMs) has been suggested to improve methane (CH₄) production by enabling electron conduction between fermenters and methanogens. This enhancement has been attributed to the electrical conductivity of some PCMs. Properties other than conductivity have received little attention, leaving uncertain the primary drivers of the observed improvements. Accordingly, the objective of this study was to relate the electrical conductivity, adsorptive behavior, pH effects, and surface properties of PCMs to CH₄ production rates and CH₄ recoveries in swine wastewater-fed, short-term anaerobic batch reactors. Three types of PCM particles [graphite, biochar, and activated carbon (AC)] in two size ranges and three particle loadings were tested and compared to non-conductive glass particles and no-particle controls. Graphite amendments resulted in higher CH₄ production rates and CH₄ recoveries than the no-particle controls and showed improvements with increased particle loadings. The impact of biochar and AC amendments depended on particle sizes and loadings, with granule amendments generally exceeding or matching the controls. Powdered biochar and AC amendments substantially decreased the performance. Material conductivity was a poor predictor of CH₄ production rates, and bulk solution pH changes due to particle amendments could not explain reactor performance discrepancies. Adsorption was a critical material property controlling the fate of wastewater chemical oxygen demand (COD). Powdered biochar and AC, which adsorbed more COD than graphite and glass, resulted in lower COD to CH₄ conversion. These results indicate that PCM properties other than conductivity, such as adsorption, can strongly impact short-term bioreactor performance. These properties should be taken into consideration when selecting and optimizing PCMs for biological-based technologies.