Methane Recovery from Cattle Manure: Role of Granular Activated Carbon Amendment into Anaerobic Digestion-Microbial Electrolysis Cells (AD-MECs) Integrated Systems

Anaerobic digestion (AD) of manure is commonly applied, yet the low methane (CH4) production yield and rate are among the limitations of the process. AD integration with microbial electrolysis cells (MECs) and amendment of conductive materials to the digesters can compensate for the limitations via the enrichment of electroactive microorganisms. For the first time in the literature CH4 production performances of cattle manure-fed AD-MECs were assessed under varying conditions, namely, by placing biofilm-attached electrodes (bioelectrodes) versus bare electrodes, the amendment of biofilm-attached granular activated carbon (BioGAC) particles versus fresh GAC particles under two different media (phosphate buffer saline (PBS) vs salt medium). The CH4 yield of the AD reactor in PBS medium was notably low (50 mL/g VSadded) due to a severe P inhibition as compared to 247 mL of CH4/g VSadded in the salt medium. AD-MECs alleviated inhibitory effects, that is, increased the yield and rate of CH4 production and reduced the start-up time. Of all the reactors, the performance of BioGAC-amended AD-MECs were superior with a 4.4 times higher yield (similar to 223 mL CH4/g VSadded) than the control with PBS and a 1.3 times higher yield (similar to 318 mL CH4/g VSadded) than the control with salt medium. Despite requiring an electrical energy input, BioGAC-amended AD-MECs remain energy-positive compared to the control.