Hydraulic retention time drives changes in energy production and the anodic microbiome of a microbial fuel cell (MFC)

Abstract

The fish-canning industry generates large quantities of wastewater that typically contains high concentrations of organic matter and salts. However, little is known about the potential valorization of this type of industrial wastewater using the microbial fuel cell (MFC) technology operated in a continuous flow mode. This study investigated the impacts of three different hydraulic retention times (HRT) on the performance, energy production, and prokaryotic and eukaryotic anodic microbiome of an MFC inoculated with activated sludge from a seafood industry and fed with synthetic wastewater that mimics fish-canning effluents. Three consecutive HRTs were studied: 1 day (HRT1), 3 days (HRT3), and 6 days (HRT6) for 30 days, 21 days, and 21 days, respectively. Voltage, current density, and power density were significantly greater at HRT1 compared to HRT3 and HRT6, whereas no differences in coulombic efficiency (CE) were detected among HRTs. Decreases in the efficiency of removal of organic compounds and increases in the abundance of archaeal communities with increased HRT was related to limited energy production at greater HRT. The increased energy production at HRT1 was tightly linked to increased and decreased absolute abundances of bacterial and archaeal communities, respectively. Variations in the HRT significantly impacted the diversity and composition of the prokaryotic community with critical impacts on energy production. The proliferation of known and diverse electroactive microorganisms, such as those belonging to the genera Geobacter, Shewanella, Arcobacter, and Clostridium, was related to increased energy production at HRT1. However, HRT3 and HRT6 enhanced the growth of archaeal methanogens (mainly Methanosarcina sp.), which negatively impacted current production. The eukaryotic community showed less sensitivity to changes in HRT and no significant impact on current production. The carbon oxygen demand and organic matter removal % increased from approximately 20% at HRT1 to almost 60% at HRT6. This study shows there is a critical balance between the HRT and prokaryotic microorganisms contributing to organic removal rate and increases and decreases in energy production in an MFC treating wastewater from the fish-canning industry and operated in a continuous mode.