Performance and toxicity assessment of an up-flow tubular microbial fuel cell during long-term operation with high-strength dairy wastewater

Abstract This study describes the development of an up-flow tubular air-cathode microbial fuel cell (MFC) bioreactor inoculated with a consortium of Shewanella oneidensis and Clostridium butyricum, fed with dairy wastewater (DW). Due to the high organic load of dairy wastewater, the MFC was operated using different strategies for the start-up (30 days) and the subsequent treatment phases (phase 1: 75 days; phase 2: 30 days). The removals of total chemical and biochemical oxygen demand (TCOD and TBOD) were 94 and 96%, under external resistance of 1.0 kΩ (phase 1), and 96 and 97%, under external resistance of 0.3 kΩ (phase 2). In addition, the MFC treatment provided high efficiency for the removal of organic nitrogen (47%), phosphorus (95%), nitrate (100%), and sulfate (75%). The MFC bioreactor delivered maximum power density of 3.5 W m−3 and current density of 1.1 A m−3 at the end of phase 1, and 2.4 W m−3 and 2.4 A m−3 in phase 2, but achieved modest Coulombic efficiency of 2.1% (phase 1) and 4.4% (phase 2), probably due to other metabolic pathways, besides the electrogenic one. Evaluation of the toxicity of the treated DW at two different trophic levels revealed no acute toxic effects towards Daphnia similis (immobilization less than 20%), but statistically significant chronic toxic effects towards Raphidocelis subcapitata (60% inhibition, using a concentrated sample). The findings indicated that the MFC constructed here had excellent potential for dairy wastewater treatment and energy recovery, and that toxicity assessment should be integrated with the MFC process.