Environmental Science and Engineering Seminar in Conjunction with the Young Investigators Lecture Series sponsored by the Caltech Division of Engineering & Applied Science

The paradigm of wastewater treatment is changing from one of mere contaminant removal to one of resource and energy recovery. In order for Water Resource Recovery Facilities (WRRFs) to achieve net-zero energy status, new treatment technologies are required. One promising new means of energy recovery is bioelectrochemical system (BES) technology, which uses microorganisms to catalyze oxidation and/or reduction reactions at solid electrodes to take advantage of the flow of electrons from anode to cathode. This seminar will describe BES applications including microbial fuel cells (MFCs), microbial electrolysis cells (MECs) and microbial electrosynthesis cells (MESs), and explore how wastewater treatment may be revolutionized with BES technology. Original research results will be presented on BES conversion of carbon dioxide (CO₂) to methane (CH₄) for the production of carbon-neutral fuel from CO₂ generated during wastewater treatment. While anaerobic digestion is currently used to recover heat and/or energy from the breakdown of high-strength wastewater, the produced biogas (a mixture of CO₂, CH₄ and trace gases) contains 25-45% CO₂, lowering the biogas energy content. BES technology is able to upgrade anaerobic digestion biogas by using Archaea to catalyze the reduction of CO₂ to CH₄ at the cathode, while wastewater is oxidized by exoelectrogenic Bacteria at the anode. Batch studies conducted at an applied voltage of 0.8 V (vs. SHE) were capable of producing natural-gas quality biogas (>97% CH₄). Furthermore, the addition of zero valent iron improved biocathode performance and assisted in the removal of sulfide, a highly-corrosive gas present in raw biogas. Significant advantages of BES biogas upgrading over traditional methods include less energy input, greater net energy recovery and the potential for simultaneous treatment of wastewater in the anode. However, BES technology is still in the early stages of development and challenges remain; thus, the future and outlook of BES application to wastewater energy recovery will be discussed.