What are Bioelectrochemical Systems?
A bioelectrochemical system (BES) is similar to an electrochemical cell, with an anode and cathode, but microorganisms act as an inexpensive, self-renewing catalyst at one or more electrodes. By inoculating with specific microbial communities and controlling the cathode reducing environment with an applied potential, biocathode output (e.g., methane (CH4,) multi-carbon compounds, metal precipitates, etc.) may be selected. One research focus of this lab is BES energy recovery, a process in which biogas carbon dioxide (CO2) is converted to CH4 by pairing an organics-oxidizing bioanode with a CO2-reducing biocathode. By applying a small potential (<1 V vs. SHE) to the cathode, CO2 is converted to CH4, producing carbon-neutral fuel for energy recovery. Anaerobic digesters produce a biogas with a mixture of CO2 and CH4, but without specialized equipment capable of utilizing high-CO2 fuel, many facilities flare the biogas. Instead, a BES is capable of directly converting biogas CO2 to CH4, producing natural gas quality fuel (>98% CH4) that may be used with existing infrastructure. Furthermore, a BES bioanode is able to oxidize organics under anoxic conditions; thus, reducing energy requirements for aeration during aerobic treatment.
BES technology is relatively young and (bio)cathodic processes have only recently gained great interest, as their potential applications have become more clear. One core area that requires more research is the microbial interactions between different species at the cathode and how these interactions affect the desired cathodic reaction. For example, it has been demonstrated that the biocathode bacterial community structure significantly affected the archaeal CH4 production. By selecting for bacteria that belonged to certain groups (e.g., exoelectrogens to recycle cell debris into electron equivalents for CO2 reduction, putative producers of redox shuttles, etc.), biocathode CH4 production could be increased nearly three-fold (Dykstra and Pavlostathis, 2017). Therefore, to optimize biocathode performance, more research needs to be done to understand the interactions between cathode microorganisms.
Because BES technology is based on a redox reaction, it is applicable to a wide array of other environmental engineering processes and systems. For example, BES technology may be applied to resource recovery from wastewater treatment. Phosphorus (P) is an essential nutrient in fertilizer and is typically produced from phosphate rock, a finite resource. However, most P is wasted during mining/processing or as runoff, with only about 20% of the P in phosphate rock reaching food supplies. Wastewater, a readily renewable resource, contains significant amounts of phosphate, which must be removed prior to discharge. Recovery of this phosphate as an organic P compound, such as struvite, may be achieved through precipitation in a BES. Therefore, the P from wastewater could be recycled into struvite for agricultural land application, allowing WRRFs to achieve P treatment goals while producing a valuable product. However, to date, few studies have examined the potential for P recovery using BESs and many questions remain unanswered.
BES technology may also be applied to non-engineered environments. Sediment microbial fuel cells, which take advantage of the natural difference in redox potential between upper and lower layers of sediments, may be developed to operate as biosensors to monitor specific sediment parameters or contaminants of concern. However, BES biosensor technology is still in its infancy and future research is needed to develop it into a scalable, robust system for remote-monitoring of specific targets.