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Microbial Fuel Cells: Now Let us Talk about Energy Zhen He* Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States better link between organic contents and electricity generation than Coulombic efficiency. It is just a simple step beyond the power calculation, but will allow us to really start thinking and talking about energy in MFCs. With energy data we can construct an energy balance, which is an effective approach for understanding energy in MFCs. Interestingly, we have intensely focused on mass balances such as carbon and nitrogen, but ignored energy balances. To build an energy balance, we need to know the energy production and consumption in an MFC. The energy production is relatively easier to calculate from power, operating time, and the volume of the treated wastewater or the removed COD within that operating time. This is based on the assumption that electric energy is the only energy produced in an MFC; in fact, when MFCs treat high-strength wastes, methane is produced and should be included in the energy production, and if other energy contents such as algal biomass are coproduced,2 they should be included, as well. It requires additional efforts to study how much of the produced energy can actually be useful (energy loss will occur during transfer and use), but for an initial energy balance, it will be acceptable to assume 100% energy efficiency (from an MFC to an end-user such as a icrobial fuel cells (MFCs) have been thoroughly studied pump). in the past decade and much understanding of their The tough part seems to be the estimation of energy microbiology, electrochemistry, and reactor architecture/ consumption due to the generally small sizes of MFC reactors. operation has been obtained through intensive research and For an MFC that is run by pumps, the estimate of energy thousands of journal publications.1 MFCs are attractive as a consumption is not “mission impossible”: the energy new platform (e.g., micro-MFCs) to study microbial interaction consumption is mainly due to the pumping system, and thus with an electron acceptor/donor and the electron-transfer we can estimate this consumption by theoretic calculation mechanisms involved in this interaction, power sources for involving flow rates and hydraulic pressure head, as described previously.3 A precise estimate of energy consumption in smallremote sensors (sediment MFCs), or more popularly as an scale MFCs could be difficult, but at least it provides us with a alternative approach for wastewater treatment and bioenergy good starting point to work with. For example, with this production from wastes. Direct electricity generation in MFCs approach, we have built an energy balance in an MFC is an obvious advantage over other energy recovery methods, containing hollow-fiber membranes and found that it produced such as anaerobic digesters that require biogas collection, and more energy than it consumed when treating a synthetic conversion. Therefore, it is not unexpected that almost every solution, but it had a negative energy balance when fed with paper about MFCs treating wastewater starts its introduction actual domestic wastewater.4 That gives us a sense how far we with the energy benefits of MFCs. However, are we really are from achieving an energy-neutral treatment process by showing any energy performance of the MFCs that we have using MFC technology. developed? “Yes” in very few studies, but “No” in the majority. Building an energy balance enables us to better understand When we describe the electricity-producing performance of the energy benefits of MFC technology. Previously, we an MFC, the most commonly used parameters include voltage, interpreted the energy advantage of an MFC by focusing on current, and power (or power density), none of which is an how much energy it can produce (though not showing the energy parameter. Energy is expressed in either joule (J) or related energy data). Now, with an energy balance, we can see kilowatt hour (kWh), which can be calculated by multiplying that an MFC cannot produce much energy (
