Microbial Fuel Cells: Now Let us Talk about Energy

Abstract

M fuel cells (MFCs) have been thoroughly studied in the past decade and much understanding of their microbiology, electrochemistry, and reactor architecture/ operation has been obtained through intensive research and thousands of journal publications. MFCs are attractive as a new platform (e.g., micro-MFCs) to study microbial interaction with an electron acceptor/donor and the electron-transfer mechanisms involved in this interaction, power sources for remote sensors (sediment MFCs), or more popularly as an alternative approach for wastewater treatment and bioenergy production from wastes. Direct electricity generation in MFCs is an obvious advantage over other energy recovery methods, such as anaerobic digesters that require biogas collection, and conversion. Therefore, it is not unexpected that almost every paper about MFCs treating wastewater starts its introduction with the energy benefits of MFCs. However, are we really showing any energy performance of the MFCs that we have developed? “Yes” in very few studies, but “No” in the majority. When we describe the electricity-producing performance of an MFC, the most commonly used parameters include voltage, current, and power (or power density), none of which is an energy parameter. Energy is expressed in either joule (J) or kilowatt hour (kWh), which can be calculated by multiplying power by time. The factor of time is always important to our data expression, especially for batch operation. For a wastewater treatment process, it will be beneficial to express energy density in kWh/m (treated wastewater), or more accurately in kWh/ kg COD removed due to the fluctuation of organic contents in different wastewaters. Energy density makes it possible to compare the performance of different MFCs and it serves as a 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, 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 pump). The tough part seems to be the estimation of energy consumption due to the generally small sizes of MFC reactors. For an MFC that is run by pumps, the estimate of energy consumption is not “mission impossible”: the energy consumption is mainly due to the pumping system, and thus we can estimate this consumption by theoretic calculation involving flow rates and hydraulic pressure head, as described previously. A precise estimate of energy consumption in smallscale MFCs could be difficult, but at least it provides us with a good starting point to work with. For example, with this approach, we have built an energy balance in an MFC containing hollow-fiber membranes and found that it produced more energy than it consumed when treating a synthetic solution, but it had a negative energy balance when fed with actual domestic wastewater. That gives us a sense how far we are from achieving an energy-neutral treatment process by using MFC technology. Building an energy balance enables us to better understand the energy benefits of MFC technology. Previously, we interpreted the energy advantage of an MFC by focusing on how much energy it can produce (though not showing the related energy data). Now, with an energy balance, we can see that an MFC cannot produce much energy (<0.04 kWh/m, or <0.1 kWh/kg COD from our current research), so it may not