1. Introduction

7, Microbial fuel cells (MFCs) are a type of bioreactors that utilize the metabolic activity of exoelectrogenic biofilms to convert chemical energy stored in biodegradable materials into an electric current [ 1 ]. MFCs have several advantages as a wastewater treatment technology including increased efficiency through mitigated aeration costs, power generation, and decreased sludge production by relying on primarily anaerobic bacteria and fixed biofilms [ 2 3 ]. When compared to conventional activated sludge treatment systems, the use of MFC technology results in positive energy output while reducing sludge production by more than 60% [ 4 5 ]. When compared to anaerobic digestion, MFCs is more suitable for lower strength wastewater than sludge, and the produced electricity can be directly used without further conversion. MFCs use electroactive bacteria to catalyze the oxidization of organic and inorganic electron donors in the anode chamber and deliver electrons to the anode. The electrons are transferred to the cathode through an external circuit, where they can be harvested as current. MFCs have been used to generate electricity from many different substrates. In addition to simple sugars and derivatives, many complex waste materials, such as different wastewaters, starch, protein, even cellulose and landfill leachates, have also been utilized for simultaneous waste treatment and energy generation [ 6 8 ]. Although MFCs show great promise, high material cost is one of the major limitations to larger scale MFC systems due to the unpreventable fouling of the electrode surface [ 9 ].

Popular tubular MFC reactors are comprised of two electrode chambers, including an anaerobic anode and an aerobic cathode, each separated by an ion exchange membrane. Of the total construction cost, 20%–50% can be attributed to the electrode material [ 10 11 ]. This has promoted research into finding low cost electrode materials such as activated carbon (AC). Newly developed AC cloth can significantly reduce reactor volume for stacked MFC applications [ 12 ], but granular activated carbon (GAC) is a more popularly-used packed bed adsorbent in other wastewater treatment processes, such a biologically activated carbon (BAC) filters. BAC filters have shown to be highly effective at contaminant removal through the combination of both carbon adsorption using GAC and biological degradation [ 13 ]. The high adsorption capacity for GAC is primarily attributed to its high surface area and microporosity and its rough surface lends itself to rapid bacterial colonization. However, in both MFC and BAC systems, prolonged use of GAC tends to foul the adsorbent surface, lowering efficiency and requiring replacement [ 14 ]. An alternative approach could be to use a similar carbon adsorbent material with a larger pore structure (>100 nm), helping to limit clogging and prolonging service life.

Wood-derived biochar (BC) is formed from the pyrolysis or gasification of woody biomass that has recently shown great potential as a GAC replacement material for contaminant removal [ 15 ]. When this type of biomass is thermally converted it maintains an interconnected three-dimensional structure resembling its original physical morphology [ 16 ]. Pyrolysis at higher temperatures (>800 °C) results in the lignin comprised cellular structure to be converted to conductive graphite [ 17 ]. These features make BC a competitive electrode material for MFCs at significantly lower cost than traditional materials such as GAC [ 18 19 ]. BC has also been shown to have additional carbon sequestration and agronomic benefits when used as a soil amendment [ 20 ]. This raises the possibility of using spent BC electrodes as an agricultural amendment, further reducing the environmental burden of MFC operation. Although BC has been evaluated as both anode and cathode in an MFC, its performance during real wastewater treatment and concurring nutrient recovery has yet to be tested.