**12. Challenges in Using Microbial Fuel Cells**

In recent years, the evolution of MFC technology has raised many concerns due to its unrealized potential for simultaneous bioenergy production and wastewater treatment. There is also a rising concern for environmental waste management and the amount of organic matter being released to the environment in the form of untreated effluent, which affects both terrestrial and marine life. However, these effluents and waste can be used

for the production of energy. Some companies have launched MFC-based wastewater treatment systems; although, there are numerous challenges still associated with the use of such technology. One of the paramount shortcomings of MFC technology is the problem of high operating costs and low power output. Other challenges associated with MFC technology are the selection of appropriate and suitable substrate thus wastewater and the complexity of the molecular structure of the agro-waste identified as a suitable substrate for MFCs and its resistance to oxidation, which will, in turn, affect its treatability and organic constituents' removal which will affect the MFC working principle. However, the pH of the substrate and its sudden alteration while performing some remediation activities must be considered for substrate conditioning as the changes in pH may lower the activity of microorganisms if the optimum is altered, which will, in turn, affect the MFC performance and quality of the wastewater being treated.

Another factor affecting the performance of the MFC system is the CE when the MFC is alternatively fed with low strength wastewaters, which will affect electrode performance due to the diversion of electrons into non-exoelectrogenic growth when using both plant and animal waste materials, resulting in the rapid depletion of the substrate along with the process of metabolism, which will lower processes such as methanogenesis thus low electron transfer efficiency. Scaling-up is another challenging factor that needs to be considered, which requires an economic evaluation with appropriate safeguards for a simple wastewater treatment-MFC set-up that can be maintained effectively and easily to generate a high-power output. Considering the capital cost of MFC based on simplified designs and their configuration and agro-waste treatment capability, this technology it is said to be more promising for long energy security than the sole use of conventional wastewater treatment systems for domestic wastewater [117]. Most of the expensive electrode materials used, such as catalyst and membrane materials, may result in the high capital and operational cost implications of MFC [175]. A high potential loss has been observed at the surface of the electrodes, leading to a reduction in current density when the upscaling of the MFC technology from a few milliliters to hectoliters is considered [6]. Furthermore, the inability of the MFC to recover heavy metal ions is a great challenge, also affecting its selection as a preferred technology. Overall, the bio-toxicity of certain heavy metals would negatively affect the performance of MFCs, imparting low energy production rates and limited wastewater remediation efficiencies [176].
