# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 4 Techno-Economic Feasibility of BES for Metal Recovery  
A conceptual framework for evaluating the energy efficiency of bioelectrochemical systems compared to conventional (abiotic) electrochemical technologies for cathodic processes, such as metal recovery (examples illustrated in Figure 3), is presented in Figure 5. This framework illustrates the operational regions where BES, driven by anodic bioelectrocatalysis, can outperform classical electrolysis technologies. The diagram highlights critical intersections in cell voltage vs. current density and power density vs. current density curves, comparing BES systems with their abiotic counterparts where the anodic reaction is the oxygen evolution reaction (OER) using either carbon felt (without any electroactive biofilm) or precious metal catalysts based (e.g., Pt, IrO\({}_{2}\), RuO\({}_{2}\)) anode materials. These intersections delineate two key regions that define the economic feasibility of each technology. In region (a), BES demonstrates a clear operational advantage, characterized by lower power consumption or even power production within the MFC operation region, making it particularly suitable for applications prioritizing energy efficiency and reduced operational costs. The MFC operation spans from open circuit voltage (OCV) to short-circuit conditions, leveraging the spontaneity of the redox processes. Beyond this region, BES transitions into MEC mode, where an external voltage is applied to drive non-spontaneous reactions, enabling the system to exceed the natural operational limits of MFCs and achieve higher reaction rates. A key advantage of BESs lies in their significantly lower CAPEX, as carbon-based anode materials (e.g., carbon felt) used for the development of the electroactive biofilm are far less expensive than the platinum group metals used in conventional systems for OER catalysis. Additionally, carbon-based materials avoid long-term stability challenges, such as catalyst deactivation or dissolution, often faced by noble metal-based anodes. These factors further enhance the economic feasibility of BES configurations. In contrast, region (b) represents operational conditions where classical electrolytic technologies, particularly those based on catalyzed OER, become more advantageous due to their higher efficiency at elevated current densities. The improved energy balance of BES systems over conventional electrochemical technology can be further expanded by optimizing system design and operation. For instance, increasing the anode-to-cathode surface area ratio or improving reactor geometry enhances the performance of BES, enabling it to reach the extended regions (a\({}^{\prime}\)) and (b\({}^{\prime}\)). These improvements allow BES to operate more efficiently under a broader range of conditions (expanded region (a\({}^{\prime}\))), maintaining competitiveness and reducing energy demands. However, despite these advantages, BESs face inherent limitations when compared to conventional systems, particularly at larger scales. The microbial catalysis that powers BESs while reducing energy demand or driving spontaneity under certain conditions is constrained by slower reaction kinetics. When BESs are pushed to higher current densities or productivity levels, their performance plateaus due to these kinetic limitations. Consequently, at high current densities (or productivity per electrode surface area), the efficiency of BESs is surpassed by their abiotic electrochemical counterparts, transitioning from region (a) to region (b), as depicted in Figure 5, where pure electrochemical technologies become more efficient. To maintain operation within region (a) under milder conditions, BESs would require a larger number of cells in a stack to achieve the same productivity as other technologies. This increases the complexity of practical implementation and significantly impacts CAPEX, posing challenges to the scalability of BESs for industrial applications.