# Metal Recovery from Wastes: A Review of Recent Advances in the Use of Bioelectrochemical Systems
## 4 Techno-Economic Feasibility of BES for Metal Recovery
Addressing these limitations requires innovative strategies to enhance microbial kinetics and reactor configurations, ensuring BESs remain competitive for large-scale metal recovery and related processes. This diagram provides a versatile tool for evaluating bioelectrochemical systems and identifying strategies to enhance their applicability, facilitating decision-making to maximize BES performance for real potential applications. BESs excel in low-current-density scenarios, where their energy efficiency and lower CAPEX, enabled by inexpensive and stable carbon-based anodes, make them ideal for decentralized and small-scale operations. However, at higher current densities, BES performance is constrained by slower microbial kinetics, leading to plateaus that necessitate larger stacks to achieve comparable productivity to abiotic systems, thereby increasing CAPEX and operational complexity. Although  
Figure 5: Conceptual performance and economic analysis of the BES and abiotic electrochemical technologies. Shaded regions indicate profitability zones: (a) where BES outperforms abiotic systems, and (b) where abiotic electrochemical reactors (e.g., utilizing the catalyzed oxygen evolution reaction, OER, as the counter-reaction at the anode) are more advantageous. Typical OER catalysts include platinum group metals such as Pt, IrO\({}_{2}\), or RuO\({}_{2}\). The extended regions (a\({}^{\prime}\)) and (b\({}^{\prime}\)) represent operational improvements achievable through reactor design optimization, such as increasing the anode-to-cathode surface area ratio or improving mass transfer, further enhancing BES efficiency. Adapted from reference [125]. abiotic technologies demonstrate greater efficiency at higher current densities, BESs remain competitive in niche applications that prioritize energy efficiency, sustainability, and cost-effectiveness over large-scale productivity. BES systems demonstrate significant advantages in low-current-density scenarios and specific applications where energy efficiency, low maintenance, and inexpensive materials are critical; however, they face inherent limitations when reaction rate is the performance indicator. Although BESs have a lower CAPEX at modest current densities, their costs increase more steeply compared to pure electrochemical reactors as current densities rise. However, BESs remain competitive for modest-scale, domestic, or localized applications where slow reaction rates are acceptable, simplicity is valued, and the use of catalyst-free, inexpensive materials is an advantage. The primary limitation of BES systems is the reaction rates of electroactive biofilms, which are constrained by the limiting current density imposed by the inherent kinetics of microbial processes. Further polarization of the cell--such as increasing the anode potential beyond +0.4 V vs. Ag/AgCl--can disrupt biofilms due to oxygen generation and a drop in the local pH within the biofilm [126]. To mitigate this, larger anode surface areas relative to the cathode may be required (e.g., tubular reactors with the outer part as the anode). In some cases, decoupling electrochemistry from bioprocesses offers a more effective alternative. In some cases, decoupling electrochemistry from bioprocesses offers a more effective alternative. While BES systems rely on direct extracellular electron transfer facilitated by bioelectrocatalysis, in some cases, Electrified Biological Systems (EBS) offer an alternative approach by decoupling electrochemistry from biological processes, making them suitable for certain applications. For instance, the bioreduction of Cr6+ to Cr3+ or Au3+ to Au0 can be achieved using electrogenerated H2 as an electron donor in an EBS configuration (as discussed in Section 2 and displayed in Figure 3c, thereby bypassing the need for an electroactive biofilm developed on the cathode and addressing some of its associated challenges.