## 5 Application of eNO\({}_{3}\)R to Brackish Groundwater: A Case Study
### Challenge to Move from Lab Scale to Pilot eNO\({}_{3}\)R  
The eNO\({}_{3}\)R reactors in most of all previous research do not exceed 500 mL. However, large-scale application requires several factors to ensure successful applicability, specifically for brackish groundwater. One of the initial considerations in designing a reactor is selecting between divided and undivided reactors. For nitrate removal from brackish groundwater, the choice between a single chamber cell (SCC) and a double chamber cell (DCC) is directly linked to the objective. If the goal is to convert nitrates into gaseous nitrogen, the choice should be an Undivided Flow electrochemical reactor (UFECR). However, if the objective is ammonium production and an inert anode for chlorides is used, as discussed in the previous section, the preference remains for UFECRs. On the other hand, if an anode with higher CIOR is used, then the choice is a Divided Flow Electrochemical reactor (DFECR), as explained in Figure 16.  
Studies employing continuous mode which allows for easier scale-up and industrial application are few. Typical continuous reactors are illustrated in Figure 17. For example, [PERSON] et al. [220] used a homemade flow cell with a porous modified copper cathode and platinum anode to convert nitrate to ammonium; the results showed a maximum NH\({}_{4}^{+}\) selectivity of 96% and a high faradaic efficiency for NH\({}_{4}^{+}\) formation (FE 72%) was recorded with a solution of pH 7.2, and a flow rate of 2 mL min\({}^{-1}\)[220]. Even though the water matrix used differs from that of brackish groundwater, the results remain significant and encourage the development of DFECRs for ammonia production. Another study by [PERSON] et al. [221] demonstrated significant results regarding the use of continuous mode; however, the application of periodic current reversal complicates the use of these processes for large-scale applications, particularly for natural waters, due to cathode dissolution [221]. Another study utilized a 500-L pilot continuous-flow reactor for nitrate reduction to ammonia [222]. The stable operation achieved a NH\({}_{4}^{+}\)-N selectivity of 92.9% at a hydraulic retention time (HRT) of 12 h and a current density of 20 mA cm\({}^{-2}\). However, the long hydraulic retention time required for a significant decrease in the initial NO\({}_{3}^{-}\)-N concentration is one of the weaknesses of this operation. This is due to non-uniform flow and the presence of numerous dead zones in the reactors, demonstrating that using a continuous-flow reactor for nitrates is not always straightforward [222]. However, the major challenge is transitioning to continuous mode with good hydraulic flow and minimal dead zones to ensure uniform nitrate reduction. Therefore, selecting the appropriate geometry and operating conditions for electrochemical reactors (ECRs) to achieve high current efficiencies (close to 100%) toward nitrate reduction to nitrogen or ammonium in brackish groundwater, requires intensive research into transport phenomena such as hydrodynamics, potential-current distributions, heat, and mass transfer. Mathematical modeling and Computational Fluid Dynamics (CFD) simulations have facilitated the design, characterization, and scaling up of ECRs. Fluids dynamics phenomena, fluid pressure drop, and current-potential distribution for ECRs have been widely studied using CFD. However, mass transfer phenomenon is an important aspect to be analyzed with CFD for eNO\({}_{3}\)R [223]. Many mass transfer issues in electrochemical reactors have been solved by neglecting the migration term, which simplifies practical assumptions, as in a recent study by [PERSON] et al. [224] who utilized a rotating cylinder electrode reactor (RCE) for nitrate reduction. The results indicate that 90% nitrate was removed in 10 min for a solution comprising 10 mM NO\({}_{3}\)\({}^{-}\) and 500 mM K\({}_{2}\)SO\({}_{4}\), at 1000 rpm and 25 \({}^{\circ}\)C confirming the RCE reactor's effective performance for nitrate reduction.