Nitrogenous (N) compounds are valuable resources, particularly nitrate and ammonium, with the primary application being in the agricultural sector as fertilizers, resulting in their occurrence in surface water and groundwater. Concentrations above the World Health Organization (WHO) guidelines may be harmful to public health and the environment. Removal of N-compounds from brackish water by electrodialysis (ED) is possible in different configurations, including batch and single-pass, with the option of a multistage operation. A comparison of single-pass multistage ED (SPM-ED) and batch ED (B-ED) was conducted with varying feedwater quality (salinity) and process parameters (electrical potential) for N-compound removal, solute flux, specific energy consumption (SEC), and current efficiency (CE). Results showed that increasing the salinity of the feed solution reduced N-compound removal in SPM-ED from 99% to 73% and in B-ED from 95% to 89%. The observed difference in N-compound and TDS removal between the B-ED and SPM-ED systems arises from both the operational mode and stack geometry. An increase in the driving force from 5 to 25 V in SPM-ED did not significantly impact N-compound removal, whereas removal was 45% lower at 5 V in B-ED. This was attributed to the larger system size (membrane area), which allows operation at a lower current density. At varying electrical potentials (5–25 V), SPM-ED achieved a lower SEC of 0.4–2.0 kWh/m3 and a higher CE compared to 0.5–6.9 kWh/m3 in B-ED. A similar observation was made when varying the feed salinity, with SPM-ED having a lower SEC of 0.01–0.8 kWh/m3 and B-ED having an SEC of 0.3–7.0 kWh/m3. Although, SPM-ED achieved lower SEC and higher CE but at the expense of a reduced specific treatment capacity (STC, 556 L/h·m2), implying a larger membrane area and higher investment cost. In contrast, B-ED provided a higher STC (750 L/h·m2), indicating greater throughput, albeit with a higher SEC due to increased stack resistance associated with a longer effective ion transport path. This highlights the inherent trade-off between energy efficiency and system capacity in operating these specific ED systems.
Adu-Boahene et al. (Wed,) studied this question.