**6. Conclusion**

CDI technology shows a great potential for brackish water desalination due to its simple design, cheap components, low energy-consumption (fairly low potential and recoverable energy), economical feasibility, high efficiency, safety and environmental friendliness. The classical EDL theory explains the concept behind the CDI which is simply associated with electrosorption of ions at the surface of a pair of electrically charged electrodes; counterions will occupy pores inside the carbon particles due to the presence of the Coulomb force. Salt rejection occurs due to both physical adsorption and electrical adsorption contributions. ACC-CDI systems should have feed water salinity between 100 and 1000 ppm (typically between 5 and 50 mM and/or 0.5 and 5 mS/cm) and applied potential must be in the range 0.6–1.6 V DC (typically 1.2 V). Desalination/regeneration time can have any duration/time, from very short ~ 4 min, with little adsorption, to very long > 90 min, depending on when equilibrium concentration becomes stable. ACC-CDI rejections could reach up to 25 and 35% for plain ACC and ACC deposited with ZnO nanorods.

CDI performance metrics and equations, which identify the system feasibility, have been discussed and included the following: desalination efficiency, charge efficiency, SAC, ASAR, specific capacitance and Langmuir isotherm. Selected parameters when choosing an ideal electrode must involve high specific surface area, high electrical conductivity, high stability, high hydrophilicity, low costs and scalability. It was found that traditional CDI design has advantages over newer designs due to its simplicity and lower costs. However, other CDI architectures showed higher system's efficiency (e.g., MCDI), but with a significant added cost. Generally, removal efficiency increases at low salt concentrations and low flow rates. Though high feed concentrations would result in higher SAC, higher applied potentials, higher surface areas and longer CDI desalination times are favored for better CDI performance. Observed salt rejections and SACs of various non-composite ACC and composites ACC/ZnO and ACC/titania showed that the composite electrodes have much higher numbers. The highest achieved rejections were 25, 35 and 50% for ACC, ACC/ZnO and ACC/ titania, respectively, and the maximum observed SACs were 5.8, 8.5 and 8.1 for ACC, ACC/ ZnO and ACC/titania, respectively.
