Stop-flow discharge operation aggravates spacer scaling in CDI treating brackish hard water

Brackish water desalination has become a major opportunity to fulfill the rising clean water needs globally. Capacitive deionization (CDI) is an attractive electrochemical separation technology to treat water salinity, and the innovative discharge operation, termed stop-flow (SF) discharge, has demonstrated high efficiency to minimize the brine volume. This work systematically compared the desalination performance of CDI when single-pass and SF discharge operation modes were respectively used. Results showed that while the SF discharge operation could achieve remarkably higher water recovery (>80 %), this inevitably resulted in the deterioration of CDI performance when the feed contained higher concentrations (5 mM) of Caz} and Sat. Compared to the electrodes, the spacer and the desalination chamber could be more vulnerable to scaling in brackish water desalination. With the formation of glauberite and gypsum in SF discharge operation, the carbon materials from the electrodes constantly existed on the scaled spacer, which contributed to the blocking of the flow path and a significant increase in pressure loss. This study provides insight into the scaling mechanism for SF discharge in multi-cycle operation, which is crucial for the design of CDI materials and optimization of operating protocols for brackish water desalination.

Automated summary

Brackish water desalination is an important opportunity to meet the increasing global demand for clean water. Capacitive deionization (CDI) is an attractive electrochemical separation technology for treating water salinity, and the innovative stop-flow (SF) discharge operation has shown high efficiency in minimizing brine volume. This study compared the desalination performance of CDI with single-pass and SF discharge operation modes, revealing the trade-off between water recovery and CDI performance deterioration caused by higher feed concentrations. The study also provides valuable insights into the scaling mechanism during SF discharge operation, which is crucial for CDI material design and operating protocol optimization in brackish water desalination.