Performance of microbial desalination cells with different cathode types in treating saline wastewater

Microbial desalination cells (MDCs), as an emerging desalination technology, have attracted increasing attention in recent years due to their ability to simultaneously achieve salt removal and wastewater treatment without the need for external energy input. In this study, the performance of two MDC systems with different cathode types-a biocathode (MDC1^#) and a permanganate cathode (MDC2^#)-was comparatively evaluated for the treatment of saline wastewater, with a particular focus on voltage output, desalination efficiency, and chemical oxygen demand (COD) removal. Experimental results showed that the average output voltage of MDC2^# reached 742.02 mV, which was significantly higher than that of MDC1^# (695.6 mV). Its maximum power density was as high as 6.22 W/m3, approximately six times that of MDC1^#. Moreover, MDC2^# exhibited a higher average chloride removal rate in the desalination chamber (32.34 mg/h), compared to 17.13 mg/h for MDC1^#, indicating superior desalination performance. However, in terms of electron recovery, MDC1^# achieved a much higher average Coulombic efficiency (28.8 ± 18.7%), nearly three times that of MDC2^#, suggesting more efficient electron utilization with the biocathode. Regarding ammonium removal, MDC1^# demonstrated a higher initial removal efficiency within the first 96 h (74.3%, with an average rate of 4.17 mg/h), but this declined sharply over time, with the later-stage rate dropping to only 0.32 mg/h (less than 10% of the initial rate). In contrast, MDC2^# maintained a relatively stable ammonium removal rate throughout the operation (ranging from 0.58 to 3.27 mg/h, with an average of 1.92 mg/h). In addition, both systems achieved stable COD removal at the anode, with efficiencies consistently above 85%. Overall, the permanganate cathode is more suitable for applications that require high voltage output and efficient desalination, whereas the biocathode shows significant advantages in organic pollutant removal and energy recovery. This study provides a theoretical foundation for the rational selection of cathode types based on the characteristics of saline wastewater, offering valuable guidance for optimizing MDC system performance.