Comparative studies of the electricity generation, desalination and wastewater treatment performance of the Three-Chamber and newly developed Five Chamber Microbial Desalination Cells

Poor wastewater treatment and limited potable water supply in the country are problems that require sustainable solutions. Available technologies to solve these problems are expensive especially to a developing country like Ghana. A recently developed technology, Microbial Desalination Cell (MDC) is considered a less expensive alternative to conventional wastewater treatment and desalination technologies. In MDCs, wastewater treatment occurs in the anode chamber and desalination in the desalination chamber. A third chamber, the cathode chamber is present to create a potential difference across the anode chamber and itself for electricity production. One setback of MDC systems however is that, most operate with chemical oxidants and buffers which can make this technology expensive. Thus, this research work sought to build MDCs which operate on alternative non-chemical oxidants sources and also, MDCs which are able to buffer electrolytes without the use of chemical buffers. Another objective of this study was to investigate the effects of rhamnolipid on MDCs electricity generation, desalination and wastewater treatment performances. The interactive effect of rhamnolipid and stirring on a five-chambered MDC was also investigated and ion exchange membranes of selected MDCs examined for the occurrence of fouled layers. A comparison between the electricity generation, desalination and wastewater treatment performances of three-chamber MDCs operating on different electron acceptor sources showed that, the threechamber chemical catholyte (3 C C) MDC performed better than the three-chamber water catholyte (3 W C) MDC and three-chamber plant-supported (3 P C) MDC. The 3 C C MDC produced the highest peak voltage of 282.91± 0.09 mV, coulombic efficiency of 78.61% and could desalinate 46.66% of saltwater. Its power (0.35 ± 0.10 W/m3 ) and current (1.24 ± 0.35 A/m3 ) densities were also the highest amongst the three-chambered MDCs. It produced a 42.81% COD reduction, 1.14% nitrate reduction and a 10.71% phosphorus reduction. On the other hand, the 3 P C MDC produced the least performance. The highest voltage recorded from it was 193.99 ± 0.80 mV. The 3 P C MDC could desalinate 23.34% of saltwater, produce 0.25 ± 0.06 W/m3 of power, 1.09 ± 0.11 A/m3 of current density with coulombic efficiency (CE) of 10.03%. It achieved a percentage COD reduction of 41.79%, highest percentage nitrate removal of 1.05% and phosphorus reduction of 9.97%. Apparently, the pH stabilization ability of neutralization chambers was lower than that of potassium phosphate buffer they were compared to. For instance, with the neutralization chamber, pH-change in the anolyte of the five-chamber chemical catholyte (5 C C) MDC was 1.66 ± 0.11 whiles pH change in the 3 C C MDC was 1.52 ± 0.15. Nonetheless, the 5 C C MDC produced a comparatively higher voltage of 343.57 ± 0.25 mV, percentage desalination of 50.01%, but a lower CE of 25.20%. Its power and current densities were 0.62 2 ± 0.13 W/m3 and 1.61 ± 0.21 A/m3 respectively. It was able to reduce COD concentration by 63.42%, nitrate concentration by 2.12% and phosphorus by 3.85%. The supply of rhamnolipids to anolytes of the five-chamber MDCs increased their performances. Voltage production of the 5 C C MDC increased from 343.57 ± 0.25 mV to 630.60 ± 1.44 mV, percentage desalination from 50.01% to 67.63% and COD, from 63.42% to 72.78%. In the case of the five-chamber water catholyte (5 W C) MDC, its voltage increased from 164.5 ± 0.11 to 623.7 ± 1.32 mV, percentage desalination from 43.74% to 63.21% and percentage COD reduction from 48.74% to 65.31%. Stirring (60 rpm) of water catholyte alone, could increase the percentage desalination of 5 W C MDC from 48.74% to 65.29% and voltage production from 164.5 ± 0.11 to 567.27 ± 18.06 mV. However, the interaction effect of rhamnolipid and stirring on the performance of the 5 W C MDC was insignificant. Analysis of membranes of the 5 W C MDC showed that, the membranes were fouled on both sides. And a comparison between the degree of fouling on cation exchange membranes (CEMs) of the 5 C C MDC and 5 W C MDCs revealed that, the CEM of the 5 C C MDC was more fouled though used for a lesser period of time. Having assessed the core performances and material requirements of MDCs investigated in this study, the 5 W C MDC was found to be the ideal MDC for possible future use in Ghana. Thus, the 5 W C MDC when integrated with conventional wastewater treatment and desalination technologies can aid these technologies achieve higher treatment qualities aside the additional benefit of ‗free‘ electricity the 5 W C MDC can provide.

Source: KNUST