Biocathode Microbial Desalination Cell Driven Capacitive Deionization For Salt Water Desalination

Global desalination capacity rose significantly and fresh water from desalination grew by 30% per year since 1980. However, desalination as a way to increase drinking water yield costs a huge energy consumption. In reverse osmosis desalination plants, the energy consumption is about 3 to 4 k Wh/m3. The shortage of energy and the high cost of desalination operation limite its application. In some special cases, such as islands far away from land and ships on the seas, it is hard to get fresh water from sea water due to the lack of elentricity. In 2009, the invention of microbial desalination cell(MDC) realized the desalination process without electricity consumption. During 5 years’ research, the performance of MDCs on the desalination efficiency had greatly increased, however, the high cost of MDC operation, due to the use of expensive Pt in air cathode as catalyst or recycling of potassium ferricyanide solution as the cathode electron acceptors, still limits its expansion and the possibility of practical application. Along with salt removal in the desalination chamber, the internal resistance of MDC systems increases, thus decrease the desalination performance. Therefore, MDCs are not suitable for the desalination of low salt concentration salt water.To solve the above problems, a detailed research on developing the microbioal cathode catalyst and combining MDC with membrane captive deionization(MCDI) for deep desalination was carried out. The biological cathode MDC that can realize simultaneous desalination and electricity production was proposed and a self energy supported biocathode MDC-MCDI system was developed to improve the desalination rate.MDC with aerobic bacteria as the cathodic catalyst was developed and named as bio-cathode MDC. Compared with the aeration cathode without microorganisms, the bio-cathode MDC could effectively reduce the cathodic internal resistance and improve the MDC voltage and power output. As a result, the desalination rate improved significantly. When the initial salt concentration was 35 g/L, the maximum voltage output achieved 570 m V with an external resistance 1000 Ω, and the effluent salt concentration reached lower than 1 g/L with salt removal efficiency higher than 90% after 480 h operation. However, when salt concentration was lower than 1 g/L, the salt removal rate was only 16% of that with salt concentration 35 g/L. When the initial concentration was 1 g/L in the desalination chamber of the MDC, its desalination rate reduced to 0.57 mg/h, and its internal resistance rose from 217 to 793 Ω.MDC was used as the power supply to drive MCDI for further salt removal in this research. The salt removal performance of the MCDI was better than that of the CDI when the salt concentration was 1 g/L. MCDI has more electric adsorption capacity when MDC was used as the power supply compare to the 0.8Vconstant current. The adsorption capacity of the MCDI with two MDCs in parallel connection mode was 1.6 times higher than that with 0.8V constant current. The coupled MDC-MCDI system not only increased the desalination effect of the MDC, but also realized the continuous operation of MCDI for desalination. This process improved the desalination rate by 36.2%. For the synthetic seawater with salt concentration 35 g/L, using MDC-MCDI system could obtain fresh water after 18 cycles treatment.After 5500 h operation, the power density, coulomb efficiency and desalination rate of biological cathode MDC decrease by 71%, 44% and 27%, respectively. The main reason was the micorbes deposit on the anion exchange membrane and the cation exchange membrane which led to the decline of membrane permeability and increased the internal resistance of the MDC. Changing the ion exchange membrane of the biological cathode MDC could effectively recover the performance of the MDC. Compared with the reported researches on MFC, the anodic microbial diversity of the MDC was less than those of MFCs, but the amount of Proteobacteria from the MDC was 30% more than that from MFCs anode. However, the amount of Planctomycetes in the cathode biofilm was much more than the one deposited on the CEM, which illustrated that Planctomycetes may be associated with oxygen reduction in the cathode. The growth of microorganisms in the desalination chamber was observed after the long term operation, the microbial community structure of the sample taken from the desalination chamber was similar to that on the cathode or on the AEM surface..In order to solve the problems of organic pollution in the desalination chamber, four chamber MDCs(PMDC/CMDC) with a buffer chamber were developed. Compared to the MDC, the voltage output of the PMDC and CMDC with 200 Ω external resistance decreased by 33.8% and 37.4%, respectively. According to the EIS determination, the internal resistance of the MDC, PMDC and CMDC were 96.7, 186.9 and 225.7Ω, respectively. The PMDC was more suitable for desalination under high initial salt concentration, it could effectively prevent CH3COO- and PO43- transfer into the desalination chamber, however, it can not prevent total nitrogen passing into the salt water.The total amount of transferred Ac- and TP in the PMDC was 105.7±26.3 and 28.8±9.8 mg/L, which were 15.8% and 35.6% of those transfered in the conventional MDC.Cl- concentration could reduce to drinking water level(<250 mg/L) when salt water with the initial salt concentration 35 g/Lwas desalinated by the coupled MDC-MCDI system. However, the effluent still needs further treatment for organics and P removal. Although there are many issues need to be resolved, the MDC-MCDI system make it possible to get fresh water from sea water in places out of energy sulpply such as islands far away from land and ships in the ocean.