In fifteen years, two out of every three persons on the globe may be living in water-scarce conditions (WHO, 2009). Desalination is one of the solutions to provide freshwater. It is estimated that in 2015, the worldwide desalination capacity will increase from 44.1 to 97.5 million cubic meters of water per day (Kristen, 2007; Lattemann et al., 2009). Concurrently, the demand for oil is expected to exceed production within the same time frame. This poses a major problem when we know that energy accounts for 40 percent of the total cost of desalination (Figure 1). Worldwide efforts are being undertaken to tackle climate change and reduce the production of greenhouse gases. In their report on energy, Barack Obama and Joe Biden asserted: “We believe we have a moral, environmental, economic, and security imperative to address our dependence on foreign oil and tackle climate change in a serious, sustainable manner.” The U.S. government will invest $150 billion over 10 years in renewable energies.

Microbial fuel cell (MFC) technologies represent a new approach to directly generate electricity from biomass using bacteria. In an MFC, microorganisms oxidize organic matter on the anode producing electrons. The electrons flow through the external circuit to the cathode where the reduction of oxygen occurs (Figure 2) (Logan et al. 2009). In 2009, a group of researchers from Tshinghua University, China in collaboration with Penn State, built a novel device called a microbial desalination cell (MDC) that could simultaneously desalinate water and produce energy (Cao et al., 2009). The concept is similar to water electrodialysis (Shaposhnik et al., 1997) but instead of applying an external source of energy, the MDC uses the electrical energy produced by the bacteria.

Fig 1: The world’s largest desalination plant is the Jebel Ali Power and Desalination Plant located 35 kilometres southwest of Dubai in the United Arab Emirates

The microbial desalination cell (MDC) consists of a MFC that was modified by adding a desalination compartment in a middle chamber. The saline water is placed between an anion (AEM) and a cation exchange membrane (CEM). The AEM is placed next to the anode and the CEM next to the cathode. Naturally occurring bacteria are used on the anode, and they grow using organic matter (acetate) and produce electrical current and release protons into the water. Protons cannot move to the cathode because they cannot diffuse through the AEM as only negatively charged ions can pass through this membrane. In order to maintain the charge balance, an anion (Cl–) flows from the middle desalination chamber to the anode. At the cathode, protons are removed from water and so sodium ions (Na+) in the desalination chamber move to the cathode chamber to balance charge. As a consequence sodium chloride salt (NaCl) in the middle chamber is removed; thus water is desalinated (Figures 3 and 4).
 

Fig 2: Schematic of a two-chambers air-cathode microbial fuel cell

In the original MDC developed by Cao et al., they used organic matter as a fuel and ferricyanide as a catholyte to desalinate water. The use of this chemical catholyte was not sustainable and would therefore be impractical for large scale systems. In addition, they replaced the solutions in the anodic and cathodic compartments several times over a cycle, resulting in the use of 200 mL of anolyte and 100 mL of catholyte to desalinate 3 mL of water. We therefore conducted additional experiments to demonstrate the feasibility of using an air-cathode in an MDC to avoid the use of ferricyanide, with the cathodic chamber filled with phosphate buffer (Mehanna et al. 2010a). We also investigated the extent of desalination in a single batch cycle, using equal volumes of solutions in all of the three compartments. We operated our system in batch-mode, and the solutions in all the compartments were changed at the end of each cycle. Several parameters were investigated: the nature of the inocula (pre-acclimated suspension of bacteria from a running MFC or domestic wastewater from the Primary clarifier of the Pennsylvania State University Wastewater Treatment Plant), the salt concentration in the desalination chamber (5 g/L and 20 g/L NaCl) and the types of the membranes (commercial and custom-made membranes).

Fig 3: Schematic of an air-cathode microbial desalination cell

Using an air-cathode MDC, we achieved up to 60±7 percent salt removal in the presence of 5 g/L NaCl in the middle compartment (Figure 5) and 50±7 percent with 20 g/L NaCl (Mehanna et al. 2010a). The MDC produced a maximum power density of 580 mW m-2. Coulombic efficiencies, defined as the percentage of electrons recovered as electricity versus that in the starting organic matter, reached 68±11 percent. To further improve the desalination, we tested custom made membranes by researchers in the Department of Materials Science and Engineering at Penn State. They made AEMs and CEMs that were ten time thinner and had twice as much ion exchange capacity compared to the commercial membranes. The “home-made” membranes improved the desalination performance by 26 percent with salt removal reaching 63±2 percent when the middle compartment contained 20 g/L NaCl (Mehanna et al. 2010a).

Results showed that the type of inoculum used did not affect the performance of the MDC. Community analysis of MDCs anodic microbial populations by 16 S RNA clone library showed that the anode population was dominated by Geobacter sulfurreducens regardless of the type of the initial inocula (Mehanna et al. 2010b). This is not surprising since G. sulfurreducens is known to produce high current densities in bioelectrochemical systems.

Fig 4: Picture of two running air-cathode microbial desalination cells

The use of the MDC provided an added benefit of wastewater treatment and power generation, while achieving a noticeable extent of desalination.  The conductivities of the desalinated water were higher than those of drinking water. Therefore, it may be that the most efficient use of wastewater as a source of organic matter for desalination in an MDC is to use it to pre-treat water that is then used in a downstream reverse osmosis (RO) process. Among the common desalination methods, RO is one of the least energy intensive methods yet it still requires 3.7 kW h m-3. The energy needed for RO decreases with a reduction in the salinity of the treated water, and therefore reducing the conductivity of the saline water by more than 60 percent would greatly benefit energy consumption in the RO process. The combination of all these factors suggests that MDCs may be very useful as a pre-treatment method for water desalination using RO. Besides that, the MDC gains further importance when we know that about 82 percent of the present desalination capacity of 44.1 million cubic meter per day is produced from saline water (63 percent from seawater, 19 percent from brackish water, and 5 percent from waste water sources) (Lattemann et al., 2009). Being able to notably reduce the salinity of the saline water without the need for any external source of energy is a promising approach to reducing energy costs for water desalination.

Fig 5: Conductivities in each compartment. The anolyte contains 2 g/L acetate. The middle compartment contains 5 g/L NaCl. The catholyte is 50 mM phosphate buffer. Data from Mehanna et al. (2010a).

There are several advantages for desalinating water using an MDC. This method doesn’t require an external, non renewable power supply. The source of energy is the biodegradable organic matter. Electrical energy is generated while the water is desalinated. The produced energy can be used to get additional energy for water pumping, for example. The next step would be ultimately to run this MDC in continuous mode. The main next challenge is to bring this technology out of the laboratory to achieve desalination at larger scales.
 

References

•    World Health Organisation (WHO). “Water, Health, and Ecosystems.” 03 March 2009. http://www.who.int/heli/risks/water/water/en/index.html. paragraph 6-7.
•    Kristen, K. (2007) Environmental costs of desalination. Environ. Sci. Technol. 41(16) 5576-5579.
•    Lattemann, S., Kennedy, M., Schippers, J. and Amy, G. (2010) Chapter 2 Global Desalination Situation, Sustainability Science and Engineering 2, 7-39.
•    Logan, B.E., Aelterman, P., Hamelers, B., Rozendal, R., Schröeder, U., Keller, J., Freguiac, S., Verstraete, W. and Rabaey, K. (2006) Microbial Fuel Cells: Methodology and Technology. Environ. Sci. Technol. 40(17) 5181-5192.
•    Cao, X., Huang, X., Liang, P., Xiao, K., Zhou, Y., Zhang, X. and Logan, B.E. (2009) A new method for water desalination using microbial desalination cells. Environ. Sci. Technol. 43(18), 7148-7152.
•    Shaposhnik, V.A. and Kesore, K. (1997) An early history of electrodialysis with permselective membranes. J. Membr. Sci. 136(1), 35-39.
•    Mehanna, M., Saito, T., Yan, J., Hickner, M., Xiaoxin, C., Huang, X. and Logan, B.E. (2010a) Using microbial desalination cells to reduce water salinity prior to reverse osmosis. Energy Environ. Sci. 3(8), 1114-1120.
•    Mehanna, M., Kiely, P.D., Douglas, F.C. and Logan, B.E. (2010b) A microbial electrodialysis cell for simultaneous water desalination and hydrogen gas production Environ. Sci. Technol. Under revision.