Simultaneous water desalination and electricity generation in a microbial desalination cell with electrolyte recirculation for pH control Youpeng Qu a , Yujie Feng a,⇑ , Xin Wang b , Jia Liu a , Jiangwei Lv a , Weihua He a , Bruce E. Logan a,c,⇑ a State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Nangang District, Harbin 150090, China b MOE Key Laboratory of Pollution Processes and Environmental Criteria, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China c Department of Civil and Environmental Engineering, 212 Sackett Building, The Pennsylvania State University, University Park, PA 16802, USA article info Article history: Received 30 August 2011 Received in revised form 5 November 2011 Accepted 12 November 2011 Available online 25 November 2011 Keywords: Microbial desalination cell Recirculation pH imbalance abstract A recirculation microbial desalination cell (rMDC) was designed and operated to allow recirculation of solutions between the anode and cathode chambers. This recirculation avoided pH imbalances that could inhibit bacterial metabolism. The maximum power density was 931 ± 29 mW/m 2 with a 50 mM phos- phate buffer solution (PBS) and 776 ± 30 mW/m 2 with 25 mM PBS. These power densities were higher than those obtained without recirculation of 698 ± 10 mW/m 2 (50 mM PBS) and 508 ± 11 mW/m 2 (25 mM PBS). The salt solution (20 g/L NaCl) was reduced in salinity by 34 ± 1% (50 mM) and 37 ± 2% (25 mM) with recirculation (rMDC), and by 39 ± 1% (50 mM) and 25 ± 3% (25 mM) without recirculation (MDC). These results show that electrolyte recirculation using an rMDC is an effective method to increase power and achieve efficient desalination by eliminating pH imbalances. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Desalination of seawater using reverse osmosis (RO) is a rela- tively energy efficient process for producing drinking water, but it still requires a considerable amount of energy (3.7 kWh/m 3 ) (Mehanna et al., 2010a; Semiat, 2008). The development of meth- ods to reduce the energy needed for desalination, and methods to produce energy from renewable sources, are essential for energy sustainability of the global water infrastructure. A new approach to partially or completely desalinate water, based on microbial desalination cells (MDCs), was recently invented. This process can desalinate water and generate electrical power or hydrogen gas by using exoelectrogenic bacteria that produce electrical cur- rent from the degradation of organic or inorganic matter (Cao et al., 2009; Chen et al., 2011; Jacobson et al., 2011; Luo et al., 2011; Mehanna et al., 2010a,b). The simplest MDC device consists of three chambers separated by an anion exchange membrane (AEM) next to the anode, and a cation exchange membrane (CEM) next to the cathode, with the salt solution in the middle chamber between the membranes. The ions in the salty water are transported through the respective AEM and CEM membranes when current is generated by the exoelectrogenic bacteria to bal- ance charge in the anode chamber from release of protons, and in the cathode chamber by the consumption of protons. This re- sults in desalination of the salty water without the use of any external energy source (Cao et al., 2009; Chen et al., 2011). Perfor- mance of an MDC can be increased by using stacks of desalination membranes to increase the transfer of charge per electron released to the circuit (Kim and Logan, 2011). One limitation to the extent of desalination is that bacteria re- lease protons into solution and lower the pH of the anode chamber, which can limit the length of the desalination cycle (Cao et al., 2009; Luo et al., 2011). In the cathode chamber, the pH is elevated due to the consumption of protons, resulting in pH imbalances in both chambers. The low anolyte pH inhibits bacterial activity (Luo et al., 2011), and the higher catholyte pH can result in poten- tial losses of 0.095 V per unit of pH (Rozendal et al., 2008; Zhang et al., 2010; Zhao et al., 2006). Therefore, it is important to mini- mize pH imbalances in the MDC in order to maximize the extent of desalination and power densities (Fornero et al., 2010; Luo et al., 2011). In a three chamber MDC, pH imbalances were reduced by increasing the anolyte volume (Cao et al., 2009) or by adding acids or bases (Cao et al., 2009; Chen et al., 2011; Jacobson et al., 2011). However, these approaches increase energy or material de- mands of the process, and adding chemicals for pH control would not be cost effective. A new type of modified MDC, called a recirculation MDC (rMDC), was developed to reduce pH variations in the electrode chambers and increase power densities in this study. In the rMDC, the solutions in the electrode chambers are recirculated through the anode and cathode chambers using thin tubing and an external pump, avoiding large changes in pH. The effect of recirculation was 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.11.045 ⇑ Corresponding authors. Tel.: +86 451 86283068, +1 814 863 7908, mobile: +86 13069891017; fax: +86 451 82373516. E-mail addresses: yujief@hit.edu.cn (Y. Feng), blogan@psu.edu (B.E. Logan). Bioresource Technology 106 (2012) 89–94 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech