A team of researchers from the U.S. and China modified a microbial fuel cell - a device that uses naturally occurring bacteria to convert wastewater into clean water producing electricity - so it could desalinate salty water. The new process cleans wastewater, generates electricity, and removes 90 percent of salt from brackish water or seawater.
A typical microbial fuel cell consists of two chambers, one filled with wastewater or other nutrients and the other with water, each containing an electrode. Naturally occurring bacteria in the wastewater consume the organic material and produce electricity.
The researchers - who included Bruce Logan of Penn State, Xiaoxin Cao, Xia Huang, Peng Liang, Kang Xiao, Yinjun Zhou, and Xiaoyuan Zhang of Tsinghua University, Beijing - changed the microbial fuel cell by adding a third chamber between the two existing chambers, and placing certain ion specific membranes (membranes that allow either positive or negative ions through, but not both) between the central chamber and the positive and negative electrodes. Salty water to be desalinated is placed in the central chamber.
Salt not only dissolves in water, it dissociates into positive and negative ions. When the bacteria in the cell consume the wastewater it releases charged ions - protons - into the water. These protons cannot pass the anion membrane, so negative ions move from the salty water into the wastewater chamber. At the other electrode protons are consumed, so positively charged ions move from the salty water to the other electrode chamber, desalinating the water in the middle chamber.