Simultaneously cleaning wastewater, generating electricity, and desalinating water seems too good to be true, but a team of researchers has created a process that does just that.
Researchers from China and the United States have modified a microbial fuel cell, which uses naturally occurring bacteria to convert wastewater into clean water while producing electricity to desalinate water, according to a Pennsylvania State University (Penn State; University Park) news release. The cell can remove 90% of salt from brackish water or seawater.
Desalination processes, such as reverse osmosis or electrodialysis, require large amounts of energy, according to the release. However, a team of scientists, supported by the King Abdullah University of Science and Technology (Jeddah, Saudi Arabia) and the Ministry of Science and Technology of the People’s Republic of China, converted a microbial fuel cell that not only desalinates water but also cleans water and produces electricity.
The researchers hail from Penn State and Tsinghua University (Beijing). The Penn State researchers include Bruce Logan, Kappe professor of environmental engineering, and Maha Mehanna, postdoctoral fellow. The researchers from Tsinghua University are Xiaoxin Cao, Xia Huang, Peng Liang, Kang Xiao, Yinjun Zhou, and Xiaoyuan Zhang.
The typical microbial fuel cell consists of two chambers, one filled with wastewater and the other filled with water, the news release says. Each cell contains an electrode. The anode accepts electrons from microorganisms, and the cathode transfers electrons onto oxygen. As the bacteria consume organic material, oxidizing the organic material, electrons flow between the anode and cathode, producing electricity.
To add desalination abilities, the researchers added a third chamber between the two existing chambers. The new chamber holds saltwater. Between the cells, the researchers placed ion-specific membranes, which allow either positive or negative ions to pass, the release says.
In the new cell, as bacteria consume and oxidize the organic material in wastewater, they release protons into the wastewater, but these cannot pass the membrane, so chloride anions from the salty water move into the wastewater. As the cathode protons are consumed, sodium cations move from the saltwater into the cathode chamber. These two processes remove the sodium chloride, or salt, from the solution, Logan said.
The desalination cell releases ions into the outer chambers that help improve the efficiency of electricity generation, compared to typical microbial fuel cells. Conductivity of wastewater from microbial fuel cells is low, Logan explained. Since salt in the water helps the system generate electricity, as the central chamber becomes less salty, the conductivity decreases and the desalination and electrical production decrease, which is why only 90% of salt is removed, the news release says.
Salt in the seawater is reduced from 35 g/L to 3.5 g/L, which is still too salty to drink, but by starting with brackish water with a salt content of 5 g/L, the system could produce water that is safe to drink, Logan said.
The largest problem with the process is that it takes 200 mL of artificial wastewater, or acetic acid in water, to desalinate 3 mL of salty water. The system is not meant to be practical, since it hasn’t been optimized yet, but is meant to show that, using bacteria, the researchers could produce sufficient current to do this, Logan said.
Another problem with the current cell is that as protons are produced at one electrode and consumed at the other electrode, the chambers become more acidic and alkaline, the news release says. Mixing water from the two chambers together when they are discharged would produce neutral, salty water, so the acidic or alkaline water is not an environmental problem if the cleaned wastewater is dumped into brackish water or seawater, but the bacteria that run the cell might have a problem living in highly acidic environments, the release says.
During the experiment, the researchers periodically added a pH buffer to avoid the acid problem, but if the system is to produce "reasonable amounts of desalinized water," this problem will have to be considered, the release says.
"Our future goals are to reduce the water needed to desalinate the water and improve process efficiency," Logan said.
For more information about Logan’s microbial fuel-cell research, see www.engr.psu.edu/ce/enve/logan/default.htm.