Desalination plants generally employ one of two methods to produce clean water – reverse osmosis or electrodialysis. Unfortunately, both processes require large amounts of energy, but an international team of researchers has proven a process that cleans wastewater can also remove 90 percent of salt from brackish water or seawater while generating electricity.
To achieve the remarkable feat a team of researchers from China and the U.S. modified a microbial fuel cell – a device that uses naturally occurring bacteria to convert wastewater into clean water - so it could also desalinate salty water.
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’ modification added a third chamber between the two existing chambers into which the salty water to be desalinated was placed. They also placed certain ion-specific membranes between the central chamber and the positive and negative electrodes that allow either positive or negative ions through, but not both.
When salt dissolves in water it also dissociates into positive and negative ions, and when the bacteria in the cell consume the wastewater it releases charged ions, or protons, into the water. Since these protons cannot pass the anion membrane, negative ions move from the salty water into the wastewater chamber, while at the other electrode, protons are consumed, so positively charged ions move from the salty water to the other electrode chamber, thereby desalinating the water in the middle chamber.
Unfortunately, because the salt in the water helps the cell generate electricity, as the central chamber becomes less salty, the conductivity decreases and the desalination and electrical production decreases. This is why only 90 percent of the salt is removed. However, a 90 percent decrease in salt in seawater would produce water with 3.5g of salt per liter, which is less than brackish water. Brackish water would contain only 0.5g of salt per liter.
"When we try to use microbial fuel cells to generate electricity, the conductivity of the wastewater is very low," said Bruce Logan, Kappe Professor of Environmental Engineering, Penn State. "If we could add salt it would work better. Rather than just add in salt, however in places where brackish or salt water is already abundant, we could use the process to additionally desalinate salty water, clean the wastewater and dump it and the resulting salt back into the ocean."
"Our main intent was to show that using bacteria we can produce sufficient current to do this," said Logan, "however, it took 200ml (6.7oz) of an artificial wastewater - acetic acid in water - to desalinate 3ml (0.1oz) of salty water. This is not a practical system yet as it is not optimized, but it is proof of concept."
Another problem with the current cell is that, as protons are produced at one electrode and consumed at the other electrode, these chambers become more acidic and alkaline, which could pose a threat to the bacteria that run the cell which might struggle to survive in highly acidic environments.
To overcome the acid problem the researchers periodically added a pH buffer during their experiment, but they admit this problem will also need to be addressed if the system is to produce reasonable amounts of desalinized water.