New process can remove 90 percent of salt from seawater

An international team of researchers from China and the US has determined that a process that cleans wastewater and generates electricity can also remove 90 percent of salt from brackish water or seawater.

Clean water for drinking, washing and industrial uses is a scarce resource in some parts of the world. Its availability in the future will be even more problematic.

Many locations already desalinate water using either a reverse osmosis process - one that pushes water under high pressure through membranes that allow water to pass but not salt - or an electrodialysis process that uses electricity to draw salt ions out of water through a membrane.

Both methods require large amounts of energy.

“Water desalination can be accomplished without electrical energy input or high water pressure by using a source of organic matter as the fuel to desalinate water,” according to the researchers.

“The big selling point is that it currently takes a lot of electricity to desalinate water and using the microbial desalination cells, we could actually desalinate water and produce electricity while removing organic material from wastewater,” said Bruce Logan, Kappe Professor of Environmental Engineering, Penn State.

The team 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.

“Our main intent was to show that using bacteria we can produce sufficient current to do this,” said Logan.

“However, it took 200 milliliters of an artificial wastewater - acetic acid in water - to desalinate 3 milliliters of salty water. This is not a practical system yet as it is not optimized, but it is proof of concept,” he added.

“When we try to use microbial fuel cells to generate electricity, the conductivity of the wastewater is very low,” said Logan.

“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,” he added.

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, which is why only 90 percent of the salt is removed.

However, a 90 percent decrease in salt in seawater would produce water with 3.5 grams of salt per liter, which is less than brackish water. Brackish water would contain only 0.5 grams of salt per liter.