Microbes could provide low-energy desalination

Researchers say they’ve found a way to use microbes to remove salt from water without the high energy input required by standard desalination methods.

The team of scientists from the US and China created the system by modifying a microbial fuel cell, a device that uses naturally occurring bacteria to convert wastewater into clean water and electricity.

Low-energy desalination would benefit many parts of the world where clean water for drinking, washing and other uses is already in scarce supply. It also holds great promise as water resources around the globe are threatened by climate change.

Currently, desalination relies on either reverse osmosis — in which water under high pressure is pushed through membranes that filter out the salt — or electrodialysis, which uses electricity to draw salts 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,” the research team writes in an online issue of Environmental Science and Technology.

“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, a professor of environmental engineering at Penn State. “Our main intent was to show that using bacteria we can produce sufficient current to do this.”

While the process works, however, it’s not yet practical in real life.

“(I)t took 200 milliliters of an artificial wastewater — acetic acid in water — to desalinate 3 milliliters of salty water,” Logan said. “This is not a practical system yet as it is not optimised, but it is proof of concept.”

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 altered the microbial fuel cell by adding a third chamber between the two existing chambers and placing certain ion-specific membranes — that is, membranes that allow through either positive or negative ions, but not both — between the central chamber and the positive and negative electrodes. Salty water to be desalinated is placed in the central chamber.

Seawater contains about 35 grams of salt per liter and brackish water contains 5 grams per liter. Salt not only dissolves in water, it dissociates into positive and negative ions. When the bacteria in the cell consume the wastewater, charged ions — protons — are released 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.

The desalination cell releases ions into the outer chambers that help to improve the efficiency of electricity generation compared to microbial fuel cells.

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

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 per cent of the salt is removed. However, a 90 per cent decrease in salt in seawater would produce water with 3.5 grams of salt per liter. Brackish water, by comparison, contains only 0.5 grams of salt per liter.

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. Mixing water from the two chambers together when they are discharged would once again produce neutral, salty water, so the acidity and alkalinity are not an environmental problem, assuming the cleaned wastewater is dumped into brackish water or seawater. However, the bacteria that run the cell might have a problem living in highly acidic environments.