Electricity and Desalination from Wastewater
In most part of the world safe and clean drinking water is unavailable for daily consumption and industrial use. Currently to desalinate water two kinds of technologies are being used. First is known as reverse osmosis and the second is electro-dialysis. Both of these processes need huge amount of energy. A team of scientists from China and U.S.A are working to eliminate ninety percent of the salts from seawater or brackish water. They are also trying to generate electricity from wastewater. “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,” reported in a recent online issue of Environmental Science and Technology.
Bruce Logan, Kappe Professor of Environmental Engineering, Penn State talks about the main highlights of the project, “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.”
The team is putting its efforts on a modified a microbial fuel cell for desalinating salty water. Microbial fuel cell is a device that cleverly utilizes naturally occurring bacteria to convert wastewater into clean water and producing electricity in the process.
Currently they are testing the theory and not trying to do something on commercial scale but practical results are quite encouraging for the team. Logan explains the purpose of the whole experiment, “Our main intent was to show that using bacteria we can produce sufficient current to do this. 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.”
A distinctive microbial fuel cell has two chambers. One chamber is filled with wastewater or other nutrients. The second chamber has water. An electrode was inserted in both the chambers. Naturally occurring bacteria becomes active in the wastewater, devours the organic material and generates electricity.
Later on the research team modified the microbial fuel cell by adding a third chamber between the two existing chambers. They also put certain ion specific membranes between the central chamber and the positive and negative electrodes. The ion specific membranes permit either positive or negative ions to pass but not both. Now they place salty water to be desalinated in the central chamber.
About 35 grams of salt per liter is found in seawater and brackish water contains 5 grams per liter. We know that salt dissolves in water and beaks down into positive and negative ions. When the bacteria start consuming the wastewater they also ionize the water. They release charged ions in water known as protons. These protons cannot get through the anion membrane. Therefore the negative ions move from the salty water into the wastewater chamber. What happens at the other electrode? Protons are being consumed so positively charged ions move from the salty water to the other electrode chamber. This way water is desalinated in the middle chamber. The desalination cell discharges ions into the outer chambers. This perks up the efficiency of electricity production compared to microbial fuel cells.
Logan is explaining how to kill two birds with a single stone, “When we try to use microbial fuel cells to generate electricity, the conductivity of the wastewater is very low. 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.”
Though this method has some problems we can hope that the research team will tackle those in recent future.