A description of the technology—microbial capacitive desalination—was recently published in an open access paper in the RSC journal Environmental Science Water Research & Technology as the cover story.

One approach to accomplish sustainable produced water management is to develop technologies that remove both organic contaminants and salts without external energy consumption or potential net energy gain. In this context, recently developed microbial desalination systems (MDS) may provide a niche in the market. MDS is based on the fundamental work on bioelectrochemical systems (BES), which employ microorganisms to breakdown organic or inorganic sources of electrons and transfer those electrons to a terminal electron acceptor such as oxygen through a pair of electrodes. The internal potential generated between the anode and the cathode drives additional salt removal, and the energy can be harvested for electricity and chemical production. Different reactor configurations have been reported, such as a microbial desalination cell (MDC), in which three chambers were separated by a pair of ion exchange membranes and salt removal was accomplished by migrating ions from the middle chamber to the anode and cathode chambers.

This study used a newly developed microbial capacitive desalination cell (MCDC) to demonstrate its efficacy in removing both organic contaminants and salts from produced water collected from a shale gas field and its energy recovery during the operation. MCDC alleviates salt migration problems associated with MDC through the integration with capacitive deionization (CDI). CDI is a desalination method where an electrical potential is applied to high surface area electrodes to adsorb charged organic and inorganic species for desalination. CDI is a dynamic process of salt removal and recovery. When the electrical potential is removed, the capacitively desalinated salts can be removed and captured for beneficial use, and part of the electrical charge can be recovered.

CDI only requires a small voltage (<1.4 V) to form the electric double layer, so it can be externally powered by an MFC [microbial fuel cell]. Previous studies showed that such an MFC–CDI system could achieve a desalination rate of 35.6 mg of TDS per liter per hour, with a desorption rate up to 200.6 mg of TDS per liter per hour.

—Forrestal et al.

This microbial electrochemical approach takes advantage of the fact that the contaminants found in the wastewater contain energy-rich hydrocarbons, the same compounds that make up oil and natural gas. The microbes used in the treatment process eat the hydrocarbons and release their embedded energy. The energy is then used to create a positively charged electrode on one side of the cell and a negatively charged electrode on the other.

The MCDC system consists of three chambers, the anode, cathode and middle chambers which contain the electrodes for CDI.

The microbial capacitive desalination cell was able to remove total dissolved solids (TDS) at a rate of 2760 mg of TDS per liter per hour and chemical oxygen demand (COD) at a combined rate of 170 mg of COD per liter per hour—18 times and 5 times faster than the traditional microbial desalination cell (MDC), respectively. The MCDC had a coulombic efficiency of 21.3%, and during capacitive deionization regeneration, 1789 mJ g⁻¹ activated carbon cloth (ACC) was harvested.

Market background. Some oil and gas wastewater is currently being treated and reused in the field, but that treatment process typically requires multiple steps—sometimes up to a dozen—and an input of energy that may come from diesel generators.

Because of the difficulty and expense, wastewater is often disposed of by injecting it deep underground. The need to dispose of wastewater has increased in recent years as the practice of hydraulic fracturing, or “fracking,” has boomed. Fracking refers to the process of injecting a slurry of water, sand and chemicals into wells to increase the amount of oil and natural gas produced by the well.

Injection wells that handle wastewater from fracking operations can cause earthquakes in the region, according to past research by CU-Boulder scientists and others.

The demand for water for fracking operations also has caused concern among people worried about scarce water resources, especially in arid regions of the country. Finding water to buy for fracking operations in the West, for example, has become increasingly challenging and expensive for oil and gas companies.

Ren and Forrestal’s microbial capacitive desalination cell offers the possibility that water could be more economically treated on site and reused for fracking.

The beauty of the technology is that it tackles two different problems in one single system. The problems become mutually beneficial in our system—they complement each other—and the process produces energy rather than just consumes it.

—Zhiyong Jason Ren, senior author

To try to turn the technology into a commercial reality, Ren and Forrestal have co-founded a startup company called BioElectric Inc. In order to determine if the technology offers a viable solution for oil and gas companies, the pair first has to show they can scale up the work they’ve been doing in the lab to a size that would be useful in the field.

The cost to scale up the technology also needs to be competitive with what oil and gas companies are paying now to buy water to use for fracking, Forrestal said. There also is some movement in state legislatures to require oil and gas companies to reuse wastewater, which could make BioElectric’s product more appealing even at a higher price, the researchers said.

Ren and Forrestal have received funds from the National Science Foundation to work on scaling up the water treatment cell. The grant came after the pair participated in NSF’s Innovation Corps Program—aimed at pushing NSF-funded research beyond the lab—and took first place in their class.

Ren and Forrestal also worked with researchers Zachary Stoll and Pei Xu at New Mexico State University. Stoll and Xu are also co-authors of the article.

Resources

• Casey Forrestal, Zachary Stoll, Pei Xu and Zhiyong Jason Ren (2015) “Microbial capacitive desalination for integrated organic matter and salt removal and energy production from unconventional natural gas produced water” Environ. Sci.: Water Res. Technol. 1, 47-55 doi: 10.1039/C4EW00050A