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Microbial capacitive deionization (MCD) shows promise as a sustainable, low-cost treatment solution for produced water. By Dr. Zhiyong (Jason) Ren W astewater management during unconventional oil and gas exploration and production is one of the biggest challenges for the industry and society. Wastewater is the industry's largest byproduct, with the annual amount generated estimated at nearly 900 billion gallons (~21 billion barrels). Unlike municipal or other industrial wastewater, this wastewater (also called produced water) is highly variable in quality and quantity, and its physicochemical composition can be very complex. The water contains dissolved and suspended organics, total suspended solids (TSS), total dissolved solids (TDS), chemical additives, microbes, and naturally occurring radioactive materials. The water is considered hazardous waste and must be treated or disposed of safely. Currently about 90 percent of such water is disposed of through deep underground injection. However, this method leads to the loss of a precious water resource, and the safety and health concerns have been widely reported in recent years, such as frequent seismic activities around the injection area. The transportation of large volumes of water to injection wells and centralized treatment facilities has also generated problems in neighboring communities. Therefore, treatment and reuse of this water is an imminent need, especially in western states where drinking and irrigation water are lacking and water acquisition can be difficult. Current water management costs already account for more than 10 percent of a well's operating expense, so it is difficult for the industry to implement even more expensive treatment technologies while maintaining profitability. In order to remove both organic contaminants and salts, traditionally multiple technologies are combined because biological treatment can be effective in organic degradation but is not suitable for salt removal; meanwhile, membrane-based desalination technologies are ineffective in organic removal and require extensive pretreatment to protect system components. Furthermore, both processes consume high energy for aeration and pumping, and previous studies showed 10 to 100 kWh of electricity is needed for the treatment of 1 m 3 of water. A Simpler And Cheaper Solution One approach to accomplishing sustainable produced water management is to develop technologies that remove both organic carbon and TDS without consuming external energy — or that potentially gain net energy. In this context, recently developed microbial capacitive deionization (MCD) may provide a market niche. MCD is based on the fundamental work derived from a platform called microbial electrochemical technology (MET). MCD employs microorganisms to break down organic or inorganic sources of electrons in the wastewater, and the electrons (i.e., current) flow through an external circuit to specially-designed membrane assemblies in the middle chamber and finally combine with protons in the cathode chamber to generate water. The current can be directly harvested as electricity or used for chemical production in 26 wateronline.com n Water Innovations New Tech Offers Low-Energy Oil And Gas Wastewater Management The main value proposition of MCD is that it offers a simpler solution for oil and gas water management because it can simultaneously remove hydrocarbons, salts, and metals in one reactor.