By Dr. Zhiyong (Jason) Ren, Associate Professor of Environmental Engineering, University of Colorado at Boulder
Microbial capacitive deionization (MCD) shows promise as a sustainable, low-cost treatment solution for produced water.
Wastewater 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 m3 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 the cathode chamber, and the electrical potential generated between the anode and cathode drives the removal of salts, heavy metals, and charged organic matter for water purification and desalination. Electrical energy can be generated during ion discharge from the electrodes, similar to a rechargeable battery. The MCD process has been tested at lab- and pilot-scale, and a mobile trailer system with a capacity of 5 gallons per minute is being developed for field testing. Using actual produced water obtained from Denver-Julesburg Basin, which has chemical oxygen demand (COD) ranging from 1,100 to 2,600 mg/L and TDS ranging from 16,000 to 28,000 mg/L, the lab systems were able to remove 10,200 to 66,240 mg TDS/L/day and ~4,000 mg COD/L/day. In addition to water treatment, the system generates 89 to 131 W/m3 of electricity, which is harvested and stored for powering online sensors.
The figure below shows the pilot system, developed by the University of Colorado at Boulder, which is stackable and can be mounted on a truck. The inset shows the water before and after treatment. Preliminary techno-economic analysis shows the MCD system is inexpensive to operate ($0.10 to 0.60/ barrel depending on treatment need) and credited with low energy consumption; moreover, extra electricity and water are produced due to the use of sodium percarbonate as an electron acceptor.
Clear results: The MCD pilot shown with influent-effluent comparison.
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. This not only reduces system capital costs by eliminating multiple units, but it also reduces operational costs by reducing energy consumption and producing renewable energy and water. MCD can also be integrated with other treatment units with complementary functions, so overall efficiency can be improved. For example, MCD has been connected with electrocoagulation (EC), with EC removing TSS while MCD removes COD and TDS. In the meantime, MCD can provide the electricity needed by EC, making the system energy-neutral. If further treatment is needed for certain reuses, MCD can also be integrated with membrane technologies such as reverse osmosis or forward osmosis to provide high-quality effluent.
While microbial electrochemical processes show good potential for oil and gas wastewater treatment and reuse, there are many challenges ahead. For extra-high-saline produced water like that generated at the Marcellus Shale in Pennsylvania or the Bakken formation in North Dakota, MCD may not be very efficient due to its limitation on adsorption capacity; rather, it may serve as a low-cost pretreatment for membrane distillation. Similar to the cost of other desalination technologies, the cost of TDS removal by MCD is still high for the industry compared with deep well injection, so further development and new incentives for external water reuse such as irrigation are needed for possible market adoption. There are several new articles covering the technology and market, and critical comments from experts can be found there: Microbes Could Help Clean Up After Fracking (CBS News) and New Technology Could Make Treatment of Oil and Gas Wastewater Simpler, Cheaper (University of Colorado Boulder).
About The Author
Dr. Zhiyong (Jason) Ren is an Associate Professor of Environmental Engineering at the University of Colorado at Boulder. He co-founded Bioelectric Inc., a cleantech startup focusing on innovative water and energy solutions. An expert in microbial and electrochemical processes for energy and environmental applications, Ren has been funded by NSF, DoD, EPA, and private sponsors to conduct water-energy R&D. He has published more than 100 journal and conference articles and has filed four patent disclosures. His research findings have been featured by NPR, ABC, and CBS. More info is available at http://spot.colorado.edu/~zhre0706/.