By Laura Martin
Oil and gas production is booming — and isn’t slowing down anytime soon. The International Energy Agency predicts that global oil production will increase to almost 100 million barrels per day by 2035. And as the oil industry grows, so does the amount of produced water that must be treated. A variety of technologies are available for produced water treatment, and more are emerging every day. Fifty-four of these were considered in “The Technical Assessment of Produced Water Treatment Technologies,” a new study created by the Colorado School of Mines (CSM), a research university focused on the development and stewardship of the earth's natural resources. We’ve selected 10, some common and some up-and-coming, and shared the advantages and disadvantages reported in the study side-by-side.
Media filtration — most commonly using sand, anthracite coal, or walnut shells — is a simple yet effective method for treating produced water. Filtration can effectively remove oil and grease and total organic carbon (TOC), and be used on highly salty water without issue. Energy consumption is minimal. However, the expected lifetime of filtration media is lower than other methods, with replacement required frequently, depending on media type and feed water quality. The process also requires a vessel to contain the media and pumps and plumbing to implement backwashes. Chemicals may also be required to increase particle size, enhance separation, and for media regeneration.
Adsorption is commonly used for the treatment of produced water, as it can remove more than 80 percent of heavy metals and results in nearly 100 percent product water recovery. A variety of materials are used for adsorption, including zeolites, organoclays, activated alumina, and activated carbon, which can remove iron, manganese, TOC, and other contaminants. Chemical use is minimal. However, the adsorbent can be easily overloaded with large concentrations of organics, so this process is not always ideal for primary treatment. The media also eventually become consumed with contaminants and must be disposed or regenerated using chemicals. Regeneration creates a liquid waste product that must be disposed. Media may require frequent replacement or regeneration depending on type and feedwater quality.
Oxidants like chlorine, chlorine dioxide, permanganate, oxygen, and ozone are also frequently used to treat produced water. Oxidation can be used to remove organics and some inorganic compounds like iron and manganese. No pretreatment is required, but solid separation post-treatment is often necessary to remove oxidized particles. Oxidation can sometimes be a more expensive method, as chemical costs may be high and the purchase of chemical metering pumps is required for dosing. Oxidation equipment has a life expectancy of 10 years or greater, as long as periodic calibration and maintenance is performed.
Ceramic membranes show great potential for the treatment of oil-contaminated wastewaters. While the technology has yet to become commonplace, ceramic membrane are currently being used successfully in a full-scale facility in Wellington, CO, to treat oilfield produced water, as well as on smaller scales throughout the country. Ceramic ultrafiltration and microfiltration membranes are made from oxides, nitrides, or carbides of metals such as aluminum, titanium, or zirconium. Ceramic membranes are more resilient than other types of membranes and are mechanically strong, chemically and thermally stable, and can achieve high flux rates. After ceramic microfiltration or ultrafiltration is performed, the resulting water is free of suspended solids and nearly all non-dissolved organic carbon is removed. Energy requirements for ceramic membranes are lower than for other types of membrane filtration, despite having similar infrastructure requirements. Ceramic membranes can be more expensive than other membrane technologies, but costs are expected to drop as more research is conducted and they become more widely used.
Electrodialysis (ED)/Electrodialysis Reversal (EDR)
ED and EDR, both electrochemical-charge-driven separation processes, have been tested for produced water treatment at the laboratory-scale and are already being used for seawater and brackish water desalination and wastewater reclamation. The technologies utilize “dissolved ions which are separated from water through ion permeable membranes under the influence of an electrical potential gradient,” according to the CSM report. ED/EDR membranes are not as susceptible to degradation by chlorine and can treat surface and wastewaters that have high concentrations of organic materials and microorganisms without significant fouling. The technologies can withstand harsh conditions and are fairly flexible to varying water quality. However, ED/EDR have limited ability to remove non-charged constituents, including organics molecules, silica, and boron. A high level of skilled labor is also required to operate ED/EDR systems.
Capacitive Deionization (CDI)
Capacitive deionization (CDI) is an emerging technology for produced water treatment that has been studied at the University of California since the 1980s. The CSM technical assessment describes how it works: “In CDI, ions are adsorbed onto the surface of porous electrodes by applying a low voltage electric field, producing deionized water. The negative electrodes attract positively charged ions such as calcium, magnesium, and sodium, and the positive electrodes attract negatively charged ions such as chloride, nitrate, and silica. The major mechanisms related to the removal of charged constituents during water treatment are physisorption, chemisorption, electrodeposition, and/or electrophoresis.”
To enhance the performance of CDI, an electronic water purifier can be used. Together the two can achieve a 90 percent removal of total dissolved solids (TDS) when treating produced water. It has a very low infrastructure requirement, a small footprint, and is mobile. The technology also requires low levels of monitoring and control and skilled labor. It works best with produced water containing less than 3,000 mg/L TDS. Studies have shown that CDI results in a poor removal of uncharged substances such as boron and organics.
Brackish Water Reverse Osmosis (BWRO)
With the right pretreatment methods, BWRO can be an excellent method for treating produced water. It can achieve a high removal rate of divalent ions like calcium, magnesium, sulfate, iron, and arsenic, and a moderate to high removal rate of monovalent ions like [maybe substitute “such as”?] sodium, potassium, and chloride. Substantial removal of organic compounds can also be achieved with BWRO. BWRO is highly sensitive to organic and inorganic constituents in the feedwater. The technology works best for produced water with TDS ranging from 500 to 25,000 mg/L. Most BWRO systems are automated, which lessens the demands for labor. However, a skilled technician is required to perform routine system maintenance. Depending on operating conditions, BWRO membranes will require replacement within three to seven years.
Forward Osmosis (FO)
FO is not commonly employed for produced water treatment, but studies have shown promise for its use in industrial waste stream treatment and enhanced water recovery during brackish water desalination. FO membranes are less susceptible to irreversible fouling and scaling and therefore perform more efficiently with feed streams containing high concentrations of sparingly soluble salts. Like BWRO, FO can achieve high removal of many monovalent, divalent, and multivalent inorganic contaminants, and high removal of organic contaminates. FO doesn’t require the application of hydraulic pressure on the feed stream, making it a more energy-efficient method than RO. Membranes used for FO are dense, non-porous barriers, similar to RO, but composed of a hydrophilic, cellulose acetate active layer cast onto either a woven polyester mesh or a micro-porous support structure.
Multi-Stage Flash Distillation (MSF)
MSF uses flash evaporation, a process that evaporates water by reducing the pressure. “The MSF distillation process is based on the principle of flash evaporation in which water is evaporated by reducing the pressure as opposed to raising the temperature with additional heat/energy. In MSF the heated feed water flows into a stage with lower pressure and immediately boils or flash into steam. The high efficiency of the MSF process is achieved by preheating new feed water through capturing of the heat of condensation in each flash chamber or stage,” reports CSM in their technical assessment.
There are several advantages to MSF. Because the elevated process temperatures automatically sterilize the water, there is no need for the use of biocides. Compared to membrane technologies, MSF requires less-rigorous pretreatment and feed conditioning. MSF can also be easily adapted to work with a variety of water qualities. It is currently a common technology for seawater and brackish water desalination, and is viable as a produced water treatment method. MSF does require significant infrastructure commitments. Typically, a large physical plant is required, and because the technology relies on the availability of low-pressure steam, an adjacent power plant that is willing to participate in a cogeneration arrangement may also be necessary.
Freeze/Thaw Evaporation (FTE)
FTE is an up-and-coming technology that is just beginning to be used for produced water treatment. There are two FTE plants operating commercially in Wyoming — one in the Jonah gas field south of Pinedale and the other in the Red Desert near Wamsutter — as well as a few other smaller scale plants elsewhere. The treatment capacity of the two Wyoming plants is more than 40,000 GPD.
The FTE process, which was developed by the Energy & Environment Research Center and B.C. Technologies, Ltd., combines a freezing and thawing cycle with conventional evaporation technology. The CSM technical assessment describes the process: “When the ambient air temperature is below 32 degrees F (0 degrees C), the saline water (feedwater) is sprayed or dripped onto a freezing pad to create an ice pile. Relatively pure ice crystals form and an unfrozen solution (brine) containing elevated concentrations of the dissolved constituents drains from the ice. The runoff can be diverted to a brine storage facility or back to the feedwater storage facility for recycling. When the temperatures rise, the ice melts and the runoff from the freezing pad is highly purified water that can be diverted to a treated water storage facility for beneficial uses or surface discharge.” This process is done outdoors when temperatures are below 32 degrees. During the non-winter seasons, FTE operations are converted into conventional evaporation ponds. The FTE process can remove over 90 percent of produced water constituents, including TSS, TDS, total recoverable petroleum hydrocarbons (TRPH), volatile organic compounds (VOC), semi-volatile organic compounds, and heavy metals. The process cannot treat wastewater that has more than 5 percent methanol. Because the process requires cold conditions, it can be the significantly limited by climate conditions.
This is just a sampling of the produced water treatment technologies available. Which do you think have the most potential?