Salt is the key parameter that determines how produced water is managed onshore. If produced water is injected underground for enhanced recovery or disposal (see separate fact sheets), the salt contents does not pose a major issue. However, reuse and discharge to an onshore surface water body require that the salt content is low enough to prevent salt-related problems. This fact sheet describes several processes that remove salt and other inorganic chemicals from produced water by using membranes. Membrane processes are the most commonly used treatment technology for removing salt from produced water.

Filtration is common in many industrial applications. Filters can be designed of different materials and in different configurations. The figures show samples of different types of filter membranes.

Samples of different filter membranes; Source: J. Veil, Argonne National Laboratory.	Samples of different filter membranes; Source: J. Veil, Argonne National Laboratory.
The blue filter is a spiral wound membrane and the tan filter is a bundle of hollow fiber membranes; Source: J. Veil, Argonne National Laboratory.	Module of hollow fiber membranes; Source: J. Veil, Argonne National Laboratory.

The filtration process involves passing liquids through a membrane that has a minimum pore size. Suspended and dissolved particles that are larger than the membrane pore size are blocked by the membrane while the water and smaller particles pass through. This fact sheet presents summary information on membrane processes. For more background and details, readers are referred to Wagner (2001), an excellent handbook on membrane filtration, and to IOGCC and ALL (2006), which discusses applicability of membrane processes to produced water.

Filtration membranes and processes are subdivided by pore size ranges. The various categories, from largest to smallest pore size, include: microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Wagner (2001), IOGCC and ALL (2006), Cartwright (2006), and other authors offer side-by-side comparisons of the four. It should be noted that, while the relative relationships between membrane filtration technologies are not subject to much controversy in the literature, pore size cutoffs and other details vary by author. The following table represents a composite summary distilled from the three references listed above.

Texas A&M treatment trailer; Source: J. Veil, Argonne National Lab.

Texas A&M treatment trailer; Source: J. Veil, Argonne National Lab.

Reverse osmosis system; Source: J. Veil, Argonne National Laboratory.

As membrane pore size decreases, the energy required to push the water solution through the membrane increases. In addition, the tendency to foul the membrane increases as the pore size decreases. Hence, membrane filtration is often conducted in stages. A pretreatment module removes the larger constituents before they reach the membranes. The pretreatment stage can include many types of processes. This depends on the different materials in the wastewater influent. If reverse osmosis is required in the final treatment step for removing salt or metals, the pretreatment module could include microfiltration or ultrafiltration. For example, Texas A&M University constructed a produced water treatment trailer. The final treatment step is reverse osmosis, but several pretreatment steps are used to avoid plugging the reverse osmosis membrane. These vary, but have included microfiltration, hydrocyclones, and organoclay adsorbent filters. More information on the Texas A&M trailer can be found in the fact sheet on reusing produced water for domestic use.

In the pretreatment stage, operators typically add different chemicals to the treatment system for cleaning, anti-fouling, or other process control purposes.

Nicolaisen and Lien (2003) survey membrane filtration of produced water for reuse, discharge, and other onshore and offshore applications. New filtration technologies, membranes, and cleaning processes continue to enter the market place, as water treatment companies become increasingly aware of the huge potential market for produced water cleaning services.

Electrodialysis
Hayes (2004) and IOGCC and ALL (2006) report on another type of membrane process for removing salt from produced water. Electrodialysis is a separation process using a stack of alternating anion- and cation-selective membranes separated by spacer sheets. Water is passed through the stack of membranes. Electrical current is applied to the cell, causing the anions to migrate in one direction and the cations in the other direction. As the migrating ions intersect the selectively permeable membranes, alternating cells of concentrated and diluted solutions are produced in the spaces the membranes.

	Schematic of the electrodialysis process; Source: U.S. Department of Energy, National Energy Technology Laboratory.

A related process -- electrodialysis reversal -- operates in a similar fashion. In contrast to conventional electrodialysis, however, the polarity of the electrodes is reversed several times per hour, and the flows are simultaneously switched so the brine channel becomes the clean water channel and vice versa. The reversal feature is useful in breaking up films, scales and other deposits, and flushing them out of the process before they can foul the membranes.

Electrodialysis offers lower energy consumption than reverse osmosis because it is conducted at lower pressures (IOGCC and ALL 2006). In practice, conventional electrodialysis and electrodialysis reversal can reduce salt concentrations to less than 200 mg/L of TDS before the internal resistance of the solution to current flow - and therefore, power demand - rapidly increases (Hayes 2004).

References
Cartwright, P.S., 2006, "Water Recovery and Reuse - A Technical Perspective," presented at the 2nd Annual Desalination Workshop, Texas A&M University, College Station, TX, August 6-8.

Hayes, T., 2004, "The Electrodialysis Alternative for Produced Water Management," GasTIPS, Summer, pp. 15-20.

IOGCC and ALL, 2006, "A Guide to Practical Management of Produced Water from Onshore Oil and Gas Operations in the United States," prepared for U.S. Department of Energy, National Energy Technology Laboratory, by the Interstate Oil and Gas Compact Commission and ALL Consulting, October. Available at: http://www.all-llc.com/IOGCC/ProdWtr/ProjInfo.htm.

Nicolaisen, B., and L. Lien, 2003, "Treating Oil and Gas Produced Water Using Membrane Filtration Technology," presented at the Produced Water Workshop, Aberdeen, Scotland, March 26-27.