CBM produced water that usually receive the most focus, although treatment for additional constituents, including fluoride, barium, ammonia, bicarbonate, and some trace elements, may be necessary to meet NPDES, UIC, state, and/or tribal regulatory requirements for surface discharge or subsurface reinjection. Some amount of pretreatment may be required as a compliment to treatment for SAR and EC or for disposal by subsurface drip or deep-well reinjection; pretreatment techniques may include degassing, settling, filtration, coagulation, flotation, and/or flocculation. These techniques are not discussed further.
Table 6.1 presents the more commonly occurring constituents in CBM produced water and the treatment technologies that are able to effectively remove or substantially reduce the concentration of these constituents: (1) ion exchange; (2) reverse osmosis; and (3) Freeze/Thaw Evaporation (FTE). The table also includes adsorption by cation exchange using zeolites and phytoremediation techniques although these techniques are not in common use for treating CBM produced water at this time. Although organics and biological agents are not known to be present in CBM produced water to any significant degree, they are included in the table for purposes of comparison between technologies. Table 6.2 provides a summary of the principles of operation, advantages, disadvantages, limitations, and relative costs of these treatment systems for CBM produced water. Each of the treatment techniques is then reviewed in detail.
Ion exchange treatments have been developed specifically in response to the need to reduce the SAR in the sodium concentration of produced water. Ion exchange systems function by capturing and removing a specific ion type within the CBM produced water. The specific purpose of ion exchange is to remove sodium by replacement with a different cation. By this fact alone, SAR will be reduced. Ion exchange resins capture specific dissolved ions and release other (like-charged) ions. Thus, the concentration of a specific ion of concern (e.g., sodium in agricultural areas) can be substantially reduced. The adsorption characteristics and saturation configuration of an ion exchange resin are specific to the ion targeted and a function of the resin composition. In as much as the fixed- and fluid-bed resin exchange technologies that are being used employ primarily sodium cation-specific resins, these treatment systems do not remove substantial proportions of anions or other cations in the produced water stream.
Exterran Water Management Services has developed ion exchange water treatment technologies that use a modification of a Higgins Loop CCIX technology—a patented process exclusively licensed from Severn Trent Services—referred to as continuous countercurrent ion exchange systems for removing sodium and other cations from produced water. Higgins Loop is the most widely used ion exchange technology for CBM produced water treatment (RPSEA, 2009). Approximately 18 percent of all permitted discharge