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<br />1.0 Introduction <br /> <br />This report presents the consensus status and thinking of the Water Quality Forum <br />Membrane Treatment Workgroup (MTW) as of July 2007. The group has spent the last <br />eighteen months exploring issues associated with the proper disposal of concentrate <br />from membrane treatment systems. This has included holding over a dozen meetings, <br />several presentations by experts working on projects in Colorado and other states, and <br />convening of two subcommittees (regulatory and technical) that have each held <br />numerous meetings and conducted significant research. This report is by no means a <br />final presentation of the group's recommendations regarding options for the proper <br />disposal of membrane treatment concentrate. Rather, it is a compilation of what the <br />group has learned to date, provides a summary of information that will feed into its final <br />recommendations, and identifies areas requiring further study and analysis before a final <br />report can be prepared. <br /> <br />2.0 Background <br /> <br />2.1 General <br /> <br />Over the last several years, the Colorado Water Quality Control Division (Division) has <br />received requests for discharge permits from several existing and potential dischargers <br />of brine from membrane treatment systems. Membrane treatment systems are <br />advanced drinking water technologies that produce high quality water along with a <br />concentrated waste stream ("brine" or "concentrate") containing a concentrated mix of <br />the contaminants in the source water that require disposal. At least one of these <br />discharge permit requests included effluent limits based on assimilative capacity of the <br />receiving stream during times when flows were greater than low flow conditions as <br />defined in Section 31.9(1) of Colorado's Basic Standards and Methodologies for Surface <br />Water. When this issue was raised before the Water Quality Control Commission <br />(Commission) during its May 2005 meeting, a determination was made to establish a <br />workgroup to evaluate potential regulatory changes to accommodate such requests by <br />dischargers. This workgroup was named the Assimilative Capacity at Flows Greater <br />Than Low Flow Workgroup. The Commission scheduled a rulemaking hearing for July <br />of 2006 to consider any proposal that the workgroup developed. <br /> <br />The threshold question for the Assimilative Capacity workgroup was the following: <br />"Should flow conditions that are higher than critical (regulatory) low flow levels be used <br />for discharge permitting and, if so, what criteria should govern their use?" The <br />workgroup determined, at the outset, that it would be appropriate to prepare a set of <br />"white papers" to elucidate the many complex issues embedded in this question. <br />Altogether, eight white papers were developed, which are available at <br />http://www.coloradowaterqualitV.com/ro/index.htm. <br /> <br />Many significant issues and concerns were raised in these papers about the prospect of <br />allocating assimilative capacity (AC) at flows greater than regulatory low flows. By <br />December of 2005, the general consensus of the workgroup was that the Division did <br />not have the resources to develop such complicated permits and oversee their <br />compliance under this type of permitting approach. Furthermore, the group agreed that <br />implementing this type of system could result in significant or unknown negative impacts <br /> <br />3 <br />