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<br />EXECUTIVE SUMMARY <br /> <br />Introduction <br /> <br />Drinking water providers increasingly have to make use of lower quality water sources <br />and in many cases have turned to membrane treatment, a recognized technology that <br />can treat water to meet or surpass most water quality objectives. Disposal of the waste <br />stream (concentrate) from membrane treatment systems is often the critical issue. <br />Reverse osmosis and nanofiltration membranes (RO/NF) may produce a concentrate <br />with higher amounts of organic and inorganic constituents and total dissolved solids <br />(TDS). Discharge options for the concentrate are typically limited by lack of assimilative <br />capacity and, in one instance, an application for a permit to discharge concentrate has <br />been denied. Therefore, drinking water systems that serve the most populated areas of <br />the state, or systems treating impaired water sources, zero liquid discharge (ZLD) of <br />concentrate will likely be the only long-term solution. Existing ZLD technology is energy <br />intensive, relatively costly and has not been demonstrated on a broad scale or for larger <br />systems. Initiation of research and development of ZLD technologies for use in <br />Colorado is imperative and it is appropriate for these efforts to receive significant political <br />and financial support from the state. <br /> <br />Source Water Supply and Treatment Challenges <br /> <br />Providing sufficient quantities of drinking water that meets the standards of the Safe <br />Drinking Water Act is vital to the public health and economic well being in Colorado. <br />Public drinking water utilities have become increasingly reliant on lower quality source <br />waters to meet increasing demand, which the Colorado Water Conservation Board has <br />estimated to be more than an additional 600,000 acre-feet per year by 2030. <br />Conventional treatment technologies may not be adequate to treat lower quality water <br />and will not remove TDS, however, RO/NF technology can remove all or most commonly <br />found contaminants, including TDS. As pollutant loadings or concentrations increase <br />and/or higher quality water supplies decrease, the need for advanced treatment <br />technologies such as RO/NF, will be required to remove contaminants of concern. <br /> <br />Concentrate Disposal Challenges <br /> <br />Disposal of RO/NF concentrate is more difficult than disposing of residuals from <br />conventional treatment systems, due to higher concentrations of contaminants. RO/NF <br />membrane treatment processes typically run at an 85% recovery rate, which produces a <br />concentrate volume of 15% of the total flow of raw water treated. The concentrate has <br />all the contaminants that were removed from the raw water during treatment, resulting in <br />an increase of approximately 6 times the original concentration of pollutants. At 95% <br />recovery, the concentrate has 20 times the raw water concentration of pollutants. <br /> <br />Concentrate Disposal Options <br /> <br />Discharqe to Surface Water: The regulatory framework for discharges to surface waters <br />involves an analysis of the capacity of the receiving stream to assimilate the discharge <br />during a low flow event without exceeding applicable water quality standards. This is <br />known as the "assimilative capacity" (AC). In order to have adequate AC, a sufficient <br />low flow volume and/or minimal upstream concentrations of contaminants is necessary. <br /> <br />1 <br />