Membrane Treatment of Rio Grande Water for Municipal Water Production during the Non-Irrigation Season

Using impaired waters to meet increasing municipal demands has become nearly mandatory in many locations. This pilot scale project was undertaken to determine the feasibility of using brackish irrigation return flows (that also contain a small percentage of treated wastewater) to satisfy part of El Paso's municipal water demand during the non-irrigation season. The first step was to determine if a sufficient quantity of water is available during the non-irrigation season to provide 10 MGD of potable water. Historical data obtained from the United States Geological Survey for the years 1936 through 2004 revealed that a flow of at least 20 MGD is available 89% of the time. A simple microfiltration system (MF) was installed ahead of the reverse osmosis (RO) and nanofiltration (NF) membranes for suspended solids removal. The desalting results showed that both the RO and NF systems performed very well, with salt rejections in the 98% range. The lowest cost blended water was $0.76 per thousand gallons, produced in the NF unit when operating at 170 psi with a flux of 25 gallons per square foot per day. This cost includes disposal of concentrate through re-blending with available irrigation return flows for possible irrigation of onions, the major winter crop in El Paso County, Texas.