With high-quality water supplies increasingly under stress across the United States, municipal utilities are challenged to seek more sustainable and reliable water supply solutions. The East Cherry Creek Valley (ECCV) Water and Sanitation District of Aurora, Colorado faced this situation when its potable groundwater supplies began to decline in the early 2000s. CDM Smith worked with ECCV staff to develop a state-of-the-art four stage reverse osmosis (RO) system that produces potable water at an extremely high recovery rate from a brackish water source. The team applied lessons learned from industrial projects, a deep understanding of the technology, and a significant amount of creativity; in the words of CDM Smith drinking water expert Doug Brown, “We were confident enough to take an approach that a few years ago was limited to academic research and small-scale industrial systems.”
Colorado is one of the fastest-growing states in the nation. The Front Range region, where ECCV is located, is considered an arid climate, with only 15-20 inches of precipitation per year. Water rights for high quality surface water in the Front Range are completely allocated to historical users. As its pumping levels of deep, non-replenishable groundwater declined, it was apparent to ECCV that obtaining high-quality surface water supplies might not offer the best value proposition. CDM Smith water resource and process engineers based in Denver were retained to explore options that would reduce ECCV’s dependence on non-renewable groundwater. While brackish water sources were available, there was a significantly higher concentration of total dissolved solids (TDS) and hardness compared to ECCV’s existing groundwater supplies. Consequently, the team looked to use reverse osmosis technology to purify the brackish source water before blending it with the groundwater, leading to the development of a $27 million RO and UV disinfection treatment and high service pumping facility.
“The major constraint when using RO is disposal of the concentrated residual stream,” said CDM Smith project manager Tim Rynders. “We had to bring all our past project and R&D experience to figure out a way to make it work.” Disposal of the RO brine (salts and suspended solids) that would be removed from the brackish water by the RO system was highly constrained. Additionally, surface and sanitary sewer disposal options were not available and/or were cost prohibitive. To solve this significant challenge, CDM Smith designed a $6 million deep-injection well system capable of pumping brine 10,000 feet below ground surface in an EPA Class I non-hazardous disposal well. ECCV was one of the first municipalities to attempt high pressure brine disposal into tight shale and sandstone formations. Since startup, biannual data analysis has indicated that the well has behaved in a positive and predictable manner.
What Can Reverse Osmosis Do For You?
To minimize the amount of brine that would need to be disposed, and to maximize the amount of potable water squeezed from every gallon of feed water, the team pushed the process chemistry and the RO equipment beyond conventional limits. “This project set the benchmark nationally for high recovery with a relatively straight-forward approach,” said Brown. “We continually asked ourselves, ‘can we drive recovery higher?’” For a municipal utility, the four-stage membrane system is essentially unprecedented. The ECCV facility is achieving an overall recovery of 94-96%, far above the 75-85% of the typical brackish recovery facility. This equates to a 70% reduction in brine volume at ECCV compared with a typical facility, achieved for a highly cost-effective $2.80/1,000 gallons of product water. This unit cost is roughly half the industry benchmark standard when utilities consider brackish water RO for a component of their water supply.
Part of the team’s success came from thinking differently: “Industry ‘best practices’ suggest you should design in a way that will make your equipment last to or beyond its manufacture-suggested lifespan,” said Rynders. “We didn’t do that. We identified that RO equipment was not the high cost variable, brine disposal was. We focused on maintaining high recovery and maintaining pressure throughout the RO system, saving energy. Now, we are focused on continually improving the RO element clean in place processes, decreasing the labor cost for maintaining the system. Buying new RO membranes for Stage 3 and 4 every 3-5 years in a case like this is actually much more economical than making it last for 7-10 years, considering what the owner gets back in terms of potable water recovery and decreased disposal costs.”
ECCV’s successful and innovative project demonstrates that a complex and interconnected RO system can be cost-effective and achievable for a municipal utility. “Everyone else in the region was into deep water or surface water, so we looked at brackish water and that was a game-changer,” said Rynders. Now, ECCV has the capacity to support future growth in the region, and has established benchmarks for the rest of the water industry to follow.
We had to bring all our past project and R&D experience to figure out a way to make it work.