East Cherry Creek Valley's Sustainable Supply Solution
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 beginning in 2008 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.”
Phase 1: 2008-2012 10 mgd Total Treated Water Capacity
Phase 2: 2018-2022 20 mgd Total Treated Water Capacity
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 the pumping levels and production rates of deep, non-replenishable groundwater declined, it was apparent to ECCV that purchasing the limited high-quality surface water supplies in the area would be expensive and might not offer the best value proposition. CDM Smith water resource and process engineers based in Denver explored options that would reduce ECCV’s dependence on non-renewable groundwater. While brackish groundwater 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 used reverse osmosis technology that had been implemented by the team at over ten sites in the western US to purify the brackish source water before blending it with the groundwater, leading to the development of the Phase I $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 two $6 million EPA Class I non-hazardous deep-injection wells and high pressure injection pumps capable of pumping RO concentrate or brine 10,000 feet below ground. 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 as predicted resulting in a cost-effective and reliable disposal alternative.
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 mineral solubility, scale inhibitors and the RO equipment beyond conventional limits. “This project set the benchmark nationally for high recovery with a relatively straight-forward approach of reducing pH without softening of the feedwater,” 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 a third of 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 the RO membranes last as long as possible which reduces in a conservative design,” said Rynders. “We didn’t do that. We identified that RO equipment was not the high-cost variable, brine disposal was. 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.” We focused on maintaining high recovery and permeate production throughout the RO system as the concentration of dissolved solids increases, reducing energy requirements per gallon of treated water. In Phase 2 we are focused on improving the RO element clean-in-place processes by automating all the cleaning valves to decrease the labor cost for maintaining the system. CDM Smith also incorporated a redundant 4th stage in the Phase 2 RO equipment that allows soaking the RO membranes in low pH permeate for extended periods of time to dissolve precipitated minerals on the surface of the membranes to gently restore permeability. Having a redundant 4th stage also means you don’t lose any treatment capacity when the 4th stage needs to go off-line for cleaning.
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.