The City of Sterling was established in 1881, in response to the arrival of the Union Pacific Railroad. For more than a hundred years, Sterling pumped its drinking water directly to tanks, where it was disinfected. But in 2008, the city faced multiple EPA violations of the national primary drinking water standards, including contamination by uranium and trihalomethanes, a byproduct of chlorine disinfection. The Colorado Department of Public Health and Environment issued the City with an enforcement order to address the problem.
Sterling’s raw water source also contained high levels of sulfates, total dissolved solids, and high hardness. Most local homes and businesses used costly point-of-use systems to treat their water, including ion exchange water softeners and under-the-sink reverse osmosis units.
The City wanted to install a new reverse osmosis water treatment plant, but disposing of the resulting brine was problematic. Colorado maintains strict standards for the quality of its scarce surface and groundwater resources. In addition, brine from the treatment process would contain uranium, posing additional cost and safety factors for disposal.
Mott MacDonald was retained as the project manager and lead design engineer for the City of Sterling Water Treatment System Project. We provided planning, pilot testing, preliminary design (civil, process, and structural design), permitting assistance, services during bidding, and services during construction.
During planning, design, and construction, we worked directly with the City, the EPA, and the state health department to address the requirements of the enforcement order and devise a sustainable solution.
Based on an extensive cost-benefit analysis, we determined that the most viable solution was a 9.6 million gallon per day (36 million liters) reverse osmosis treatment plant, coupled with EPA Class 1 deep injection wells to dispose of uranium-contaminated brine. Typically used in the oil and gas industry, deep injection wells allow brine to seep into natural rock formations more than one mile (1.6km) below the surface.
The capacity of the injection wells is impacted by the potential of the waste brine to form particulate matter and foul the wells. A successful solution required a balance between finished water, waste stream, injection well capacity, and the chemical makeup of the brine and the rock formations where the brine would be injected.
The marriage of municipal engineering with oil and gas technology presented unique challenges in permitting, designing, and bidding. Even with the help of a specialty consultant, the permitting process for the Class 1 deep injection wells took more than a year.
Our out-of-the-box thinking enabled the City to meet its water quality goals, provide 14,000 residents with safe, clean, and aesthetically pleasing drinking water, and protect waterways, ground water, farmland, and the natural environment from contamination.
In August 2014, Sterling’s Utilities Superintendent Jeff Reeves wrote that because our expertise, "the City now benefits from the effectiveness of RO without incurring the high costs and safety risks associated with the disposal of uranium contaminated waste."
Reeves wrote that Mott MacDonald "exceeded our expectations" in providing an environmentally friendly method of uranium disposal, cost savings to clients through reduced need for water softeners, removal of contaminants from the river, and project completion under budget.
"Additionally," he wrote, "our centralized RO water treatment has reduced the amount of water softener salt released to the river by approximately 1 million pounds per year, which allows us to be a good neighbor to the downstream communities who use the water."
In November 2014, we received an Engineering Excellence Award from the American Council of Engineering Companies of Colorado for the project. (See video.)