Mott MacDonald delivered out-of-the-box solutions enabling the City of Sterling to meet all its water quality goals and to offer around 14,000 residents with safe, clean, and aesthetically-pleasing drinking water.
- By drawing on deep well injection techniques from the oil and gas industry Mott MacDonald was able to design a sustainable and cost-effective way to dispose of brine waste produced by the treatment process.
- Though its population is less than 15,000, the City of Sterling is the biggest community in northeast Colorado.
For more than a hundred years Sterling pumped its drinking water directly to tanks, where it was disinfected. But in 2008, the city faced EPA violations of the national primary drinking water standards including contamination by uranium and trihalomethanes, a byproduct of chlorine disinfection. A sustainable solution was urgently required.
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 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 because of the method’s effectiveness in removing contaminants. Disposing of the resulting brine in a sustainable way posed a challenge. Colorado maintains strict standards for the quality of its scarce surface and ground water resources. As well as this, brine from the treatment process would contain uranium, posing additional cost and safety factors for disposal.
We provided planning, pilot testing, preliminary design (civil, process and structural), permitting assistance and advisory services during bidding as well as construction.
Our team worked directly with the City, the EPA, and the state health department to address requirements and devise an effective, sustainable solution.
Based on an extensive cost-benefit analysis, we determined that the most viable solution was a 9.6 million gallon-per-day reverse osmosis treatment plant, coupled with EPA Class 1 deep injection wells to dispose of uranium-contaminated brine. In this way, Sterling could ensure its supply of clean drinking water while protecting the natural environment.
Deep injection wells allow brine to seep into natural rock formations more than 1mi below the surface, much deeper than the wells used to dispose of municipal wastewater. These wells are typically used in the oil and gas industry. Because of their great depth, pilot testing was prohibitively expensive – estimates and simulations were developed during pre-design.
The marriage of municipal engineering with oil and gas technology presented challenges in permitting, designing, and bidding. The capacity of the injection wells is impacted by the potential of the waste brine to form particulate matter and foul 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.
Value and benefits
Our specialist team enabled Sterling to meet its water quality goals – to provide 14,000 residents with safe, clean, and aesthetically pleasing drinking water and to protect waterways, ground water, farmland and the natural environment from contamination.