By Jason Schaefer
Facing source water issues that demanded a new water treatment plant (WTP), the City of Thornton turned to progressive design-build to deliver a modern, flexible fix.
Like many communities in Colorado, the City of Thornton has long faced challenges in maintaining high water quality standards due to a range of issues affecting raw water sources.
In 2005, Thornton unveiled the upgraded and expanded Wes Brown Water Treatment Plant, a facility that has become a model throughout the industry for innovative treatment standards utilizing ultrafiltration membrane technology and other advanced treatment processes to eliminate impurities. At the time of its completion, it was recognized as the largest submerged membrane treatment plant in the U.S. and the largest membrane retrofit in the world.
Now, Thornton is midway through the next phase of its ongoing commitment to high-quality water with construction of the new Thornton Treatment Plant (TTP), a 20-MGD facility that will replace an existing 16-MGD treatment facility that is still operating but is well past its original design life.
New Baseload Capacity
By the time Thornton and its design-build team of Garney Construction and Burns & McDonnell began design on the new TTP in March 2017, it was clear that further investment in treatment processes at the existing facility would be less cost-efficient than building an all-new facility. With completion scheduled for July 2020, the 68,000-square-foot treatment facility will serve as the city’s baseload treatment plant, operating at or near full capacity year-round, even during winter conditions. Located on a 13-acre site adjacent to the existing treatment facility, the new TTP had reached 50 percent completion by the end of August of last year.
Though the existing plant meets all current regulatory standards for water quality, the new TTP will be a much more flexible plant that can adjust to treat several source water conditions and exceed existing water treatment standards. In addition, the TTP has been designed with hydraulic capacity for emergency treatment during high-flow periods.
The TTP will treat water from Thornton’s two primary existing surface water sources: Standley Lake and the South Platte River. Water from Standley Lake presents few quality issues, though seasonal fluctuations sometimes create reduced volumes of water available from that source.
Water obtained from the South Platte is a different story. Thornton has faced chronic challenges with taste and odor, a problem stemming primarily from the presence of organic matter, algae blooms, and other constituents. Though it is not a health hazard, it can create perceptions about water quality. So, rather than continuing to invest in treatment processes at the existing plant, the decision was to address the issue within the treatment trains at the new TTP.
Commitment To High-Quality Drinking Water
The City of Thornton has consistently maintained high-quality drinking water standards and the TTP will continue to elevate that effort. The city has adopted policies that will adhere to higher treatment standards than required by current regulations, and design features of the new TTP allow Thornton to meet its Partnership for Safe Water, Phase IV goals.
The Partnership for Safe Water, Phase IV is a voluntary self-assessment program administered by the American Water Works Association that assists in the optimization of water treatment plants and distribution system performance. It focuses on rigorous data collection and reporting as a means of holding the reporting entity to the highest standards of public health and safety for water quality.
Flexibility, Reliability, And Ease Of Operations
To provide operational flexibility, the new TTP will be able to isolate treatment of the respective water sources coming into the plant. This gives operators the flexibility to isolate or blend the two raw water sources as needed, optimizing chemical use and potentially saving costs. The inclusion of multiple chemical injection points throughout the treatment train also aids in operational flexibility. This feature, combined with strategically placed instrumentation, will allow the city to quickly respond to significant variations in water source quality.
The new facility will tie into an existing raw water and distribution system connected to the existing plant, so very little new pipe will be required outside the footprint of the new plant. This arrangement allows treatment processes to roughly follow existing grade, thereby minimizing the amount of excavation required and allowing gravity flow through the facility.
The piping configuration at the plant is designed with enough space to accommodate the addition of a third water supply source in future years. All design layouts for treatment areas, storage, and maintenance have been designed for easy access for operators and staff as well as convenient access for routine equipment maintenance.
The TTP is designed to treat the wide range in water quality provided by the city’s multiple source waters. The raw water intake area is designed to allow isolation or blending of the raw water sources before the water is sent to the pretreatment processes that reduce turbidity caused by smaller particles and condition the water for filtration.
Flash mixing, the first component of the pretreatment stage, involves injection of chemicals via a nozzle within the raw water pipeline to destabilize particles suspended within the water. From there, the water moves to the flocculation stage, a mixing process with three zones of decreasing intensity, allowing the destabilized particles to combine and form larger particles that will more easily settle to the bottom.
Next, the water moves into a sedimentation zone with stainless-steel plate settlers designed to separate solids via gravity. The plate settlers are designed with an incline to increase the settling rate for optimal separation. After flowing through the plates, the water is then ready for ozone injection in an intermediate treatment stage.
The ozone injection process will address MIB (2-methylisoborneol) and geosmin, naturally occurring compounds that are the primary causes of taste and odor in the raw water supply. The ozone oxidizes these and other organics as well as pharmaceutical compounds and algal toxins. Additionally, ozone provides disinfection that reduces the formation of chlorinated disinfection byproducts, if they are present.
Following ozone injection, water moves to a biological filtration stage consisting of granular media filters without chlorination or other compounds that could deter growth of the microscopic beneficial bacteria. Functioning similarly to conventional granular media filters, biological filtration is an additional treatment stage that typically is used within wastewater treatment facilities as a way of removing remaining impurities.
The final treatment stage will be a chlorine disinfection process to remove Giardia lamblia, a microorganism that can cause intestinal distress if present in large amounts, as well as viruses. The entire treatment process will produce drinking water that exceeds existing state and federal water-quality standards.
Having a strict budget and timeline for completing the new plant, Thornton quickly determined that it would not be able to meet its cost or schedule objectives using traditional design-bid-build delivery. Progressive design-build became the preferred option because of its inherent flexibility to select the right treatment technologies installed for the plant, as it allowed construction to begin prior to finalizing the design, and because it allows flexibility to accommodate future expansion as well as any number of current conditions.
With the costs of capital projects spiraling for many municipalities, collaborative project delivery methods like progressive design-build are increasingly emerging as effective ways to meet the high expectations of stakeholders. All those factors were in play in Thornton.
Beginning with pilot testing and engineering design in March 2017, the Garney and Burns & McDonnell project team began collaborating daily with Thornton staff and the owner’s advisor to identify and mitigate risks that could adversely affect the city’s cost, schedule, and treatment goals.
The team presented multiple engineering design packages as part of the process for reaching agreement on the optimal design. Because of the flexibility of this design-build approach, foundation work began after only 60 percent of the design had been finalized. With the design’s completion in August 2018, all remaining phases of construction are now in full swing with commissioning on several key plant components set to begin late in 2019. Substantial completion will be reached by July 2020, if not sooner.
Water Security For The Future
Having the ability to effectively treat water from multiple sources and consistently provide clean, great-tasting water to the community will be an invaluable asset to the city for decades to come.
About The Author
Jason Schaefer, PE, leads the drinking water group for Burns & McDonnell in the Rocky Mountain region. As a senior project manager, he is responsible for managing, planning, designing, and troubleshooting conventional and advanced water treatment facilities. He has experience with greenfield water treatment facilities and complex water treatment facility retrofits and expansion projects, most of which are completed using design-build or other collaborative delivery methods.