PROJECT

Thornton Water Treatment Plant Replacement

Innovative technologies such as ozone and biological filtration solve taste and odor challenges at the Thornton Water Treatment Plant, which provides drinking water to more than 160,000 area residents. In partnership with Garney Construction, we provided design-build services for the new plant with a treatment capacity of 20 million gallons per day (MGD).

Each spring, as the snow in the South Platte River Basin begins to melt, its water flows into that river, the city’s primary water source. This snowmelt carries potentially harmful algae blooms, as well as other organic matter and natural additives that can sometimes create unfavorable taste, color and smell. To address both water quality and palatability, the city decided to introduce advanced treatment processes that would give operators the flexibility to either isolate or blend water from nearby Standley Lake or a third potential future source to produce quality drinking water with optimal taste. The design accommodates a future potential capacity increase up to 30 MGD.

The team executed this project from developing final design criteria through pilot testing, design and construction. A six-month pilot project was conducted with a goal of achieving a variance for an increased filter loading rate, which would allow a smaller footprint for the plant, as well as confirming design criteria values and testing emergency scenarios and mitigation strategies. The pilot began during basis of design and ended near the time of the 30% design stage.

Our team was instrumental in maintaining the budget and overall schedule. The design-build team collaborated on value engineering ideas that ultimately saved $5.3 million. The project included development of a greenfield site and connection to the existing distribution system.

Client

City of Thornton

Location

Thornton, Colorado

Region

Southwest

Services

Municipal Water & Wastewater

Advanced Water & Wastewater Treatment

Construction

Design-Build for Water & Wastewater Infrastructure

Industry

Water

The overall project schedule allowed design and construction to happen in parallel. The project was broken into multiple work packages for construction and permitting, which saved 15 months on the schedule and allowed for early procurement of equipment and construction to start before the full design was complete.

Corrosion Modeling

As part of the design submittal to Colorado Department of Public Health and Environment (CDPHE), we developed corrosion models for various blending ratios from the multiple water sources that the plant treats. This study included a review of similar systems, including those in the adjacent facility, which was being replaced. The assessment reviewed the calcium carbonate precipitation potential, aggressiveness index, chloride-to-sulfate mass ratio, the Ryznar index, and dissolved inorganic carbon. The results of this assessment indicate that the water was not aggressive.

Corrosion impacts were part of the design and unit process selection. For example, ferric sulfate was used in lieu of ferric chloride to lower the chloride-to-sulfate mass ratio, which is a key indicator for water corrosivity. Additionally, a caustic soda feed system was put in place for pH and alkalinity adjustment.

Treatment Processes

The plant’s treatment system was designed to handle the wide range of water quality from the city’s multiple water sources. The raw water intake area allows isolation or blending of the raw water sources before the water is sent to the pretreatment processes, both to reduce turbidity caused by smaller particles and condition the water for filtration.

Pretreatment: Flash mixing is the first component of pretreatment. It 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 more easily settle to the bottom.

Sedimentation: Stainless steel plate settlers separate solids via gravity. The plate settlers are designed to be at an incline to increase the settling rate for optimal separation. After flowing through the plates, the water is then ready for ozone injection.

Intermediate treatment: The ozone injection process addresses naturally occurring compounds — 2-methylisoborneol and geosmin — that are the primary causes of taste and odor issues in the raw water supply. The ozone oxidizes these and other organics as well as pharmaceutical compounds and algal toxins. Ozone also provides disinfection that reduces the formation of chlorinated disinfection byproducts.

Biological filtration: The granular media filters permit the growth of microscopic beneficial microscopic bacteria to remove impurities including naturally occurring taste and odor compounds.

Final treatment: A chlorine disinfection process removes giardia lamblia, a microorganism that can cause intestinal distress if present in large amounts, as well as viruses.

The entire treatment process produces drinking water that exceeds existing state and federal water quality standards.

Case Study

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Progressive Design-Build Approach Accelerates Work on a Water Treatment System