Denver Water, the provider of drinking water for 1.5 million people in central Colorado, recently achieved a key milestone on its $600 million North System Renewal Project. A critical component of the utility’s ongoing efforts to revamp its infrastructure and improve system reliability and sustainability, the project entails the design and construction of the new 75 mgd Northwater Treatment Plant, the installation of a new 8.5 mi long pipeline, and the redevelopment of its nearly 85-year-old Moffat Treatment Plant.
Begun in 2018, construction on the Northwater Treatment Plant is now 40% complete with the pipeline construction slated to wrap up later this year.
Separate systems
Denver Water maintains two separate gravity-fed systems for collecting and conveying raw water, nearly all of which originates as mountain snowmelt. The South System provides 80% of the utility’s water supplies and 90% of its storage. The much smaller North System collects runoff from the western slope of the Continental Divide and conveys it through the Moffat Tunnel to the eastern slope, where it eventually passes through the Gross Reservoir and on to the Ralston Reservoir, which is north of Golden, Colorado. From there, the water is sent through conduits to the Moffat Treatment Plant, in Lakewood, Colorado.
Despite its smaller contribution, the North System affords Denver Water a fair amount of flexibility, particularly in the event that a hazardous event disrupts operations within the South System, says Robert Mahoney, P.E., the chief engineering officer for the utility. “If we have a problem like a fire in the South System and we get a lot of debris in the reservoirs, we can pull water from the North System,” Mahoney says. “It’s good to work them in conjunction with each other.”
Rehabilitation and renewal
Originally completed in 1937, the 185 mgd Moffat Treatment Plant was expanded in the 1950s and the 1970s. Similarly, the rest of the North System was constructed either in the 1930s or the 1950s, prompting Denver Water to begin undertaking rehabilitation efforts several years ago. “We’re replacing and upgrading different parts of our system,” Mahoney says.
Although built in a rural setting in the 1930s, the Moffat Treatment Plant today is surrounded by development, precluding an expansion of the facility. “There are homes around the property now, not horses,” Mahoney says.
As a result, Denver Water opted to construct the Northwater Treatment Plant as a greenfield facility near the Ralston Reservoir in Jefferson County, Colorado. Scheduled to begin operations in the first quarter of 2024, the Northwater facility will feature gravity separation, flocculation and sedimentation, filtration, and disinfection. Finished water will be held in two clear wells, each of which will have a volume of 10 million gal., before entering the new pipeline that is designed to convey flows to the existing Moffat Treatment Plant, where the treated water will enter the distribution system.
Designed for the future
The Northwater facility was designed with future needs in mind, Mahoney says. The treatment plant was sized such that its capacity can be doubled to 150 mgd and additional treatment steps can be added if regulatory developments require them. “We made room for future regulations and future expansion,” Mahoney says.
Among its more notable features, the Northwater facility will be capable of operating essentially “off the grid,” Mahoney says, thanks primarily to its proximity to the hydropower facilities at the nearby Ralston Dam. At the same time, Denver Water and its team of consulting engineers went to great lengths to reduce the energy demands associated with the treatment plant. “We looked at every electrical load on the site,” Mahoney says. “We’re very, very efficient on our energy use.”
That said, Northwater will have backup generators and be connected to the electrical grid in case of emergencies. Meanwhile, Denver Water has plans to install photovoltaic panels as well as 10 hydrokinetic units on-site, further increasing the potential for power generation. As a result of the hydropower and other generation facilities, the utility expects the Northwater plant to be a “net energy producer,” Mahoney says. “We can pump (electricity) back into the grid and actually make money.”
A collaborative design approach
Rather than select a single consulting engineering firm to design the Northwater facility, Denver Water took a more “innovative” approach, Mahoney says. “We decided to pick the best firm for process engineering, the best firm for electrical engineering, the best firm for architecture and civil engineering,” he explains. “We had different design packages, and we picked different firms to do different pieces.”
To promote collaboration, Denver Water then had the participating design firms work together in a single office. “We told them to check their egos and their firms at the door,” Mahoney says. “They were now part of the Northwater project.” Ultimately, delivering the design by means of multiple design packages “worked very well,” he says.
As for the consulting engineering firms that participated on the project, AECOM designed the site and civil improvements and the architectural and building systems. Jacobs was responsible for the design of the treatment processes and structures. Carollo Engineers Inc. designed the electrical, instrumentation, and control systems. And Brown and Caldwell and Bates Engineering Inc. designed the post-tensioned tanks, while Burns & McDonnell was responsible for the design of the improvements to the Moffat Treatment Plant. Denver Water and AECOM designed the outlet works for the Ralston Dam.
As for the contractor, Kiewit was selected to serve as the construction manager at risk on the project. “Kiewit did a really good job of working with us and all of our designers through the (design) process,” Mahoney says.
Denver Water opted for a similarly collaborative approach for the design of the Northwater Pipeline, which will convey treated flows from the Northwater plant 8.5 mi to the Moffat Treatment Plant. After receiving strong design proposals for the project from consulting firms HDR and Dewberry, the utility persuaded the two competitors to collaborate on the pipeline design.
Initially, Denver Water planned on installing 84 in. diameter, cement-mortar-lined steel pipe along the entire conduit length. However, the utility subsequently decided to continue operating the Moffat treatment facility for the foreseeable future but at a reduced treatment capacity. As a result, a “good deal of the pipeline” was downsized to a diameter of 66 in., Mahoney says, reducing the project cost by $10 million.
Phased pipeline construction
Begun in 2017, construction of the pipeline has been conducted in four phases. An early phase, conducted by the Michels Corp., involved tunneling under several major highways and rail lines and installing slightly more than 1 mi of pipeline by means of open-cut construction. The rest of the work has been conducted as part of three phases, all of which have entailed open-cut construction. Completed by Garney Construction in 2019, the west phase involved constructing approximately 1.7 mi of the pipeline. Earlier this year, S.J. Louis Construction Inc. completed the central phase, which consisted of 2.8 mi of pipeline. Meanwhile, Garney is expected to complete the 1.25 mi long east phase later this year.
As for the Moffat treatment facility, Denver Water plans to repurpose some of the plant’s flocculation and sedimentation basins and use them as storage facilities. However, one of the facility’s treatment trains will undergo improvements in order to remain available for use, mainly as a backup.
