Agencies Find Unique Solutions for Water Upgrades Across the Region

Cities around the Northwest are upgrading water systems to meet the specific needs of area residents.

By Lia Steakley Dicker

Municipalities in the Northwest are investing in multimillion dollar wastewater projects in an ongoing effort to eliminate pollution in waterways and protect natural resources.

In Lake Oswego, crews are constructing a one-of-a-kind buoyant gravity sewer line. The $110-million floating pipeline features a unique S-shape design secured to the lakebed with anchors and wire tethers.

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In Seattle, construction is starting on a $44-million project to rehabilitate twin wood-stave siphons and build an 85.5-in.-diameter pipeline below the Lake Washington Ship Canal.

Near Spokane, the city of Airway Heights recently broke ground on a $44-million facility to recycle wastewater to the most rigorous water-quality standards and replenish groundwater supplies.

Each project encountered numerous environmental and technical challenges, forcing engineers and contractors to develop inventive designs and come up with innovative construction methods.

Lake Osewgo Solution

Construction is well under way on the largest phase of a $110-million project to construct a one-of-a-kind submerged, buoyant, gravity-flow sewer pipeline across Oswego Lake, in Lake Oswego, Ore.

The project involves replacing 18,000-ft of pile-supported, 24- to 33-in.-diameter interceptor sewer with 12,000 ft of submerged, buoyant pipeline and 6,000 ft of pile-supported pipe; repairing or replacing connecting sewer lines; and constructing two sewage pump stations by January 2012.

Contractors are creating innovative projects to ease the region’s water issues.

Built in the early 1960s and supported by steel and wood pilings, the interceptor carries effluent east to a treatment plant on the Willamette River. The aging concrete system is undersized and unsound, and prolonged rains can cause it to back up, which prompted the Oregon Department of Environmental Quality to mandate its replacement.

Lake Oswego officials initially explored the possibility of constructing a pipeline around the shoreline of the lake rather than an underwater system. But a lengthy analysis proved the in-lake interceptor sewer design was the most cost-effective and environmentally friendly choice.

“The around-the-lake option was quite a bit more expensive because it required an extensive property acquisition effort and the construction of an additional 35,000 ft of pipe and six new pump stations,” says Joel Komarek, Lake Oswego Sewer Interceptor project director. “The in-lake option transports effluent using gravity so its energy costs are significantly lower than maintaining and operating six pump stations.”

Designed for a severe marine environment, the replacement interceptor will be constructed from high-density polyethylene pipe with over 3-in. walls and held in place using rock anchors attached with stainless steel wire rope. About 4,000 ft of the pipeline will be supported on piles in shallow areas where water depths are below 10 ft and incapable of supporting a buoyant system.

In creating a reliable, durable and environmentally safe system that floats 8 to 17 ft below the lake’s surface, designers at Brown and Caldwell of Portland solved a range of challenges including thermal expansion of the pipe, earthquakes and future maintenance of the underwater system.

“The concept of a gravity buoyant sewer pipe is completely new, and as with most things that are new, there ended up being more complexity to everything,” says John Holland, Brown and Caldwell engineer and project manager. “Because this system is underwater and it’s carrying sewage, the ramifications of the system failing are incredibly costly, so we really had to work to mitigate or limit any risks.”

Engineers choose an S-shaped interceptor to compensate for seasonal temperature changes in the lake, which can vary about 40 degrees and cause the pipe to shrink or grow. A twin buoyant solution was used to make the pipeline more flexible and prevent it from capsizing during earthquakes or when lake levels are down.

Buoyant submerged manholes were selected to replace existing interceptor manholes that extend above the lake’s surface.

In July, Portland-based Advanced American Construction began building a large trestle dock near Oswego Lake Dam as part of the $26-million, in-water portion of the project.

The trestle and barges will be used to install the piles, anchors and tethers and fuse together lengths of pipe into 1,500-ft segments to be floated into place.

“Installation of the ground anchors is going to be technically challenging because the anchors are drilled into the bedrock, and irregular ground conditions could complicate the process,” says Dee Burch, president of Advanced American Construction.

Once the in-lake work is completed in September 2010, the lake level will be lowered to allow the pile-supported segments of the new pipeline to be connected to the submerged, buoyant system and to the existing interceptor system at the west and east ends of the lake that does not need replacement.

Airway Heights Aims for Class A Water

Earlier this year, the city of Airway Heights broke ground on a $44-million water reclamation plant that aims to be the first facility in Washington to treat wastewater to class A-plus water reclamation and reuse standards and replenish crucial groundwater supplies.

In Lake Osewgo. Ore., the city is building a floating sewage pipe to improve its system.

Airway Heighs, a town of 5,500 located 6 mi west of Spokane, is like many communities in arid Eastern Washington that are looking for a sustainable solution to meet growing water needs.

The city’s primary water source, the West Plains Aquifer, is shrinking because nearby communities are growing and using water faster than the aquifer can naturally recharge. In 2006, Airway Heights officials approved plans to build a facility to recycle the city’s wastewater and recharge the ailing aquifer.

“We felt the need to provide an alternative stable water supply, and that required adding another $5 million to the cost of the project to achieve the highest level of treatment,” says Albert Tripp, Airway Heights city administrator. “Without adding that additional level of treatment, we would not have been able to recharge the aquifer.”

Designed by Spokane-based Century West Engineering Corp., the plant uses advanced biological treatment, coagulation and membrane filtration, U.V. disinfection and nitrogen removal to treat wastewater.

When the plant begins operating in 2011, up to a million gallons of water per day will be pumped back into the aquifer. The facility will also improve the water quality of the Spokane River, because Airway Heights will no longer send its wastewater to Spokane’s Riverside Park Water Reclamation Facility, which discharged effluent into the river.

“The design challenges we had to overcome and the combination of solutions that we’ve come up with make the Airway Heights plant pretty unique,” says Dennis Fuller, Century West Engineering president.

Site location was the project’s first obstacle. Constructing the plant above a permeable paleochannel, an ancient underground streambed filled with unconsolidated sediments, was critical to the water reclamation process because filtration through its sandy soil treats water to Class A-plus standards and allows it to re-enter the aquifer.

After a lengthy search, a 75-acre parcel situated above a 125-ft paleochannel was selected for the plant. But the property’s close proximity to Fairchild Air Force Base and the Spokane International Airport posed a new challenge.

Airport and military officials feared a surface-percolation system, where wastewater is discharged into ponds allowing effluent to seep through soil into the water table, would attract birds and become hazardous for air traffic.

“To resolve the issue, we developed a subsurface percolation system involving a network of perforated pipe that is completely below grade,” Fuller says.

Construction of the facility is occurring in two phases.

Robert B. Goebel General Contractor Inc. of Spokane is nearing completion on the $7.3-million initial phase of the project, which includes construction of sewer lines, administrative buildings, tanks, an anaerobic process basin and a 2-million-gallon reclaimed-water storage tower.

“It’s fairly basic construction but it’s still a big job,” says Steve Goebel, president of Goebel General Contractor. “The interior walls of the tanks are 21 ft tall, and there are a lot of interior walls, so those took several months to complete.”

The contract for phase 1B of the Airway Heights project has been awarded to Bellingham-based IMCO General Construction. The bid amount was $27 million. Construction on phase 1B is expected to begin in mid-September. The project is slated to be complete in July 2011.

Wooden Pipes in Ballard to be Replaced

Nested deep in the sediment of the western end of the Lake Washington Ship Canal lays the Ballard Siphon, one of the last remaining wooden sewer pipelines in King County, Wash.

The twin 1,365-ft long pipelines were built in 1935 using methods similar to old-fashion barrel construction. Wood staves were carved from trees logged from the Pacific Northwest’s plentiful forests and fastened together with rebar.

The floating sewage pipe will be tethered to the bottom of Lake Oswego.

Today, the twin 36-in diameter siphons carry up to 60 million gallons a day of wastewater to King County’s Westpoint Treatment Plant.

A routine sonar inspection of the siphons in 2005 showed deformation and signs of fatigue and prompted the King County Wastewater Treatment Division to launch an effort to upgrade the antique system.

Later this year, King County will embark on a $44 million project to rehabilitate the siphons and build an additional 85.5-in diameter pipeline stretching 2,000 ft underneath the canal to meet future wastewater demand.

“When we first became aware of this problem the initial plan was to lay replacement pipes on the bottom of the canal,” says King County Wastewater Treatment Division project manager Adé Franklin. “But permitting and other regulatory difficulties we encountered proved that this solution would be extremely difficult.”

A lengthy permitting process, tribal fishing rights issues, community impacts and environmental concerns about upsetting salmon migration or disturbing contaminated sediments restricted potential methods for replacing the siphons.

Design options were further complicated because the hydraulic requirements and space constraints of the project site ruled out horizontal directional drilling and micro-tunneling methods to install replacement pipes beneath the floor of the Ship Canal.

During the assessment of design solutions, engineers cleaned the siphons by pushing a large polyethylene cylindrical device through the pipe, a method known as “sewer pigging,” and used closed-circuit television videos.

“Pigging the system gave us the confidence that it would be possible to line the existing pipes with 30-in high-density polyethylene pipe and reuse the remaining capacity instead of abandoning it,” says Franklin. “We were able to design around the environmental issues by tunneling underneath the canal rather than laying pipe on the bottom.”

In addition to circumventing design challenges, the slip-line approach reduced project costs by $3 million because maintaining the existing siphons eliminated the need to construct extra pipelines to insure the system would accommodate a 20-year storm event.

Based on soil and groundwater conditions, alignment and shaft locations, pipeline elevation and site constraints an earth pressure balance (EPB) tunneling method was selected instead of micro-tunneling to dig a 2,000-ft canal crossing with a diameter of 104 inches to house the new pipeline.

“We choose to use an EPB tunneling method because if a micro-tunneling machine hits an obstruction then you have to drill down in front of the machine to access the cutterhead and remove the obstruction, which would have been very expensive and a permitting nightmare,” says Franklin. “But with an EPB machine we can back it up, remove the obstruction and keep moving forward.”

The deadline for project bids was July 26. At press time the contract had not been awarded. Construction is scheduled to be completed in late 2012 or early 2013.