The Bonnybrook Wastewater Treatment Plant upgrades and expansion program

By Paul Adair

In spite of its critical importance –especially when you need to flush the toilet or turn on the faucet – wastewater and water treatment is something that isn’t regularly considered. That is, unless you happen to be a city experiencing growth like Calgary.

Sitting on the historic Bow River, Calgary is currently home to three wastewater treatment facilities: Fish Creek, Pine Creek, and Bonnybrook. Originally built in the 1930s, the largest of the three, the Bonnybrook Wastewater Treatment Plant (WWTP), is one of the largest biological nutrient removal (BNR) plants in Canada, and is the largest cold-weather BNR plant in the world.

But after several significant upgrades and modifications made over the years (the last occurring in the early 1990s) to meet its expanding needs and increasingly stringent treatment standards, it was becoming clear that the plant was nearing its capacity. As such, the City of Calgary made the decision to invest more than $1 billion into the Bonnybrook Wastewater Treatment Plant Upgrades and Expansion Program – a series of extensive upgrades and expansions that will protect the environment and accommodate Calgary’s expected growth well into the future.

When complete in 2024, the 10year program will expand the plant’s capacity from 950,000 to 1.37 million people, while increasing overall energy efficiency and resiliency.

Plant D expansion

By far, the largest component of the Bonnybrook WWTP program has been the Plant D Expansion, which is in of itself comprised of several significant projects. Stantec is the consultant for the program, with Graham Construction as the construction manager.

Stantec’s website states that the Plant D Expansion will include the installation of three additional secondary treatment bioreactors and six additional secondary clarifiers to provide additional secondary treatment capacity, as well as improvements to primary effluent conveyance, BNR processes, secondary clarification, effluent filtration, ultraviolet (UV) disinfection, plant outfall, chemical and process utility systems, secondary sludge thickening, secondary sludge dewatering, thermal hydrolysis of dewatered waste activated sludge, and sludge digestion.

As of 2022, the program has approximately doubled the capacity of the plant’s existing co-generation facility, which captures biogas from the solids treatment process to create electricity and heat for the facility. This upgrade has allowed the Bonnybrook WWTP to seasonally take itself off the grid, further lowering its greenhouse gas (GHG) emissions and allowing the plant to reduce its overall operating costs.

Challenges overcome

The ground conditions on site consisted of sands interbedded with deposits of gravel, and the depth to bedrock ranged from four to 14 metres (13 to 46 feet) with groundwater being encountered four metres (13 feet) below working grade.

These ground conditions posed a challenge as the new facilities’ stringent settlement criteria required a deep foundation solution that could support the effluent tunnel, installed in a shallow excavation underlain with existing active utilities. Because of the low-headroom and tight space working conditions, the project’s geotechnical engineer recommended the use of micropiles – a deep foundation element constructed using high-strength, small-diameter steel casing and/or threaded bars.

In 2020, Geotechnical engineering company Keller designed, supplied, and installed nine micropiles with a permanent casing up to 14 metres (46 feet) in length. To ensure the ground conditions would not impact the installation, special care had to be taken with the selected drilling method.

“The soil conditions were quite gravelly and cobbly,” says Keller – North America Project Manager Justin Bekkers. “When you drill with air and water in loose sands and gravels, materials can come up the casing and create voids, meaning that we would sometimes have to partially drill the hole, grout it, and then drill it again.”

Another challenge was related to the abundance of historic infrastructure in the area, which necessitated several micropile locations to be redesigned to work around the utilities.

“When a problem like an obstruction was identified, it required quick decisions to be made on the fly,” says Bekkers. “We would coordinate through Graham and with Stantec to collaboratively come up with solutions to relocate piles, which allowed us to finish the job safely and on time.”

Flood protection

In 2013, Calgary and the Bonnybrook WWTP were heavily impacted by extensive flooding across southern Alberta, which caused more than $13 million in damage to the facility. This is something the expansion program hopes to avoid in the future.

A new 830-metre long (2,720-feet), 3.5-metre (11-feet, 6-inches) high earth flood berm was constructed along the eastern side of the facility to prevent overland flows from the river. The program then relocated several storm sewer pipes, removing an existing storm outfall and re-routing trunk sewers. In addition, the existing treated effluent outfall has repositioned downstream and embedded into the riverbed to prevent the back-up of river water levels.

The Stantec team then developed a groundwater dewatering system, including a groundwater dewatering pipe, to reduce the impacts of high groundwater levels on existing structures and the site as a whole. These flood mitigation measures will help to ensure that the Bonnybrook WWTP will no longer be affected by rising waters along the Bow River and continue to provide uninterrupted services for years to come. l