As solids are broken down by bacterial and physical actions in sewage treatment, large volumes of gases are generated. Known as “biogas,” these gases which are full of impurities, have long been allowed to vent into the atmosphere, contributing to the malodorous air associated with wastewater treatment. Thankfully, more modern wastewater facilities are capturing and safely incinerating or flaring their biogas, and in recent years, some plants have begun adding portions of their biogas to commercially sourced natural gas and burning the mixture to produce steam or modest amounts of electricity, while still flaring most of their biogas. Calgary’s Bonnybrook Wastewater Treatment Facility is one such site.
Today, Bonnybrook is taking biogas handling to the highest level: Using all of its biogas as the primary fuel source to meet its needs for electric power and industrial steam for its advanced treatment process. A first in Canada, the Bonnybrook Cogeneration Expansion Project is currently undergoing commissioning and system operational testing in preparation for handover to the City of Calgary.
Collaboration and complex processes
The ambitious $60 million, 3-year project custom designed by Stantec and general contracted by Graham, not only introduced a new operations concept for the Bonnybrook wastewater team, it also required vendor-packaged equipment sourced from around the world during the height of the COVID-19 pandemic. Meeting these challenges and keeping the project moving forward became a case study in applied cooperation among the design, construction, and City of Calgary teams.
“This facility is designed with a certain intent, and it’s going to be run by the client, so the collaborative approach we took to ensure we had a positive outcome is one that I’ll take forward to future projects,” says Andrew Buchner, Stantec’s B.C. Practice Lead for Water and resident engineer for the project. “It’s the only way I see to commission a complicated project, especially one that is unique to all of the key parties.”
Although Bonnybrook’s new cogeneration facility is small by power plant standards, its technical complexity is greater than that of a commercial gas-fired electrical generating station with 100 times the output. The power plant provides both combined-cycle power generation and cogeneration of power and heat.
“Cogenerations of power and steam is totally different from wastewater treatment, which is the facility’s core business,” says Jifan Liu, the City of Calgary’s Project Manager. “It is technically complicated and there are a lot of components that all need to work together. Our technology goals were significant, and we are using the most advanced gas turbine, steam turbine, and the latest biogas conditioning and water treatment systems.”
The intricacies include the capability to handle an adjustable fuel mix of biogas and commercial natural gas. Because biogas production varies greatly along with wastewater treatment volume, a low-pressure biogas storage bubble was included. The power plant must also satisfy regulatory requirements that it always operate with at least some biogas. Even so, some natural gas will be required from time to time.
A Solar Turbines Inc. Centaur 50 gas turbine-driven generator set (CTG) gas combustion turbine is coupled to a generator producing 4 MW of power. All waste heat from the CTG is cycled through the heat recovery steam generator (HRSG) where steam is produced. That steam is then either sent to the steam turbine generator (STG) to create additional power or will feed the future thermal hydrolysis process (THP) facility — part of Bonnybrook’s Plant D expansion — that will make waste solids more digestible and, in turn, produce even more biogas. The steam surface condenser (SSC) condenses all excess steam not used by the STG or THP back into feed-water. A gas-fired steam boiler is included to provide a backup steam source. Incoming water for steam production also requires complex purification. Gas compression and chemical feed systems are further elements.
“This plant has a lot of interesting features,” says Kyle Jensen, Stantec’s Electrical and Process Design Engineer. “Using biogas as a fuel is interesting in its own right. It only contains about half the energy of commercial natural gas. And it comes with a host of impurities, including highly toxic hydrogen sulfide, or H2S, which requires careful handling.” For safety and to avoid damaging the turbine, the process design includes sophisticated gas conditioning, cleaning, and dehydration to remove impurities such as siloxane, housed inside a robust blast-proof gas conditioning room. H2S, however, remains part of the fuel inlet stream and is safely incinerated during engine combustion.
The complex design, in turn, required sophisticated electrical and control systems, including a motor control system, switch-gears, and cabling room. The new plant had to be integrated in three different ways: with Bonnybrook’s wastewater treatment system, with its existing reciprocating-engine cogeneration plant, and with the facility’s power system, which draws from Alberta’s electrical grid. “You have three different power sources all producing electricity, that all have to talk to each other through the power control system,” notes Cedric McIntosh, Graham’s Project Manager.
Set up for success
Construction began in October 2019 and the technical complexity immediately made itself felt. “There was a lot of research and development as we went, there were a lot of changes as we went, and we had to do a lot of work to optimize the schedule to fit the changes and work with the consultant and owner to end up with a product that is useful,” recalls McIntosh. This stemmed from the fact that a lot of key technical information wasn’t initially available to Stantec.
“The team solved a lot of problems during the project,” says McIntosh. “It required people recognizing the challenges, where they came from, and how we could all work together in good faith to solve each challenge towards a favourable outcome. A lot of that success has to do with the relationship we have with the City and with Stantec.”
The pandemic’s arrival in March 2020 caused immense problems. In addition to the by-now familiar productivity-sapping effects of working under COVID-19 protocols, supply chain issues hit this project particularly hard. “We had a truly international supply chain, and certain factories around the world simply shut down in the pandemic, so there were serious challenges in getting equipment,” says McIntosh. In addition to serious delays in fabricating and shipping the physical items, vendor reviews of shop drawings and vendor site inspections were all delayed, cancelled, or had to be moved online.
“The supply chain issues related to COVID were some of the biggest challenges,” agrees Stantec’s Buchner, who has over a decade of experience working on wastewater facility design. “The issue-for-tender design was based on certain vendor packages and assumed pieces of equipment, but we did not even get pricing from some of the intended vendors.” Having to switch vendors midstream contributed to significant delivery delays on some of the large equipment packages.
Creative thinking was applied to re-sequence equipment installation and other key tasks. Under normal sequencing, large equipment packages are placed on-site and the building envelope is later constructed around them. But when delays on certain packages like the steam turbine generator threatened to halt the entire project, Graham decided to leave a large gap in the building’s second-floor envelope. Once the package finally arrived, it was craned into position, inserted and placed inside on its skid mounts, after which the building was enclosed.
As construction proceeded, another critical focus was setting up the Bonnybrook team for successful plant operations. “The seriousness of the process—moving from handling waste sludge to producing high-pressure steam—is a step-up for the Bonnybrook operators,” notes Jensen. “Keeping the operators safe for the long run became a key part of project planning.”
Planning began early to create a comprehensive commissioning and handover process with extensive hands-on training. “Just getting handed a facility at the end of commissioning by a contractor, which often happens, would not work here,” says Buchner. “Here, the whole team is going through commissioning and training together.” This has proved to be a key decision, says Jensen.
While delivering the Bonnybrook Cogeneration Expansion Project has been a complicated and difficult process, Liu is excited by what the new facility will deliver once it is fully operational in July. “This project will enable us to consume all of the biogas produced by the plant,” he says. “It will create energy savings for the facility and greatly reduce external consumption of electricity, which will reduce our power bill, and also improve the overall reliability of our power supply.” If a civil disaster such as another major flood were to hit Calgary, for example, Bonnybrook could continue operating under its own power. As Liu sums it up: “I’m optimistic that we will achieve the objectives we set at the beginning of the project.”
George Koch is a researcher and writer based in Calgary who has written extensively about construction and public infrastructure.