Great Danes: Making Wastewater Treatment Energy Positive

As the second-largest city in Denmark, Aarhus is perhaps best known to outsiders for its Viking history, Nordic-classical architecture, and annual, 10-day art festival. It is also, unknown to many, the home of a forward-thinking wastewater treatment project.

In Ega, a suburb of the Danish city, lives a treatment plant that is expected to produce 50 percent more electricity than it consumes for daily operation.

“Hopefully, our project will help set new standards for how to build wastewater treatment plants in the future,” said Jan Tøibner, a representative of Aarhus Vand, the Danish water utility responsible for the project. “Climate change makes it important to innovate in energy efficiency and energy production in order to reduce carbon emissions and energy consumption.”

The plant, which is in the process of being converted from an energy user into an energy creator, typically treats 5.8 MGD and serves 120,000 people. Aarhus Vand doesn’t expect the Ega plant to reach its full energy-recovery potential until 2020, but has a vision (and a 10-million-euro budget) for the combination of technologies it will rely on.

It all starts with an anaerobic digester.

“However, the digester alone will not be sufficient,” Tøibner said. “It will require optimization of both the energy-producing process taking place in the anaerobic digester and the energy-consuming process taking place in the activated sludge tanks… Further, the optimization cannot be seen as two independent tasks, as the energy production and energy consumption are closely linked through the mutual carbon source available in the raw wastewater flowing to the treatment plant.”

The developers also plan to utilize a shortcut nitrogen removal process in the main process tanks, sludge liquor treatment, a carbon harvest and control system, and an organic rankine cycle process. While most of these technologies are fairly common, it’s the combination that will allow the Ega plant to achieve such lofty energy savings.

“Optimal control of the interaction between the different systems results in an overall reduction in energy consumption and an increase in energy production,” Tøibner said. “The overall success criterion of the project is to develop and demonstrate that such new technologies can work individually and in interaction with each other, as well as with well-proven energy technologies such as combined heat and power generation from extracted gas.”

Design and build for the recovery technologies was completed in October 2016, but there are still unknowns about how long it will take for the biological processes to fully integrate.

“Most of the new technologies are now implemented and we have started the running-in process,” Tøibner said. “It will, however, take some time before we achieve our goals as we are talking about biological processes, which need to be developed.”

While Aarhus Vand claims that the promised 50 percent net energy will be a historic high for wastewater treatment, it has already achieved close to that mark in nearby Marselisborg.

“We produced about 30 percent more energy at Marselisborg wastewater treatment plant than we used and everybody thought it was impossible to go any further,” said Tøibner. “But we challenged the contractors and consultants and started the process with an idea competition, where everyone was invited to come with their idea of how to build the wastewater treatment plant of the future.”

Aarhus Vand received 66 ideas from students, consultants, and contractors, ultimately implementing two of them: a solids separator from Salsnes Filter and an anammox system from EnviDan.

As most of the technologies used at the Ega plant are widely available, it seems to be only a matter of budget and desire that keeps more communities around the world from pursuing similar efficiency goals. If and when the Danish project proves successful, hopefully it is an idea that will catch on.