Powerful sludge

Danish wastewater treatment plants are some of the most sustainable in the world. Not only do they produce electricity and heat from the wastewater they also extract nutrients from the sludge as a resource. A DTU PhD project helps them assess which technologies are most climate neutral.

The wastewater treatment plant Marselisborg Renseanlæg in Aarhus has produced energy from the wastewater for many years and are now producing 150% of the amount of electricity they consume. Excess electricity is sold back to the grid. Photo: Aarhus Vand

Most significant gains in laughing gas

If the treatment plants are to reduce their CO2 footprint significantly, Maria Faragò's analyses show that laughing gas (nitrous oxide) is one of the areas with the best payoff. During the purification of the wastewater, nitrogen is converted into laughing gas through biological processes, a greenhouse gas almost 300 times more potent than CO2.

According to the Danish Environmental Protection Agency, laughing gas emissions from treatment plants account for 0.4 percent of Denmark's total climate impact. But by monitoring when the amount of nitrogen in the wastewater peaks, the treatment plants can adjust their operations to ensure that the wastewater stays longer in the process tanks, the concentration of the biomass is being increased, or the aeration is adjusted, thus reducing the amount of laughing gas.

"Sensors to measure laughing gas are quite cheap. Online measurements and management of laughing gas emissions can also provide the greatest climate benefits," says Maria Faragò.

An environmental balancing act

These kinds of conclusions are sweet music in the ears of Morten Rebsdorf. He is a senior project manager at Aarhus Vand focusing on treatment plants, and Aarhus Vand is one of the utilities that has been producing energy from their treatment plants for many years. They are doing that so well that some plants produce 150% of the energy they consume. But in order for the water utility to achieve their goal of becoming energy and CO2 neutral by 2030, it also requires that they extract other resources from the wastewater. Aarhus Vand is also in the process of building a large, new treatment plant, which is to be completed by 2028, so they need to decide which technological solutions they will invest in.

"It is a complex decision, so we need a systematized method to make these decisions," says Morten Rebsdorf.

"It may well be that we will choose a solution that reduces laughing gas and methane emissions, which may use more energy but result in lower emissions. It is a balancing act of what has the least net impact on the environment.”

The days when a treatment plant 'just' had to clean the water are over, but expectations will also be higher in the future.

"There are more and more cleaning requirements and if, for example, the government makes it mandatory to also remove drug residues from the wastewater, that will result in another cleaning step which requires more energy. So it may be increasingly challenging to stay energy neutral in the future," says Morten Rebsdorf.

  • One of the water industry’s largest and most important conferences in the world.
  • Takes place from September 11-15 in Bella Center Copenhagen.
  • This year’s theme is 'water for smart liveable cities' with a focus on solutions for everything from wastewater to groundwater and extreme weather that enable cities to adapt to climate change.
  • Around 10,000 participants, including researchers, businesses, authorities, politicians, utilities, and NGO’s from the water sector.
  • Read more on www.worldwatercongress.org