ARES - Active Reduction of Emissions from Wastewater Treatment Plants

One of Aarhus Vand’s central focus areas is to minimize CO2 emissions and ensure a sustainable water cycle. The Danish water sector emits up to 100,000 tons of CO2 each year from the production of drinking water and the treatment of wastewater. And when the Danish government adopted a new climate law in 2020 with a goal of reducing CO2 emissions, an additional vision for a CO2-neutral Danish water sector by 2030 was established.

Problematic greenhouse gases 

To achieve a CO2-neutral water sector, it is necessary to consider all emission sources in the supply chain. One of the major challenges is emissions from sewer systems and wastewater treatment plants (WWTP), where the greenhouse gases methane (CH4) and nitrous oxide (N2O) are emitted. These two gases are far more harmful than CO2; for example, nitrous oxide is almost 300 times more damaging than CO2.

Therefore, the R&D project ARES was initiated in collaboration between Aarhus Vand, project owner VandCenter Syd, as well as EnviDan, BIOFOS, Aarhus University, and Technical University of Denmark, with support from the Environmental Technology Development and Demonstration Program (MUDP).

Testing measurement technologies and developing control principles

The ARES project contributes new knowledge to handle the pressing challenge of greenhouse emissions and the resulting climate changes. The purpose has been to provide new knowledge about the formation of and emissions from methane (CH4) and nitrous oxide (N2O) in sewer systems and at WWTPs.

The project had four main objectives:

• Develop and demonstrate a covered sludge storage with built-in gas collection on a full scale
• Use advanced measurement technologies to identify and quantify the main emission sources at WWTPs and in sewer systems
• Develop control strategies and treatment technology to reduce N2O emissions from the biological processes at WWTPs
• Perform Life Cycle and economic assessment of the tested technical principles.

Understanding the biological treatment processes

In the ARES project, Aarhus Vand has focused on the emission of nitrous oxide. One of the major challenges with this gas is that it is difficult to measure the emissions. The understanding of the formation of nitrous oxide in wastewater processes is complex. There are two essential biological treatment processes, nitrification and denitrification, which we use in wastewater treatment today. These remove nitrogen from wastewater but also risk forming nitrous oxide. When nitrification occurs too fast, or too much ammonium is added at once, the risk of forming nitrous oxide increases. Similarly, a lack of carbon during denitrification can also lead to increased production of nitrous oxide. To minimize environmental impact, it is therefore crucial to balance these processes carefully.

To address these challenges, Aarhus Vand’s focus in the project has been to create a better understanding of the two biological treatment processes, which can form the basis for the development of control strategies and treatment technology to reduce their nitrous oxide emissions.

Measurements of Nitrous Oxide at Egå WWTP

A series of tests have been conducted at Aarhus Vand’s treatment plant in Egå. The focus has been on comparing measurements of the gases released during water treatment and the liquid wastewater. At modern Danish treatment plants, emissions are typically measured in the liquid phase, which can make it difficult to obtain accurate data on the formation of greenhouse gases such as nitrous oxide. But by measuring what happens between the liquid phase and the gas phase, it is possible to verify the existing measures and improve controls.

The research at Aarhus Vand’s facilities have revealed a high turnover of ammonium in one of the tanks at Egå WWTP, which requires a new control algorithm. This is now being tested at the facility.

Next steps towards a CO2-neutral water sector

The ARES project has provided important new knowledge that supports the efforts to achieve Denmark’s and the water sector’s climate goals. This is an important step towards more sustainable wastewater treatment.

The next step for Aarhus Vand involves optimizing the control at Egå WWTP. At the same time, the ARES partners wish to submit a new MUDP application, which continues the efforts to reduce greenhouse gases and investigates further measures for more sustainable wastewater treatment