The state of knowledge on GHG emissions in the wastewater sector

By Jeanette Agertved Madsen, EnviDan A/S. The blog post is part of a series of perspectives leading up to the IWA World Water Congress & Exhibition in Copenhagen in September 2022

The latest IPCC report made it very clear – once again – that the planet is under severe stress and urgent climate action is needed immediately. It is necessary for everyone to contribute to the sustainable reduction of greenhouse gas (GHG) emissions, including all sectors in all countries. This calls for ambitious national and international policies and collaborations across borders and sectors – water included.


Energy and climate-neutral water sector in Denmark by 2030

The Danish Government implemented a new climate law in 2020 with the overall goal of reducing the national CO2 emissions by 70 percent in 2030 (compared to 1990 levels). This law was followed by several other initiatives, including a climate plan for a green waste sector and circular economy. This climate plan includes goals to reduce nitrous oxide (N2O) emissions from wastewater treatment plants by 50 percent, and to implement a limit for emissions from WWTPs larger than 30,000 person equivalents (PE), no later than 2025. These national goals have pushed the Danish wastewater sector from focusing mainly on energy to implement tangible climate actions on other fronts.

A national model, inspired by Paris Agreement on GHG reductions, was introduced in 2021 by the Danish EPA. In this model, all water and wastewater utilities are urged to report CO2 emissions and their plans for reducing these in the next 15 years. One of the results of the model is the ambitious goal of energy and climate neutrality by 2030. You can read more on this here:

The CO2 emission from the production of Danish electricity is decreasing by the day due to more and more renewable energy production. This means that contributions to the GHG footprint coming from the wastewater sector are taking over as the biggest individual contributors – especially N2O from biological nitrogen removal processes, and methane (CH4) from the biogas system. Thus, it is no longer enough to focus on reducing energy consumption and increasing renewable energy production. More climate measures need to be taken to fulfil the national goals.


Comprehensive data and knowledge on N2O emissions should be shared

The International Greenhouse Gas Protocol sets standards for measure and manage emissions, categorising them in three different ‘Scopes’. These are exemplified in a water sector context in Figure 1. Most often Danish utilities report on Scope 1 and Scope 2, whereas Scope 3 is less frequently addressed, despite covering a very substantial part of GHG emissions in water.

Figure 1: Definition of scope 1, 2 and 3 based in the water sector (figure is developed by EnviDan based on the Greenhouse Gas Protocol Standards).

The Danish EPA has funded several projects related to N2O emissions, providing a good starting point for reporting direct emissions in Scope 1. This is one of the reasons why Denmark has come far in gathering knowledge on N2O emissions, and several control strategies are now being tested and used at full-scale at Danish utilities.

The VARGA project ( is an example of one of these projects. It showcases the transformation of a traditional WWTP to a Water Resource Recovery Facility (WRRF) and can be visited as part of the World Water Congress & Exhibition technical tours programme: Tour 2, Avedøre WWTP (

More than three years of online data on N2O emissions have been collected, and a range of control strategies have been tested on-site. The results showed reductions of 30-80 percent (depending on the strategy) without compromising the nitrogen removal.

These experiences, together with the ambitious upcoming implementation of a limit on N2O in Denmark, are key tools for effective emissions reductions that can serve as inspiration for other countries on the road to Net Zero.


Don’t get lost in operation

As mentioned earlier, Scope 3 is now starting to gain traction in the wastewater sector. Here in particular we can find CO2 emissions from construction. We know from other sectors that this has a huge impact, especially when building completely new WWTPs. So far, Danish utilities have focused mostly on CO2 from operations as part of their reporting activities.  But now is the time to start expanding our gaze to include wider climate goals and also consider indirect emissions.


More information and resources

IWA is holding a series of webinars on process emissions, focusing on monitoring, management and mitigation of GHG emissions, including process emissions, carbon dioxide, nitrous oxide and methane. Masterclass 3 on Methane in Wastewater is scheduled for 23 June, 11:00 UK time. Find out more and register here.

IWAP has recently published a book on Quantifying and modelling of fugitive greenhouse gas emissions from urban water systems.

Finally, IWA looks forward to sharing ideas and knowledge at the IWA World Water Conference & Exhibition in Copenhagen, 11-15 September 2022. The Congress will offer many opportunities for water professionals and utilities to learn from emerging research on climate adaptation, mitigation, and much more!

Registrations for the Congress are now open. Secure your seat at the world’s leading event for water professional

Jeanette Agertved Madsen

Head of R&D, EnviDan A/S
Jeanette Agertved Madsen is Head of R&D within wastewater in the Danish engineering and consulting company EnviDan A/S. Jeanette has more than 25 years of experience as a consultant within the wastewater sector from planning to handover of full-s... Read full biography