Climate positivity – wastewater treatment the Copenhagen way

Model of the VARGA project.© BIOFOS

Water experts will be heading to Copenhagen for the IWA World Water Congress & Exhibition on 11-15 September. Registration is now open. To highlight some of the latest developments in sustainable wastewater treatment in the area, Miriam Feilberg hears from Dines Thornberg, head of R&D at BIOFOS.

BIOFOS is responsible for the treatment of wastewater from 15 municipalities in the greater Copenhagen area. With a goal of delivering climate positive and resource efficient wastewater treatment to the city, the company is engaging in a programme to transform its existing treatment systems.

How did you get started on the road to delivering sustainable, resource-conscious wastewater treatment?

Dines Thornberg: We see our activities as an element within the circular economy and we realised that we could contribute on several issues. We can recover energy, reduce GHG emissions (not least nitrous oxide), recover phosphorus, and produce organic fertilizer. To strengthen our activities within this area of development, we started a project called VARGA. The aim of the project was to transform the existing wastewater treatment plant at Avedøre, south of Copenhagen, into a water resource recovery facility (WRRF).

What was the driver and purpose of the project?

DT: From the beginning the push-factors were coming from our strategy and from dedicated employees. It is our vision to promote the green transition and circular economy, through ambitious objectives for the environment, sustainability, and climate neutrality. This is closely related to the implementation of the Sustainable Development Goals, which is a core focus for our company. We investigated our operations and realised that we could do better in terms of climate and sustainability.

What were the challenges of this project?

DT: We have been producing energy for some years, but we wanted to be able to produce more energy and to become climate positive. We also realized that nitrous oxide is probably one of our key issues related to greenhouse gas emissions. We needed to measure and reduce our emissions, but only focusing on nitrous oxide would have been too narrow, we also wanted to look into carbon harvesting and phosphorus recovery. One of our digesters had some excess capacity, which could allow us to produce more biogas by taking in some organic household waste. We designed the VARGA project as a package looking at several opportunities and new technologies to transfer the existing plant into a resource recovery plant.

What have you learnt from this project that would be relevant in other countries?

DT: The process of transferring an existing plant into a water resource recovery facility is important. We have demonstrated that this is possible. This is important because there are many wastewater treatment plants like Avedøre all over the world. Just looking at Europe, there are 250 existing wastewater treatment plants like Avedøre.

The existing plants were designed to deliver a certain treatment quality without looking at energy consumption, circularity, or carbon emissions. We have learnt that it is possible to transform a wastewater treatment plant like Avedøre to the new demands without lowering treatment quality. Our experience is that we must develop a strategy for systematic transformation of processes and technologies to secure this process. We have worked with a combination of existing technologies in new applications, alongside the successive introduction of new technologies; by doing so, we have secured a transformation within economic boundaries, providing secure treatment.

What are the project’s key achievements?

DT: We have made the most progress within three areas: energy, nitrous oxide, and phosphorus recovery. We have not yet finished all of the activities planned for the project, but we are working well, and will have very interesting demonstration sites to present when the IWA World Water Congress and Exhibition comes to Copenhagen in September 2022.

We have also learned during the process that we probably need to concentrate more on climate positivity and less on energy production, as energy becomes greener in the future. But the energy from wastewater can still serve as part of the energy mix.

Energy recovery – carbon harvesting

BIOFOS is working with dynamic control of the plant to improve energy recovery by using organic household waste in the process and harvesting carbon. The pilot project has demonstrated that it is possible to produce more biogas from household waste and, as a spin-off, it has been possible to produce proteins that can be used for animal feed. The digestate from the food waste process can even be used as organic fertiliser. A main element of VARGA is the introduction of a pre-filter after the sand trap and before aeration. A pilot test has shown that it is possible to recover 20-30% more organic matter from wastewater, and to make it drier. The next steps in the process are to see if this drier sludge can produce more energy and provide more efficient use of the digesters. The knowledge gained from this project is particularly relevant for plants that are not yet carrying out preclarification. In this instance, the new filters will lower energy consumption significantly. The experiments and knowledge gained through this project will also be important to future water resource recovery facilities that may be more membrane based.

Nitrous oxide from wastewater treatment

Nitrous oxide is a greenhouse gas that is 298 times as potent as carbon dioxide and is among the most significant emissions from wastewater treatment. The Danish water sector aims to become 100% energy and carbon neutral by 2030 and, therefore, needs to manage nitrous oxide emissions, along with methane and carbon dioxide.

For a company such as BIOFOS, emissions of nitrous oxide may be half of its total greenhouse gas emissions, but these can’t be tackled until the extent of the problem is established. BIOFOS has been measuring emissions with different technologies and has, of now, three years of continuous measurement. One of the lessons learned is that there is huge seasonal variation, with large emissions from March to August. It may be possible to control emissions with different processes related to the dosage of carbon, amount of sludge and the transformation of bacteria. There may be a microbiological cause for these fluctuations, which makes it relevant to also look at DNA footprints related to the emissions.

Phosphorus recovery at Avedøre facility

Phosphorus is a scarce resource, but much needed in agriculture. In the Danish water sector, the goal is to recover phosphorus from wastewater and many projects for this are in development. BIOFOS is involved in two, testing and comparing the efficiency of different technologies. Results will be shared with IWA Congress participants visiting the plant in 2022.

Tests are taking place to recover phosphorus using electricity and membrane technology. This does not require the use of chemicals and, by using green electricity, offers the possibility of chemical- and carbon-free recovery of phosphorus. The target is to be able to recover more than 80% of phosphorous.

Another project is using easy-mining technology, where the ashes are taken from incinerated sludge, hydrochloric acid added, and phosphorus extracted by using calcium. This produces very clean phosphorus as a powder that can even be used in animal feed.

See wastewater and resource recovery in action during IWA Congress

BIOFOS believes the ‘future treatment plant’ is a water resource recovery facility. Visit the living lab, the Avedøre Wastewater Treatment Plant, where the focus is on sustainable circular economy. If you come to Copenhagen for the IWA World Water Congress & Exhibition on 11-15 September, there is a chance to join a guided tour to see and hear about:

  • Recovery of phosphorous from sludge ashes
  • Carbon harvesting by prefiltration to increase biogas production and reduce energy consumption
  • The Biogas Park, where biogas is upgraded to bio-natural gas and carbon dioxide is methanised for use by the public grid
  • Nitrous oxide minimisation by online control and the test facility for treatment of hazardous substances in wastewater


Sustainable Development Goals – a beacon for activities at BIOFOS

BIOFOS decided to prioritise seven out of the 17 Sustainable Development Goals (SDGs) in its strategy. The company has selected the goals that are aligned with its strengths and where it believes it can make the most difference, locally, regionally and globally. The SDGs are not used to set quantitative targets, but to choose areas of action and to prioritise resources. In this way, employees feel confident that their daily activities are contributing to global sustainability. For instance, recovery of phosphorus contributes directly to SDG12: ‘Responsible consumption and production’, and energy recovery activities contribute to SDG 7: ‘Affordable and clean energy’. The company has operational targets on treatment quality, but carrying out activities based on the SDGs ensures it also focuses on climate targets. BIOFOS has prioritised the following SDGs:

6. Clean water and sanitation

7. Affordable and clean energy

11. Sustainable cities and communities

12. Responsible consumption and production

13. Climate action

14. Life below water

17. Partnerships for the goals


BIOFOS is Denmark’s largest wastewater utility, treating the wastewater of 1.2 million people living in the Greater Copenhagen Area at its three treatment plants: Lynetten, Avedøre and Damhusåen. The wastewater is treated for organic and inorganic solids together with nitrogen and phosphorus. The treated wastewater from Lynetten and Damhusåen is discharged via two 1.5 km-long pipes leading into the sea. The treated wastewater from Avedøre is discharged into Køge Bay via two 1.1 km-long pipes. BIOFOS has a positive energy balance, at 177%. The company is owned directly or indirectly by 15 municipalities in the capital area of Copenhagen, treating wastewater from all 15 owners.

Miriam Feilberg is climate change manager at DANVA