National wastewater surveillance of SARS-CoV-2

The national wastewater surveillance of SARS-CoV-2 in Denmark covers 49% of the population. Wastewater samples are collected from 29 wastewater treatment plants distributed across the country, allowing the development of SARS-CoV-2 to be monitored both nationally and regionally.

SARS-CoV-2 status in wastewater as of 03.09.2025

In week 35 (August 25 - August 31) the national SARS-CoV-2 concentration in wastewater, indicating the amount of SARS-CoV-2 circulating in the population, is at a medium level.

In Capital Region of Denmark the concentration is at a low level and in Central Denmark Region, North Denmark Region, Region Zealand and Region of Southern Denmark the concentration is at a medium level.

Since week 26, technical uncertainties in the laboratory have likely led to an overestimation of the growth during this period.

The results for the wastewater concentration of SARS-CoV-2 should therefore be interpreted with caution for this period, and the national growth rate, which is the average weekly growth over the past three weeks, will not be published this week.

National concentration of SARS-CoV-2 in wastewater over time and the current growth rate

The figure below shows the concentration levels of SARS-CoV-2 in wastewater across all sampling locations, as well as this week's growth rate.

The figure illustrates the development of the weekly average concentration of SARS-CoV-2 in wastewater over the past 16 months. The concentration is adjusted for the amount of feces in the wastewater to account for dilution and changes in the number of people contributing to the wastewater. The concentration is categorized in five levels (very low, low, medium, high, and very high), marked as horizontal bands in the figure. The growth rate, which represents the average weekly percentage change based on the last three weeks of measurements, is also shown in the figure. The growth rate is color-coded and ranges from a very strong decrease to a very strong increase across seven categories.

Regional concentration levels of SARS-CoV-2 in wastewater and development over time

The figure below shows the level of SARS-CoV-2 in wastewater for each region, along with the regional development of SARS-CoV-2 concentration over time.

The figure presents a map of Denmark divided into regions, which are color-coded according to the latest weekly level of SARS-CoV-2 in wastewater. Additionally, the figure includes a graph for each region displaying the development of the weekly average concentration of SARS-CoV-2 in wastewater over the past 16 months. The concentration is adjusted for the amount of feces in the wastewater to account for dilution and changes in the number of people contributing to the wastewater. The concentration is categorized in five levels (very low, low, medium, high, and very high), which are marked as horizontal bands in the figure.

Download wastewater data

Data from the national wastewater surveillance of SARS-CoV-2 (03.09.2025)

Information on the National Wastewater Surveillance of SARS-CoV-2

How is the national wastewater surveillance of SARS-CoV-2 conducted?

This figure illustrates the setup of the National Danish wastewater surveillance system and the different steps involved. Bacteria and viruses that cause disease (pathogens), such as SARS-CoV-2, may be shed and released into wastewater from households across the country. The wastewater is directed to wastewater treatment plants, where samples of wastewater are collected at the inlet (i.e., before any treatment of the wastewater takes place). The wastewater samples are transported under refrigeration to Statens Serum Institut. In the laboratory at Statens Serum Institut, the wastewater is purified, and genetic material from the pathogen is extracted. Polymerase chain reaction (PCR) is used to detect and/or quantify the pathogen. Additionally, next-generation sequencing may be performed to identify variants of the pathogen. The laboratory results are processed, and epidemiological trends are analyzed. Every week, epidemiological trends are published on Statens Serum Institut’s website. The wastewater results are integrated with data from other surveillance systems, such as hospital admissions and clinical tests.

Where is SARS-CoV-2 measured in wastewater?

In the national surveillance of SARS-CoV-2 in wastewater, 29 untreated wastewater samples are collected from treatment plants across Denmark.

This figure shows a map of Denmark with catchment areas (geographical area where wastewater is collected) for the 29 wastewater treatment plants marked. The catchment areas are color-coded by region to visualize the representation all five regions. For the North Denmark Region, four treatment plants are included (Hjørring, Aalborg Øst, Aalborg Vest, and Himmerland), covering 47% of the region’s population. For the Central Denmark Region, six treatment plants are included (Aarhus Egå, Aarhus Marselisborg, Aarhus Åby, Aarhus Viby, Søholt, Herning, and Horsens), covering 40% of the region’s population. For the Region of Southern Denmark, five treatment plants are included (Vejle, Fredericia, Kolding, Esbjerg Vest, Odense Nordvest, and Odense Ejby Mølle), covering 36% of the region’s population. For the Capital Region of Denmark, five treatment plants are included (Hillerød, Målev, Lynetten Nordre, København Damhusåen, Lynetten Søndre, and Hvidovre Avedøre), covering 68% of the region’s population. For the Region Zealand, five treatment plants are included (Holbæk, Mosede, Ringsted C, Køge, Slagelse, and Næstved), covering 32% of the region’s population. For all of Denmark, the total population coverage is 47%.

How is SARS-CoV-2 measured in wastewater?

  • Particles of the SARS-CoV-2 virus are shed in the feces of approximately 50 % of infected individuals and can therefore be detected in wastewater.
  • Wastewater samples are collected using an automatic sampler over a 24-hour period. Personnel at treatment plants collect the samples and record relevant data associated with the sampling.
  • The wastewater samples are transported to the Statens Serum Institut (SSI), Department of Virology and Microbiological Special Diagnostics, where they are pre-processed and analyzed using real-time quantitative polymerase chain reaction (RT-qPCR) to estimate the number of SARS-CoV-2 virus particles per liter of wastewater.
  • The naturally occurring plant virus Pepper Mild Mottle Virus (PMMoV), which is ingested through food and excreted in feces, is used as an indirect measure of the amount of fecal matter in wastewater. By measuring the concentration of PMMoV, adjustments can be made for wastewater dilution (e.g., due to rainwater) and the number of people using the sewage system in each catchment area.
  • The laboratory results are analyzed by the Department of Infectious Disease Epidemiology at SSI.

How are the wastewater measurement results reported?

The wastewater results are compiled weekly as:

  1. A weekly weighted average concentration of SARS-CoV-2 in wastewater, which is categorized in one of five levels, ranging from very low to very high concentration. This is done for each region and for the entire country.
  2. A growth rate that describes the change in the national concentration level based on the past three weeks of data. The growth rate is, however, not calculated or published if the SARS-CoV-2 concentration is at a very low level or if sample testing is associated with high uncertainty due to significant dilution.

Where can I learn more about the national wastewater surveillance?

Read a detailed description of wastewater surveillance (PDF)

 

Publications

Predicting hospital admissions due to COVID-19 in Denmark using wastewater-based surveillance. Gudde, A., Krogsgaard, L. W., Benedetti, G., Schierbech, S. K., Brokhattingen, N., Petrovic, K., Rasmussen, L. D., Franck, K. T., Ethelberg, S., Larsen, N. B., & Christiansen, L. E. (2025). The Science of the total environment, 966, 178674. https://doi.org/10.1016/j.scitotenv.2025.178674

A survey of the representativeness and usefulness of wastewater-based surveillance systems in 10 countries across Europe in 2023. Benedetti, G., Wulff Krogsgaard, L., Maritschnik, S., Stüger, H. P., Hutse, V., Janssens, R., Blomqvist, S., Pitkänen, T., Koutsolioutsou, A., Róka, E., Vargha, M., La Rosa, G., Suffredini, E., Cauchie, H. M., Ogorzaly, L., van der Beek, R. F., Lodder, W. J., Madslien, E. H., Baz Lomba, J. A., & Ethelberg, S. (2024). Euro surveillance: European communicable disease bulletin, 29(33), 2400096. https://doi.org/10.2807/1560-7917.ES.2024.29.33.2400096

Results from the SARS-CoV-2 wastewater-based surveillance system in Denmark, July 2021 to June 2022. Krogsgaard, L. W., Benedetti, G., Gudde, A., Richter, S. R., Rasmussen, L. D., Midgley, S. E., Qvesel, A. G., Nauta, M., Bahrenscheer, N. S., von Kappelgaard, L., McManus, O., Hansen, N. C., Pedersen, J. B., Haimes, D., Gamst, J., Nørgaard, L. S., Jørgensen, A. C. U., Ejegod, D. M., Møller, S. S., ... Ethelberg, S. (2024). Water Research, 252. https://doi.org/10.1016/j.watres.2024.121223

Detection of Sequencing Reads from 5’-End Genomic and Subgenomic SARS-CoV-2 RNAs in Wastewater Sampled, Extracted and Sequenced Directly from Aircrafts, Airport Buildings, or Regular Wastewater Treatment Plants. Tang, M.-H. E., Bennedbaek, M., Gunalan, V., Qvesel, A. G., Thorsen, T. H., Rasmussen, L. D., Krogsgaard, L. W., Rasmussen, M., Stegger, M., & Alexandersen, S. (2023). Heylion 2024, May 15. https://doi.org/10.1016%2Fj.heliyon.2024.e29703

Early detection of local SARS-CoV-2 outbreaks by wastewater surveillance: A feasibility study. Nauta, M., McManus, O., Træholt Franck, K., Lindberg Marving, E., Dam Rasmussen, L., Raith Richter, S., Ethelberg, S. (2023) Epidemiology & Infection, 151, e28. https://doi.org/10.1017/S0950268823000146

First cases of SARS-CoV-2 BA.2.86 in Denmark, 2023. Rasmussen, M., Møller, F. T., Gunalan, V., Baig, S., Bennedbæk, M., Christiansen, L. E., Cohen, A. S., Ellegaard, K., Fomsgaard, A., Franck, K. T., Larsen, N. B., Larsen, T. G., Lassaunière, R., Polacek, C., Qvesel, A. G., Sieber, R. N., Rasmussen, L. D., Stegger, M., Spiess, K., Tang, M. E., … Jokelainen, P. (2023). Euro surveillance: European communicable disease bulletin, 28(36), 2300460. https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2023.28.36.2300460

Wastewater Surveillance in Europe for Non-Polio Enteroviruses and Beyond. Bubba, L., Benschop, K. S. M., Blomqvist, S., Duizer, E., Martin, J., Shaw, A. G., Bailly, J. L., Rasmussen, L. D., Baicus, A., Fischer, T. K., & Harvala, H. (2023). Microorganisms, 11(10), 2496. https://doi.org/10.3390/microorganisms11102496

SARS-CoV-2 Variants BQ.1 and XBB.1.5 in Wastewater of Aircraft Flying from China to Denmark, 2023. Qvesel, A. G., Bennedbæk, M., Larsen, N. B., Gunalan, V., Krogsgaard, L. W., Rasmussen, M., & Rasmussen, L. D. (2023).Emerging Infectious Diseases, 29(12). https://doi.org/10.3201/eid2912.230717

Predicting COVID-19 Incidence Using Wastewater Surveillance Data, Denmark, October 2021-June 2022. McManus, O., Christiansen, L. E., Nauta, M., Krogsgaard, L. W., Bahrenscheer, N. S., von Kappelgaard, L., Christiansen, T., Hansen, M., Hansen, N. C., Kähler, J., Rasmussen, A., Richter, S. R., Rasmussen, L. D., Franck, K. T., & Ethelberg, S. (2023). Emerging Infectious Diseases, 29(8). https://doi.org/10.3201/eid2908.221634