One of the ways to monitor the spread of the coronavirus (SARS-CoV-2) is by measuring the number of virus particles in sewage. When the virus was first detected in the Netherlands in 2020, this research project started on a small scale. RIVM now receives a sewage sample from over 300 locations in the Netherlands several times a week, for research purposes.
This means that the sewage of virtually every Dutch household (over 17 million people) is examined. Sewage research is a supplement to the other studies that RIVM is conducting to monitor the coronavirus, such as testing people for COVID-19 through the Municipal Public Health Services (GGDs).
The aim is for sewage surveillance to enable early detection in the event of localised outbreaks and to monitor the spread of the virus, regardless of how often people get tested. This research also makes it possible to detect and recognise new variants.
How does it work?
Virus particles can enter the sewage system via human faeces from people who are infected. About half of infected people have the virus in their faeces. That includes people with and without symptoms. Sewage flows through the sewer system to the over 300 sewage treatment plants in the Netherlands. Samples of untreated sewage are taken at these plants, refrigerated and sent to RIVM. Researchers at RIVM analyse the samples and figure out how many coronavirus particles they contain. The results are shown on the Coronavirus Dashboard provided by the national government. The results can also be viewed as open data on the RIVM website.
Watch the video on coronavirus monitoring in sewage research
View infographic: Sewage research related to COVID-19
The possibilities of sewage surveillance
Coronavirus monitoring in sewage research in the Netherlands is made possible by a partnership between RIVM, the Association of Regional Water Authorities, and the 21 regional water boards, as mandated by the Ministry of Health, Welfare and Sport (VWS). Together, these partners are exploring deployment of sewage surveillance for broader applications.
For more information about the possibilities of sewage research, see the RIVM page on sewage research.
Virus particles in faeces
Sewage research (National Sewage Surveillance)
Results of the sewage research
Virus particles in faeces and sewage
Is the virus in the sewage contagious?
There is no evidence of human infections transmitted by sewage.
Do all people who are infected actually excrete coronavirus particles in their faeces (poop)?
No. About 50% of infected people also have detectable virus particles in their faeces (poop). Some people who are infected have higher levels of virus particles in their faeces than others. These levels are not determined by whether a person has many or few symptoms, or is asymptomatic or presymptomatic.
Are there additional risks to people working with sewage?
People who work with sewage should avoid direct contact with the People who work with sewage should avoid direct contact with the sewage, and should avoid ingesting, swallowing and/or breathing in spray or mist from the water. Personal protective measures for people working with sewage offer effective protection against the various types of pathogens in sewage, including coronaviruses like SARS-CoV-2. All these protective measures for safe working practices involving wastewater are listed in the occupational health and safety catalogue of the water management authorities. If you have questions about the Health and Safety Catalogue, please contact your Health and Safety Coordinator. It is also important that employees at the sewage treatment plant follow the hygiene protocols.
Coronavirus monitoring in National Sewage Surveillance: how and why?
How is RIVM conducting the study?
Employees from all of the over 300 sewage treatment plants across the Netherlands take sewage samples. Researchers at RIVM analyse the samples and figure out how many coronavirus particles they contain. This is how they do it: genetic material (RNA) from the virus is isolated from the sewage sample. A molecular detection method known as PCR is then performed. This method enables the researchers to determine how much RNA from the coronavirus SARS-CoV-2 is present in the sewage.
RIVM then calculates the number of virus particles in sewage per 100,000 inhabitants, based on data published by Statistics Netherlands (CBS). RIVM publishes the findings in an open database. The Ministry of Health, Welfare and Sport (VWS) processes the data and calculates the weekly averages for the Coronavirus Dashboard nationally and at the security region and municipality levels, according to a predefined method designed jointly by RIVM and the Ministry. The results are shown on the Coronavirus Dashboard provided by the national government. A detailed description of the calculation method is provided here.
Which locations are included in the study?
In the Netherlands, RIVM analyses sewage from almost the entire country. Sewage samples taken from all of the over 300 sewage treatment plants all over the country are sent to RIVM.
Where can I find the sewage research results?
The sewage research results can be found on the Coronavirus Dashboard provided by the national government. The data from each sewage treatment plant is available as open data. A map with a current overview of the measurements is available on this RIVM page about sewage research in the Netherlands.
Can the results also be converted into the number of people who tested positive for COVID-19?
No. We do not currently have any way of converting the number of virus particles into the number of infected people. This is partly because we do not know exactly how much of the virus could be excreted by an infected person. In addition, some people could continue shedding the virus for a long time. Based on the sewage research results, it is possible to arrive at a reasonable estimate of the expected number of hospital admissions. Over time, it may be necessary to review whether these estimates also apply to any new variants that emerge.
Is it possible to trace patients on the basis of this research?
No. Sewage research investigates sewage from many different people, which cannot be traced back to one individual or household. However, the information can be traced back to the supply area of a sewage treatment plant. That means that it is possible to see which areas have high levels of virus particles in sewage, but not to identify the precise households where those particles originated. Detecting the origin is not the aim of this research.
Are the results affected by heavy rainfall?
When rainfall increases, there are fewer virus particles per millilitre of sewage. By using a predefined sampling method, we can prevent the amount of precipitation from influencing the results of the analysis. A small sample of sewage is taken periodically over a 24-hour period. The size of the sample depends on the volume of sewage flowing through the sewage treatment plant. This total volume of sewage is known as the flow rate or discharge.
If there is a lot of rain, and thus a lot of water flowing through the plant, a larger sample is taken than if there is little or no rainfall. By combining and mixing all these small samples, it is possible to achieve a sample that is representative of the amount of sewage that has passed through the sewage treatment plant in the last 24 hours. From that mixed cumulative sample, a smaller sample is then taken for analysis by RIVM. As a result, the amount of rainfall does not affect the sewage sample.
Can we compare the figures (on the number of virus particles in sewage) from different municipalities and security regions?
Yes, that is possible. In all municipalities and security regions, the sewage measurements are calculated and displayed in the same way: the average number of virus particles in sewage per 100,000 inhabitants.
Are the sewage measurements for all the municipalities in the Netherlands displayed on the Coronavirus Dashboard? Even the small municipalities that do not have their own sewage treatment plant?
Yes, research results are available for every municipality – even if they do not have their own sewage treatment plant. If that is the case, the municipality (or part of it) is connected to a sewage treatment plant in a nearby municipality. Statistics Netherlands (CBS) keeps track of population distribution statistics, making it possible to see how many people are in the supply area of a sewage treatment plant that processes sewage from multiple municipalities. That means we can also calculate the number of virus particles in sewage for municipalities without their own sewage treatment plant.
Why is sewage research important?
Sewage research is a supplement to the other studies that RIVM is conducting to monitor the coronavirus, such as testing people for COVID-19. This is especially relevant in periods when there is less testing at GGD locations. During these periods, sewage surveillance is particularly important to keep track of the spread of the virus. Trends in sewage surveillance figures can inform us about the increase or decrease of virus particles in sewage from people who are infected, with or without symptoms.
Does sewage surveillance provide early indicators?
Coronavirus monitoring in sewage research can offer early indicators of a surge in infections. Early detection would enable the Municipal Public Health Services (GGDs) in that region to investigate further. To that end, RIVM has worked with the GGDs, water boards and municipalities to develop an action plan with clearly defined steps.
Sometimes a specific location shows very high levels of virus particles in sewage surveillance. What could be the explanation for that?
High figures in sewage surveillance indicate high levels of virus particles in sewage. That means that the virus is circulating in that area. When sewage surveillance figures for a specific location are exceptionally high, RIVM always does an extra check to see if the measurement was carried out correctly, if the data is correct and if the calculations were done properly.
During the holidays, sewage surveillance often shows high levels of virus particles in tourist areas, such as the Wadden Islands and the coastal towns in Zeeland without a significant increase in the number of people who test positive for COVID-19 in those municipalities. Why is that?
Tourist destinations often attract many more people during the holidays than in off-peak periods. Tourists who spend time in the area obviously also use the toilets there. If some of the tourists are infected with the coronavirus SARS-CoV-2, they may contribute to higher levels of virus particles in the sewage from the tourist area. The virus particles that are measured in the sewage are displayed per 100,000 inhabitants. This is calculated based on the number of permanent residents living in that area. If there are many tourists in the area, the number of people using the toilet will be significantly higher than the number of registered inhabitants. As a result, sewage surveillance figures may be higher, especially for municipalities with relatively few inhabitants.
Can sewage research also be used to identify different variants? Since January 2021, RIVM has also been investigating sewage to detect new variants of the coronavirus, using a specialised method known as sequencing. All the current variants and sub-variants have also been detected in sewage surveillance.
Sewage research: the future
How long will sewage research related to COVID-19 continue?
RIVM, the water boards and the Ministry of Health, Welfare and Sport (VWS) have agreed to continue coronavirus monitoring in sewage research until at least the end of 2025. Keeping track of the virus remains important.
Can sewage research be deployed in a broader context? For example to detect and monitor other diseases?
Sewage research is not new. RIVM is currently testing sewage to check for the coronavirus SARS-CoV-2, but also does similar testing for the poliovirus and for antibiotic-resistant bacteria. RIVM is working closely with the Ministry of Health, Welfare and Sport (VWS) and the water boards to explore the directions in which sewage research can be expanded in the future.
For more information about the possibilities of sewage research, see the RIVM page on sewage research in the Netherlands (only in Dutch).