Where do the numbers come from?
The figures for wastewater testing are supplied by the National Institute for Public Health and the Environment (RIVM) as open data.
- The national waste water numbers RIVM (in Dutch)
- The municipality waste water numbers RIVM (in Dutch)
- The data per waste water treatment plant
How are the numbers calculated?
Wastewater samples are collected at waste water treatment plants (WWTP) across the country, so that data is available for all of the Netherlands. The samples of untreated waste water are kept chilled during transport to RIVM. RIVM researchers analyse the samples and determine how many coronavirus particles each sample contains. For each location, the researchers analyse an average of 1.4 times per week (all locations 1x, 133 locations 2x) waste water collected in 24 hours.
It takes several days before the outcome of an analysis is known. That is why the dashboard does not show measurements from the most recent days.
The number of samples that can be analysed successfully differs from one week to the next. The open data file shows the date and location of successful measurements. If the number of virus particles in a sample is too low to be measured, the value is recorded as 0. This monitoring method is still under development.
In collaboration with the water authorities, Statistics Netherlands (CBS) has mapped out the number of connected residents per WWTP. This enables RIVM to calculate the number of virus particles per 100,000 inhabitants. To be able to compare municipalities with each other, we look at how many virus particles there are on average per 100,000 people. If exactly 100,000 people lived in 1 specific municipality, the average number of virus particles displayed on the dashboard of that municipality x 100 billion.
- View the explanation of the method for calculating the average number of virus particles in wastewater
Explanation big changes in percentages
If few virus particles are found in sewage, small increases and decreases in the number of virus particles can lead to large percentages of increases and decreases. It is therefore important to always look at the changes together with the absolute numbers of virus particles.
Adjustments and corrections
From 2024 Change in the number of measurements per week
For each location, the researchers analyse sewage collected in 24 hours on average twice a week. From 2024 onwards, this will remain the case for a selection of 133 WWTPs, and for the rest of the WWTPs this will happen once a week.
July 2023 Change in the number of measurements per week
The National Institute for Public Health and the Environment (RIVM) has reduced the number of measurements per location from 3 to 2 per week. This is enough to keep track of the virus.
Removing extremely high readings from datasets
In a number of cases, RIVM and the Ministry of Health, Welfare and Sport have decided to extract measurements from the open data. The results of these measurements were very high and deviated strongly from the regional and national trends in wastewater research.
Other indicators, such as the number of positive tests, also showed a different picture. It has been extensively investigated by RIVM and remains an ongoing investigation. The RIVM has established a protocol for dealing with these kinds of high values.
6 February 2023 Change in number of measurements per week
RIVM has reduced the number of measurements per location from 4 to 3 per week. This is sufficient to keep track of the virus.
30 December 2022 New database of CBS residents
RIVM uses a new version of the database of Statistics Netherlands (CBS), which describes the number of inhabitants per catchment area of waste water treatment plants. The new figures in this version have been retroactively applied to all waste water measurements from 2022.
26 September 2022 Rainfall causes deviations in measurements at all WWTP's
According to RIVM, the results of the waster water measurements of Monday, September 26, 2022 do not provide a good picture of the amount of virus particles in the sewer. This is because a lot of rain fell throughout the country while the samples were being taken. Therefore, these measurements will not be published in the open data.
18 August 2022 Removal of high measurements of all data at WWTP Katwoude
From 18 August 2022, there will no longer be any wastewater measurement for the Katwoude WWTP. The wastewater measurements of WWTP Katwoude are also no longer included in the calculations with retroactive effect.
4 May - 24 June 2022 No measurement data WWTP Zeewolde
Between May 4 and June 24, 2022, no measurement data are known from the Zeewolde wastewater treatment plant due to technical maintenance.
3 February 2022 Removal of high measurements from dataset
- 8 November 2020 WWTP Beilen
- 21 February 2021 WWTP Dokhaven
- 21 March 2021 WWTP Veenendaal
- 22 July 2021 WWTP Ameland
- All data from WWTP Woerden
31 August 2021 Deviating measurement location Dinther
On 31 August 2021, the dashboard published a measurement of August 26, 2021 from measurement location Dinther with a very high outcome. This measurement also caused a large increase in the national figures. Because the value was so extreme and RIVM did not find an explanation after quality checks and research, RIVM removed the measurement from the data on September 1, 2021. RIVM has conducted further research into other measurements from the same period. The results of samples from August 25, 2021 and August 30, 2021 are on the dashboard.
4 March 2021 Calculations for municipalities without their own WWTP
From 4 March 2021, the dashboard will also calculate how many virus particles are in the wastewater for municipalities that do not have their own wastewater treatment plant. We do this by using the measurements of the wastewater treatment plants to which these municipalities are connected and by looking at the population figures of Statistics Netherlands. Other calculations were also adjusted at the same time. See the paragraph 'How are the figures arrived at' for the calculation method.
5 October 2020 and 7 December 2020 Closure of location Aalst and Lienden
In 2020, two measuring locations were closed and their areas were taken over by two other locations. In the week of October 5 2020, location Aalst was taken over by Zaltbommel, and in the week of 7 December 2020, location Lienden was taken over by Tiel. As a result, before these weeks a reduced number of inhabitants is calculated for the Zaltbommel and Tiel location.