Concentration of coronavirus RNA in sewage falls faster than recorded patient numbers

In search of the explanation for a striking phenomenon

Since early February, KWR’s microbiologists have been studying levels of SARS-Coronavirus-2 RNA in sewage in collaboration with the water authorities and their research institute, STOWA. We have been taking samples at a range of wastewater treatment plants in the Netherlands, including large plants (Amsterdam-West), obvious candidates (Schiphol Amsterdam airport) and peripheral plants (the island of Terschelling). We found no traces of the virus in early February but there were initial indications that it was present in the subsequent measurements on 5 March. That pattern became even clearer in the samples taken on 16 March. On Monday, 24 March, we posted our study online.

Because the sharing of knowledge is so important and given the enormous level of interest in the wider national and international scientific community, we published the experimental method on 30 March in a “pre-print” version. Since then, we have given a large number of interviews in Dutch and international media. On 20 May, our scientific article Presence of SARS-Coronavirus-2 RNA in sewage and correlation with reported COVID-19 prevalence in the early stage of the epidemic in the Netherlands was published in Environmental Science & technology. The article provides a transparent and thorough description of how we look for SARS-Cov-2 RNA and the concentrations we have found over time.

The samples from the sewage water that were collected and analysed by KWR help to understand the spread of the virus through the entire population (of specific cities) better and so they are a valuable complement to the numbers of COVID-19 patients registered by the Dutch municipal health services.  We are going to collaborate on a major European project to analyse and compare samples from a large number of cities in the European Union.

Can the sewage system serve as an early warning system?

Because it is impossible to test every individual for coronavirus, sewage monitoring can be used as a tool to determine the ‘status’ of entire populations (a city or a smaller town) and to provide administrators (such as the national government or municipal authorities) with a useful guideline as a basis for additional preventive measures or for deciding at some point in the future to ease the lockdown ‘intelligently’.

The press conference given by the Dutch Prime Minister Mark Rutte and Minister of Health Hugo de Jonge on Tuesday, 19 May showed that sewage surveillance will be included in the ‘corona dashboard’ that the cabinet is developing to monitor the spread of the virus. In this context, the sewage system could be used as an early warning system. If the concentration in sewage falls, the spread of the virus in the general population using the sewer will also fall (in the case of a city or a smaller town). If the level rises again unexpectedly, it can be concluded that the level of infection is rising and that additional measures are needed.

An important question relating to the use of sewage data in the dashboard is whether the levels of SARS-Cov-2 RNA in wastewater treatment plants correlate with the numbers of registered patients. In the series of video recordings below, the leading researcher of the COVID-19 research team, Gertjan Medema, describes what he and his colleagues have discovered in the meantime.

Questions for Gertjan Medema

KWR has been measuring coronavirus in sewage since February. What can we see?

How do the results found by KWR relate to the number of cases reported to the municipal health services?

  1. We found a nice correlation between sewage data and cases registered by the municipal health services in the phase during which the spread in the general population was on the increase.
  2. Now, when the number of cases is falling, levels in sewage are falling faster than the number of reported cases. We have no clear explanation for this phenomenon. Is it related to variations in the testing policy? Or is it because patients who report symptoms have been ill for several days and are excreting the virus in stools? We need to conduct further research.

What role can sewage surveillance play in the exit strategy?

Monitoring sewage to see whether the virus levels are falling, staying at a low level or increasing again.

What reactions has KWR received to studying coronavirus in sewage?

Is water (and sewage water in particular) a transmission route for the coronavirus?

What is KWR planning as the follow-up to this study?

In the Rijnmond area, we want to measure concentrations in both the sewage system and at the GP monitoring stations.

Regular updates

In the weeks to come, we will be continuing to share our knowledge about SARS-Cov-2 RNA regularly with our alliance partners and society at large.


If you have any questions about our sewage research, visit our frequently asked questions page.