Frequently asked questions on the monitoring of SARS-CoV-2 in wastewater

Sampling at WWTP Werdhölzli, Zurich (Photo: Eawag, Andri Bryner)

Sampling at WWTP Werdhölzli, Zurich
(Photo: Eawag, Andri Bryner)

 

The following answers to frequently asked questions on the detection of SARS-CoV-2 in municipal wastewater were compiled jointly by the Eawag Departments of Urban Water Management and Environmental Microbiology and the Environmental Chemistry Laboratory at EPFL.

FAQs

What is the significance of epidemiological analyses based on wastewater in relation to SARS-CoV-2?

  • Genetic material from the SARS-CoV-2 virus can be excreted in the stool of infected individuals. This genetic material can be detected regardless of whether the virus is still intact or whether only gene fragments are present.
  • If SARS-CoV-2 genetic material is detected in wastewater, this indicates infections within the population.
  • The detection of SARS-CoV-2 in wastewater can help to define the priorities for affected regions (e.g. with regard to materials, testing intensity, behavioural measures, etc.).

What can wastewater-based epidemiology achieve?

  • Wastewater analyses allow the monitoring of large populations (see Figure 1).
  • A wastewater sample that tests positive for SARS-CoV-2 can serve as an alert to trigger the introduction of measures in the region.
  • Regular samples can confirm, but currently not predict, the dynamics of an epidemic (rise and levelling-off of infection numbers). Data from the second wave in Zurich and Lausanne reveal agreement between wastewater analyses and reported case data (see graphs updated regularly on project website.

  • Once widespread clinical testing seizes (e.g., after a significant decrease in transmission due to the introduction of a vaccine), wastewater analyses can inform on the residual circulation of the virus in a catchment.

A wastewater sample that tests positive for SARS-CoV-2 means that

  • … recently recuperated or currently infected individuals have contributed to the analysed wastewater sample.
  • … a threshold value of infected individuals has been exceeded (see Figure 1).
Conceptual model of the limit of detection for SARS-CoV-2 in wastewater as a function of the size of the population served by the wastewater treatment plant. Example interpretation: whereas it is possible to detect a very small number of individuals excreting SARS-CoV-2 in an old people’s home of 100 people ...
Figure 1. Conceptual model of the limit of detection for SARS-CoV-2 in wastewater as a function of the size of the population served by the wastewater treatment plant. Example interpretation: whereas it is possible to detect a very small number of individuals excreting SARS-CoV-2 in an old people’s home of 100 people, the limit of detection at WWTP Lausanne, which serves 240,000 people, is equal to around 100 individuals excreting SARS-CoV-2 (based on the few clinical studies available so far, this would correspond to 10–20 new cases per day in the example of Lausanne).

How is SARS-CoV-2 genetic material detected in wastewater?

  • A sample of untreated sewage is generally collected from the inlet of a wastewater treatment plant (routine, flow-proportional 24h composite sample) and therefore encompasses a large number of toilet users in the catchment area of the wastewater treatment plant.
  • The detection and quantification of SARS-CoV-2 in wastewater is based on real-time quantitative PCR (qPCR) or droplet digital PCR (ddPCR). These are the same RNA replication and analysis methods that are used for clinical tests.
  • As the wastewater contains only very low concentrations of the target gene sequences (N1 and N2), the samples must undergo an elaborate preparation process involving filtration, concentration and RNA extraction prior to detection.
  • For quality control, known concentrations of known viruses are also added to the samples.  Differences between the amount of virus added and the amount of virus measured help inform us if and how well the measurements work.

How long does it take to detect SARS-CoV-2 genetic material in wastewater?

  • With the method currently used by EPFL and Eawag (for more information, see project website), the process of sample preparation, analysis and reporting takes two working days.

What can’t wastewater epidemiology do in relation to SARS-CoV-2?

  • Wastewater samples do not reflect every positive case – due to the limits of detection (Figure 1) and because some infected individuals do not excrete SARS-CoV-2 genetic material.
  • The analysis of wastewater samples does not distinguish between new cases of infection, on the one hand, and people who have recovered but are still excreting SARS-CoV-2 genetic material, on the other.
  • Wastewater samples are no substitute for the swabs used to identify individual cases.
  • Wastewater samples cannot be used to predict the further development of an outbreak.
  • Wastewater samples can only provide indications of – but not confirm – whether the precautionary measures adopted have been successful (given the limits of detection, the impact of commuters and tourists or infected individuals who do not excrete SARS-CoV-2 genetic material).

Further information and links

Project website

Application of Wastewater-based Epidemiology to SARS-CoV-2 Detection

Video

EPFL/Eawag video: “COVID-19: using wastewater to track the pandemic” (3 min)

SRF Einstein broadcast of 30 April 2020. “Der Lockdown: Das Virus und sein Impact” (Lockdown: the virus and its impact; 36 min).

Broadcast

SRF Einstein broadcast of 30 April 2020. “Der Lockdown: Das Virus und sein Impact” (Lockdown: the virus and its impact; 36 min).

News

April 30, 2020

The novel coronavirus has been successfully detected in wastewater – even at low concentrations, in samples collected at an early stage of the outbreak. Researchers at EPFL and Eawag are now working to optimise the method. The aim is to develop a system which could warn of a resurgence of cases earlier than clinical diagnostic tests.

Read more

April 3, 2020

Researchers want to make use of the fact that coronaviruses excreted by infected individuals can be found in wastewater. If the viruses can be detected successfully, this may provide a much faster way of spotting a wave of infections than via testing of symptomatic individuals. To find out more, we interviewed Eawag researcher Christoph Ort.

Read more

August 28, 2020

Nature Sustainability has just published a review article on possible risks that COVID-19 viruses could cause in wastewater. Prof Eberhard Morgenroth (Head of the Eawag Department of Process Engineering) is a co-author of the review. We asked him five questions.

Read more

Eawag/EPFL experts

Dr. Christoph OrtGroup LeaderTel. +41 58 765 5277Send Mail
Dr. Tim JulianGroup Leader of Pathogens and Human HealthTel. +41 58 765 5632Send Mail