Wastewater Surveillance Frequently Asked Questions
What is the National SARS-CoV-2 (COVID 19) Wastewater Surveillance Programme?
How does SARS-CoV-2 get into wastewater?
What is the aim of the NWSP?
How will the results of the NWSP be used?
What does it mean if the virus is detected in a sample?
Can you tell how many people have COVID-19/SARS-CoV-2 infection based on the results of the NWSP?
What does it mean if the concentration of the virus was below the limit of detection?
Does the NWSP test for variants of SARS-CoV-2?
Which wastewater catchment areas are involved and why were they selected?
How are the samples collected?
How are the samples analysed?
What does Below Quantifiable Limits (BQL) mean?
What does flow rate mean?
Do the results account for visitors or people who work in the wastewater catchment area?
How are the results reported?
Can I be infected with COVID-19/SARS-CoV-2 from contact with faeces / urine or wastewater?
What is the National SARS-CoV-2 (COVID 19) Wastewater Surveillance Programme?
The National SARS-CoV-2 (COVID-19) Wastewater Surveillance Programme (NWSP) is a partnership between the HSE, Irish Water, the National Virus Reference Laboratory (NVRL), and University College Dublin (UCD). It involves weekly sampling of the influent of wastewater from 68 wastewater catchment areas across Ireland. These samples are tested for the presence of genome of SARS-CoV-2, the virus that causes COVID-19.
How does SARS-CoV-2 get into wastewater?
People with COVID-19/SARS-CoV-2 infection shed the virus in their faeces and urine when they go to the toilet.
What is the aim of the NWSP?
The NWSP is an additional tool in our response to COVID-19/SARS-CoV-2. The NWSP aims to complement our case-based surveillance systems (i.e. monitoring the number of people testing positive or presenting to a healthcare provider with symptoms) through acting as a potential early warning system for the circulation, or the increased circulation, of SARS-CoV-2 in an area.
Case-based surveillance systems rely on people presenting for medical care and/or being tested. These surveillance systems may not capture all people with SARS-CoV-2 infection, in particular asymptomatic people. In addition, there can be a time lag between when someone is infected and when they present for care, or are tested, which means that case-based data can be slightly delayed in reflecting virus circulation. Wastewater surveillance offers the advantage of not being dependent on people presenting for care or being tested.
How will the results of the NWSP be used?
Since the onset of the COVID-19 pandemic, analysis of wastewater for the presence of SARS-CoV-2 has been taking place in many European countries and elsewhere as recommended by the European Union. Results of the NWSP will be shared with key stakeholders in the response to COVID-19/SARS-CoV-2 pandemic in Ireland, including the HSE Public Health Departments, the Irish Epidemiological Modelling Advisory Group (IEMAG), and the National Public Health Emergency Team (NPHET). The findings will help inform testing strategy and the initiation of preventive public health measures. The NWSP will likely be of greatest value when the circulation of SARS-CoV-2 and testing rates are lower, when a detection of SARS-CoV-2 in wastewater may be the first indication of circulation in a catchment area.
What does it mean if the virus is detected in a sample?
If the virus is detected in a wastewater sample it means that there are people in the wastewater catchment area with SARS-CoV-2/COVID-19 infection. However, a person can shed the virus for several weeks, even when they are no longer infectious. Therefore, detection of the virus in wastewater may not mean that there is someone with currently infectious COVID-19/SARS-CoV-2 infection in the catchment area. People living or visiting the catchment area should still follow public health advice to reduce the spread of SARS-CoV-2/ COVID-19.
Can you tell how many people have COVID-19/SARS-CoV-2 infection based on the results of the NWSP?
There is evidence that the quantity of SARS-CoV-2 in wastewater is closely correlated with detected infections in the wastewater catchment area. However, at present we are not able to reliably estimate the prevalence of COVID-19/SARS-CoV-2 infection, i.e. the number of people who currently have COVID-19/SARS-CoV-2 infection, in the wastewater catchment area based on the results of the NWSP. This is an ongoing area of research worldwide. A better understanding of how much virus people shed in their faeces and how that changes over the course of the infection is needed to understand the relationship between the quantity of the virus in wastewater and the prevalence of infection in the wastewater catchment area.
What does it mean if the concentration of the virus was below the limit of detection?
The tests used by the NWSP, like other analytical tests, have a limit of detection. This limit of detection is the lowest concentration of viral material that can be detected in a wastewater sample. If the concentration of viral material in a sample is below the limit of detection, it does not mean that there is no virus present. Viral material may be present, but it cannot be detected by the test due to its very low concentration
Does the NWSP test for variants of SARS-CoV-2?
Currently the NWSP tests for the presence of SARS-CoV-2 in wastewater but may be expanded to detect specific variants. The role of NWSP in identifying emerging variants of SARS-CoV-2 in wastewater catchment areas undergoes regular review.
Which wastewater catchment areas are involved and why were they selected?
Ireland has over 1,100 public wastewater catchment areas where wastewater is collected and conveyed for treatment. As it is not feasible to include them all in the NWSP, catchment areas were selected to provide the greatest coverage of the population and to represent catchment areas in all counties. Two catchment areas from each county have been included in the NWSP. In addition, we included all wastewater catchment areas greater than 10,000 population equivalent (PE) in size. The 68 wastewater catchment areas included in the NWSP cover 80% of the population connected to public wastewater treatment plants.
How are the samples collected?
Staff of the participating wastewater treatment plants collect 24-hour composite samples on specified days. In some instances, incomplete composite, or grab samples may be collected.
How are the samples analysed?
Following arrival of the wastewater samples in the laboratory, the virus is concentrated and the genetic material is extracted. The genetic material (RNA) is quantified using reverse transcriptase quantitative PCR which is used to determine the concentration of the virus in wastewater. The amount of viral genetic material per day entering a wastewater treatment plant can be calculated based on the concentration of the virus and the flow rate.
What does Below Quantifiable Limits (BQL) mean?
BQL stands for below quantifiable limits. The limit of quantification is the lowest concentration of viral genetic material that can be reliably quantified. A result which is BQL means that viral RNA was detected but the concentration cannot be accurately determined.
What does flow rate mean?
The ‘flow rate’ used in this programme is the total volume arising from the wastewater catchment area in the 24hours the sample is taken (m3 per day) as measured at the inlet of the wastewater treatment plant.
Do the results account for visitors or people who work in the wastewater catchment area?
Yes, visitors to the wastewater catchment area who use toilet facilities while in the area contribute to the results.
How are the results reported?
Results are presented as the numerical value of the concentration of SARS-CoV-2 in the wastewater sample and as a descriptive category. The descriptive categories used are ‘positive’ when SARS-CoV-2 RNA was detected and could be quantified; ‘weak positive’ when SARS-CoV-2 RNA was detected but was below the quantification limit (BQL); ‘undetected’ when the numerical value was below the detection limit (BDL); and ‘unavailable’ if a result was not available because either a sample was not received or could not be fully analysed e.g. if the amount of wastewater was insufficient.
Can I be infected with COVID-19/SARS-CoV-2 from contact with faeces / urine or wastewater?
Stool or urine are not recognised sources of transmission for SARS-CoV-2. Standard hygiene measures should be used after using the toilet. Infectious viable SARS-CoV-2 has not been detected in untreated or treated sewage. The WHO recommend that sanitation workers follow standard best practices, including PPE for those working in proximity to wastewater. WHO guidance is available here. https://www.who.int/publications-detail/water-sanitation-hygiene-and-waste-management-for-covid-19
Version 1.0
Created: 31 August 2021