Wastewater Surveillance Frequently Asked Questions

Wastewater Surveillance Frequently Asked Questions

What is the National Wastewater Surveillance Programme?

The National Wastewater Surveillance Programme (NWSP) is one of HSE’s Surveillance programmes. The NWSP is conducted in a partnership between the HSE’s Health Protection Surveillance Centre (HPSC), Uisce Éireann, UCD National Virus Reference Laboratory (NVRL) and the School of Biomolecular and Biomedical Science at University College Dublin (UCD). It involves weekly sampling of the influent of wastewater from 30 wastewater catchment areas across Ireland. The NWSP was initially established in 2021 during the COVID-19 pandemic as an additional tool to monitor COVID-19/SARS-CoV-2 activity in the community. However, recognising the power of the network to detect other pathogens of concern, the programme was expanded to include testing for poliovirus in 2023.

What is the aim of the NWSP?

The NWSP aims to complement other case-based surveillance systems which monitor the spread of infectious pathogens of concern in the community.

People infected with certain pathogens can shed the pathogen, or fragments of the pathogen, in their urine or faeces. These can then be detected in wastewater. This means that wastewater surveillance can be used to monitor the circulation of these pathogens in the population.

In contrast, case-based surveillance typically monitors the number of people testing positive for a pathogen or presenting to a healthcare provider with symptoms. As case-based systems rely on people presenting for medical care and/or being tested, they therefore do not capture all people infected with the pathogen of interest, in particular asymptomatic people. In addition, there can be a time lag between the point at which someone is infected and when they present for care or are tested; this means that case-based data can be slightly delayed in reflecting pathogen circulation. Wastewater surveillance offers the advantage of not being dependent on people presenting for care or being tested.

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 practicable coverage of the population, and to include different areas of the country.

Since January 2023, the programme has included 30 wastewater catchment areas across the country and covers approximately 70% of the population connected to public wastewater treatment plants.

Of note, the programme did originally include 67 catchment areas and covered approximately 80% of the population connected to public wastewater services. However, by reducing to 30 areas, we can maintain good population coverage, whilst also ensuring the programme is sustainable and cost-effective.

How often are samples collected?

Samples are collected on a weekly basis, with the exception of Ringsend wastewater catchment area in Dublin, which is sampled twice a week. Ringsend wastewater catchment area is sampled more frequently because it is the largest wastewater catchment area in Ireland, covering a residential population of approximately 1.3 million people.

How are the samples collected?

Staff at the participating wastewater treatment plants collect 24-hour composite samples on specified days. In some instances, incomplete composite, or grab samples may be collected. A composite sample consists of numerous individual discrete samples taken at regular intervals over a period of 24 hours. A grab sample is collected at a particular time and place and can represent only the composition of the source at that time and place. Composite samples are the preferred sample type as they reflect wastewater passing through the sampling point for an entire 24-hour period.

SARS-CoV-2 (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 wastewater surveillance for SARS-CoV-2?

The NWSP monitors trends in the amount of SARS-CoV-2 in wastewater. This is an indicator of the level of circulation of COVID-19 in the community.

How are the samples analysed for the presence of SARS-CoV-2?

Following arrival of the wastewater samples in the laboratory, the sample undergoes a special filtration and concentration method to concentrate any SARS-CoV-2 virus material that might be present. After this process is complete, the genetic material of the virus (RNA) is extracted. The RNA is then quantified using reverse transcriptase quantitative PCR (RT-qPCR) which is used to determine the concentration of the virus in the original wastewater sample.

The amount of SARS-CoV-2 genetic material (the viral load, reported as gene copies per day) entering a wastewater treatment plant is then calculated from the viral concentration and the flow rate for the catchment area. Finally, population-normalised viral loads are reported as gene copies/day per person residing in the catchment area.

What does flow rate mean?

The ‘flow rate’ is the total volume arising from the wastewater catchment area during a 24-hour period (reported as cubic metres per day) as measured at the inlet of the wastewater treatment plant. Where possible we use the flow rate on the day the sample was collected in our calculations. When this is not available, we use the average annual flow rate for that catchment area.

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) of the assay/test; ‘undetected’ when the numerical value was below the detection limit (BDL) of the assay/test; 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.

What does “Below quantification limit” (BQL) mean?

BQL stands for “below quantification limit” and refers to results where the concentration of SARS-CoV-2 in the wastewater sample are below a defined threshold known as the limit of quantification. This is an important performance metric in RT-PCR that defines the sensitivity and reproducibility of our analysis. The limit of quantification is the lowest concentration of viral genetic material that can not only be detected but can also be reliably and accurately quantified. A result which is BQL means that viral RNA was detected but the concentration is too variable between replicates for an accurate concentration to be determined. All results that are reported as “positive” are above this threshold, which means that the concentration of SARS-CoV-2 RNA will be similar every time that sample is retested.

What does “Below the detection limit” (BDL) mean?

Similar to the BQL, BDL refers to results where the concentration of viral material is below the limit of detection. This limit of detection is the lowest concentration of viral material that our analysis can reliably detect 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. The viral material may be present, but at a level that is too low for the test to pick up. As such, BDL can mean one of two things: either (i) there is no viral material present; or (ii) there is viral material present at a level that we cannot detect with our current analysis. At present it is not possible to distinguish between these two possibilities, so results are reported as “BDL” and rather than “negative” to avoid over interpreting very low results or potential false negatives.

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 correlated with the number of detected person 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 also needed to understand the relationship between the quantity of the virus in wastewater and the prevalence of infection in the wastewater catchment area.

Does the NWSP test for variants of SARS-CoV-2?

In October 2023, sequencing of SARS-CoV-2 genomic material in wastewater was added to the NWSP and is currently performed on samples taken from five wastewater catchment areas. This facilitates unbiased evaluation of viral lineages circulating in the community and may help to reduce the time to detection of a novel variant or lineage.

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 (i.e. live) SARS-CoV-2 has not been detected in untreated or treated sewage. That being said, the World Health Organization recommend that sanitation workers follow standard best practices, including the use of personal protective equipment (PPE) for those working in proximity to wastewater. World Health Organization (WHO) guidance is available here. https://www.who.int/publications-detail/water-sanitation-hygiene-and-waste-management-for-covid-19

Poliovirus

How does poliovirus get into wastewater?

Poliovirus belongs to the enterovirus family of viruses, which primarily live and reproduce in the intestinal tract: as such, people infected with poliovirus shed the virus in their faeces. In addition, people who have recently received the oral poliovirus vaccine also may shed traces of vaccine-like poliovirus in their faeces. While the oral polio vaccine is no longer administered in Ireland, it is still used in other countries around the world. People in Ireland receive an inactivated injected polio vaccine as part of the routine childhood immunisation schedule.

What is the aim and objectives of wastewater surveillance for poliovirus?

Wastewater surveillance for poliovirus aims to detect the circulation of poliovirus in the community.

There have been no cases of polio detected in Ireland since 1984. However, detections of poliovirus in wastewater in the UK and the reporting of a clinical case in New York State, USA in 2022, in addition to more recent wastewater detections in a number of European countries during 2024 and 2025 have highlighted the importance of surveillance systems for poliovirus detection. Non-case-based surveillance is especially important as the majority of individuals infected with poliovirus will not have any symptoms but will still excrete the virus in their faeces. Testing wastewater for the presence of poliovirus can provide an opportunity to detect resurgence of poliovirus before symptomatic cases occur and allow public health authorities to implement measures to prevent its spread.

What does wastewater surveillance for poliovirus detect?

The wastewater surveillance programme initially focused on testing for type 2 poliovirus (PV2) based upon recent detections of this type of poliovirus internationally. However, since October 2025, testing also includes type 1 and 3 polioviruses (PV1 and PV3).

Which areas are included?

Testing for poliovirus was initially undertaken in the catchment area of the Ringsend wastewater treatment plant serving the greater Dublin area, which as previously mentioned is the largest wastewater catchment area in Ireland. This testing was expanded in February 2025 to five catchment areas; Ringsend, Cork City, Galway, Limerick and Letterkenny. This covers 46% of the Irish population connected to public wastewater treatment plants.

How are samples analysed for the presence of poliovirus?

Following arrival of the wastewater samples in the laboratory, the genetic material is extracted. The genetic material (RNA) is tested by digital PCR testing to determine if poliovirus is present.

What does it mean if poliovirus is detected in a sample?

Finding poliovirus RNA in wastewater indicates that there may be an individual in the population shedding poliovirus. However, it is important to note that a detection of poliovirus in wastewater is not necessarily a cause for concern. For example these detections can occur when an individual recently vaccinated with the live oral polio vaccine sheds traces of the vaccine virus in their faeces.
If poliovirus RNA is detected in a sample, further investigations are required to determine the significance of the result.

Following detection of poliovirus RNA in a wastewater sample, the sample is sent to the National Virus Reference Laboratory, Ireland’s WHO-accredited National Polio Laboratory, to attempt to grow poliovirus on cell culture in accordance with WHO protocols, to confirm the result. If the culture is positive for poliovirus further laboratory investigations will be carried out to obtain more information, including the type of polio virus detected. The final result will be officially reported to WHO, if it meets the criteria for notification under International Health Regulations (2005).

Can I be infected with poliovirus from contact with faeces / urine or wastewater?

Poliovirus can spread from contact with faeces, with infection occurring usually by faecal-oral transmission. The majority of people living in Ireland are vaccinated against poliovirus during childhood. The poliovirus vaccine provides excellent immunity and protects against serious illness. Although poliovirus can be detected in wastewater samples, the amount of virus present within such samples is usually well below the estimated level required to cause infection. Wastewater collected in closed sewage systems is therefore not considered to be a source of poliovirus infection or transmission for the general population. It is however recommended that sanitation workers in proximity to wastewater and laboratory workers handling wastewater samples follow standard best practices, including the use of PPE. Specific WHO guidance for facilities collecting, handling or storing materials potentially infectious for poliovirus is available on WHO.int

For further information on poliovirus please visit https://www.hpsc.ie/a-z/vaccinepreventable/polio/factsheet/ 

Version 4.0
Last updated: 29 January 2026