Figure 6 below is a phase plot that traces each catchment area forward in time in terms of the population-normalised viral load each week and the percent change from the previous week. This allows us to more easily identify catchment areas where the recent change in viral load has been noteworthy, and compare that to the history of such changes over the duration of the programme.
Figure 6 SARS-CoV-2 viral load phase plot by wastewater catchment area, National Wastewater Surveillance Programme, week 19 2021 to week 46 2023
Acknowledgements
Sincere thanks are extended to all those who are participating in the collection and processing of samples, processing data and reporting of data used by the NWSP. This includes the staff at participating wastewater treatment plants, Irish Water, laboratory staff (UCD and NVRL), HSE COVID-19 Contact Management Programme (CMP), Health Intelligence Unit (HIU), surveillance scientists, microbiologists and administrative staff.
Report prepared by the COVID-19 Epidemiology Team, HPSC
Glossary
Below detection limit (BDL): The 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.
Below Quantifiable Limits (BQL): 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.
Composite Samples: Wastewater sample consists of numerous individual discrete samples taken at regular intervals over a period of 24 hours.
Grab sample: A wastewater sample collected at a particular time and place can represent only the composition of the source at that time and place.
Flow rate: 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.
Population data: Population normalised results use 2020 estimates of the residential population of the wastewater catchment area provided by Irish Water.
Technical notes
Wastewater sample collection: Staff of the participating wastewater treatment plants collect 24-hour composite samples on specified days. In some instances incomplete composites, or grab samples, are used instead.
Laboratory analysis of wastewater samples: Carried out by UCD. 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.
Selection of wastewater catchment areas for inclusion in the NWSP: Catchment areas were selected to provide the greatest coverage of the population and to represent catchment areas in all counties.At least one catchment area from each county have been included in the NWSP, with two catchment areas included for most counties. In addition, we included all wastewater catchment areas greater than 10,000 population equivalent (PE) in size. The 30 wastewater catchment areas included in the NWSP cover 70% of the population connected to public wastewater treatment plants, and are shown in Figure 1.
Smoothing: Smoothed trends were fit to observed data with generalised additive models using restricted maximum likelihood estimates of smoothing parameters with interpolation for weeks with missing sample data. The resulting trend lines should only be viewed as one reasonable, visual summary of the observed data. Smoothed trends should be reviewed with caution for areas with several consecutive weeks of missing data.
Data uses
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. The NWSP will work with Departments of Public Health and other stakeholders on how the data can best support the public health response to SARS-CoV-2/COVID-19 in Ireland. At the current stage of the pandemic, circulation is widespread with high incidence rates across Ireland. 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.
The NWSP will also be of value in monitoring for the presence of new variants of SARS-CoV-2 as they emerge.
Data limitations and uncertainties
In general, higher levels of SARS-CoV-2 in wastewater suggests more people with SARS-CoV-2 infection in the wastewater catchment area. However, people shed different amounts of virus during the period that they are infected which declines are as they are clearing the virus. For this and other reasons it is therefore difficult to reliably estimate the number of people with SARS-CoV-2 infection in the catchment area based on analysis of SARS-CoV-2 RNA in wastewater.
Aside from the number of people with SARS-CoV-2 infection, and the amount of virus they shed, there are other factors which influence the amount of SARS-CoV-2 detected in wastewater. For example, wastewater treatment plants also take in surface waters from rainfall, which alters the flow rate of influent entering the treatment plant. The concentration of SARS-CoV-2 is therefore adjusted to take the daily flow rate of the plant into account. When a flow rate is not available for the day the composite sample was taken, the average annual flow rate is used instead. Composite samples are the preferred sample type as the reflect wastewater passing through the sampling point for a 24-hour period. However, composite samples are not always possible. The NWSP will continue to monitor its methodology as part of its quality improvement process.
NWSP data should be reviewed in conjunction with data from other case-based surveillance systems.
Further information
A technical report by the Joint Research Centre (JRC) and the Directorate-General for Environment (DG-ENV) of the European Commission on wastewater surveillance for SARS-CoV-2 is available here.
Links to dashboards of other SARS-CoV-2 wastewater surveillance programmes are available here.
Appendices
Supplemental Table 1 National Wastewater Surveillance Programme sampling results by wastewater catchment area, week 46 2023
| County | Wastewater catchment area | Sample type | Sample date | Result category |
|---|---|---|---|---|
| Carlow | Carlow | 24 Hrs | 14/11/2023 | Positive |
| Cavan | Cavan | 24 Hrs | 13/11/2023 | Positive |
| Clare | Ennis North | Unknown | 12/11/2023 | Positive |
| Cork | Ballincollig | Unknown | 13/11/2023 | Positive |
| Cork City | 24 Hrs | 13/11/2023 | Positive | |
| Cork Lower Harbour | 24 Hrs | 13/11/2023 | Weak Positive | |
| Donegal | Buncrana | 24 Hrs | 14/11/2023 | Positive |
| Letterkenny | Grab | 14/11/2023 | Positive | |
| Dublin | Balbriggan | Grab | 13/11/2023 | Positive |
| Portrane Donabate | 24 Hrs | 13/11/2023 | Positive | |
| Ringsend 1 | 24 Hrs | 12/11/2023 | Positive | |
| Ringsend 2 | NA | NA | NA | |
| Shanganagh | Unknown | 14/11/2023 | Positive | |
| Swords | 24 Hrs | 13/11/2023 | Positive | |
| Galway | Galway | 24 Hrs | 12/11/2023 | Positive |
| Kerry | Tralee | 24 Hrs | 13/11/2023 | Positive |
| Kildare | Lower Liffey Valley Regional Sewerage Scheme (Leixlip) | 24 Hrs | 14/11/2023 | Positive |
| Upper Liffey Valley Sewerage Scheme (Osberstown) | Grab | 14/11/2023 | Positive | |
| Kilkenny | Kilkenny | 24 Hrs | 12/11/2023 | Positive |
| Laois | Portlaoise | 24 Hrs | 12/11/2023 | Positive |
| Limerick | Limerick City | 24 Hrs | 13/11/2023 | Positive |
| Louth | Drogheda | Grab | 13/11/2023 | Positive |
| Dundalk | 24 Hrs | 13/11/2023 | Weak Positive | |
| Mayo | Castlebar | Incomp | 12/11/2023 | Positive |
| Meath | Navan | 24 Hrs | 13/11/2023 | Positive |
| Monaghan | Monaghan | 24 Hrs | 13/11/2023 | Positive |
| Sligo | Sligo | 24 Hrs | 14/11/2023 | Positive |
| Tipperary | Clonmel | 24 Hrs | 13/11/2023 | Positive |
| Waterford | Waterford | 24 Hrs | 12/11/2023 | Positive |
| Westmeath | Mullingar | 24 Hrs | 14/11/2023 | Positive |
| Wexford | Wexford | 24 Hrs | 12/11/2023 | Positive |
Report prepared on 22/11/2023
Version 1.1
Dates of epidemiological weeks are available in the technical notes and at: https://www.hpsc.ie/notifiablediseases/resources/epidemiologicalweeks/↩︎