COVID-19: Wisconsin Wastewater Monitoring Program

Note: Wastewater surveillance is an evolving science, and DHS may update methods and visualizations to improve the understandability of wastewater data.

This interactive dashboard includes:

• A waffle chart showing the number of sewersheds within each concentration category (outlined below).
• A summary of the number of sewersheds that have had a significant increase (see below for criteria) within the latest week of their sampling.
• A graph showing average SARS-CoV-2 levels across all participating sewersheds statewide.
• Concentrations of SARS-CoV-2 virus gene copies in the wastewater, normalized by sewage flow and sewershed population.
• Wisconsin sewershed locations and boundaries that are included in the monitoring network.

Sewershed: An area of land where raw sewage from homes and businesses flows through a series of sewer pipes into a single downstream point, where it enters a wastewater treatment facility.

Processing time: Dates on the graph indicate the day that the sample was collected at the treatment plant. With shipping and processing time, it typically takes five to 10 days for data to be included on the dashboard. The statewide average also has a 5-10 day lag to allow data to be received from sites. The statewide average concentration is not final until all data are received (indicated by grey area labeled “preliminary data”).

Category: The colored squares on the dashboard (SARS-CoV-2 concentration categories) show the category of SARS-CoV-2 levels in each community (very low to very high). Categories are calculated by averaging the SARS-CoV-2 concentrations of the three most recent samples at a given facility, and comparing this current level to past levels measured at that facility, which are sorted into five categories:

• Highest 20% (very high)
• 60th - 80th percentile (high)
• 40th - 60th percentile (moderate)
• 20th - 40th percentile (low)
• Lowest 20% (very low)

The current category displayed on the dashboard for each site indicates how recent levels at that facility compare to the historical range of concentrations from that facility.

The length of the reference period used to determine the cut-off levels for each category depends on the laboratory method used and how long the facility has been submitting samples. The start of the reference period will be indicated by a vertical line on the graph for each facility, and will be fixed to the specific date when current methods were implemented.

Trajectory: Significant increases in SARS-CoV-2 levels in wastewater are defined when recent measurements show an increasing trend and new values are higher than recent measurements. Increasing trends are determined from a linear regression over the past five measurements. A high measurement is defined if the average of the three most recent data points is above the 80th percentile of all data from the past 30 days.

An increase in virus gene copies over time shows that cases may be increasing in the community. Because each community is different, you should not compare viral gene copy numbers between communities. Looking at trends over time in a specific community can be used to help understand whether cases or hospitalizations are likely to increase in the future.

Scale: The wastewater concentration of SARS-CoV-2 figure can now be viewed with a linear or a logarithmic (log) scale. These two options are displaying the same values, but with different scales on the y-axis of the figure.

• A linear scale can be most helpful when data points are in a narrow range and there isn't much variability between them.
• A log scale can be most helpful when there is a large range of values in the data. Because there is a large range of values in the wastewater data, a log scale may be more helpful to see trends over time in certain sewersheds.

Statewide Average: The statewide average concentration (orange line) represents SARS-CoV-2 levels across all sites in Wisconsin over the past seven days. This is calculated as the average amount of SARS-CoV-2 virus in the wastewater among participating sites, displayed on the graphs as millions of gene copies per person per day. Individual sites may have levels and trends that are different from the statewide average, so click on your local sewershed for a detailed view.

The statewide average is calculated with the following formula:

• For each site, we calculate the average SARS-CoV-2 concentration over the past seven days. This one-week average uses a rolling geometric mean of all samples at that site from the past seven days normalized to the flow rate and population size of the sewershed.
  • Before averaging across sites, concentrations submitted by UW-Milwaukee laboratory are multiplied by a fixed amount to account for known differences in laboratory methods between that lab and other labs.
• A statewide average for each day is then calculated using the geometric mean of all sites combined.
• The daily statewide average is smoothed with a seven-day rolling average (arithmetic mean).
• The statewide average will not be displayed on the graph until there are at least 75% of sites represented in the calculation (75% of sites with a measured concentration in the previous seven days). The average typically lags about four to five days behind the current calendar day to ensure data quality and completeness.

Benefits of monitoring for SARS-CoV-2 in wastewater

Early detection

• People with COVID-19 begin to have the virus in their feces shortly after they catch the virus, sometimes days before they would begin to show symptoms or get tested.
• Increases of viral gene copies in wastewater have been seen up to a week before corresponding increases in diagnosed cases seen through testing.
• By seeing a rise in cases earlier, local public health interventions can be implemented sooner. This can then help limit COVID-19 spread in a community.

"Pooled" sampling

• Rather than test every single person individually, this approach allows monitoring of entire communities at the same time.
• It captures those without symptoms and people that may not go in for testing. This provides a more complete picture of COVID-19 activity in a community.

Limitations of monitoring for SARS-CoV-2 in wastewater

There are several limitations to wastewater surveillance, which is why we recommend following trends over time:

• Population migration: The people that contribute to a sewershed may vary from day-to-day, as they travel in or out of a sewershed for work or tourism.
• Uncertainties in shedding of the virus: The amount and how long people shed the virus can vary from person to person. Because of this, it is very difficult to estimate the specific number of people with COVID-19 based on a given wastewater concentration.
• Variability of samples and analysis: The laboratories analyzing results from the sampled sites work closely with the CDC to ensure best methods are used, but specific concentration values can vary based on the laboratory and methods used. Therefore, concentrations should not be compared across sites and testing laboratories.

We are still learning about when and how much virus is in the feces of people with COVID-19. Therefore, you should only look at the trends of viral gene copies over time, not the absolute number of gene copies in the results. The data in the dashboard shows the total number of viral gene copies detected in the area from which the wastewater was collected.

Because people can have the virus in their feces for 20 to 30 days after they are no longer contagious, decreases in the number of viral gene copies in wastewater might lag behind decreases in cases in a community. You should take the number of viral gene copies in wastewater into account along with other COVID-19-related data to inform decisions about taking actions to help limit COVID-19 spread.

Can I compare results from one community with another?

We do not recommend comparing the number of gene copies found between communities. The flow of wastewater is not all the same, and sources of wastewater flow are different in each community (for example: industrial discharges or rainwater). It is better to use data from a single location to see trends over time. However, you may compare trends between different locations.

The data in this dashboard will be updated at least twice per week.

Data sources

SARS-CoV-2 concentration data are from the Wisconsin COVID-19 Wastewater Surveillance System.

Additional resources

• CDC National Wastewater Surveillance System
• Wisconsin State Lab of Hygiene COVID-19 Wastewater Surveillance System

Beginning August 31, 2023, DHS will no longer publicly update case counts for COVID-19 in Wisconsin. During 2020 and 2021, daily counts of confirmed COVID-19 cases effectively estimated community spread and predicted trends in hospitalization and death. However, since late 2021, at-home rapid antigen tests have become widely available, and fewer people are getting tested at a clinic or local testing site.

At-home antigen tests are not generally reported to public health and are not represented in confirmed case counts published by DHS. As a result, published case counts have grown increasingly unreliable and no longer accurately represent community spread in Wisconsin.

While accurate case data are no longer available, there is still a need to track COVID-19 activity in Wisconsin communities. Since early 2020, the Wisconsin Wastewater Monitoring Program has developed methods to track SARS-CoV-2 levels in sewage treatment plants throughout the state. These methods are used alongside our other respiratory surveillance data to monitor COVID-19 activity. 

Multiple studies from around the world demonstrate that wastewater monitoring can provide reliable and valid information on community spread and predict COVID-19 outbreaks before hospitalizations begin to increase. In Wisconsin during the first two years of the COVID-19 pandemic, case counts closely follow wastewater trends (Figure 1) and analyses have found that cases are closely correlated with wastewater levels in participating sewersheds.¹

Figure 1. Comparing average SARS-CoV-2 levels in wastewater to COVID-19 case rates

Wastewater may also be an early indicator of COVID-19 surges. A 2023 analysis of North Carolina wastewater data found that an algorithm derived from three wastewater metrics (detectability, percent change, and flow-population normalized viral concentrations) reliably identified COVID-19 outbreaks approximately four to five days before case data throughout the Delta and Omicron waves,² providing communities extra time to prepare for the coming surges.

While wastewater monitoring has been used for at least 20 years, its widespread use for surveillance of respiratory viruses is relatively new. Wastewater has limitations and absolute measures are influenced by laboratory techniques, flow rates, and unexplained variation. Absolute wastewater levels should not be compared across geographic regions or time points. However, three years of evidence demonstrate wastewater’s utility as a marker of community spread of COVID-19. The Wisconsin Wastewater Monitoring Network is a sustainable, robust replacement for case counts to track COVID-19 in Wisconsin, and alert communities when levels are rising so that they can act quickly to prevent further spread of the virus.

References

1. Feng S, Roguet A, McClary-Gutierrez JS, Newton RJ, Kloczko N, Meiman JG, McLellan SL. Evaluation of Sampling, Analysis, and Normalization Methods for SARS-CoV-2 Concentrations in Wastewater to Assess COVID-19 Burdens in Wisconsin Communities. ACS ES&T Water. 2021/08/13 2021;1(8):1955-1965. doi:10.1021/acsestwater.1c00160
2. Keshaviah A, Huff I, Hu XC, et al. Separating signal from noise in wastewater data: An algorithm to identify community-level COVID-19 surges in real time. Proceedings of the National Academy of Sciences. 2023;120(31):e2216021120. doi:doi:10.1073/pnas.2216021120