Frequently Asked Questions


What is wastewater?

Wastewater, also known as sewage, is used, dirty water from homes (toilets, sinks, kitchens, and laundries) and from other facilities like restaurants, schools, and office buildings that is collected through a series of pipes and manholes and directed to a wastewater treatment plant. The sewage is treated by the wastewater treatment plant so that it may be safely returned to the environment. Wastewater is not the same as clean drinking water, which is water from a stream or well that is highly treated by a water treatment facility before flowing into homes and businesses.

What is wastewater surveillance for SARS-CoV-2?

Wastewater surveillance is the monitoring of wastewater for the viral particles of SARS-CoV-2, the virus that causes COVID-19. When a person is infected with a SARS-CoV-2 virus, the virus is shed into the person’s digestive tract and out with their feces. Wastewater samples are collected at the wastewater treatment facilities and analyzed in the lab for SARS-CoV-2. Public health can use this information to estimate whether the levels of community infection are going up or down.

What are the benefits of wastewater surveillance for COVID-19?

This surveillance can let us know how much COVID-19 might be in the community. This means a single sample can give an estimate of infection across the whole community and include both symptomatic (have symptoms) and asymptomatic (do not have symptoms) cases. Routinely monitoring these locations can give approximate infection trends in the community. It is cost effective as it does not require clinical patient testing.

How do you measure COVID-19 trends using wastewater surveillance?

SARS-CoV-2 (the virus which causes COVID-19) has RNA. The Polymerase Chain Reaction (PCR) technology can be used to determine the amount of viral RNA in the wastewater. By routinely measuring how much virus is in the wastewater, we can look at how the virus amount changes over time and look for trends. The amount of the virus in the wastewater is related to how many people have COVID-19 and are shedding the virus in their feces, whether they feel sick or not. A single sample collected at a treatment plant provides an indicator of the trends of COVID-19 infections in the community served by the treatment plant (the sewershed).

What is a ‘sewershed’?

A sewershed is the entire network of sewer pipes that flows into a wastewater treatment facility. This network represents all of the connected households and businesses that flush and drain their wastewater into that wastewater treatment plant.

Why report COVID-19 infection/trends occurring in the sewershed?

Comparing trends of reported cases within the sewershed to trends in wastewater can help understand the amount of COVID-19 in a community over time.

Where is wastewater surveillance happening in Virginia? Is my city participating in the Virginia Sentinel Monitoring Program?

We are currently sampling wastewater from 25 sites in Virginia… You can check to see if your community is participating on the Sentinel Monitoring Map.

How is wastewater data being used to support the COVID-19 response?

Studies have shown that the wastewater viral load will increase about a week before there are increases in patient cases.  That’s because people with COVID-19 start shedding the virus within 2 to 3 days of becoming infected, even before they feel sick. 

What happens if wastewater surveillance shows an increase in COVID-19?

State and local public health officials review the data along with other COVID-19 metrics to determine the need for action, which may include:

  • Adding more testing in the community.
  • Increase public health communication and outreach about how individuals can protect themselves from COVID-19 (social distancing and masking for people who are not fully vaccinated, handwashing, avoiding crowds and enclosed spaces, encouraging vaccination).
  • Alert hospitals, clinics and local physicians that an increase in the virus that causes COVID-19 has been observed, which could indicate an increase in cases.
  • Provide recommendations to community and public health leaders Take steps to increase access to COVID-19 vaccines.

What do the “Viral Copies per Day” units mean in the figure and why do you use that as your measurement?

The virus can be detected by a technology that counts how many copies of the virus are present (i.e., viral copies) in a sample. In order to know how many viral copies entered the wastewater plant on a given day, the number of gene copies counted in the sample is multiplied by the total flow measured at the plant on the sampling day. The resulting unit is “viral copies per day.” It tells us how many virus particles likely passed through the plant over the span of one day. 

Can you compare viral loads across different monitoring sites in Virginia?

No. It is best not to compare values across monitoring sites, but it is acceptable to compare trends. For instance, you could consider if multiple sites in the same region peaked during the same week. There are various factors that contribute to the spread of the virus in a community, so each site should be considered as unique. These factors may include: the amount/percent of individuals served by a given wastewater treatment plant, shifts in population, international vs domestic travel, transient populations, commuter populations, etc.

Are there any limitations of wastewater surveillance?

There are several limitations to using wastewater surveillance.

Most significantly, we can only collect information about the portion of the population whose sewage flows to its respective wastewater treatment plant. Portions of the community served by onsite septic systems are not included in those wastewater samples. Approximately 30% of the population in Virginia, especially in rural areas, uses septic systems to dispose of their wastewater.

Additionally, scientists are still working to understand how much SARS-CoV-2 RNA is shed in the feces by infected individuals and for how long. This may change based on variant or individual factors like vaccination status. Collection and laboratory methods may also affect analysis values, as would site changes like heavy rains that may dilute the sewage.

Changing populations may also be a limitation, as testing results may not adequately reflect tourism or other transient populations. For example, infected individuals may travel for vacation and influence wastewater values, but they may not get diagnosed clinically until they return to their hometown.

Finally, wastewater surveillance is not a substitute for clinical testing. While there are limitations to all forms of data collection, including clinical testing, this data set should be used in tandem with best public health practices.

Are there any potential long-term benefits of wastewater surveillance?

Wastewater surveillance can serve as a complementary tool in understanding and predicting the number of COVID-19 infections in the communities. This tool can help in correlating and understanding factors affecting prevalence of disease. Wastewater surveillance can also support mitigation strategies and effectiveness of public health interventions like executive orders, mask mandate, etc. It has been used in other disease surveillance in the past like Polio, and scientists are exploring potential applications of wastewater surveillance for other pathogens like influenza, antibiotic resistance, adenovirus, norovirus, etc.

What is suppression and why use it?

Health data can include identifying information like patient names, addresses, and health status. VDH cannot publish information like this so that patient confidentiality is maintained. This identifying information is specifically omitted from our public-facing datasets. In situations where case numbers may be very low (i.e., between 1 and 4), it is possible that a small community may be able to identify those individuals. We therefore change all case numbers between 1 and 4 to a value of 2, so that we may blur the lines a bit to prevent patient identification.

Where else is wastewater surveillance happening?

Wastewater surveillance is being conducted by a number of other states who are also participating in the CDC NWSS program.  All the data from the various participating states can be found on the CDC website.  A number of colleges have implemented dorm level wastewater surveillance to identify sick students.  This type of surveillance has also been used in institutional living settings such as correctional facilities.

Can I get wastewater surveillance in my neighborhood?

Pending funding availability, VDH hopes to add more sentinel monitoring sites in the next year.  If your neighborhood is served by a wastewater treatment plant, the utility that runs the treatment plant can apply to participate in the program when that expansion occurs.

How often is the wastewater surveillance dashboard updated?

Once a week.

How long will the wastewater surveillance dashboard stay up?

For as long as VDH continues to participate in statewide wastewater surveillance.

Can I contract COVID-19 from my drinking water?

No, drinking water from a public utility is disinfected to remove disease causing organisms (to include viruses such as SARS-CoV-2).  SARS-CoV-2 is very susceptible to standard disinfection methods. As a reminder, drinking water is the water that enters our home and wastewater is the water that leaves it.

If you can measure COVID-19 in wastewater, does that mean it’s in our waterways? Can I contract COVID-19 by swimming in a river?

The virus that causes COVID-19 is no longer active once it passes through the gut of humans.  There have been no reported cases of individuals being infected with COVID-19 from handling sewage.  However, remember sewage is still treated at the wastewater plant and the final step is disinfection before it enters any body of water. This disinfection step destroys harmful organisms. 

Is collecting sewer water dangerous?

The virus that causes COVID-19 is inactivated as it passes through the gut of humans.  There have been no reported cases of transmission of COVID-19 from sewage. However, human sewage can still contain a large amount of harmful organisms found in feces prior to being treated, and should absolutely be handled only by trained individuals using personal protective equipment (PPE) and sterile collection methods.  

What does it mean if the viral loads are increasing?

 Assuming everything else stays the same (sample location, analytical methods, sewershed population, etc.) an increase in viral load would generally mean there are more infected individuals shedding the virus in the sewershed.

What does it mean if the viral loads are decreasing?

Assuming everything else stays the same (sample location, analytical methods, sewershed population, etc.) a decrease in viral load would generally mean there are less infected individuals shedding the virus in the sewershed.  

There have been cases where a community has shown an increase in viral load that quickly decreased.  It was linked to a large event that brought a lot of visitors to the area.  Once the visitors left, the load dropped, so it did not reflect a true change in the community’s health. 

Why does it say that the data is “subject to change”?

There are many data sources that we use for this dashboard, some of which can be updated or even (retroactively) changed. For example, the patient case data that is used to show sewershed patient case numbers is routinely updated by VDH as new more accurate information is obtained. We pull the cases fresh each week to give the most accurate version possible, so it’s likely that each week may show different total case numbers than were shown previously. It’s also possible that some of our sewershed areas themselves may be updated, as we work with the utilities to get the most accurate version possible. If we update the underlying area for the sewershed, that may change the linked number of cases for that sewershed.  The population counts may shift as well which would affect any population-normalization graphs which we may generate. Generally the wastewater viral load data is unlikely to change, but occasionally errors or missing values, such as flow, could affect the reported values as those errors are corrected.

Due to the nature of running fresh analyses routinely on the dashboard, any updates, corrections, or changes we make to any of the underlying data elements may result in minor changes to the entire dataset. Thus, the data may change over time.