Frequently Asked Questions

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.

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.

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.

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).

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.

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.

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.

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. 

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.

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. 

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.

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.

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.

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.

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.

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.

Once a week.

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

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.

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. 

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.  

 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.

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. 

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.

While states were ramping up their wastewater surveillance efforts, CDC contracted with a commercial laboratory to initiate monitoring for SARS-CoV-2 at wastewater treatment plants not currently involved in a state monitoring program.  Biobot is the current commercial laboratory under contract.  As state programs are developed, sites are transitioned from the commercial contract to the state program to be part of a comprehensive state program.  The Virginia Department of Health started the Virginia wastewater monitoring program in 2021.  Over half of the sites that initially started with Biobot in Virginia have transitioned to the state program to date.  It is anticipated that all of the commercial contract sites in Virginia will transition as CDC has indicated that future use of the commercial contract will be limited to states/territories without a state/territory program.

Biobot reports viral concentration which is the number of RNA fragments measured in a given amount of wastewater.  VDH measures viral concentration as well but reports viral load.  Viral load is the total amount of viral fragments that came into the wastewater treatment plant during the day the sample(s) was collected.  The load is calculated based on the measured concentration and the flow in million gallons per day reported by the wastewater treatment plant.  Some wastewater treatment systems are subject to groundwater and rainwater entering the collection system during rain events.  This produces a very high flow at the treatment plant, and it dilutes the wastewater.  The concentration would be measured as lower because it’s diluted by the extra water in the system.  By using load, VDH accounts for the excess water and only reports the total load which is not affected by the dilution from rain/groundwater. Additionally, VDH uses the geographic boundary of the collection system to define the area that contributes wastewater to the sample. VDH is able to match different elements in its various public health databases to the actual area being measured in order to produce more accurate data such as current patient cases.  The VDH data is now on a public-facing dashboard here.