Wastewater Data FAQs


What is wastewater data? 

Wastewater is the used water from our showers, dishwashers, sinks, toilets and washing machines. Samples from wastewater can provide insight into the health of the population by measuring concentrations of viruses like polio and influenza, or identifying geographic hotspots in the opioid crisis.  Similarly, wastewater data can provide a snapshot of SARS-CoV-2 variant (also called COVID or COVID-19) levels in our communities.

Why is wastewater data important now? 

Wastewater data is one way to track COVID activity. It is performed independently of individual testing and hospital monitoring.

In the past several months, many states have stopped reporting daily COVID cases and are now reporting weekly or bi-weekly case counts. These case counts don’t include unreported results from at-home rapid tests. Additionally, the CDC’s community levels and community transmission reports weigh COVID hospitalizations more heavily than test positivity and case counts, making it harder to accurately estimate the risk of COVID infection in our communities. With the frequent emergence of new and more contagious variants, we must find other ways to estimate the presence of COVID beyond case counts. 

Individuals and community decision-makers deserve to know how much COVID they will be exposed to so that they can make informed decisions. Wastewater data is one way to understand how much COVID is in communities in the absence of valid, reliable, and timely case counts.

Where does wastewater data come from?  

Biobot Analytics is a private company that analyzes wastewater across the US to produce commercial COVID data. Raw data for the CDC’s National Wastewater Surveillance System (NWSS) are provided by Biobot, but NWSS and Biobot use different methods to calculate the concentration of COVID in the wastewater. The People’s CDC uses Biobot Analytics wastewater monitoring in the weekly Weather Report to give a better understanding of COVID’s continued presence and risk to public health.

How is wastewater data collected and analyzed?

COVID virus is found in fecal matter. Biobot samples are collected from wastewater treatment plants nationwide. They report viral load based on the concentration of virus and feces in the sample, whereas the CDC reports COVID viral load based on the amount of virus, water flow, and population in the area where the sample was collected. The source of the water to each treatment plant (neighborhoods, schools, business district, hospitals, etc.) influences the concentration of virus found in the sample. For more detailed information about how COVID is measured in wastewater, click here.

What do wastewater graphs show?

The graph from Biobot above shows individual days on the x-axis (horizontal axis). On the left-hand side of the y-axis (vertical axis) the scale shows the number of copies of Covid virus per mL of sewage (also known as viral load), represented by the dark blue line on the graph. On the right-hand side the scale shows the number of new cases of Covid per day from individual tests reported by the CDC, represented by the light blue line.  

When there was widespread PCR testing, the viral load in dark blue closely corresponds to the number of new cases per day, shown in light blue. As visible on the right hand side of the graph, as at-home testing became more accessible and new variants emerged, a gap between viral load and reported cases began to grow. The gap between the curves cannot be interpreted as missing cases because viral load varies across infected individuals and communities depending on numerous factors including but not limited to community vaccination rates and severity of illness.

Can you estimate the number of cases from the viral load? 

Not yet, but we might be able to in the future

The concentration of SARS-CoV-2 in fecal matter is higher in someone with severe illness than someone who is asymptomatic. Therefore, areas with more unvaccinated people who are at risk of severe disease, may have a higher viral concentration in the wastewater than areas with more vaccinated people. The overall concentration of COVID in an area’s wastewater is known as community viral load. Community viral load is a great crude indicator of the average amount of COVID present within a community. 

Can you become infected with COVID-19 from wastewater feces or poop? 

While we can detect SARS-CoV-2 in wastewater, there are no known cases of COVID being transmitted via fecal matter.

What are the advantages of wastewater data?

Wastewater data is anonymous and does not depend on self-reported test results. As free testing is removed, people who have access to tests are more likely to be higher income, which would bias testing data.

Wastewater is promising as a leading indicator, meaning it can alert us to the spread of disease in advance of hospitalizations.

It also tracks community viral load. Unlike case counts that do not reflect asymptomatic and low-symptomatic cases, these are still captured by wastewater testing, albeit with a smaller impact than a severe case. Because the amount of virus you’re exposed to influences how sick you get and your chance of getting Long-COVID, knowing the community viral load is an important piece of information for personal and community decision-making.

What are the disadvantages with wastewater data? 

Wastewater data requires sewer and testing infrastructure. For privacy reasons, septic systems of private homes or smaller neighborhoods cannot and should not be tested.  

Wastewater testing is not performed in every community. Thus, it is not a perfect measure of national trends or of what is happening in communities where it is not conducted. For the CDC’s tracking, Illinois has widespread testing, whereas California has testing only near major cities, and Mississippi has no testing at all. Areas of concentrated testing have more influence on the national trend line, whereas areas without any testing are left in the dark. If there is a surge in Mississippi, it will not show up on the national wastewater graph. Conversely, if there is a surge in Illinois, this will falsely suggest that there is a nationwide surge. 

A map of the US with states outlined. Guam, Alaska, Hawaii, Puerto Rico, and US Virgin Islands are included on the bottom. DC and New York City are on the right side. The map shows dots where there are wastewater testing sites. There’s a concentration around Ohio, Illinois, Indiana, Michigan, Wisconsin, and Missouri. New England, the Mid-Atlantic, Colorado, and the western coast also have good coverage. Guam, Puerto Rico, US Virgin Islands, Mississippi, North Dakota, and Wyoming have no sites.

This week’s map from the CDC: https://covid.cdc.gov/covid-data-tracker/#wastewater-surveillance

There are also some concerns with wastewater data on a county level. If your county tests a site with a high proportion of hospital wastewater, it will no longer be useful as a leading indicator. Testing only affluent work-from-home neighborhoods will appear to have a lower ratio of virus in the wastewater than testing a working class neighborhood where people are more exposed to the virus at work. When too few locations are tested, increases in cases at one or two sites can cause spikes that aren’t representative of the whole community. Because of privacy concerns, information about which plants are tested is not publicly available. This lack of transparency, while reasonable, also makes it difficult to discern the limitations of the data. 

Spikes can also occur because of how we analyze the samples. This is a new field, and analysis will improve with further research. 

Neither individual COVID tests nor wastewater data test DNA, however they both inadvertently collect this material. Any science that collects DNA material, even if it’s not used, must follow ethical standards with this material. Wastewater is no exception. More about ethical concerns of wastewater testing, including non-COVID testing. 

What’s next?

Wastewater has a promising future in outbreak monitoring. However, it’s not ready to stand on its own. It should be used with other indicators such as individual tests and hospitalizations. In addition to much-needed research, we also encourage development such as:

  • Increasing the number of communities represented in wastewater testing, especially in underrepresented states. 
  • Standardizing testing methods
  • Development of ethical standards for the field

We want to hear your thoughts on wastewater data. 

This document was prepared by People’s CDC volunteers with analytical and public health backgrounds. If there is anything we missed, please feel free to contact us here. Fill out this form if you’d like to join the team. 

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