Frequently Asked Questions

How does wastewater monitoring work?

Our partners at wastewater agencies collect 24-hour composite samples from the entrances to wastewater treatment plants, which receive sewage from whole communities. They mail the sample overnight to our laboratory at UC Berkeley, where we extract and concentrate the genetic material (RNA) from the sample and then measure the amount of SARS-CoV-2 RNA present using PCR (similar to how clinical samples are tested by PCR). Results are uploaded to the dashboard within 48 hours of sample arrival to our laboratory.

Is this information shared with public health departments?

Our team works closely with local county public health officials and the California Department of Public Health. Public health officials have access to the data on this dashboard, along with additional detailed, real-time information about each wastewater sample.

What laboratory method do you use to analyze wastewater samples?

We use the 4S method, which was developed by our research team because it is economical, highly sensitive, and uses readily accessible materials that are less likely to be subject to supply-chain delays. See here for the research article describing the method and our laboratory protocols.

How should I interpret the data?

Wastewater data reflects the amount of SARS-CoV-2 viruses shed by the whole population in the sewershed. Trends in the wastewater data reflect whether COVID-19 infections in the community are increasing, decreasing, or remaining constant.

Research on our laboratory method has shown it is very sensitive, able to detect a positive signal when there are as few as 2 reported daily cases per 100,000 people (see our team’s research article on the topic here). At very low concentrations (below 10 gene copies/mL of wastewater), the detection method is less accurate at counting the viral RNA. Any data point between 2-10 gene copies/mL should be considered a positive signal, but the number of gene copies may be inexact.

Does rainfall affect the data?

Yes. In San Francisco, any rain that falls in the city is directed into the sewer system. In many other cities, rain leaks into sewage pipes during big storms. That means that on very rainy days, the SARS-CoV-2 signal would look artificially low, because it is being diluted by rainwater.

What does the wastewater data tell me about the number of SARS-CoV-2 cases in my region?

Trends in the wastewater data reflect whether the infection levels in the community are increasing, decreasing, or remaining constant. Many research studies have shown that these wastewater trends mirror COVID-19 case data in the sewershed. At some points in the pandemic and in places when there has been less availability of clinical testing, research has shown that the SARS-CoV-2 wastewater trends can precede case data trends by as much as two weeks.

In situations when the wastewater data show an increasing trend but the case data do not, this may be an indication that people who are infected with COVID-19 are not accessing clinical testing (or have not yet gotten tested).

It is also worth noting that wastewater sewersheds often don’t line up with city or county boundaries, such that making comparisons to case counts is not straightforward.

It is not yet possible to relate the concentration of SARS-CoV-2 in wastewater to the number of infected individuals in the community, but this is an active area of research. Factors that may influence this relationship include the amount of virus that each person sheds in their feces over time, which may vary according to the intensity of infection, the SARS-CoV-2 variant, and whether an individual is vaccinated, as well as factors related to the wastewater (see next question).

What are sources of uncertainty in the wastewater data?

Wastewater is a heterogeneous mixture and samples from different locations may be affected by different factors. For example, factors that may affect the SARS-CoV-2 signal include the amount of time sewage takes to reach the treatment plant, the presence of other chemicals in sewage (like soap and bleach) that may affect the viral signal, human mobility patterns, and the flow patterns of water and solids in the sewage pipes. Rain can also enter the sewer system, diluting the viral signal from sewage during stormy days. We account for some of these factors by taking a 24-hour composite sample and by testing each sample for inhibition by other materials in the wastewater that could obscure the viral signal. To account for human mobility, our data visualizations show the concentration of SARS-CoV-2 (gene copies per milliliter of sewer). This assumes that each person uses roughly the same amount of water indoors each day.

Is wastewater monitoring for SARS-CoV-2 dangerous for the people collecting the samples or working in the lab?

Research has shown that SARS-CoV-2 found in feces is likely not infectious. Our laboratory measures the viral RNA, not the live virus itself. That being said, wastewater contains many other germs that can make people sick. The sample collection teams and laboratory staff are trained to handle this biohazardous material and wear personal protective equipment. Our laboratory is certified to handle biohazardous material (BSL2+), and includes protective features like negative air pressure and biosafety cabinets.

Is it possible to identify who is infected with SARS-CoV-2 from the wastewater data?

No. Each of the sewersheds shown on this dashboard combines the wastewater from at least 10,000 people. There is no way to trace a SARS-CoV-2 signal in the wastewater to any particular household or person.