SARS-CoV-2 (the virus that causes COVID-19) is shed in feces by infected individuals and can be measured in wastewater. More cases of COVID-19 in the community are associated with increased levels of SARS-CoV-2 in wastewater, meaning that data from wastewater analysis can be used as an indicator of the level of transmission of COVID-19 in the community..
Wastewater analysis measures the levels of non-infectious RNA (Ribonucleic Acid) in wastewater, not the viable virus. There are no known cases of transmission resulting from exposure to wastewater.
Wastewater-based epidemiology (WBE) is the study of a population’s exposure to chemicals or pathogens, like SARS-CoV-2, by measuring their presence in wastewater.
Advantages of WBE
The use of WBE has several potential advantages:
- It includes asymptomatic individuals and people who are unable or unwilling to obtain clinical tests, for a variety of reasons.
- It provides a mechanism to monitor the level of community transmission as clinical testing declines, and other, more convenient testing takes place (such as home-based rapid antigen tests) that are not reported to the Public Health Department. Wastewater analysis continues to be a part of our ongoing strategy to monitor the level of community transmission.
- Wastewater information is available sooner than information from clinical testing, which means that monitoring SARS-CoV-2 in wastewater can serve as an early indicator of increasing or decreasing COVID-19 infections in the community.
- During an increase, this early information could be used to enhance public health messaging in the affected communities to reinforce safe practices, promote more clinical testing, and highlight strategies the public can take to help stop a surge in new cases.
- It can help confirm current trends of COVID-19 infections in the community that are based on clinical data.
- It can increase confidence that clinical testing results are not biased by availability, time lags, and other factors.
Treatment Plants & Locations
The wastewater samples used for SARS-CoV-2 monitoring are collected at consolidated treatment plants, and the samples collected from these plants represent the entire community that the plant services. All four wastewater treatment plants within Santa Clara County have been participating in this project.
The plants are located in San José, Gilroy, Sunnyvale, and Palo Alto. The plants each serve between approximately 100,000 and 1,500,000 people living in their service areas, representing most of the population of Santa Clara County.
Santa Clara County is able to observe trends in SARS-CoV-2 levels at each plant individually. Samples are collected seven days a week from the wastewater treatment plants, and results are typically ready within 24-48 hours of sample drop-off and are uploaded to this page.
SARS-CoV-2 Measurements in Wastewater
The plots shown here provide an overview of the results of SARS-CoV-2 measurements in wastewater over time. These are the results for two SARS-CoV-2 genes: N gene, and S gene. When reviewing these graphs, it is important to look at the overall, sustained trends, instead of measurements on any given day. Wastewater samples tend to have some variability simply due to the nature of environmental samples, but the general trends over time have proven accurate and consistent with our clinical case trends.
Credit: Sewer Coronavirus Alert Network (SCAN) project by Stanford University
Partners and Population size within each sewershed:
San Jose-Santa Clara Regional Wastewater Facility: 1,458,017
Palo Alto Regional Water Quality Control Plant (PARWQCP): 213,968
PARWQCP- Owned and operated by the City of Palo Alto, the Plant treats wastewater for the communities of Los Altos, Los Altos Hills, Mountain View, Palo Alto, Stanford University and the East Palo Alto Sanitary District.
Sunnyvale - Donald M. Somers Water Pollution Control Plant: 169,000
Gilroy – South County Regional Wastewater Authority: 110,338
Total Santa Clara County population covered: 1,921,730
For additional information on Wastewater Based Epidemiology, see the following links:
Centers for Disease Control and Prevention: https://www.cdc.gov/healthywater/surveillance/wastewater-surveillance/wastewater-surveillance.html
CDC COVID Data Tracker:
California Department of Public Health:
Detailed Data Notes
Protocol of Sample Collection & Testing: Samples for testing consist of “settled solids” from wastewater entering each treatment plant. Samples are collected seven days a week from the wastewater treatment plants and are transported to a commercial lab for analysis daily. Laboratory staff process the samples to quantify pieces of the SARS-CoV-2 viral genome in the wastewater. These procedures use methods to concentrate solids and extract RNA and detect viral RNA. Results are typically ready within 24 hours of sample drop-off and are uploaded to this page. The quick turnaround and posting are one of the major advantages of utilizing this data.
Interpretation of Plots: The plots shown here provide an overview of the results of SARS-CoV-2 measurements in wastewater (settled solids) over time. These are the results for two SARS-CoV-2 genes (N gene and S gene), and the concentrations are “normalized” by the concentration of a plant virus that is harmless to humans but is shed in stool (pepper mild mottle virus, or PMMoV). Normalizing by PMMoV adjusts for changes in the amount of feces in the sample and the efficiency of the procedures from day to day.
Wastewater typically contains waste from a variety of sources, such as from your shower or a commercial process. These types of waste do not have stool (which is the source of SARS-CoV-2) so they dilute the sample. Therefore, we adjust this sample to the known amount of feces in the wastewater to get an accurate result.
The curves displayed are the “5-day trimmed average” of the sample results. This is done by using 5 consecutive samples, eliminating the maximum and minimum among the 5 samples, and then taking the mean. The smoothing is centered. The y-axis shows a linear scale. Trends in wastewater have been shown to track trends in COVID-19 incidence. The grey area at the bottom of the charts shows values below the approximate method detection limit. Trimmed averages may appear within this range when they include samples where the target was not detected, as these are recorded as 0.
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