Environmental monitoring of wastewater (regularly testing sewage) is another tool Healthy Davis Together is using to learn about COVID-19 and opportunities to prevent its spread in our community.
Did you know that the virus that causes COVID-19 (SARS-CoV-2) is found in the excrement (feces and urine) of individuals who have COVID-19, including asymptomatic and pre-symptomatic individuals? Wastewater testing can be used as an early warning detection system for the virus in a regional area (e.g., neighborhood) so that action can be taken to provide support and reduce the spread.
Healthy Davis Together (HDT) wastewater testing can help slow and prevent the spread of COVID-19 in two ways. First, it’s an inexpensive way to get information on city-wide infection trends over time. Trend data indicate whether existing efforts to combat COVID-19 are working and whether more needs to be done. Second, wastewater testing can identify areas where COVID-19 might be on the rise and therefore where we can put additional focus on direct support like free testing and quarantine support.
Through HDT, UC Davis and the City’s Department of Public Works have set up wastewater sample collection locations at 25 key points (“nodes”) in the City’s sewage (“wastewater”) collection system. These nodes also draw directly from the City’s Wastewater Treatment Plant (WWTP). Each node pools the wastewater from a broad geographic region of the City (illustrated in the map below; note that some regions overlap), while the WWTP pools all city wastewater. There are some residential regions not listed on the map. Wastewater samples in those areas are part of the WWTP samples. This type of sampling provides a regional overview but does not isolate individual households. The HDT wastewater monitoring team will continue to add more samplers to capture more regions served by the city wastewater collection system.
How to read regional wastewater graphs
After collection, wastewater samples are analyzed in a UC Davis lab to determine the concentration of virus copies present. Concentration data are normalized (to adjust for things like rainfall and other factors that could affect the sample dilution) and plotted against the City’s Wastewater Treatment Plant numbers (the city-wide average). A two-week moving average is calculated to smooth out noise in the data, Finally, a linear fit is applied to data from the past two weeks to determine whether the amount of virus in a region is going up, going down, or staying about the same.
By comparing regional data (in blue) to the city-wide average (in green), we can see how trends compare. These regional wastewater graphs are updated every Saturday.
Variability is expected in samples collected from sewer systems. Environmental factors and the timing of sample collection occasionally yield unusually high results. For this reason, wastewater data is best used to assess trends: that is, to monitor whether concentrations of SARS-CoV-2 in a particular area are persistently low, high, increasing, or decreasing over multiple days. The City of Davis does not make public-health decisions based on isolated (i.e., single-day) sampling results.
On 3/1/21, processing switched from a sample-concentration method based on ultrafiltration to a method based on magnetic bead capture. Internal tests showed these methods to be comparable.
Until 4/8/21, the regional wastewater plots utilized a log scale. Since 4/9/21, the plots have utilized a linear scale. The underlying data remain the same.
Because the UC Davis campus is on a different sewer network than the City of Davis, data from the two regions are not directly comparable. The City-wide wastewater data is therefore omitted from the UC Davis plot.
Davis Regional Testing Graphs
Wastewater sampling week of April 24-30
The wastewater data indicate that levels of COVID-19 in Far West Davis are going up, while levels of COVID-19 in the City overall are staying about the same. Healthy Davis Together encourages everyone to remain vigilant and follow guidance to prevent the spread of COVID-19 and all its variants.
Health Davis Together encourages everyone to remain vigilant and follow guidance to prevent the spread of COVID-19 and all its variants, including:
- Wearing a mask/face covering; making sure it fits properly
- Keeping physical distance
- Practicing good hand hygiene
- Getting tested at least weekly for COVID-19; even if you are asymptomatic
- Avoiding large crowds, opt for outdoors
- Staying home if you feel sick
- Getting vaccinated
Wastewater testing is just one of the many tools and resources that can help keep Davis healthy. To learn more about wastewater surveillance as a strategy for containing COVID-19, please visit the Centers for Disease Control and Prevention (CDC)’s webpage on the topic. If you have questions, please email wastewater@HealthyDavisTogether.org.