Frequently Asked Questions (FAQs)

Check out answers to some of the most frequently asked questions received about wastewater surveillance and the California WWS Center of Excellence.

  • Wastewater surveillance, also known as wastewater-based epidemiology (WBE), is a public health tool that looks for fragments of infectious pathogens (e.g., viruses, bacteria, fungi, etc.) in untreated wastewater. Samples for wastewater surveillance are generally collected as the sewage or wastewater enters a wastewater treatment plant (WWTP) and before it begins the treatment process in which microbes, contaminants, and debris are removed from the wastewater.

    Testing these samples provides additional understanding about the presence or spread of diseases within a community to supplement other sources of public health information.

    By collecting a single sample of wastewater at the WWTP and looking for how much pathogenetic material is present, wastewater surveillance can quickly provide public health with information about how much a particular pathogen, such as COVID-19 is impacting that community. Wastewater surveillance can provide useful information as individuals change their testing habits over time and can even provide an early warning of changing disease activity within a community.

  • Once a sample is collected, it is sent to a laboratory for processing and tested for pathogens of interest. First, the genetic material (RNA or DNA) of a target pathogen is concentrated and extracted from wastewater. Then, a technique called polymerase chain reaction (PCR) is used to detect if a pathogen is present in a sample, and if it is, how much of it there is. For more information, visit our FAQs on lab methods below.

  • The National Wastewater Surveillance System Centers of Excellence serve as regional leaders in wastewater surveillance implementation and coordination. They foster innovation, develop and conduct trainings, and provide technical assistance to jurisdictions.

    The six Centers of Excellence are led by public health departments and are in California, Colorado, the City of Houston, New York, North Carolina, and Wisconsin. For more information about the CoEs visit the CDC NWSS Centers of Excellence webpage.

General Information

Data Interpretation

  • Wastewater surveillance data are used by a wide range of stakeholders to make informed decisions - including public health professions, health care providers, as well as the public.

    The CDPH Cal-SuWers Epidemiology Team routinely generates pathogen reports that track changing trends in pathogens such as SARS-CoV-2, influenza, RSV, norovirus and others across wastewater sites statewide. These reports are shared with public health leadership, subject matter experts, and network partners, which include wastewater utilities, local health departments and lab partners. Wastewater data are utilized by local partners at county health departments to provide situational awareness and additional context into the health of their communities.

    The public can stay informed about changing wastewater trends through publicly accessible dashboards. The U.S. Centers for Disease Control and Prevention (CDC) National Wastewater Surveillance Systems (NWSS) include these data in a national dashboard. Data is also shared with the public on the California Department of Public Health Wastewaters Surveillance Dashboard.  

Wastewater Sample Collection

  • Sampling frequency can vary depending on the surveillance program, ranging from once to seven times per week. Most commonly, samples are collected two to three times per week. Increasing the sampling frequency improves the ability to detect changes in community trends more accurately and in near real-time. However, determining the right sampling frequency requires balancing data needs with available resources and cost for all stakeholders.

    Testing these samples provides additional understanding about the presence or spread of diseases within a community to supplement other sources of public health information.

    By collecting a single sample of wastewater at the WWTP and looking for how much pathogenetic material is present, wastewater surveillance can quickly provide public health with information about how much a particular pathogen, such as COVID-19 is impacting that community. Wastewater surveillance can provide useful information as individuals change their testing habits over time and can even provide an early warning of changing disease activity within a community.

  • Two main types of samples are collected for wastewater surveillance: raw liquid influent and settled solids. Liquid wastewater samples are taken from untreated sewage that flows into a treatment plant. These samples are typically collected at the plant’s main influent headworks.

    Settled solids are the solids that settle out of the wastewater during what is known as “primary treatment” at a wastewater treatment plant. Solid samples are commonly collected from a primary clarifier, a large tank that allows wastewater to separate heavier solids from liquids.  Solids can also be obtained in a laboratory by centrifuging liquid samples to separate the heavier particles.  

  • There are two main types of sample collection methods used in wastewater surveillance: composite and grab samples.

    Composite samples consist of a mixture of several individual samples collected at regular and specified time periods, often over the duration of a day (24-hours). Composite samples are often collected from an automated device known as an autosampler. An autosampler can be deployed at either the main headworks of a treatment plant, upstream in the collection system at a manhole, or within the collection system targeting a specific building (i.e., dormitory, hospital, correctional facility, skilled nursing facility, etc.).  A grab sample, or catch sample, consists of a single sample taken at a specific point in time.  

  • Wastewater samples are commonly collected by staff at the wastewater utility as part of the utility’s routine operations. These personnel are trained and experienced in safely collecting and handling raw wastewater. For wastewater surveillance programs, it is common practice for wastewater utility staff to collect the samples and ship them to a laboratory for analysis. In some circumstances, local county health jurisdictions have supported wastewater utilities by either collecting samples or facilitating the logistics and shipping of samples.  Contracts with external service providers can also be arranged to have the wastewater samples collected and shipped to the laboratory.

Site selection

  • California has hundreds of active WWTPs. However, due to resource constraints for utility, laboratory, and public health partners, it is neither feasible nor necessary to collect and test samples from every facility in the state.

    The goal of the current wastewater surveillance program in California is to serve as a sentinel surveillance system, with a sufficient number of wastewater treatment plants (WWTPs) participating that we have sampling across a diversity of regions. This approach aims to have data be representative of most populations and regions of the state and to provide CDPH and local health jurisdictions with useful data on disease trends.

Lab & Quantification Methods

  • One of the most common methods for quantifying viral genomic material in wastewater is polymerase chain reaction (PCR). PCR works by combining a sample with specific reagents that allow for the amplification of a target DNA or RNA fragment. The sample is then cycled through a series of temperature changes, enabling the replication of the target genetic material. The results of this amplification allow us to determine the original concentration of pathogen in the sample.

  • The methods and workflows for each of the steps necessary to quantify pathogens in wastewater are complex. The following is a simplified outline of the main steps.

    1.      Sample preparation homogenizes the sample through mixing or blending. A known amount of a matrix recovery control or process control (e.g. bovine coronavirus) may be spiked in the sample at this step.

    2.      Concentration is the subsequent step that reduces the sample volume and increases the viral or target concentration. This can be achieved through processes such as filtration or pelleting by centrifugation.  

    3.      Extraction is used to isolate RNA or DNA from the concentrated sample.  

    4. Quantification measures the amount of target RNA or DNA using either qPCR or ddPCR. The matrix recovery control or process control should also be quantified in order to estimate the percent recovery of genetic material after the concentration and extraction steps.