Best practices for sampling and analyzing wastewater for SARS-CoV-2
Wastewater surveillance provides a community-level view of SARS-CoV-2 circulation without relying on individual testing. By measuring viral RNA in sewage, public health and water professionals can identify changes in infection trends, support clinical surveillance, and help agencies prepare for shifts in demand.
Reliable results depend on much more than laboratory instrumentation. Sampling location, flow conditions, collection frequency, temperature control, concentration methods, quality assurance, and data interpretation all influence whether a result reflects actual community conditions or a sampling artifact.
Water resource recovery facilities are well positioned to support this work because operators understand collection systems, influent variability, treatment processes, and local population patterns. A coordinated program also creates opportunities for engineers, laboratory staff, epidemiologists, and agency leaders to share information quickly.
Define the surveillance question
Every monitoring program should begin with a clearly stated purpose. A facility may be tracking broad trends across a service area, comparing neighborhoods, evaluating changes after a public health intervention, or monitoring a specific institution. Each objective affects the sampling point, schedule, analytical method, and reporting format.
The sewershed boundary should be documented using current collection-system maps and known connections. Rainfall, industrial discharges, groundwater infiltration, tourist activity, commuter populations, and seasonal events can all affect wastewater composition. Normalizing SARS-CoV-2 measurements against flow or a fecal indicator can make comparisons more meaningful.
Sampling at the treatment plant influent offers a broad community signal, while upstream locations can provide more localized information. Large interceptors require careful access planning and representative collection methods; practical guidance on interceptor sewer inspection can also inform safe reconnaissance and site characterization.
Select representative collection methods
Composite samples usually provide a stronger estimate of average viral shedding over a defined period than a single grab sample. Time-weighted composites collect aliquots at regular intervals, while flow-weighted systems adjust the sample volume according to influent rate. Flow-weighted sampling can reduce bias when wastewater volume changes substantially during the day.
Grab samples remain useful when equipment, staffing, or site conditions limit composite collection. They may also be appropriate for rapid investigations or locations with highly constrained access. However, grab results can be strongly influenced by the exact collection time, short-term discharges, and uneven solids distribution.
The sampler intake should be positioned where wastewater is thoroughly mixed and where solids are unlikely to settle around the intake. Field staff should document the sampling point, start and end times, total volume collected, influent flow, weather, unusual discharges, and equipment settings. These records are essential when analysts compare results across dates or facilities.
Protect sample integrity from collection to analysis
Viral RNA can degrade when samples remain warm, exposed to sunlight, or delayed in transit. Use clean, leak-resistant containers and follow an established chain-of-custody procedure. Samples should generally be chilled immediately, protected from direct light, and transported to the laboratory as soon as practical. A documented holding-time limit should be established before routine sampling begins.
Field blanks, equipment blanks, and duplicate samples help identify contamination and collection variability. Laboratories should also include extraction controls, inhibition controls, positive controls, and no-template controls. If a control fails, the result should be flagged and investigated rather than treated as equivalent to a valid measurement.
Worker protection remains central. Sampling teams need appropriate personal protective equipment, hygiene procedures, vaccination and exposure policies, and training for confined-space or wet-well hazards where applicable. Operational continuity planning matters as well; lessons from the Hyperion treatment plant fire demonstrate why resilient procedures and clear emergency coordination are valuable at critical water infrastructure.
Choose analytical methods with quality controls
Most wastewater surveillance programs target SARS-CoV-2 RNA using reverse transcription quantitative polymerase chain reaction, commonly called RT-qPCR, or digital PCR. Assays may target one or more viral genomic regions. Using multiple targets can improve confidence when mutations or inhibition affect a particular primer-probe set.
Before adopting a method, laboratories should evaluate recovery efficiency, analytical sensitivity, precision, inhibition, and reproducibility. Concentration procedures may include membrane filtration, electronegative filtration, ultrafiltration, polyethylene glycol precipitation, or other validated approaches. No single method performs identically across all wastewater matrices, so local verification is important.
A fecal normalization marker, such as pepper mild mottle virus or a human-associated fecal marker, can help account for changes in fecal strength and dilution. Flow data, total suspended solids, ammonia, conductivity, and rainfall records may provide additional context. Normalization does not eliminate uncertainty, but it can help distinguish a real epidemiological signal from a sudden change in wastewater composition.
| Program choice | Strength | Limitation | Best use |
|---|---|---|---|
| Grab sample | Fast and simple | Sensitive to timing and short-term variability | Rapid checks or constrained sites |
| Time-weighted composite | Represents conditions over a period | Requires reliable automated equipment | Routine trend monitoring |
| Flow-weighted composite | Accounts for changing wastewater volume | More complex flow measurement and programming | Variable-flow facilities |
| RT-qPCR | Widely available and established | Sensitive to inhibition and calibration choices | Standard surveillance workflows |
| Digital PCR | Strong quantification at low concentrations | Higher cost and specialized equipment | Confirmatory or low-level analysis |
Laboratories should report process recovery, inhibition findings, replicate agreement, limits of detection, and any qualified results. A number without these details can appear precise while concealing substantial uncertainty.
Interpret trends with operational context
A single positive or negative result should rarely drive a major decision. Wastewater signals can vary because of shedding patterns, solids handling, rainfall, travel, laboratory variability, and delays between infection, shedding, sampling, and reporting. Trend analysis is more useful when it considers several consecutive observations.
Results should be reviewed alongside clinical case data, hospital indicators, testing access, vaccination patterns, and local events when available. A rising concentration may warrant closer public health attention, but it does not directly translate into a specific number of infected residents. Viral copies in wastewater are an environmental measurement, not a diagnostic result for individuals.
Statistical methods should be selected according to the sampling frequency and data quality. Rolling averages, confidence intervals, replicate analysis, and change-point monitoring can help reveal meaningful shifts. Analysts should preserve raw values as well as normalized values so that later users can evaluate how processing decisions affected the interpretation.
Clear communication is as important as accurate measurement. Reports should state the sampling date, sewershed, sample type, target assay, normalization approach, detection status, quality flags, and relevant limitations. Consistent terminology helps operators, public health officials, and community stakeholders understand what the data can and cannot show.
Build a sustainable program
Long-term surveillance requires defined responsibilities, reliable funding, trained personnel, and an accessible data-management system. Standard operating procedures should cover site selection, sampler maintenance, sample handling, laboratory analysis, result validation, corrective actions, and reporting. Periodic audits can identify drift in collection practices or analytical performance.
Facilities should also plan for equipment failure, power interruptions, extreme weather, staffing shortages, and sudden changes in public health priorities. Cross-training operators and laboratory personnel reduces dependence on one individual. Agreements among utilities, laboratories, universities, and public agencies can establish expectations for data ownership, turnaround time, confidentiality, and public release.
Professional development supports program quality. Technical presentations, workshops, facility tours, automation training, and MOC certification courses can help water professionals strengthen both field and analytical capabilities. LABS of CWEA’s professional events offer a practical setting for connecting with peers and learning how other organizations manage emerging water-quality challenges.
Practical recommendations for implementation
- Start with a written surveillance objective, mapped sewershed, sampling schedule, and documented decision process.
- Prefer flow-weighted or time-weighted composite sampling when the goal is to estimate average community conditions.
- Maintain cold-chain procedures, chain-of-custody records, field blanks, duplicates, and complete equipment logs.
- Validate concentration and RT-qPCR or digital PCR methods using the local wastewater matrix before routine reporting.
- Interpret SARS-CoV-2 results as trends supported by quality data and operational context, rather than as standalone case counts.
A well-designed wastewater surveillance program turns routine influent monitoring into actionable environmental intelligence. By combining representative sampling, rigorous laboratory controls, transparent reporting, and strong coordination among water professionals and public health partners, agencies can produce results that are timely, defensible, and useful.
LABS of CWEA members can strengthen this work by sharing field experience, attending technical programs, and building relationships across the Los Angeles Basin water community. Engage with upcoming events and professional development opportunities to advance reliable wastewater monitoring across the region.