All the Steps You Need to Know for Accurate Wastewater Sampling

Why, when, where and how — follow these guidelines to help determine the performance level of a system or component

All the Steps You Need to Know for Accurate Wastewater Sampling

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Taking wastewater samples from a septic system is an important task regularly performed by service providers, operators and maintainers along with designers and engineers to collect data on organic loadings and constituent levels. This data is sometimes used for regulatory purposes or to help us understand what we are working with. This data can be used to compare systems, rate performance and calculate loading rates. 


Samples are collected for a variety of reasons. The most common are diagnostic, troubleshooting and performance. With diagnostic testing, you are trying to figure out the status of the system and potentially what needs to be changed or designed for in order to treat a specific contaminant. Performance testing gives a real-time evaluation. Wastewater quality can be determined for the purpose of troubleshooting the system or for evaluating system performance. System diagnostics associated with troubleshooting and mitigation usually require information on both the hydraulic load to the treatment train and the sample analysis. The combined hydraulic and constituent load can provide a detailed view of system performance. In some jurisdictions, there may be regulations regarding required sampling and testing. The sample must represent the wastewater. It must be done in an analytical method that can be reproduced by another person. Documentation must be provided, and the results must be accurate so that the sample analysis is defensible, and the samples collected should be useful in meeting the objectives of why the sample was pulled.  


The timing of the sample is important to document because it can influence the wastewater characterization. The composition of the sample will probably be different if it is collected during a peak loading time rather than a resting period for the system. For example, maybe the problems in a system serving a fast-food restaurant arise over the weekend during a peak loading period, but if you don’t come and collect the sample until Thursday, the analysis of the sample may be misleading. The sample should be taken within 18 hours of the peak event. Data is helpful to properly determine the peak analysis of flow.

When you arrive at the site, remove the lids from the risers and take your sample before doing anything else so that the sample won’t be contaminated by material that you stir up while working.  

What you are testing for will also influence the optimum collection timing. You should check with the lab where you are having the sample analyzed because they may need the sample by a certain time of day or certain day of the week. For example, samples that take multiple days to process (fecal coliform or BOD) may not be accepted on certain days if the lab is closed on the weekends.  

For new system design, one sampling event is likely not sufficient. Many times, this sampling is required for commercial applications where nondomestic levels are expected. Two to four samples are recommended for this purpose, with more if the results are variable. 


Sampling locations are usually at the inlet or outlet point of a treatment component. These samples can be defined as influent and effluent samples. Influent samples are samples collected from within the treatment train. For example, an influent sample of the aeration chamber would be collected from the outlet of the trash tank. Even though this sample could be called an effluent sample following primary treatment in the trash tank, an effluent sample notation is generally reserved for the final step in the treatment train. Influent samples are usually collected from the various internal components of the treatment train to determine the influent wastewater characteristics. This information aids the maintenance provider in diagnosing, troubleshooting or mitigating the system. Effluent samples, collected following the final treatment component, may serve the purpose of diagnostic evaluation or compliance monitoring. When troubleshooting advanced treatment systems, it is helpful to have an influent and effluent sample from the unit. 

Freshly flowing water is captured during active flow from one component to the next and required by some pretreatment devices such as ATUs and media filters. Processed water is captured from the next component but not during active flow. If it is important to test active flow, water can be added on the inside of the facility to induce flow. Some purposes allow water to be captured from a downstream component (processed water) for the sample. Be sure to consult with manufacturers operations and maintenance manuals prior to pulling a sample. 

If you are pulling a sample from a system without advanced treatment, you will either sample out of the septic tank or the pump tank. If you sample at the inlet side of the septic tank, it is impossible to get a representative sample as it is highly influenced by the last activity in the establishment, so sampling from the outlet lets you know what the typical effluent is. This sampling can be impacted by both recent and overdue need for pumping. If the tank was recently cleaned, the effluent will typically be lower in contaminants than would be typical; but if the tank’s liquid depth contains more than 1/3 sludge and scum, it will be higher than typical. 


Most sampling from septic systems is classified as grab samples since they are one sample taken from one point and time, which gives us an idea of what is happening right then. This is typically what is mentioned in the regulatory requirements. 

The amount of sample needed is going to be dependent on the test or tests being performed. Typically the rule of thumb is a quart. Check with the laboratory to find out how much sample will be necessary. Some testing only requires a small amount of sample, while others need quite a bit. For the most part, the more tests being run on the sample, the more sample will be needed. Also, the cleaner the sample, the more sample will be needed. To be on the safe side, fill the container to the top for a given sample collection.

The sample can be collected using many different tools. Use the correct tool if a test calls for a specific type of container. Sample collection can be achieved with a long-handled dipper or other container. A sampling probe commonly known as a Tru-Core, Sludge Judge or Dip Stick can be used to collect a sample for analysis. The device selected may be influenced by the collection location and access port. For example, many different types of collecting devices work if you are pulling the sample from a tank, but sometimes a vacuum pump or some other type of device might be necessary if you are collecting the sample from a small sampling tee. It is important that the solids from a baffle, wall or pipe do not enter the sample container, as they can greatly impact the results. The sampling devices must be clean to prevent cross contamination of samples. If multiple diagnostic samples are being collected from the same treatment train and the sampling device will not be cleaned between sample collections, start collecting samples from the effluent end (cleanest effluent) of the treatment train. To avoid cross contamination, the sampling device should be cleaned with distilled water before collecting the next sample.

It is important to use proper personal protective equipment when collecting the wastewater. Remember:

  • Never enter confined spaces.
  • Be cautious of toxic gases.
  • Disinfect hands and equipment.

Be sure to store, transport and test according to the most current version of Standard Methods.

If these steps are followed, the data gathered during a sampling event should be useful in determining the performance of a system or component.

About the author: Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the president elect of the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to


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