Understanding Whole Effluent Toxicity Testing in NPDES
The National Pollutant Discharge Elimination System (NPDES), established in 1972 under the Clean Water Act, is enforced by the U.S. Environmental Protection Agency (EPA). Any entity holding an NPDES permit is required to monitor chemical and biological water characteristics of their effluent, and there are a range of tests that must be performed to acquire and maintain these permits. One example is Whole Effluent Toxicity (WET) testing, which measures the collective toxic effect that all pollutants in wastewater have on aquatic organisms.
What is WET Testing?
WET testing is a type of bioassay that uses living organisms to evaluate the potential toxicity of pollutants in freshwater or saltwater environments. Scientists first select representative invertebrate and vertebrate organisms from different trophic levels, algae for fresh water, and a plant, if necessary, for the environment being studied. These species are then exposed to effluent water samples over a period of time to observe potential toxicity effects based on survival, reproduction, or growth endpoints. The observed effects can then be extrapolated to predict the impact of the effluent on the natural environment.
Testing can be conducted with varying concentrations of pollutants over a wide range of durations. Acute testing durations can range from 24-96 hours and focus on short-term effects, such as organism survival. Chronic tests can range from 2-7 days and assess short-term effects in addition to longer-term impacts, such as growth and reproduction. Acute testing data can also be collected by observing data from the 48- or 96-hr marks from chronic testing.
Why is WET Testing Important?
WET testing provides facilities with a clear picture of how the wastewater they discharge would impact the local ecosystem. While municipalities and wastewater treatment plants order this testing most frequently, paper mills, power generation plants, mining facilities, steel manufacturing plants, and a range of other industries also benefit from these tests. In addition to obtaining NPDES permits and maintaining compliance, test results can help dischargers limit potential harm to the natural world and contribute to ensuring the safety of drinking water and recreational waters, such rivers and lakes.
Developing Regulations
While NPDES regulations have remained relatively consistent since the 1970s, EPA has been introducing interim permitting strategies since 2020 to monitor newer chemical pollutants of concern, such as per- and polyfluoroalkyl substances (PFAS) in wastewater. However, there are currently no available analytical methods that have been approved by EPA to analyze PFAS in wastewater, making the implementation of these measures unclear. As technology and regulations continue to develop, as discussed in a blog written by my colleague Dana Coe in April 2025, requirements are likely to change.
Spheros Environmental
Spheros Environmental integrates a diverse array of scientific disciplines, including air quality monitoring, water resources, digital technologies, climate services, permitting, and ecological sciences, to deliver scientifically robust and sustainable solutions to both private- and public-sector clients nationwide. We offer a comprehensive range of freshwater, marine, and terrestrial services, from acute and chronic WET testing often associated with NPDES permits, to toxicity testing of soils or sediments, as well as product testing that evaluates how model organisms respond to different concentration of that product. With two state-of-the-art bioassay laboratories, located in Port Gamble, WA, and Alachua, FL, Spheros Environmental provides nationwide coverage and has the capability to handle international projects as well. Our laboratories are certified under the National Environmental Laboratory Accreditation Program and by agencies in multiple states, ensuring the highest standards of testing and compliance. Spheros Environmental scientists furthermore have extensive experience working with a diverse range of species, which provides our clients with a flexible approach to evaluating potential ecological issues.
About the Author
Michelle Bennett has over 10 years of laboratory and project management experience in the field of environmental toxicology. She is highly proficient at conducting and managing the bioassays associated with the NPDES whole effluent toxicity testing field. She has managed a wide variety of effluent projects spanning the country and has experience interpreting NPDES permits, communicating project-specific details to clients, and providing data reports with interpreted results. As a project manager with EcoAnalysts, Mrs. Bennett coordinates and schedules incoming NPDES projects with clients and ensures that data quality objectives are met in accordance with the NPDES permit, the laboratory’s quality assurance protocols, and accrediting agencies.
If you have a need related to WET testing or NPDES permit testing, email Mrs. Bennett at Michelle.Bennett@spherosenvironmental.com.