Whole effluent toxicity (WET) tests are key components of many wastewater discharge permits that assist facilities in meeting the goals of the Clean Water Act. WET tests, which may be acute or short-term chronic (STC), are designed to supplement chemical-specific analyses and prevent the discharge of “toxic pollutants in toxic amounts”, thus helping to ensure the health of aquatic ecosystems. Cases of significant WET failure with both Ceriodaphnia dubia and Pimephales promelas in effluents have been traced to the presence of microorganisms in the effluent. To track down the suspect toxicant, Toxicity Identification Evaluation (TIE) methods were modified by inclusion of manipulations effective in reducing or eliminating biological interference (e.g., UV, chlorination). Wastewater samples were cultured in an aerobic environment using, primarily, MacConkey, TSA, and Columbia agars.
Results/Conclusions
While several bacterial taxa have been identified in wastewater, Pseudomonas and Aeromonas are the prime suspects in causing significant biological interference. Effects to the test organisms were replicated by reintroduction of captured (via filtration) particulates into clean lab water. Biological interference in WET tests with final effluents generally disappears once air temperatures drop in the autumn. Recent cases, however, have occurred later in the year when it is suspected that higher, sustained temperatures have allowed microorganisms to continue to flourish either in the discharged wastewater itself and/or on surfaces in the wastewater plant. Longer and more intense periods of warm and/or very hot weather may result in more cases of biological interference in WET tests. The impacts, if any, on receiving water bodies has not been thoroughly explored, but could be of consequence especially in arid regions where treated water is in high demand for multiple uses.