Introduction. Industrial facilities that discharge directly to surface water receiving streams must have NPDES permits, which typically include a requirement to measure the toxicity of a wastewater effluent sample. Effluents from permitted facilities are monitored, and WET limits are established if the effluent could reasonably exceed numeric toxicity criteria
Purpose of the study. Permittee discovers a toxicity problem, a toxicity reduction evaluation (TRE) may be used to identify and reduce or eliminate the sources, whether or not the NPDES permit contains WET limits. Regulators also may require the permittee to perform a TRE via special permit conditions or an enforcement action.
Materials and methods. COMMON TOXICS. The following pollutants are typically found in wastewater treatment system effluents:
- Chlorine (at concentrations between 0.05 and 1.0 mg/L);
- Ammonia (at 5 mg/L as NH3 -N);
- Nonpolar organics (e.g., organophosphate insecticides);
- Metals (e.g., cadmium, copper, iron, lead, nickel, and zinc) at various concentrations;
- Chemical treatment additives (e.g., dechlorination chemicals and polymers);
- Surfactants; and
- Total dissolved solids (at concentrations between 1000 and 6000 Amhos/cm).
- For more information on toxic pollutants.
Results and discussion. TESTING APPROACH. Toxicity can be identified via two approaches: conventional and toxicity-based. In the conventional approach, an effluent sample is analyzed for the 126 "priority pollutants" to try to identify the substance(s) responsible for the effluent's toxicity. If any were found to be present, analysts then compare the concentration(s) in the sample to the known reference toxicity data for that pollutant. Unfortunately, this approach often fails to pinpoint sources of toxicity for two reasons:
The 126 priority pollutants are a tiny fraction of the chemicals that could be toxic to aquatic organisms, and
This approach does not take into account a chemical's bioavailability [the synergistic effect of other factors (e.g., TSS, pH, hardness, and alkalinity) can affect a toxicant's bioavailability and, thus, its toxicity[1-4].
In the toxicity-based approach, the effluent sample is subjected to various physical and chemical treatment methods that categorize the nature of the toxic sub- stance(s). Each physical and chemical treatment test method that is applied to the wastewater sample narrows the field of possible toxicants. Once the screening test procedures have been applied, a list of suspect toxicants is developed and further specific chemical analysis may then pinpoint the cause of toxicity. This latter approach has proven to be more efficient and effective.
TEST METHODS. The U.S. Environmental Protection Agency has issued guidance on the TIE test program (U.S. EPA, 1999), and modified WET procedures to reduce the testing procedures' time and cost burdens. The decision to use the acute or short-term chronic tests depends on NPDES permit requirements and the effluent's toxicity. The initial TIE testing should be performed using the test organism shown to be most sensitive to the effluent. If several organisms are equally sensitive, analysts should select the one that is easiest to use.
The effluent's toxicity is initially characterized via the so-called Phase I approach, in which several treatment methods are used to indicate the types of compounds responsible. These treatment methods include:
Filtration—removes insoluble compounds.
Aeration—batch aeration at acid, neutral, and basic pH removes essentially all volatile organics, as well as ammonia at high pH.
EDTA addition—this chelation test removes combined cationic metals.
Zeolite resin—zeolite ion exchange removes ammonia.
Sodium thiosulfate—reduces any oxidants (e.g., chlorine).
Biodegradability—biological treatment almost completely oxidizes biodegradable organics. After each treatment step, analysts test both treated and untreated samples for toxicity.
Conclusions. Process of elimination and knowledge of the facility's manufacturing processes and operations, analysts can discover the specific chemical or chemicals responsible for the toxicity. Then, control alternatives are identified and evaluated, and the appropriate controls are selected. Finally, the toxicity control method or technology is implemented and monitored to ensure that it achieves the TRE objectives and meets permit limits.
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