The NPDES program, established under the 1972 Clean Water Act, Section 402, allows the EPA to issue permits to entities that discharged effluent to navigable waterways of the United States. The original permits were consistent with the pollution control mechanisms of the CWA, specifically Sections 301 (numerical effluent limitations), 303 (water quality standards), and 304 (quality criteria) and were, for the most part, technology-based.
At that time, toxicity testing was used only sporadically and there was no consistent policy for application of WET. It became clear, however, that technology-based limits were not completely effective in protecting receiving systems, and the existing permit system was not meeting the CWA goal of preventing the discharge of 'toxic pollutants in toxic amounts.' As a result of the new data, in 1984 EPA issued the 'Policy for Development of Water Quality-based Permit Limitations for Toxic Pollutants' indicating that EPA would use WET tests to complement chemical-specific analysis.
While WET tests provide a means of determining whether an effluent might pose a risk to receiving stream organisms, they do not inform the discharger what is causing the toxicity. Without that information, a treatment plant manager cannot identify and eliminate the toxicity. This shortcoming of the WET process was identified early on and the need for a mechanism to identify and eliminate toxicity was established.
TIEs as Part of the TRE Process
A toxicity reduction evaluation (TRE), as defined by EPA's Technical Support Document for Water Quality-Based Toxics Control, is a Site-specific study conducted in a stepwise process to narrow the search for effective control measures for effluent toxicity.
TREs identify the cause of effluent toxicity, isolate the sources of the toxicant(s), evaluate toxicity control options, and confirm toxicity reduction in the effluent. TREs can involve many steps and are seldom the same for all situations. The major components of a TRE include (EPA TRE manual for municipal POTWs - EPA/833B-99/002):
- Information and Data Acquisition
- Facility Performance Evaluation
- Toxicity Identification Evaluation
- Toxicity Source Evaluation
- Toxicity Control Evaluation
- Toxicity Control Implementation
Some or all of these components may be important in identifying and eliminating toxicity. In some cases it may only be necessary to examine recent activities in the process stream to determine if any significant changes (e.g., addition of a new treatment chemical) occurred immediately before the toxicity was detected. This kind of investigation can be conducted internally with minimal cost. Information concerning atypical plant operations is often most effectively obtained from plant engineers who have knowledge of day-to-day facility operations.
In many situations, however, simply examining operational records is of little value until the toxicant has been identified. In these cases, it is necessary to complete a systematic process of toxicity identification via effluent manipulation. Treatment and control options usually increase, and control costs decrease, when the precise cause of toxicity is known. (The remainder of this article will discuss the toxicity identification evaluation [TIE]).
Typical Toxicity Identification Evaluation Phase 1: Manipulations and Target Toxicants
Phase 1 Tests
pH Adjustment Tests Those affected by extreme changes in pH (e.g., sulfide, cyanide)
Filtration Tests Filterable solids, or those whose solubility is affected by acidic or basic conditions (e.g., cationic metals under basic conditions)
Aeration Tests Voltive, oxidizable, or sublatable compounds (e.g., chlorine, surfactants)
Solid-Phase Extration (SPE) with C18 Non-polar organic compounds (e.g., pesticides, VOCs)
EDTA Chelation Test Cationic metals (e.g., copper, cadmium, zinc, nickel)
Oxidant Reduction Test (addition of Sodium Thiosulfate) Oxidants, such as chlorine or peroxide; certain cationic metals (e.g., copper, cadmium, mercury, silver)
Ulva lactuca Addition Ammonia (marine only)
Graduated pH Test pH-sensitive toxicants (e.g., some metals, ammonia)
A TIE separates toxicants based on their reaction to various chemical and physical manipulations. In Phase I of a TIE, effluent is subjected to manipulations (see table below) before being retested for residual toxicity. The data are interpreted by comparing the results of the manipulated sample tests to the results of the baseline test, conducted with unaltered effluent. For example, if solid-phase extraction removes toxicity, then non-polar organic toxicants would be expected. Alternatively, if toxicity is removed by pH 11 filtration and the addition of EDTA, toxicity can be attributed to one or more cationic metals (e.g., zinc and/or nickel).
Through the results of Phase I studies, the general characteristics of the final effluent toxicity are more clearly defined. With this knowledge, Phase II Toxicity Identification studies are conducted, which focus and build on Phase I results to identify specific causative toxicants. For example, if organic toxicants are indicated, isolation and concentration steps (using SPE and High Pressure Liquid Chromatography [HPLC]), followed by chemical analysis, are the typical Phase II procedures. Conversely, if metals are suspected, Phase II may simply involve analyzing the effluent sample for the presence of metals. Another Phase II study that might be conducted involves the use of mock effluents. These may be particularly useful in situations where toxicity due to TDS ions is suspected.
The final step in a TIE is Phase III, Toxicity Confirmation. The goals of Phase III are to: 1) confirm that the causative toxicant(s) has (have) been correctly identified, and 2) confirm that the causative toxicant(s) is (are) not changing over time. Given these goals, Phase III Confirmation could involve many different types of studies. Usually, these studies are designed to quantitatively correlate measured toxicity with the concentration of the suspect toxicant.
It Can't be Toxic . . . All Numerical Limits are Being Met!
The appearance of WET is often frustrating for plant managers, especially if all of the numerical limits for priority pollutants specified in the permit are being met. The permitted parameters seldom are the cause of toxicity since they are already controlled to meet numerical requirements.
In closed-loop systems, the cause of toxicity might be the addition of a new chemical, a change in the feed levels of existing chemicals, improper calibration of metering or monitoring equipment, or corrosion leading to the mobilization and release of toxic materials. In open systems, such as municipal treatment plants, upstream sources can result in any number of potential toxicants. Some of the more common toxicants identified at the ENSR toxicology laboratory include:
- Organophosphate pesticides (e.g., diazinon, malathion, chlorpyrifos)
- Metals (e.g., copper, nickel, hexavelent chromium)
- TDS ions
When toxicity is first observed in a routine WET test, the required response is usually spelled out in the permit. The discharger may have to immediately begin a TRE/TIE, conduct accelerated WET tests, write a specific TRE/TIE plan to be presented to the authorities, or a combination of these. Typical wording in some permits might state:
If WET failure occurs, the permittee must provide written notification of the failure. . . along with a statement as to whether 1) the Preliminary Testing Incident (PTI)/ Toxicity Identification Evaluation (TIE) or 2) accelerated testing is being performed.
If a pattern of toxicity is found after accelerated testing, the permittee has to begin a TIE anyway. Because of this, many dischargers may choose to begin a TIE immediately after the original toxicity is found. The effort needed to complete a TIE can be variable.
Ceriodaphnia dubia, or the water flea, commonly used in toxicity tests, including TIEs.
Although the response of some materials to effluent manipulations is highly characteristic of specific toxicants, other chemicals can be frustratingly cryptic and may defy identification through several testing iterations. The number and complexity of required tests can, therefore, be quite different between effluents. As a result, TIE costs can vary dramatically. However, a toxicologist with extensive TIE experience can often identify patterns that others might miss, thus reducing the number of required tests and forestalling unnecessary costs.
The cost of a TRE/TIE can also vary with state and region. California, Nevada, and Arizona (EPA Region 9), for example, require industrial and municipal dischargers to prepare an extensive Toxicity Reduction/Toxicity Identification Evaluation Workplan even before toxicity is observed. Other states, such as Colorado in EPA Region 8, simply require a discharger to proceed with a TIE or preliminary investigation when toxicity is observed. The monetary difference between these two options can be substantial (e.g., $25,000-$40,000 to complete a workplan vs $2,000-$4,000 for Phase I). In any case, the goal of a TIE should be to provide the discharger with accurate information on the cause of toxicity as quickly and cost-effectively as possible, and the key to reaching this goal is experience, both from facility engineers and from the scientists conducting the TIE.