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Two different approaches can be taken in conducting a TIE (Toxicity Identification Evaluation).  One approach is a conventional approach in which the effluent sample is analyzed for series of known toxicants in an attempt to identify the substance (or substances) responsible for the observed toxicity in the effluent.   The second approach is a toxicity based approach in which various physical and chemical manipulations of the sample are used to first characterize the toxic substance before attempting to identify it.   Of these two approaches, the latter almost always proves to be the most effective and efficient. 

Conventional Approach

Traditionally when an effluent was found to be toxic to aquatic organisms, a sample of the effluent was analyzed for 126 "priority pollutants".  If any of these priority pollutants were found to be present, their concentrations were subsequently compared to toxicity data for that toxicant.

This approach, however, often fails to pin-point the cause of toxicity in an effluent for two main reasons.

1. The so-called "priority pollutants" represent only a tiny fraction of all the potential chemicals that can cause toxicity to aquatic organisms.  The cost to analyze an effluent for every possible toxic chemical compound would be prohibitive.
2. This approach tells us nothing about the bio-availability of chemical substances found in the effluent.  Thus, even if one or more of the "priority pollutants" are found in the effluent sample, that does not necessarily mean that they are responsible for the observed toxicity.  This is because other factors such as total suspended solids, pH, hardness and alkalinity can change the bioavailabilty and therefor the toxicity of a given toxicant. 

Toxicity Based Approach

This approach first uses the responses of test organisms to show changes in toxicity as the sample is subjected to various chemical and/or physical manipulations.  In this way, knowledge of the physical/chemical characteristics (such as solubility, volatility, filterability, pH dependence, etc.) of the toxic substance is gained, and the number of possible toxic agents is greatly reduced before actual chemical analysis begins. 

A toxicity based TIE typically has three phases:  characterization, tentative identification, and confirmation.  In the first phase, the toxicant is characterized ( i.e. general physical/chemical characteristics of the toxic substance are determined).   In the second phase, toxicant-specific methods are used to isolate, analyze and tentatively identify the toxicant.  In the third phase, confirmation tests are carried out to confirm that the identified toxicant is the substance responsible for the effluent toxicity.

To be successful, toxicity-based TIEs  require that toxicity be consistently present in the effluent so that repeated testing can be carried out to characterize, identify, and confirm the toxicants.  Speed in conducting the tests is usually important because effluents may decay during storage. The TIE tests should be conducted by a multi-disciplinary team of experienced toxicologists and chemists who are aware of the many factors that can affect test results.

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