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- Water Effect Ratio (WER) & Recalculation Procedure

- Alternate Species Studies

ETT is the leader in Site Specific Studies. We conduct the first Water Effect Ratio (WER) Study approved by the USEPA for a South Carolina permitted discharger, and South Carolina is among the first states in the nation to allow the use of Site Specific Studies. The quality control requirements of site specific studies are rigorous, and only laboratories with the necessary expertise can successfully complete these tests.

   How does the WER work?

Using an EPA approved method, parallel toxicity tests are conducted to compare the toxicity of a metal in laboratory water to the toxicity of the same metal in site downstream water. Of course the toxicity in downstream water is the real life situation. Copper (Cu) may be toxic at 10 ppb in laboratory water and 50 ppb in site downstream water. This gives a ratio of 50/10=5.0 which is used to adjust your effluent limit for the metal in your permit.

      Your old permit limit   =  20 ppb

      Your new permit limit = 100 ppb

We have successfully modified many of our client’s NPDES permit metal limits based on Site Specific Studies. The limits are raised, but the receiving streams are still protected!

Our Services
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TIE & TRE StudiesTIE%26TRE.html
Toxicity TestingToxicity.html
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Site Specific Studies
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