Antidegradation Trigger and Examples

1. Antidegradation Trigger and Examples June 25, 2008

2. What triggers a Tier II Antidegradation Review? • A new or increased discharge of a pollutant that requires a new or increased permit limit which also results in a discharge of the pollutant that is greater than the De minimis Lowering of Water Quality.

3. When are permit limits required? • Water quality based effluent limits (WQBELs) are required when there is a reasonable potential to exceed a water quality criterion. • Technology based effluent limits are required when one has been established through either a Federal effluent guideline or by IDEM using the rule 327 IAC 5-5-2(b).

4. Reasonable Potential to Exceed a Water Quality Criterion • When the Projected Effluent Quality (PEQ) for a pollutant is greater than the Preliminary Effluent Limits (PEL) for a pollutant, the permit shall include a WQBEL for that pollutant.

5. Projected Effluent Quality • The PEQ is calculated by taking the highest value from a set of effluent samples and multiplying it times an uncertainty factor based on the number of effluent samples in the data set. A higher number of samples will result in a lower uncertainty factor.

6. Preliminary Effluent Limits • The PEL is the WQBEL calculated using the default mixing zone. For the protection of aquatic life from chronic toxicity, the default mixing zone is 25% of the stream design flow volume. The PEL utilizes approximately 25% of the unused loading capacity.

7. De minimis Lowering of Water Quality for Tier II Waters • The proposed increase in mass-based effluent limits is less than or equal to the water quality-based effluent limit (WQBEL) calculated using 10% of the unused loading capacity, or the Default Technology Based Effluent Limit (DTBEL), whichever is more stringent.

8. Tier II Demonstration Trigger • When a WQBEL or Technology Based Effluent Limit (TBEL) is required and the WQBEL or TBEL is greater than the limit based on the De minimis Lowering of Water Quality, a Tier II antidegradation demonstration is required, unless the discharger accepts the limits based on the De minimis Lowering of Water Quality.

9. Cumulative Cap on Increases • When the representative background concentration has increased by more than X % above the benchmark set at the time of the initial antidegradation demonstration or de minimis evaluation in the area of the discharge, a Tier II antidegradation demonstration is required.

10. New Industrial Discharger • A new manufacturing facility will have the following pollutants in their discharge: Copper, Lead, Nickel, TCE and Zinc. • Discharger Design Flow = 2.0 MGD • Stream Design Flow = 56.0 MGD

11. Existing Industrial Discharger • An existing manufacturing facility has the following pollutants in their discharge: Copper, Lead, Nickel, TCE and Zinc.   • Discharger Design Flow = 2.0 MGD • Stream Design Flow = 56.0 MGD • The discharger proposes to double their existing operations and their discharge flow from 2 MGD to 4 MGD.

12. New Sanitary Discharger • A new Regional Sewer District has been formed to treat wastewater from two small towns that have been on septic systems. The RSD is building a new 0.5 MGD extended aeration treatment system to treat the wastewater from the two small towns. The receiving stream has a design flow of 1 MGD.

13. Existing Sanitary Discharger • An existing community is expanding their existing extended aeration treatment system from 5.0 MGD to 7.0 MGD to treat the wastewater from the expansion of the population in the community. The receiving stream has a flow of 55 MGD.