Who is blending?
The Association of Metropolitan Sewerage Agencies conducted a survey in 2001 on blending and received 122 responses—47% of the organization's membership. Of these, 59 respondents or 48% blended during some peak flow conditions. Of the 48% of the facilities that blended, 70% of the plants were originally designed to blend, 31% of permits for these facilities recognized blending, and 33% of these communities were served by combined sewers.The EPA's 2003 proposed blending policy
The blending policy proposed by the U.S. EPA in November 2003 (and subsequently withdrawn by EPA) consisted of two components—an interpretation of the bypass provision and draft guidance on implementation of this interpretation. The blending of two effluent streams—one that has been routed around biological units or other advanced treatment processes and the other that has passed through secondary treatment—would be allowed and authorized in a NPDES permit as long as the following six principles were applied:The final discharge meets effluent limitations based on the secondary treatment regulation including applicable 30-day average percent removal requirements, or any more stringent limitations necessary to attain water quality stancards.The NPDES permit application for the POTW provides notice of, and specifically recognizes, the treatment scenario that would be used for peak flow management. This treatment scenario would be consistent with generally accepted practices and long-term design criteria and would be re-evaluated as circumstances change and at permit reissuance.The treatment scenario used for peak flow management should provide, prior to blending, at least the equivalent of primary clarification for the portion of flow routed around biological or other advanced treatment units.The peak flow treatment scenario chosen by the permit holder for use when flows exceed the capacity of storage units/equalization units, biological treatment units of advanced treatment units should be operated as designed and in accordance with the treatment scenario and conditions stated in the permit record.The permit would require monitoring sufficient to yield data representative of the final blended discharge to ensure compliance with water quality-based effluent limitations.The permit would require at a minimum that the permit holder properly operate and maintain all parts of the collection system over which the permit holder has operational control. This includes conforming to the provisions of the 1994 Combined Sewer Overflow Control Policy.
— Adapted from the EPA's NPDES Permit Requirements for Municipal Wastewater Treatment Discharges During Wet Weather Conditions.