API PUBL 4664-1998

$30.55

Mixing Zone Modeling and Dilution Analysis for Water-Quality-Based NPDES Permit Limits

Published By	Publication Date	Number of Pages
API	1998	176

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Description

In the United States, National Pollutant Discharge Elimination System (NPDES) permits for wastewater discharges to surface water include effluent limits based on available treatment technology. More stringent limits may be set on a site-specific basis to protect local receiving water quality. The derivation of water-quality-based permit limits may consider effluent dilution within a "mixing zone" near the outfall. Mathematical water quality models are generally used to estimate such dilution.

Although the concept of a mixing zone is straightforward, its application to a specific discharge situation often raises technical and policy concerns. This report presents a summary of available information on the role of dilution analysis and mixing zone models in the NPDES permitting process. It is intended as guidance for those who manage or evaluate mixing zone studies in the course of obtaining water-quality based NPDES permits. The document includes an analysis of the mixing zone regulations and policies of the United States Environmental Protection Agency (EPA) as well as 14 states. Basic concepts are presented to describe the physical interaction of effluent discharges and ambient waters. The application of these concepts to outfall design and mixing zone model selection is discussed.

Ten EPA-developed mixing zone models are presented in detail. These range from simple analytical equations to sophisticated computer programs. The discharge and ambient conditions appropriate for each model are described. A structured approach is presented for the selection, validation, and strategic use of mixing zone models in the NPDES process. Dye tracer studies are discussed as supplements or alternatives to modeling. Case histories illustrate the role of mixing zone models and tracer studies in real-world permitting situations. References are provided for model documentation as well as electronic access via the Internet.

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