Hydrodynamics of Initial Mixing Zones of Wastewater Discharges

EPA Grant Number: R826216
Title: Hydrodynamics of Initial Mixing Zones of Wastewater Discharges
Investigators: Roberts, Philip J.W.
Institution: Georgia Institute of Technology
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 1997 through September 30, 2000
Project Amount: $277,643
RFA: Exploratory Research - Physics (1997) RFA Text |  Recipients Lists
Research Category: Water , Land and Waste Management , Air , Engineering and Environmental Chemistry


The objectives of this study are to obtain experimental data on the physics of turbulent mixing processes in buoyancy-modified flows typical of industrial and municipal wastewater discharges. The data will be used to refine the mathematical models of dilution and mixing zones used by EPA, and to test the hypothesis that the end of the initial hydrodynamic mixing zone is caused by turbulence collapse under the influence of gravity forces.


Experimental studies will be conducted in a specially built laboratory and test tank using innovative non-intrusive optical methods. Fully three-dimensional mapping of the tracer concentration field will be obtained by Laser-induced fluorescence (LIF) by sweeping a laser beam with scanning mirrors in a high-speed programmed pattern through the flow. The fluoresced light will be captured by an image-intensifed, high-speed imaging system and converted to spatial concentration (and hence dilution) data. Refractive index matching will be used to remove the effects of refractive index fluctuations caused by density variations. These techniques have only recently become available due to rapid advances in opto-electronics, sensors, and high-speed video. Systematic experiments will be done over a range of conditions typical of realistic wastewater discharges into stationary and flowing, stratified and unstratified, water bodies such as lakes, estuaries, and coastal waters.

Expected Results:

The results of this study will lead to better prediction of the environmental impact and risk to human health and ecosystems of wastewater discharges containing toxic substances. Efficient initial mixing can substantially reduce this risk. The widely used mixing zone mathematical models used by EPA for prediction, however, are unable to predict the mixing which occurs due to internal hydraulic jumps and by gravitational diffusion, and the turbulence collapse that ultimately limits the inital hydrodynamic mixing. The data will be used to improve these models. Reliable predictions are needed for toxics control and NPDES permitting and for appropriate choice of wastewater treatment levels which avoids either unnecessary expense or inadequate risk containment. Finally, the present regulatory definitions of mixing zone dimensions are arbitrary and bear little relationship to the physics of the mixing processes. The results should lead to more rational, scientifically based, regulatory definitions of mixing zones.

Publications and Presentations:

Publications have been submitted on this project: View all 2 publications for this project

Supplemental Keywords:

Water, Toxics, Discharge, Mixing Zone, Mathematical Models, Dilution., Scientific Discipline, Water, Hydrology, Wastewater, Environmental Chemistry, Physics, mathematical model, dilution, estuaries, industrial wastewater, National Pollutant Discharge Elimination System (NPDES), turbulent mixing process, high speed imaging, municipal wastewater treatment, wastewater discharges, discharge, initial mixing zones, hydrodynamics, three dimensional model

Relevant Websites:


Progress and Final Reports:

  • 1998 Progress Report
  • 1999
  • Final Report