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DO Sag Curve (Streeter-Phelps)

DO Sag Curve (Streeter-Phelps). April 17, 2006. Objectives. Draw and discuss the DO sag curve Calculate DO and solve for a variety of unknowns using the DO sag equation. Biota vs. D.O. Typical DO sag curve. Initial Deficit (D a ). 10 8 6 4 2. Saturation DO (Do s ). Deficit. Critical

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DO Sag Curve (Streeter-Phelps)

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  1. DO Sag Curve (Streeter-Phelps) April 17, 2006

  2. Objectives • Draw and discuss the DO sag curve • Calculate DO and solve for a variety of unknowns using the DO sag equation

  3. Biota vs. D.O.

  4. Typical DO sag curve Initial Deficit (Da) 10 8 6 4 2 Saturation DO (Dos) Deficit Critical Point Dissolved Oxygen (mg/L) DO Concentration (DO) tc 2 4 6 8 10 Travel Time (d)

  5. What we’re trying to accomplishand how we’re going to do it • Calculate the dissolved oxygen deficit using mass balance equation • Start with simplified model and calculation • BOD • Reaeration • Initial DO reduction • Add some more components to the model • Get an equation for D (dissolved oxygen deficit) • Get an equation for tc, the “critical time” when the dissolved oxygen is at its lowest point

  6. Very Simplified Model • Assume plug-flow and define the control volume as a unit rectangle • Follow the movement of the plug which tracks the pollution downstream • Determine the initial oxygen content after mixing (La)

  7. Mass-Balance for Initial Condition Qr, Lr, Tr Qa, La, Tf (Ta) Qw, Lw, Tw

  8. Account for other DO gain/loss processes Reaeration Decay Qr, Lr, Tr Qa, La, Tf (Ta) Qw, Lw, Tw

  9. Why We Do All of This • To determine how much waste can safely be put in a river • Process • Determine minimum acceptable DO • Calculate waste load that keeps critical DO above the minimum • If discharged waste is above acceptable limits: • More treatment needed • Discharger may add dissolved oxygen to wastewater • Cautions • Be sure to make calculations for worst conditions • Remember to consider all dischargers

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