DO Sag Curve (Streeter-Phelps)

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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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### 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

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)

• Calculate the dissolved oxygen deficit using mass balance equation
• 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
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)
Mass-Balance for Initial Condition

Qr, Lr, Tr

Qa, La, Tf (Ta)

Qw, Lw, Tw

Account for other DO gain/loss processes

Reaeration

Decay

Qr, Lr, Tr

Qa, La, Tf (Ta)

Qw, Lw, Tw

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