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DO Sag Curve Streeter-Phelps - PowerPoint PPT Presentation

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)

April 17, 2006

• Draw and discuss the DO sag curve

• Calculate DO and solve for a variety of unknowns using the DO sag equation

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)

What we’re trying to accomplishand how we’re going to do it

• 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

• 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)

Qr, Lr, Tr

Qa, La, Tf (Ta)

Qw, Lw, Tw

Reaeration

Decay

Qr, Lr, Tr

Qa, La, Tf (Ta)

Qw, Lw, Tw

• 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