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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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objectives
Objectives
  • Draw and discuss the DO sag curve
  • Calculate DO and solve for a variety of unknowns using the DO sag equation
slide4
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)

what we re trying to accomplish and how we re going to do it
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
very simplified model
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
Mass-Balance for Initial Condition

Qr, Lr, Tr

Qa, La, Tf (Ta)

Qw, Lw, Tw

account for other do gain loss processes
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
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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