1. CE 3231 - Introduction to Environmental Engineering and Science
Ohio Northern University
CE Introduction to Environmental Engineering and Science
Introduction
Chemistry, Microbiology & Material Balance
Water & Air Pollution
Env Risk Management
Dissolved Oxygen Sag Curve
When organic carbon is placed into a stream environment, the dissolved oxygen drops following a characteristic pattern. The resulting “sag curve” is able to be modeled to predict the minimum oxygen concentration expected and the point along the river stretch where the deficit will occur.
Readings for This Class: 1

2. Lecture 25 DO Sag Curve – Streeter Phelps Model (Water Quality V)

3. BOD and DO
Definition of BOD
Calculating BODx from equation

4. DO Sag Curves

5. Simple Plug Flow River Model
Objective: derive a mathematical model to predict DO downstream: DO(x,t)
Wastewater discharge
(High BOD, Low DO)
Assumptions:
completely and uniformly mixed in the cross flow and vertical direction
neglect pollutant dispersion

6. Formulation: Mass balance
DO replenish from air
A control volume in the river A
A: cross section area (m2)
u: flow velocity u
DOin
DOout Δx
DO consumed due to oxidation

7. Formulation: Mass balance

8. Formulation: Mass balance

9. Formulation: Mass balance

10. Critical Point Time/Distance @ DO=Domin Minimal occurs when dDO/dt=0
Taking the sag equation with respect to t and setting it equal to zero, then solve for t
Distance downstream where DOmin occurs = tc x u
u: flow speed of the river

11. Streeter Phelps solved for Dmax
Streeter Phelps solved for critical time
In-class Problem
The initial BOD of a river just below a sewage outfall is 25 mg/L. The oxygen deficit just upstream from the outfall is 2 mg/L. The deoxygenation rate coefficient kd is 0.4/day, and the reacation rate coefficient kr is 0.7/day. The river is flowing at a speed of 20 miles/day. You have been asked to determine the impact of this single source on the river:
Find the critical distance downstream at which DO is a minimum
Find the minimum DO

12. Solution (a)
In-class Problem
The initial BOD of a river just below a sewage outfall is 25 mg/L. The oxygen deficit just upstream from the outfall is 2 mg/L. The deoxygenation rate coefficient kd is 0.4/day, and the reacation rate coefficient kr is 0.7/day. The river is flowing at a speed of 20 miles/day. You have been asked to determine the impact of this single source on the river:
Find the critical distance downstream at which DO is a minimum
Find the minimum DO

13. Solution (b)
In-class Problem
The initial BOD of a river just below a sewage outfall is 25 mg/L. The oxygen deficit just upstream from the outfall is 2 mg/L. The deoxygenation rate coefficient kd is 0.4/day, and the reacation rate coefficient kr is 0.7/day. The river is flowing at a speed of 20 miles/day. You have been asked to determine the impact of this single source on the river:
Find the critical distance downstream at which DO is a minimum
Find the minimum DO

14. What happens when there is no initial deficit?
Streeter Phelps solved for Dmax
Streeter Phelps solved for critical time
What happens when there is no initial deficit?
What happens where the organic loading increases?
What happens if the river increases velocity?