Introduction to Environmental Engineering and Science Readings for This Class 5556 O hio N orthern U niversity Introduction Chemistry Microbiology amp Material Balance Water amp Air Pollution ID: 356359
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Slide1
CE 3231 -
Introduction to Environmental Engineering and Science
Readings for This Class: 5.5-5.6
Ohio Northern University
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.Slide2
Lecture
25DO Sag Curve – Streeter Phelps Model(Water Quality V)Slide3
BOD and DO
Definition of BOD
Calculating
BOD
x
from equationSlide4
DO Sag CurvesSlide5
Simple Plug Flow River Model
Wastewater discharge
(High BOD, Low DO)
Objective: derive a mathematical model to predict DO
downstream: DO
(
x,t
)
Assumptions:
completely and uniformly mixed in the cross flow and
vertical direction
neglect
pollutant dispersionSlide6
Formulation: Mass balance
A control volume in the river
u
A
A: cross section area (m
2
)
u: flow velocity
DO
out
DO
in
DO replenish from air
DO consumed due to oxidation
Δ
xSlide7
Formulation: Mass balance Slide8
Formulation: Mass balance Slide9
Formulation: Mass balance Slide10
Critical Point
Time/Distance @ DO=DominMinimal occurs when
dDO/dt=0Taking the sag equation with respect to t and setting it equal to zero, then solve for t
Distance downstream where DO
min
occurs = t
c
x u
u: flow speed of the riverSlide11
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 minimumFind the minimum DOSlide12
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 minimumFind the minimum DOSlide13
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 k
r 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 minimumFind the minimum DOSlide14
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?