to the Environment: Pollution - Dartmouth Collegecushman/courses/engs41/Pollution.pdf · Nitrogen...
Transcript of to the Environment: Pollution - Dartmouth Collegecushman/courses/engs41/Pollution.pdf · Nitrogen...
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ENGS 41
SUSTAINABILITY and NATURAL RESOURCE MANAGEMENT
Benoit Cushman‐Roisin
19‐21 February 2020
Return to the Environment: Pollution(Source: Lynch, Chapter 6)
Environment
HumanActivities
Sustainabilityhere
And sustainabilityhere, too
Questions:1. Can a waste or residue be dumped into the environment?2. If yes, how much is enough?
What is the assimilative capacity of the environmental medium?3. What about accidental releases?
Concept:Carrying capacity
Concept:Assimilative capacity
Waste or residue dumped into the environment is a “resource in reverse.”
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(cartoon floating on the web without attribution)
A minor point – A matter of definition:
A contaminant is a material that wouldn’t be there naturally, regardless of whether it causes harm or not.
A pollutant is a material that causes harm, regardless of whether it is natural or not.
Carbon dioxide (CO2) is neither a contaminant (it is naturally occurring) or a pollutant (it causes no direct harm).
Carbon dioxide is a greenhouse gas!
Also:
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Environmental medium:Airshed, watershed,
or land area
Qin Qout
L R
Loading Removal
Volume V
Mass of material M
Budget for the material:
Budget for the medium:
dML R
dt
in out
dVQ Q
dt
Define the concentration:
' '
in outQ QM dC L RC C
V dt V V VL R aC
Dilution for a > 0
Important concepts:
Residence time:
Accumulated concentrationrelative to loading:
volume= a time
volume / time
V
Q
Restrict attention to medium in steady state (= unchanging in time):
0 0in out
dVQ Q Q a
dt
concentration mass / volume time = =
loading mass / time volumeLynch
CA
L
Advective export: mass removed mass volume removed ×
time volume time
out
R
CQ CQ
This is Lynch’s definition of assimilative capacity.
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dC L RaC
dt V V
volume insteady state
R CQ
'dC L Q C
C Ldt V V
At equilibrium (dC/dt = 0): ' 0 '
/ 1Lynch
C L V LL C L
V Q Q
C L QA
L L Q
If there is a maximum concentration allowed by regulation, say Cmax, then:
maxmax max max .
LC L C Q
Q
There is a maximum tolerable loading.
V
Q with residence time
Cushman‐Roisin’s definition of assimilative capacity:
The assimilative capacity of a fluid medium with respect to a particular pollutant is the maximum loading of that pollutant that preserves the integrity of the system, in steady state.
Integrity of the system is said to be preserved as long as the pollutant’s concentration remains below a threshold value specified by regulations based on scientific studies in the field and in the laboratory.
Thus, the assimilative capacity is the answer to the question:
max max ?C C L
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out
dML KVC QC
dt
Mass budget of pollutant
In steady state at max loading
max max0 ( )L KV Q C
max max max( ) (1 )L KV Q C K QC Assimilative capacity:
Assimilative capacity increases with
‐ Faster transformation rate (higher decay constant K),
‐ Longer residence time ,‐ Stronger flushing rate Q, and
‐ Higher tolerance of the system (higher value of Cmax).
Transformation and sequestration:
Oftentimes, the contaminant or pollutant is not passive.
While it is in the environment, it may ‐ be degraded into something else (transformation), or‐ be consumed by something (sequestration).
Examples
Nitrogen oxide (NO2) in the air may combine with water vapor (H2O) to form nitric acid (HNO3).
Some carbon dioxide is sequestered by vegetation through photosynthesis.
S
T
L
R
out
dML R T S
dtL CQ kM G
dBG
dt
in which
B
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Effect of transformation(Section 6.1.2)
'
1'
out
dML CQ kM
dt
dC CL kC
dt
L k C
Solution:
1'
( ) 1 for ( 0) 01
k tLC t e C t
k
Ultimate steady state: ( )1 1
L LC
kk
flushing
decay
If transformation saturates at high concentration values, then the rate of transformation is made to level off to some finite value
'1 / h
dC C CL k
dt C C
[Note: Textbook defines , rather unnecessary.]0 hT kC
Ultimate steady state: '1 / h
C CL k C
C C
1h
h h
LC C C
kC C C L k C
Asymptotic behaviors:
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Steady state value C(∞) as a function of the loading per volume L’
For algebraic saturation
For exponential saturation
Effect of sequestration(Section 6.1.3)
1'
1
out
dML CQ kM G
dt
dCL C kC G
dt V
dB BG gB
dt K
with
If growth becomes limited by availability of the material being sequestered
1B
G gB MK
new factor
Ex. Trees need carbon to grow; no carbon, no trees.
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Carbon scenarios: (Section 6.2)
The material is CO2, and the biomass sequestering it is vegetation on the surface of the earth.
Case 1:Initially, B = 0. It grows.Eventually, B is harvested at MSY.The biomass is eventually burned and CO2 returned to the atmosphere.
Case 2:Same but with initial B that is removed and burned before desired biomass crop is planted and grown.
Case 3:Same as Case 2 but with CO2 from burning biomass not returned to the atmosphere.
CO2
biomass
energy generated by burning biomass
CO2
CO2
biomass
biomass
energy
energy
Multiple loading: (Section 6.4)
Assume a linear relationship between the several loadings (L1 and L2) and the concentrations (Cwater, Cpark, Chabitat) at various points of concern.
1 21
1 22
1 2
water w w
park p p
habitat h h
C a aL
C a aL
C a a
park
habitat
intake for water supply
Loading 2
Loading 1
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1
2
2 0
2 1
1 3
water
park
habitat
CL
CL
C
Numerical example:
with current loadings L1 = 2 and L2 = 1 (of same contaminant).
2 0 42
2 1 51
1 3 5
water
park
habitat
C
C
C
If regulations require:
3, 2, 2,water park habitatC C C
then regulations are violated, and the loadings must be reduced.
Question: Reduce which loading by how much?
Cost of loading reduction:1 1 2 2(2 ) (1 )Z c L c L
Minimization of total cost Z is identical to maximizing1 1 2 2'Z c L c L
ci = cost of reducing Liby one unit
This is an optimization problem amenable to linear programming:
Find the variables
that maximize
under the constraints:
1 1 2 2'Z c L c L
1 2
1 2
0, 0
2, 1
3, 2, 2water park habitat
L L
L L
C C C
non‐negative loadings
loadings not larger than they are now
to meet regulations
1 2,L L
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Solution
1
2
c
c
0 1/3 2C B ASolution point as function of relative cost
Dissolved oxygen depletion in streams: (Section 6.3)
Define: BOD = Biochemical Oxygen Demand = waste that consumes oxygenDO = Dissolved Oxygen, 0 DO DOs
d
d r s
dBODk BOD
dtdDO
k BOD k DO DOdt
BOD decays by action of bacteria
DO decays by metabolic action of same bacteriaand DO regenerates by re‐aeration through surface
maximum deficit(worst time) at
1ln d
peakd r r
kt
k k k
Note time delay from release.Bacterial decay takes time.
In a stream, time becomes distance from point of release.
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Dissolved oxygen cannot go negative.When DO approaches zero, bacteria slow down their activity, to the point of no bacterial activity as DO reaches zero.
= BOD
= DOs – DO
DOs
/0 1 refDO DO
d dk k e Take decay rate, no longer constant, as:
A progressive path toward a sustainable industrial system
naive
improvement causing conflict between industrialists and environmentalists
with increasing cooperation between various stakeholders
ideal situation