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8/12/2019 Lecture : Water Quality Modelling 577l02p
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CEE 577 Lecture #2 1/21/2013
Lecture #2(modeling fundamentals & mass balance)
Updated: 21 January 2013
Chapra, L1(pp. 3-20)
Print version
Water Quality ModelingObjectives
Waste Load Allocation
to determine the environmental controls that must be instituted to achieve a specific water quality objective
focus on point sources
David A. Reckhow CEE 577 #2 2
X
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CEE 577 Lecture #2 1/21/2013
Water Quality Management
David A. Reckhow CEE 577 #2 5
Figure 1.2 from Chapra, 1997
Objectives (cont.)Toxics Modeling
to understand the fate of hazardous substances in the aquatic environment
General Understanding of the Ecosystem to understand the response of natural system to pollutant inputs
Errors?
David A. Reckhow CEE 577 #2 6
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TABLE 1.1 PRINCIPAL POLLUTION PROBLEMS, AFFECTED USES, AND ASSOCIATED WATER QUALITY VARIABLES (From Thomann & Mueller, 1987)
David A. Reckhow CEE 577 #2 7
Manifestationof problem
Water useinterference
Water quality problem
Water qualityvariables
1 Fish kills Nuisance odors, H2S"Nuisance" organisms Radicalchange in ecosystem
FisheryRecreationEcological health
Low DO(dissolvedoxygen)
BOD NH3, org N,Organic solidsPhytoplankton, DO
2 Disease transmissionGastrointestinal disturbance, eyeirritation
Water supply,Recreation
High bacteriallevels
Total coliform bacteria,Fecal coliform bacteria,Fecal streptococci,Viruses
3 Tastes and odors-blue green algaeAesthetic beach nuisances, algalmats "Pea soup" Unbalancedecosystem
Water supply,Recreation,Ecological health
Excessive plant growth,(Eutrophication)
Nitrogen, Phosphorus,Phytoplankton
4. Carcinogens in water supplyFishery closed-unsafe toxic levels,Ecosystem upset; mortality,reproductive impairment
Water supplyFisheryEcological health
High toxicchemicallevels
Metals Radioactivesubstances PesticidesHerbicides Toxic productchemicals
RatesDetermination of Mass Loading
Point Sources - General Concepts
W(t) = Q(t) c(t) Important Conversion Factors
David A. Reckhow CEE 577 #2 8
8 34.lb litersmg MG
5 39 3.seclb liters
mg ft day
2 45 3.
secKg litersmg ft day
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CEE 577 Lecture #2 1/21/2013
Rates (cont.)
Related Rates Volumetric flow rate
Q=UA c Mass Flux rate
David A. Reckhow CEE 577 #2 9
J m
tAW A
Ucc c
Refer to Example 1.2 (pg 9)
U velocity of water
Ac cross-sectional area
And therefore, W=JA c
Model Implementations The Model
concentration, c, is proportional to loading, W , by the reciprocal of an assimilation factor, a
Simulation Mode c=W/a
Design Mode I Assimilative Capacity W=ac
Design Mode II Environmental Modification a=W/c
David A. Reckhow CEE 577 #2 10
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Two Approaches to Modeling Empirical Modeling
based on inductive approach heavily dependent on statistical analysis of existing data
Mechanistic Modeling based on deductive approach more dependent on theory of underlying processes emphasized in Chapras book
David A. Reckhow CEE 577 #2 11
Mass Balance or Mass Inventory Also known as conservation of mass
Key to mechanistic WQ modeling
If sources are in balance with sinks, mass remains constant and we are at steady state:
Separate mass balances written for each substance
David A. Reckhow CEE 577 #2 12
Accumulation loadings transport reactions
Accumulation
0
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Typical Mechanistic Model
David A. Reckhow CEE 577 #2 13
Based on: Figure 1.5 from Chapra, 1997
Reactions
Loadings
Substance B
Substance A
Transport In Transport Out
Components
SystemResponse
Spatial/Temporal Resolution When spatial or temporal concentration differences are important, system may be divided into sub volumes or times
Segmentation is the process of dividing space and matter into increments
space: 1, 2 or 3 dimensions
Resolution is the degree to which space, time and matter
are segmented
David A. Reckhow CEE 577 #2 14
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Historical Development of Mechanistic
Modeling 19251960: Streeter Phelps DO modeling, based on BOD, SOD
19601970: Computerization greater complexity, resolution possible
19701977: Biology eutrophication modeling, based on N, P, light
1977present: Toxics
partitioning of
hydrophobics,
complex
physical,
chemical and biological transformations
David A. Reckhow CEE 577 #2 15
Evolving Issues
David A. Reckhow CEE 577 #2 16
Figure 1.6 from Chapra, 1997
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Economics of pollution control
As standards become more strict
Costs go up disproportionally
Errors in judgment are more costly
David A. Reckhow CEE 577 #2 17Figure 1.7 from Chapra, 1997
To next lecture
David A. Reckhow CEE 577 #2 18