Lecture : Water Quality Modelling 577l02p

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


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?



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CEE 577 Lecture #2 1/21/2013

TABLE 1.1 PRINCIPAL POLLUTION PROBLEMS, AFFECTED USES, AND ASSOCIATED WATER QUALITY VARIABLES (From Thomann & Mueller, 1987)


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


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


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



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CEE 577 Lecture #2 1/21/2013

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


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


Accumulation loadings transport reactions

Accumulation

0


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CEE 577 Lecture #2 1/21/2013

Typical Mechanistic Model


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



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CEE 577 Lecture #2 1/21/2013

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


Evolving Issues


Figure 1.6 from Chapra, 1997


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CEE 577 Lecture #2 1/21/2013

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

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Lecture : Water Quality Modelling 577l02p