water quality modeling d nagesh kumar, iisc water resources planning and management: m8l4 water...

Water Quality Modeling

D Nagesh Kumar, IIScWater Resources Planning and Management: M8L4

Water Resources System Modeling

Objectives


To discuss about water quality parameters and standards

To discuss about river water quality modeling, its

components and governing equations

To model dissolved oxygen in rivers

Introduction


Water quality maintaining and monitoring has become an important part of

sustainable development

Quality problems in water arise mainly due to its molecular structure itself which

makes it a universal dissolvent

Water quality is expressed as a function of space and time

Quality of water body shows rapid variations

Variations are more in a river, less in lakes and much less in aquifers.

Water Quality Monitoring


Monitoring indicates the long term, standardized measurement and observation of

the environment

Location of sampling station depends on the purpose of study

Sampling stations can be divided into

i. Basic stations which are located usually at the mouth of main streams, major tributaries,

downstream of river development projects, at hydrometric stations, gauge discharge sites,

industrial and urban centers and at points of water use

ii. Auxiliary stations which are meant to analyse the effect of pollutants discharged into a stream,

determination of assimilation capacity of stream etc

Stream quality is assessed by interpolating the data collected at these stations.

Samples for water quality should be taken at places where composition is uniform

across the cross-section.

Water Quality Monitoring…


Sampling points in rivers should be away from any disturbing influences

Sampling frequency depends on the purpose and relative importance of the station,

variability of data and accessibility of station

Basic stations usually collect at a sampling frequency of 3 - 4 months per year

Atleast one sample should be taken in one season

Stations located downstream of a waste outlet, should take samples weekly or bi-

weekly

If the purpose is recreational, sampling can be restricted to the season of use.

Water Quality Parameters


Many organic constituents may present in water

Measurement of all of them may be practically difficult

Organic pollution is indicated by the non-parametric tests

Carbon Oxygen Demand (COD) or

Total Organic Carbon (TOC).

Parameters to be measured are the following:

Total organic carbon, biochemical oxygen demand (BOD), cyanide, pesticides, suspended

solids, nitrogen, fluoride, cadmium, chromium, copper, lead, nickel, zinc, mercury, boron,

dissolved oxygen (DO), pH value, and coliform bacteria.

Additional information such as total hardness, alkalinity, calcium hardness, sulphate,

phosphate, sodium, potassium etc may be also needed if the objective is to evaluate the

influence of pollution control measures on the cost of water treatment.

Water Quality Standards


Usage of water depends on its quality Permissible limits depend on the intended use For example, water from a particular source may be good for irrigation, but may

not be advisable for drinking Guidelines for drinking water quality were issued by WHO in 1996 (

www.who.int/water_sanitaion_health/Water_quality/drinkwat.htm)

Organisms Guideline

All water intended for drinking

E. coli or thermotolerant coliform bacteria Must not be detectable in any 100 ml sample

Treated water entering the distribution system


Total coliform bacteria Must not be detectable in any 100 ml sample

Treated water in the distribution system


Total coliform bacteria Must not be detectable in any 100 ml sample

Bacteriological quality of drinking water

River Water Quality Modeling


Water quality can be divided into five categories based on the colour of discharge:

Two components for river water quality modeling:

Forecasting the developments in the basin and subsequent effects in the water quality

Forecasting of pollution concentration changes within the stream which includes

prediction of solute concentration at various points at various times taking solute

concentration at specified point as input and finding the dominant processes controlling

the solute concentration.

Category Colour Amount of pollution

I Blue No/ Slight

II Green Moderate

III Yellow Heavy

IV Red Excessive

V Black Zone of devastation

River Water Quality Modeling…


Forecasting of concentration depends on the stability of contaminants

Contaminants are stable if the concentration changes only through dilution and

evaporation

However, precipitation, sedimentation, adsorption etc also act as forcing factors to

change the concentrations of many organic and inorganic substances

Such processes are influenced by factors such as pH, temperature, bed

characteristics etc, which need to be treated separately

Pollutants may enter the river through a

Point source: If the source is a well-defined outlet such as industrial outlet or municipal

sewers

Non-point source: If the source is distributed along the water course such as runoff from

land

River Water Quality Modeling…


Source of pollutants can also be classified as continuous and instantaneous source

Continuous sources: Dump pollutants over a long period of time (eg. municipal waste

water plant)

Instantaneous sources: Dump pollutants for a very short interval of time (eg. Spill from a

tanker).

Pollutograph: A plot of pollute concentration with respect to time

Loadograph: Graph of the product of pollutant concentration and flow rate/time

Components of a River Water Quality Model


Governing dynamic equations of a river water quality model consider various hydrologic, thermal and biochemical processes that take place within the system These equations are basically conservation of mass, momentum and energy.

Biochemical and chemical processes in a river are influenced by hydraulic and

thermal conditions Main influencing hydraulic variables are flow velocity, depth and discharge An increase in flow velocity decreases the self-purification It also results in increased turbulence

Aids in proper mixing of oxygen Increases reaeration.

Greater depths block penetration of sunlight – Slows down the photosynthetic

process Pollutant concentration is inversely proportional to discharge since an increased

discharge increases the dilution rate.

Components of a River Water Quality Model…


River water quality model can be divided into three components

Hydraulic

Thermal and

Biochemical submodels

Hydraulic and thermal models:

Considering one-dimensional unsteady flow, the influencing variables are flow

depth and cross-section of flow

Influencing equations are conservation of mass and momentum.

Thermal model has only one variable i.e., temperature. This submodel can be

skipped by directly inputting the variable to the biochemical model.



Biochemical model:

Real life interactions may lead to a large number of variables

Make the model complex

One may reduce the number of variables by substitution or grouping of similar

variables

Pertinent indicator of water quality: Dissolved oxygen (DO)

BOD: Amount of oxygen needed for biochemical oxidation of matter in a unit

volume of water

Most of the biochemical models are simplified versions of the real processes



Geochemical processes

In addition to the physical processes, chemical and biological processes also

influence the solute transport

These geochemical reactions can be either Homogenous: Dissolved species interact with species of same phase (eg. Hydrolysis) or

Heterogenous: Species from different phases are involved (eg. Reaction between

dissolved oxygen and atmospheric oxygen)

Sorption

Process which controls the pollute concentration

Process in which a dissolved species called sorbate becomes associated with a

solid surface called sorbent

Absorption: Sorbate penetrate the sorbent

Adsorption: When sorbate interacts with surface or intersurface of sorbent



Pollutant concentration

Concentration C: Measure of the quantity of a constituent

C = M / V

where M is the mass of the constituent and V is the volume of fluid.

Transport of pollutant in rivers

Transport of a constituent is measured in terms of flux

Flux is the rate of flow of mass or energy per unit area normal to the direction of

flow

Unit: Quantity per unit area per unit time.

Pollutant is transferred through two important mechanisms:

Advection and

Dispersion.



Advection (or Convection): Transfer of a constituent due to the motion of the carrying fluidThe constituent moves with the fluid velocityFlux F due to advection is the product of velocity U and concentration C

F = U CVolume of the constituent remains same, while the location changes

Dispersion: As the constituent moves downstream, the volume increases and the concentration

decreases due to mixing upDispersion can be due to molecular diffusion and velocity variationsIn natural streams, the main forcing factor is velocity variationsFlux due to diffusion is given by Flick’s lawFlux in the x direction is

where Ex is the longitudinal dispersion coefficient or diffusivity.x

CEF xx



Time, t Time, t+1

Advection

Dispersion

Advection + Dispersion

Effects of advection and dispersion

Governing Advective – Diffusion Equation


Basic equation behind the effects of advection and dispersion on solute

concentration is the conservation of mass

Time rate of change of mass = mass inflow – mass outflow One-dimensional form commonly applied in streams

where U is the average longitudinal velocity

A cross-sectional area and

EL is the longitudinal dispersion coefficient.

Advection-dispersion equation assuming constant U and EL

Above equations can be solved analytically

KC

x

CAE

xAx

UC

t

CL

1

2

2

x

CE

x

CU

t

CL

Modeling of Oxygen in Rivers


Dissolved oxygen

Depends on temperature, pressure, dissolved solids turbulence etc.

DO in freshwaters at 1 atmospheric pressure ranges from 15 mg/L at 00C to 6 mg/L at

400C

DO level is used as an indicator of pollution by organic matter

Biochemical oxygen demand (BOD)

Amount of oxygen required for aerobic micro-organisms to oxidize the organic matter to a

stable inorganic form

A measure of the amount of biochemically degradable organic matter present in the water

Concentrations in rivers vary from 2 to 15 mg/L

BOD in unpolluted water is less than 2 mg/L

Raw sewage has a BOD of about 600 mg/L which may come down to 20 – 100 mg/L after

treatment.

Modeling of Oxygen in Rivers…


Chemical oxygen demand (COD)

A measure of the oxygen equivalent of organic matter that is susceptible to oxidation by a

strong oxidizing agent

In natural water, it is the amount of oxygen required for oxidation of a waste to carbon

dioxide and water

COD of surface water ranges from 20 mg/L (for unpolluted waters) to 200 mg/L (for

effluent added waters)



Mass balance of BOD and oxygen deficit in a stream with constant flow and geometry

can be expressed as

where U is the average stream velocity,

L is the carbonaceous BOD concentration,

x is the distance downstream from the discharge point,

kr is the total removal rate,

D is the dissolved oxygen deficit,

ka is the aeration rate and

kd is the removal rate of BOD

LkDkdx

dDU

Lkdx

dLU

da

r



cs is the saturation concentration for DO

c is the oxygen concentration

Then deficit D = cs – c

Above equations can be solved for BOD and DO concentration for a single point

source whose BOD concentration is L0 and oxygen deficit is D0 and can be

expressed as

Water quality models for catchments, lakes , reservoirs and ground water differ

from those of rivers in terms of sources, biological considerations etc.

ra

daras

r

kk

Lkx

U

kx

U

kx

U

kDcc

xU

kLL

00

0

expexpexp

exp


Thank You

water quality modeling d nagesh kumar, iisc water resources planning and management: m8l4 water...

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