6lecture;waste polution control and water quality modeling

8/17/2019 6Lecture;Waste Polution Control and Water Quality Modeling

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CE 4312- WATER AND

WASTEWATER ENGINEERING

LECTURELECTURELECTURELECTURE 6666---- Water Pollution Control and WaterWater Pollution Control and WaterWater Pollution Control and WaterWater Pollution Control and Water

Quality Modeling Quality Modeling Quality Modeling Quality Modeling

Ms. Nadeeka Miguntanna, Senior Lecturer,Ms. Nadeeka Miguntanna, Senior Lecturer,Ms. Nadeeka Miguntanna, Senior Lecturer,Ms. Nadeeka Miguntanna, Senior Lecturer,

Faculty of Engineering, University ofFaculty of Engineering, University ofFaculty of Engineering, University ofFaculty of Engineering, University of RuhunaRuhunaRuhunaRuhuna

[email protected]@[email protected]@gmail.com

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WATER PURIFICATION PROCESSES IN NATURALSYSTEMS

These systems include physical, chemical and biologicalprocesses.

The speed and completeness with which these processesoccur depend on system-specific variables.

Hydraulic characteristics - volume, rate, and turbulenceof flow,

physical characteristics - bottom and bankmaterial, variations in sunlight and temperature,

chemical nature of natural waters

In natural waters these system variables are set by nature

and can seldom be altered.

PhysicalPhysicalPhysicalPhysical Processes

DilutionDilutionDilutionDilution

Although dilution is a powerful solution to the self-cleaning

mechanisms of surface waters, its success depends upondischarging relatively small quantities of waste into large bodies of

water.

Dilution capacity of a stream can be calculated using the principalsof mass balance.

If the volumetric flow rate and the concentrations of a given materialare known in both the stream and the waste discharge, the

concentration after mixing can be calculated as,

C - the concentrations(mass/volume) of the selected material,

Q - the volumetricflow rate (volume/time)and subscripts

s, w and m - stream, wasteand mixture conditions

CCCCssssQQQQssss ++++ CCCCwwwwQQQQwwww ====QQQQmmmmCCCCmmmm

Sedimentation and Re-suspension

Sedimentation is nature’s method of removing suspended

particles from a watercourse.

The impacts are;

Anaerobic conditions are likely to be developed at the

sediment deposits and any organics trapped in them will

decompose.

Sediment deposits may fill up the pore space at the

streambed and creating unsuitable conditions for the

reproduction of many aquatic systems.

May cause dif ficulty for navigation. May reduce the reservoir storage capacity and silt harbors.

May increase flooding due to channel filling.


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Small bits of organic matter or inorganic clays and othersediments may be filtered out by pebbles or rocks along thestreambed.

As the water percolates from the surface downward intoground water aquifers, filtration occurs.

Gas transferGas transferGas transferGas transfer The transfer of gasses into and out of water is an important

part of the natural purification system.

The replenishment of oxygen lost to bacterial degradation ororganic waste is accomplished by the transfer of oxygen fromthe air into the water.

Conversely gasses evolved in the water by chemical andbiological processes may be transferred from the water to

the atmosphere.

FiltrationFiltrationFiltrationFiltration Chemical Processes

Natural waters contain many dissolved minerals and gasses

that interact chemically with one another in complex and varied

ways.

Oxidation-reduction, dissolution-precipitation, and other

chemical conversions may aid or obstruct natural processes of

natural water systems.

Note: Oxidation Reduction (Redox) reactions

Oxidation is loss of electrons and Reduction is gain of electrons.

Metal ions will replace ions of a less easily oxidized metal from solution.

Cd2+ + Fe → Fe 2 + + Cd

Redox reactions can be expressed as a sum of two half reactions

Cd 2 + + 2 e -→ Cd and Fe → Fe 2 + + 2 e -

Many of the chemical reactions involved in the self-purification processmustbe biologicallymediated.

These chemical reactions are not spontaneous but require an externalsource of energy for initiation.

In the case of biodegradable organics and other nutrients this activationenergy can be supplied by microorganisms that utilize these materials forfood and energy.

The total sum of the processes by which organisms assimilate and use foodfor subsistencegrowth and reproductionis called metabolism.

The metabolic processes and the organisms involved are a part of the self-purification process of natural water systems.

The biochemical reactions involved in metabolism are extremelycomplicated and are not yet completely understood. It is known howeverthat two types of processes each involving many steps must occursimultaneously.

Catabolism- provides the energy for the synthesis of new cells as well as for themaintenance of other cell functions.

Anabolism-provides the materials necessary for cell growth.

BioBioBioBio----chemical Processeschemical Processeschemical Processeschemical Processes

Advection, Diffusion and DispersionAdvection, Diffusion and DispersionAdvection, Diffusion and DispersionAdvection, Diffusion and Dispersion

AdvectionAdvectionAdvectionAdvection

Advection results from flow that is unidirectional and does not

change the identity of the substance being transported.

Advection moves matter from one position in space to another.

Simple examples of transport primarily of this type are the flow

of water through a lake’s outlet and downstream transport due

to flow in a river or estuary

WATER QUALITY MODELINGWATER QUALITY MODELINGWATER QUALITY MODELINGWATER QUALITY MODELING


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DiffusionDiffusionDiffusionDiffusion

Refers to the movement of mass due to random water motion

or mixing. Such transport causes the dye patch to spread out

and dilute over time with negligible net movement of its center

of mass.

On a microscopic scale molecularmolecularmolecularmolecular diffusiondiffusiondiffusiondiffusion results from the

random Brownian motion of water molecules. A similar kind of

random motion occurs on a larger scale due to eddies and is

called turbulentturbulentturbulentturbulent diffusiondiffusiondiffusiondiffusion. Both have a tendency to minimize

the gradients. That is the differences in concentration by

moving mass from regions of high to low concentrations.



DispersionDispersionDispersionDispersion

This process also causes pollutant to spread.

However in contrast to random water motion in time, dispersion

is the result of velocity differences in space.

For example, suppose the dye were introduced into water

flowing through a pipe.

In such cases, a molecules of dye near the wall of the pipe

would move more slowly than a molecule near the center due

to a velocity gradient or shear.

The net effect of these differences in mean velocity is to spread

or mix the dye along the pipe’s axis.


6lecture;waste polution control and water quality modeling

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