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Water :Resource and Characterisic

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Water resources• vary in regional/local patterns of availability• dependent on topographic & meteorological

conditions• dependent on the physical features of the

earth and on the earth’s geological structure

3 types of water resources:

1. Surface water

2. Ground water

3. Sea water

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Water quality characteristics

• Physical

• Chemical

• Biological

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Physical characteristics

• Temperature

• Turbidity

• Suspended solid

• Taste and odor

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Chemical characteristics

• pH

• Alkalinity

• Hardness

• Conductivity

• Dissolved organic, inorganic materials & gas

SCALING

CORROSION

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pH

• Indicates the hydrogen ion concentration in water

• pH > 7 scaling

• pH < 7 corrosion

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Alkalinity

• A measure of the water’s ability to absorb hydrogen ions without significant pH change

• Water alkalinity is determined by carbonate ions and hydroxyl ions (OH-)

• The carbonate system is comprised of the following chemical species:– Aqueous carbon dioxide CO2 (aq)

– Carbonic acid H2CO3

– Bicarbonate ion HCO3-

– Carbonate ion CO32-

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A series of alkalinity reaction

CO2(aq) + H2O H2CO3 H+ + HCO3-

HCO3- H+ + CO3

2-

CO32- + H2O HCO3

- + OH-

If there is a source of solid CaCO3, then the solubility reaction for CaCO3(s) applies in the system:

CaCO3(s) Ca2+ + CO32-

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• From the equations, it can be seen that CO2 and three forms of alkalinity are all part of one system that exist in equlibrium

• A change of concentration of one ions will result in a change in pH

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CO2 pH 4.5 – 8.5 HCO3

- pH 4.5 – 9.5 CO3

2- pH 8.5 – 14 OH - pH 9.0 -- 14

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Alkalinity (mol/L) = [HCO3-] + 2[CO3

2-] + [OH-] – [H+]

where concentrations in the brackets are [mol/L]

If the concentrations are measured in terms of equivalents, or in mg/L as CaCO3:

Alkalinity (meq/L) = (HCO3-) + (CO3

2-) + (OH-) – (H+)

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Analysis

• Using sulfuric acid (H2SO4)

• Alkalinity P (end point pH : 8.3)

CO3- HCO3

-

• Alkalinity M (total alkalinity, end point pH : 4.5)

HCO3- H2CO3

If P alk = 0 all of the alkalinity is bicarbonate

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Hardness

• The presence of multivalent cations, most notably Ca and Mg ions

• 2 problems:– The reaction between hardness and soap

produces a sticky, gummy deposit (soap curd)– Ca2+ and Mg2+ precipitate while heated

reduces efficiency of HE

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HardnessThere are 2 types of hardness based on the anions1. Carbonate hardness (CH) temporary

hardness. It can be removed by simply heating the water.

associated with HCO3- & CO3

2-

Ca2+ + 2HCO3- CaCO3 + CO2 + H2O

bicarbonate Ca bicarbonate

2. Noncarbonate hardness (NCH)

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Total hardness (TH) = Ca2+ + Mg2+

NCH = TH – CH

CH = HCO3- + CO32-

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Hardness commonly expresses as mg/L

CaCO3

Equivalent weight =

mg/L of X as CaCO3 =

n

weightmolecularoratom

meqmgXofweightequivalent

meqCaCOmgLmgXofionconcentrat

/

/0.50/ 3

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Hardness classification

Descriptionhardness

meq/l mg/L as CaCO3

Soft < 1 < 50

Moderately hard 1 – 3 50 – 150

Hard 3 – 6 150 – 300

Very hard > 6 > 300

* Masters, G.M. 1998. Introduction to environmental engineering and science. p281

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Conductivity

Electrical conductivity: a measure of the ionic activity of a solution in terms of its capacity to transmit current

Can be used to predict total dissolved solid

Units: micromhos/cm

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Relationship between TDS & conductivity varies according to

solution concentration

Conductivity (mhos/cm)

TD

S (

mg

/L)

Dilute solution Concentrated solution

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Water Saturation Index

Saturation Index is a means of evaluating water quality data to determine if the water has a tendency to be corrosive or to form a chemicals scale

The evaluation is based on the pH, alkalinity, hardness, total dissolved solid or conductivity

There are many type of saturation index, the most commonly used is :Langelier saturation indexRyznar saturation index

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Langelier saturation index

• The saturation index (SI) can be calculated as follows :

• Whereas pHs = theoritical pH or pH at saturation can be determined from graph or by calculation

SI = pH – pHs

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SI values and recommended treatment

www.water-research.net/corrosion.htm

SATURATION INDEX DESCRIPTION RECOMMENDATION

-5 Severe corrosion Treatment recommended

-4 Severe corrosion Treatment recommended

-3 Moderate corrosion Treatment recommended

-2 Moderate corrosion Treatment may be needed

-1 Mild corrosion Treatment may be needed

-0.5 None-mild corrosion Probably no treatment

0 Near balanced no treatment

0.5 Some faint coating Probably no treatment

1 Mild scale coating Treatment may be needed

2 Mild to moderate coating Treatment may be needed

3 Moderate scale forming Treatment advisable

4 Severe scale forming Treatment advisable

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Ryznar saturation index

• The Ryznar Stability index is an empirical method for predicting scaling tendencies of water based on a study of operating results with water of various saturation indices

• This index is often used in combination with the Langelier index to improve the accuracy in predicting the scaling or corrosion tendencies of a water.

Stability index = 2pHs - pH = pHs - Langelier's Saturation pH

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Biological characteristics

• Microbiological contaminants

• Microorganisms:– Non-pathogenic– Pathogenic

• Coliform bacteria (typically e.coli) are used as indicator organisms

• The presence of coliform suggests that the water is contaminated

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Summary

Ground water Characteristic Surface water

Low turbidity High

High mineral Vary

Low dissolved O2 Vary

Low microorganism High

Stable temp & chem Unstable

Low pollutant High