Water quality

1. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
W A T E R Q U A L I T Y
Color
 Pure water should not possess any color.
 Most water available to us is colored to some extent due to the presence of various impurities (i.e.,
iron and manganese in association with organic matter from decaying vegetation).
 Impurities may be in the colloidal form in water or it may be in suspended state.
 Color caused by dissolved and colloidal form of impurities is called true color and that caused by
suspended matter, in addition to dissolved and colloidal matters, is called apparent color.
 Ground water may show color due to the presence of iron compound.
 Color intensity generally increases with an increase in pH. For this reason recording pH along with
color is advised.
Unit
 Color is usually expressed in platinum-cobalt units (Pt-Co Units) which is based on the intensity of
color produced by a solution of Platinum and Cobalt salts that approximate the yellow-brown color of
natural waters.
 For easier comparison color discs are prepared to represent various units of color. The sample is
compared with the color discs and the color is determined.
Limit
According to Bangladesh Environment Preservation Act (1997), drinking water standard for color is
15 units.
Environmental Significance
 Colored water is not always harmful to man, but in most cases it is.
 Even if the water is not harmful, it is not preferred by people for aesthetic reasons.
 Also, disinfection by chlorination of waters containing natural organics (which produces color)
results in the formation of chloroform, other trihalomethanes, and a range of other chlorinated
organics, leading to problems which are a major concern in water treatment.
 So it is important to limit the color of water for domestic supplies.
pH
 pH is a measure of the acid or alkaline condition of water.
 It is a way of expressing the hydrogen ion concentration, or more preciously, the hydrogen ion
activity.
 pH is defined as follows:
pH = -log [H+
] (1)
Where, [H+
] is the concentration (or activity) of hydrogen ion (or proton) in moles per liter (M).

2. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
Water dissociates to form hydrogen ion (H+
) and hydroxyl ion (OH-
) according to the following
equation:
H2O=H+
+ OH-
(2)
At equilibrium, we can write,
= [H+
][OH-
]/ [H20] (3)
But, since concentration of water is extremely large (approximately 55.5 mol/L) and is diminished very
little by the slight degree of ionization, it may be considered as a constant and its activity is taken as 1.0.
Thus, Eq. 3 may be written as:
Kw = [H+
][OH-
] (4)
Where Kw = Equilibrium Constant
 For pure water at 25 C, 1Kw = 1477
101010 
 . This is known as the ion product of water or
ionization constant for water. In other words, water (dc-ionized or distilled water) at 25 0C
dissociates to yield 10-7
mol/L of hydrogen ion (H+
) and 10-7
mol/L of hydroxyl ion (OH-
). Hence,
according to Eq. 1, pH of deionized water is equal to 7.0.
 The pH is usually represented by a scale ranging from zero to 14 with 7 being neutral.
 Most natural waters are slightly alkaline due to the presence of bicarbonate and less often carbonate.
 Water with pH outside the desirable neutral range may exhibit sour tastes and accelerate the corrosion
of metallic plumbing fittings and hot water services.
 Aeration removes carbon dioxide and hence causes a rise in pH value.
Limit
According to WHO & Bangladesh Environment Preservation Act (1997), drinking water standard for
pH is 6.5 - 8.5.
Environmental Significance
 A controlled value of pH is desired in water supplies, sewage treatment and chemical process plants.
 In water supply pH is important for coagulation, disinfection, water softening and corrosion control.
 In biological treatment of waste pH is the most significant. Organisms involved in treatment plants
are operative within certain pH range.
Turbidity
 Turbidity is defined by the International Standards Organization (ISO) as the reduction of
transparency of a liquid caused by the presence of undissolved matter.
 Turbidity can be interpreted as a measure of the relative clarity of water and often indicates the
presence of dispersed, suspended solids; particles not in true solution such as silt, clay, algae and
other microorganisms; organic matter and other minute particles.
 Turbidity is the phenomena where by a specific portion of a light beam passing through a liquid
medium is deflected from undissolved particles.

3. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
 The diffusion of light caused by undissolved particles in the medium to a lesser or greater degree of
the deflection depends on:
 the type of the particles (absorbance)
 the size of particles
 the concentration (the number of particles)
 the type and shape of particles
 the wavelength of the light
 the angle of measurement
Unit
 Nephelometric Turbidity Unit
The propensity of particles to scatter a light beam focused on them is now considered a more
meaningful measure of turbidity in water. Turbidity measured this way uses an instrument called
a nephelometer with the detector set up to the side of the light beam. More light reaches the detector if
there are lots of small particles scattering the source beam than if there are few. The units of turbidity
from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). To some extent, how
much light reflects for a given amount of particulates is dependent upon properties of the particles like
their shape, color, and reflectivity. For this reason (and the reason that heavier particles settle quickly
and do not contribute to a turbidity reading), a correlation between turbidity and total suspended
solids (TSS) is somewhat unique for each location or situation.
 Jackson Turbidity Unit:
There are several practical ways of checking water quality, the most direct being some measure
of attenuation (that is, reduction in strength) of light as it passes through a sample column of water. The
alternatively used Jackson Candle method (units: Jackson Turbidity Unit or JTU) is essentially the
inverse measure of the length of a column of water needed to completely obscure a candle flame viewed
through it. The more water needed (the longer the water column), the clearer the water. Of course water
alone produces some attenuation, and any substances dissolved in the water that produce color can
attenuate some wavelengths. Modern instruments do not use candles, but this approach of attenuation of
a light beam through a column of water should be calibrated and reported in JTUs.
Environmental Significance
 Excessive turbidity, or cloudiness, in potable water is aesthetically unappealing, and may also
represent a health concern.
 Turbidity can provide food and shelter for pathogens. If not removed, turbidity can promote re
growth of pathogens in water distribution systems, leading to waterborne disease out-breaks, which
have caused significant cases of gastroenteritis throughout the world.
 Suspended solids (the particles of turbidity) provide shelter" for microbes by reducing their exposure
to disinfectants.
 Further, waters with high turbidity from organic sources may give rise to a substantial chlorine
demand.
 This could result in reductions in the free chlorine residual in distribution systems as protection
against possible recontamination.

4. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
Alkalinity
 Alkalinity is a measure of the acid-neutralizing capacity of water.
 It is an aggregate measure of the sum of all titratable bases in the sample.
 Alkalinity in most natural waters is due to the presence of carbonate (CO3
2-
), bicarbonate (HCO3),
and hydroxyl (OH-
) anions.
 However, borates, phosphates, silicates, and other bases also contribute to alkalinity if present.
 This property is important when determining the suitability of water for irrigation and/or mixing
some pesticides and when interpreting and controlling wastewater treatment processes.
 Alkalinity is usually reported as equivalents of calcium carbonate (CaCO3).
Hardness
 Hardness is most commonly associated with the ability of water to precipitate soap.
 As hardness increases, more soap is needed to achieve the same level of cleaning due to the
interactions of the hardness ions with the soap.
 Chemically, hardness is often defined as the sum of polyvalent cation concentrations dissolved in the
water. The most common polyvalent cations in fresh water are calcium (Ca++
) and magnesium
(Mg++
).
 Hardness is usually divided into two categories: carbonate hardness and noncarbonate hardness.
 Carbonate hardness is usually due to the presence of bicarbonate [Ca(HCO3)2 and Mg(HCO3)2]
and carbonate (CaCO3 and MgCO3) salts.
 Non-carbonate hardness is contributed by salts such as calcium chloride (CaCl2), magnesium
sulfate (MgSO4), and magnesium chloride (MgCl2).
 Total hardness equals the sum of carbonate and non-carbonate hardness. In addition to Ca++
and
Mg++
, iron (Fe++
), strontium (Sr++
), and manganese (Mn++
) may also contribute to hardness (APHA
et al. 1998).
Table. Principal cations causing hardness and the major anions associated with them
Cations causing
Hardness
Anion
Ca2+
HCO3
-
Mg2+
SO4
2-
Sr2+
Cl-
Fe2+
NO3
-
Mn2+
SiO3
2-
 However, the contribution of these ions is usually negligible.
 Hardness is usually reported as equivalents of calcium carbonate (CaCO3) and is generally classified
as soft, moderately hard, hard, and very hard.

5. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
 Table: Water hardness classifications (reported as CaCO3 equivalents) used by the U.S. EPA
(EPA 1986).
 It is best to report results as the actual equivalents of CaCO3 since the inclusive limits for each
category may differ between users of the information.
 Pseudo-Hardness
Sea, brackish, and other waters that contain appreciable amounts of Na+
interfere with the normal
behavior of soap because of the common ion effect. Sodium is not a hardness-causing cation, and so
this action which it exhibits when present in water in high concentration is termed pseudo-hardness.
Significance of Hardness
Hardness can also affect the utility of water for industrial purposes. Hard water is often the source of
scale formed in hot water heaters and industrial systems where water is heated. This scale results from
the precipitation of calcium carbonate, which becomes less water soluble as the temperature increases
(Snoeyink and Jenkins 1980). In these situations, water is usually softened by precipitating the CaCO3 or
by using ion exchange softening methods.
Sources of Alkalinity and Hardness
 Water alkalinity and hardness are primarily a function of
1) The geology of the area where the surface water is located; and
2) The dissolution of carbon dioxide (CO2) from the atmosphere.
 The ions responsible for alkalinity and hardness originate from the dissolution of geological minerals
into rain and ground water.
 Rainwater is naturally acidic, which tends to solubilize some minerals more easily.
 Surface and ground water sources in areas with limestone formations are especially likely to have
high hardness and alkalinity due to the dissolution of bicarbonates and carbonates.
Chemical Action:
 The interaction of CO2 with the dissolved minerals is described by the carbonate system.
 The carbonate system describes a series of chemical equilibrium with CO2(g) in the atmosphere and
various bicarbonates and carbonates dissolved from surrounding mineral deposits. CO2(g) readily
dissolves in water (Eq. 1).
 The dissolved CO2(aq)reacts with water molecules to form carbonic acid (H2CO3*), which is very
unstable and quickly dissociates, yielding H+
and a bicarbonate ion (HCO3
-
) (Eq. 2 and 3).
Soft <75
Moderately hard 75–150
Hard 150–300
Very hard >300

6. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
 At pH = 6.3, the amount of CO2 dissolved in water equals the amount of bicarbonate ion (HCO3
-
).
 Dissolved CO2 is dominant when pH < 6.3.
 At higher pH, the HCO3
-
dissociates to yield H+
and a carbonate ion (CO3
2-
) (Eq. 4).
CO2(g)↔CO2(aq) Eq. 1
CO2(aq) + H2O↔H2CO3* Eq. 2
H2CO3*↔H+
+ HCO3
-
pKa= 6.3 Eq. 3
HCO3
-
↔H+
+ CO3
=
pKa= 10.3 Eq. 4
 At pH 10.3, the bicarbonate ion concentration equals the carbonate ion concentration.
 CO3
2-
is dominant at pH > 10.3, and HCO3
-
dominates between pH 6.3 and 10.3.
 The pH of most natural waters falls in the 6 to 9 range because of the bicarbonate buffering.
Alkalinity and Hardness Relationship
 Alkalinity and hardness are related through common ions formed in aquatic systems.
 Specifically, the counter-ions associated with the bicarbonate and carbonate fraction of alkalinity are
the principal ions responsible for hardness (usually Ca++
and Mg++
) (Eq. 3 and 4).
 As a result, the carbonate fraction of hardness (expressed as CaCO3 equivalents) is chemically
equivalent to the bicarbonates of alkalinity present in water (Burton Jr. and Pitt 2002) in areas where
the water interacts with limestone (Timmons et al. 2002).
 Any hardness greater than the alkalinity represents non-carbonate hardness.

7. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
Calculating Calcium Hardness as CaCO3
The hardness (in mg/L as CaCO3) for any given metallic ion is calculated using following Equation:
Problem1.
A water sample has calcium content of 51 mg/L. What is this calcium hardness expressed as CaCO3?
Solution
Problem 2.
The calcium content of a water sample is 26 mg/L. What is this calcium hardness expressed as CaCO3?
Solution
CALCULATING MAGNESIUM HARDNESS AS CaCO3
Problem 3.
A sample of water contains 24 mg/L magnesium. Express this magnesium hardness as CaCO3.
Solution

8. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
CALCULATING TOTAL HARDNESS
Calcium and magnesium ions are the primary cause of hardness in water. To find total hardness, we
simply add the concentrations of calcium and magnesium ions, expressed in terms of calcium carbonate
(CaCO3)
Problem 4.
A sample of water has calcium content of 70 mg/L as CaCO3 and magnesium content of 90 mg/L as
CaCO3.
Solution
Total hardness (mg/L) = 70 mg/L + 90 mg/L
= 160 mg/L as CaCO3
Problem 5.
Determine the total hardness as CaCO3 of a sample of water that has calcium content of 28 mg/L and
magnesium content of 9 mg/L.
Solution
Express calcium and magnesium in terms of CaCO3:

9. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University.//Water Quality.
CALCULATING CARBONATE AND NONCARBONATE HARDNESS
As mentioned, total hardness is comprised of calcium and magnesium hardness.
Once total hardness has been calculated, it is sometimes used to determine another expression
hardness — carbonate and non-carbonate.
When hardness is numerically greater than the sum of bicarbonate and carbonate alkalinity, that
amount of hardness equivalent to the total alkalinity (both in units of mg CaCO3/L) is referred to as
the carbonate hardness; the amount of hardness in excess of this is the noncarbonated hardness.
When the hardness is numerically equal to or less than the sum of carbonate and non-carbonate
alkalinity, all hardness is carbonate hardness, and non-carbonate hardness is absent.
Again, the total hardness is comprised of carbonate hardness and non-carbonate hardness:
Total hardness = carbonate hardness + non-carbonate hardness
When the alkalinity (as CaCO3) is greater than the total hardness, all the hardness is carbonate
hardness:
Total hardness (mg/L) as CaCO3 = carbonate hardness (mg/L) as CaCO3
When the alkalinity (as CaCO3) is less than the total hardness, then the alkalinity represents
carbonate hardness and the balance of the hardness is non-carbonate hardness:
When carbonate hardness is represented by the alkalinity: