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Water supply
Chemical and Biological Characteristics of Water
Ghada Kassab, PhD
Civil Engineering Department
Faculty of Engineering and Technology
Water quality
Drinking water is expected to be
clear, colorless, odorless, and free
of harmful chemicals and
pathogenic microorganisms.
Quality is described with four categories;
• Physical characteristics
• Inorganic chemical characteristics
• Organic chemical characteristics
• Biological characteristics
Inorganic and organic chemical characteristics
General
classification
Specific constituents
Inorganic chemical
characteristic:
PH
Alkalinity
Typical concentration range
pH of natural water 5-8.5
Alkalinity in (mg CaCO3 /L)
Surface water: 20-200
Groundwater 50-1000
Major inorganic
constituents
Calcium, chloride, fluoride, iron,
manganese, nitrate, sodium, sulfur
1- 1000 mg/l
Minor inorganic
constituents
Cadmium, chromium, copper, lead,
mercury, nickel, zinc, arsenic
0.1-10 g/l
Naturally occurring
organic compounds
Naturally occurring organic matter
that is measured as total organic
carbon (TOC)
0.1-20 mg/l(SW)
0.1-2 mg/l (GW)
Anthropogenic
organic constituents
Synthetic organic chemicals and
Below 1 g/l up to tens of mg/l
emerging chemicals of concern in
industry, households, and agriculture
Chemical characteristics
pH
pH is a numeric scale that specify the acidic, neutral or
basic nature.
𝑝𝐻 = − log 𝐻+
The pH of natural water range from 5-8.5
Acidity
Acidity of water is a quantitative representation of the;
• Amount of acid present in water.
• Ability to neutralize strong bases.
Sources:
1. Dissolution of CO2
Carbon dioxide is a common constituent of all natural waters. It can access
surface water by means of:
i. Absorption from the environment
CO2 makes up 0.0355% of the atmosphere. According to Henry’s law
CO2 dissolved in water is a function of partial pressure of CO2 in the
atmosphere
𝑐𝐶𝑂2 = 𝑃𝐶𝑂2 /𝐻
𝐶𝐶𝑂2 = (0.000355 𝑎𝑡𝑚)/1.42 × 103
i.
It is one of the end products during the biological decomposition of
organic matter available in waters- especially in wastewater.
iii. Some times groundwater contains 30-50 mg/L CO2,
2. Acidity due to mineral acids (inorganic acids)
When the effluent of some industries involved in metal processes is
discharges to water streams, mineral acidity can be imparted.
For example
HCL
HNO3
H2SO4, , etc.
3. Heavy metals salts, salts of some metals like
Fe and Al
𝐴𝑙𝐶𝑙3 + 3𝐻2 𝑂 → 𝐴𝑙 𝑂𝐻 3 + 3𝐻 + + 3𝐶𝑙 −
Significance:
1. Corrosion
water containing acidity is of great concern because of it corrosive
nature, which can cause the destruction of water mains and related
equipment.
2. Water Treatment
In water treatment, acidity has influence on chemical treatments of
water. Excess CO2 interferes with the water softening process.
Alkalinity
Alkalinity of water is its ability to neutralize an acid due to its CO3,-2 HCO3and OH- content of elements such as calcium, magnesium, sodium, potassium
and ammonia.
Total alkalinity helps to resist changes in pH.
Alkalinity is determined by titrating against a standard acid, the result is
expressed in terms of calcium carbonate, mg CaCO3/L.
Alkalinity of terms of molar quantities equals:
Where
refers to concentration in moles/L
Alkalinity in terms of equivalents equals
where
refers to concentrations in terms of equivalents/L
Equivalent is a measure of reactive capacity of a given chemical species. For
example 1 M H2SO4 is 2 N for acid base reactions and 1 N for sulfate precipitation.
For alkalinity determination
Ion
Concentration
(mg/l)
MW
(mg/mmol)
n
Eq. Wt
(mg/ mEq)
Concentration
(mEq/l)
Concentration
(mg CaCO3/l)
CO3-2
100
60
2
30
3.33
166.66
(3.33*50)
HCO3-
110
61
1
61
1.8
90.2
H+
1
1
1
OH-
1.7
1
1.7
Where n is the number of equivalents per mole. Which in most cases will be the
oxidation state.
To convert from mEq/l to mg/l as CaCO3, we use the conversion factor:
For most natural waters with pH from 5-8.5, the HCO3- content represent
the major portion of alkalinity.
Inorganic constituents
Major dissolved constituents found in water
constituent
source
Problem in water supply
Range in natural water
Calcium and
magnesium
Surface and
groundwater
Above 60 mg/l can be
considered nuisance as
hardness
Ca+ 1-500 mg/l
Mg 10-20 mg/l (SW)
30-40 mg/l (GW)
Chloride
Surface and
ground water;
saltwater
intrusion
Above 250mg/l can impart Typical surface water
salty taste. Below 50mg/l is usually less than
can be corrosive to some 10mg/l
metals.
Fluoride
Surface water
and
groundwater
Toxic to humans at
concentrations of 250-450
mg; fatal at concentrations
above 4 g/l
For surface water with
TDS concentration
less than 1000mg/l,
fluoride is usually less
than 1 mg/l
Major dissolved constituents found in water
constituent
source
Problem in water supply
Range in natural water
Iron and
manganese
Surface water
and
groundwater
Taste threshold of iron for
many consumers is around
0.01mg/l. iron can impart a
brownish color to laundry
and bathroom fixtures.
In oxygenated surface
water, the concentration
of total iron is usually
less than 0.5mg/l
in groundwater that
has low bicarbonate
and dissolved oxygen,
iron concentrations can
range from 1-10 .mg/l.
Manganese ion can impart
a dark brown color. At
concentrations around 0.4
mg/l, manganese can
impart an unpleasant taste
to the water and can stain
laundry and fixtures.
The concentration of
manganese ion in
surface water and
groundwater may be
less than 1mg/l
Major dissolved constituents found in water
constituent source
Problem in water supply
nitrate
Surface and
groundwater can contain
high concentrations for
nitrate from runoff from
fertilizers found in urban
agricultural watersheds
Very high nitrate
concentrations may
produce blue baby
syndrome
Sulfur
Surface and
groundwater
Groundwater low in
dissolved oxygen can
contain reduced sulfur
compounds which impart
objectionable odors such
as that of rotten eggs.
Sulfates are also
corrosive in concrete
structures and pipes
Range in
natural water
Sulfate
concentrations
in fresh water
can approach
10mg/l.
Biological characteristics
Potable water must be free from pathogenic
microorganisms, which are disease causing organisms.
That include
•Bacteria (basic plant unit, single cell organisms that utilize soluble food)
•Viruses
•Protozoan parasites (single cell animal and it is scavengers for
excess bacteria)
•Other organisms
Biological characteristics
Because there are many different water based pathogens, monitoring
and detecting all of them would require a prohibitive amount of
resources. Consequently, indicator organism have been identified
and are used to monitor the microbial quality.
Pathogens regulated in drinking water include:
1. For protozoa
• Cryptosporidium: single cell protozoan parasite commonly found
in lakes and rivers. Cause gastrointestinal illness (diarrhea,
vomiting and cramps).
•
Giaridia Lamblia: single cell protozoan parasite, lives in
intestines of infected human and animals.
2. For Bacteria
• Escherichia coli (e-coli) of the coliform group.
E-coli was selected as indicator for the following reasons:
1. E. Coli usually inhabits the intestinal tracts of humans and other mammals. Thus
the presence of e-coli is an indication of fecal contamination of the water.
2. Even in acutely ill individuals, the number of E.coli excreted in the feces
outnumbers the disease producing organisms by several orders of magnitude.
The large number make them easier to culture
3. The coliform group of organisms survives in natural water but does not
reproduce effectively in this environment. Thus, the presence of coliforms in
water implies fecal contamination rather than growth of the organisms. These
organisms also survive in water better than most of the bacterial pathogens. This
means that the absence of coliforms is a reasonably safe indication that
pathogens are not present.
4. The coliform group of organisms is relatively easy to culture.
E-coli cause diarrhea, cramps, nausea and headache.
• legionella: bacteria found naturally in the environment, usually in water.
Grow best in warm wter, hot tubes, plumbing systems, cooling towers or
conditioning systems. If transferred in form of aerosols and inhaled it can
cause pneumonia.
3. For viruses:
Enteroviruses which are group of viruses that live in intestines of
infected human or animal.
Water quality standards
•Designated uses
• Water quality criteria
• Antidegradation policy
• General policies