ENVIRONMENTAL ENGINEERING
III Year – II Semester
CHAPTER NO. 2
CHARACTERSITICS OF WATER
M Rupas Kumar
Department of Civil Engineering,
RGUKT, RK Valley
1. Physical Characteristics: These can be determined by physical analysis (tests).
a) Turbidity: It is expressed as the amount of Suspended matter present in the water.
This suspended matter includes soil, organic material, microorganisms. These
materials are insoluble and interfere with the passage of light through the given water.
It is expressed as amount of suspended matter (mg) present in water (liters). So the
Units of turbidity are mg/l or ppm.
The standard unit is that which is produced by 1 mg of finely divided silica in 1 liter
of distilled water.
Turbidity value exceeding 5 units in easily detectable, so it is OBJECTIONABLE.
Permissible limit: 5 – 10 units
Desirable: < 5 units
Turbidity in clear lake is around 25 Units and for muddy water, it is around 100
Units.
The turbidity is measured by a turbidity rod or by a turbidimeter.
Turbidity Rod: This is a field test. The turbidity rod consists of graduated aluminium
rod, which gives the turbidity value in silica units (mg/l). A screw containing
platinum needle (1 mm dia & 25 mm long) and nickel ring are inserted at the lower
end of the rod.
In order to measure turbidity, the rod is lowered in water till the needle disappears
due to turbidity of water. The length of the rod under water gives the turbidity value.
The lesser the length, the greater will be the turbidity.
Turbidimeters: These are laboratory tests. These meters works on the principle of
measuring the interference caused by the water sample to the passage of light rays.
Jacksons Candle Turbidimeter, Baylis turbimeter are used from the past.
Jackson’s turbidimeter: Used to measure the turbidities between 25 – 1000 mg/l.
So, it is used measure raw waters of natural sources only.
Baylis Turbidimeter: Normally, used to measure the turbidities in the range of 0-10
mg/l. So, it is used to measure the turbidities of treated water supplies. It can measure
higher turbidities up to 100 mg/l.
Modern commercial Turbidimeters: These are measured in terms of NTU
(Nephelometric turbidity Units) as Naphelometers are mostly used as modern
Turbidimeters.
These are currently being used in large scale, particularly for measuring very low
turbidities of drinking water (up to less than 1 Unit).
The difference between Old and New Turbidimeters:
1. In old instruments, the light was measured after it passed straight through the
turbid water.
2. In modern instruments, the light intensity is measured at right angles to the
Incident light. These instruments use a photometer to measure the intensity of
light which is scattered at right angles. Example: Ratio Turbidimeter.
NOTE: If Formazin polymer is used as reference in place silica, then the turbidity
units are expressed as FTU (Formazin Turbidity Units).
b) Colour: Dissolved organic matter from decaying organic matter or inorganic
materials like coloured soils may impart colour to the Water. This colour is not
objectionable from health point of view, but may spoil the colour of the clothes being
washed in such waters and is also objectionable from the aesthetic and psychological
point of view.
The colour of the water can be easily detectable to naked eye. It can be measured by
comparing the colour of water sample with other standard glass tubes (Nessler tubes)
containing the solutions of different standard colour intensities.
The standard unit is that which is produced by one milligram of Platinum cobalt
dissolved in one liter of distilled water.
Ex: 1 mg of planitum cobalt dissolved in water give a colour intensity of 1ppm.
The maximum permissible colour for domestic supplies is 20 ppm (based on cobalt
scale). But the desirable (preferable) limit is 10 ppm.
For precise determination of small colour intensities, a compact instrument called
TINTOMETER is generally used.
c) Tastes and Odours: The dissolved organic materials or the inorganic salts or the
dissolved gases impart tastes and odours to the water, which generally occur together.
Caused by
1. dissolved gases like H2S, CH4, Co2, O2 combined with organic matter
2. Mineral substances like Nacl, iron compounds, carbonates, sulphates, phenols,
other tarry or oily matter.
Though drinking water should not contain any undesirable or objectionable taste &
odour, some tastes, imparted by dissolved oxygen and dissolved carbon dioxide are
generally desirable.
The odour or taste is measured by a term called odour intensity, which is related to
threshold odour.
The threshold odour number represents the dilution ratio at which the odour is hardly
detectable by human observation.
Dilution ratio: Ratio of diluted volume to original volume.
For example, 4 ml sample of water is diluted to 100 ml, then the dilution ratio or the
threshold number is 25.
For public supplies, the water should be free from odour. The threshold number
should be 1 (desirable) and should never exceed 3 (permissible).
d) Temperature: Desirable water temperature should be around 10oC and temperatures
above 25oC are considered objectionable.
e) Specific Conductivity of Water: The dissolved salts can be estimated by measuring
specific conductivity of water. This is done by means of a portable ionic water tester,
and expressed as micro-mhos per cm at 25oC.
[Mho is the Unit of conductivity and equals to ratio of 1 ampere/1 volt]. The obtained
measurement is multiplied by a coefficient (generally 0.65) so as to directly obtain the
dissolved content in mg/l.
Chemical Characteristics: These are found by chemical analysis.
a) Total Solids and Suspended Solids: The total Amount of solids (suspended &
dissolved) present in water can be determined by evaporating a sample of water
and weighing the dry residue left.
The suspended solids can be found by filtering the water sample and weighing the
residue left on the filter paper.
Dissolved Solids: Total Solids – Suspended Solids
Desirable limit of total Solids is less than 500 ppm and permissible limit
should be less than 1000 ppm.
b) pH of water: indicates the negative logarithm of hydrogen ion concentration
present in water. It is thus an indicator of acidity (pH < 7) or alkalinity (pH 7 - 14)
present in Water.
H+ ions increases Acidity, OH- ions increases alkalinity.
Acidity is caused by mineral acids, free carbon dioxide, sulphates of iron &
aluminium etc.
Alkalinity is caused by
1. The presence of bicarbonates of calcium and magnesium.
2. Carbonates or hydroxides of sodium, potassium, calcium, & magnesium.
Depending on the substance causing alkalinity, it is called bicarbonate alkalinity
or carbonate alkalinity or hydroxide (caustic) alkalinity.
Measurement of pH: pH can be measured by
1. Potentiometer: The method is very quick. This instrument measures the
electrical potential exerted by hydrogen ions and thus indicates their
concentration.
2. Colour Indicators (Dyes): These are added to water and the coloured water is
compared to standard colours of known pH values.
Set of standard sealed tubes containing coloured waters of known pH values
with different indicators are generally kept in public health laboratories.
Some indicators for measuring pH values in Water
Final Colour
Original Colour
Indicator pH range Produced in
of Indicator Dye
Water
Methyl Orange 2.8-4.4 Red Yellow
Methyl Red 4.4 – 6.2 Red Yellow
Phenolphthalein Red
8.6-10.3 Yellow
Red
Permissible pH Values: Close to 7 as possible.
Indian Standard: 6.6 to 8.5
Low pH Values cause tuberculation and corrosion while the higher values cause
instruction of pipes, sediment deposits, difficulties in chlorination
c) Hardness of Water: Caused by the presence of Calcium and Magnesium Salts in
Water.
1. If these salts are produced due to Bicarbonates and Carbonates, it is called
temporary hardness. Removed by Simple Boiling or addition of lime to water.
This is also called Carbonate Hardness.
Why Scaling Occurs in Boilers??
When the hard water (temporary Hard) is boiled, the Carbon Dioxide escpes
out and Calcium Carbonate which is insoluble is left as precipitate. This
deposition of calcium causes scaling in boilers.
INTERESTING FACT: Magnesium carbonate is soluble in water
2. If the salts are produced due to sulphates, Chlorides and nitrates, it is called as
Permanent Hardness (or) Non-Carbonate Hardness. This cannot be removed
by simple boiling and requires special treatments.
Measurement of Hardness: EDTA method is used for computing Hardness.
The amounts of calcium & magnesium ions present in water is found first by
titration. Then the hardness of water can be easily estimated as
Total Hardness in mg/l as CaCO3
= [Ca++ in mg/l × Combined wt. of CaCO3/Combined wt. of Ca++]
+ [Mg++ in mg/l × Combined wt. of CaCO3/Combined wt. of Mg++]
= [Ca++ in mg/l] × [50/20] + [Mg++ in mg/l] × [50/12]
Carbonate Hardness = Total Hardness or Alkalinity, whichever is lesser
Non-Carbonate Hardness = Total Hardness in excess of Alkalinity.
If the Alkalinity is more than the Total Hardness, then there is ZERO non-carbonate
Hardness.
Hardness upto 75 ppm ------- Soft
Hardness 75 - 200 ppm ------- Moderate
Hardness over 200 ppm -------Hard
Underground waters are generally harder as the water do not come in contact with
minerals. Their hardness, many a times, exceed over 300 ppm.
For boiler feeder waters and for efficient cloth washing, water must be SOFT. But the
SOFT water is objectionable to drinking as water becomes Tasteless.
Prescribed hardness limit for public supplies ranges from 75 – 115 ppm.
d) Chloride Content: Generally present in water in the form of NaCl (Common
Salt). This may be due to leaching of marine sedimentary deposits, Sea Water,
industrial wastes. This can be found by titrating the water with standard silver
nitrate solution using potassium chromate as indicator.
The chloride content for public usage should not exceed 250 ppm.
e) Nitrogen Content: The presence of nitrogen in water indicates the presence of
organic matter. This may occur in the form of Free Ammonia (Ammonia
Nitrogen) or Organic Nitrogen (Albuminoid) or Nitrates or Nitrates. Sometimes,
occurs in the combinations of the above.
Free Ammonia: Indicates the start of decomposition of organic matter present in
water (indicates recent pollution). Its value should not exceed 0.15 ppm for
potable waters. It can be easily measured by simple boiling of water and
measuring the liberated ammonia gas by distillation process.
Organic Nitrogen: Indicates th pollution and presence of undecomposed organic
matter. It should not exceed 0.3 ppm for potable water.
It can be determined by digestion of organic matter and measuring the released
ammonia gas. This can be done by adding strong alkaline solution of potassium
permanganate to already boiled water sample and boiling the same to liberate the
ammonia gas.
Nitrates: Nitrates represent, the fully oxidized organic matter and such waters
may not be harmful. However, excess nirates can cause a disease called
Methemoglobinemia (blue baby disease) in infants. Children suffering from this
disease may vomit, their skin may become dark, and may die in extreme cases.
The nitrate concentration should be less than 45 ppm for public usage.
Nitrites: represent partly decomposed organic matter and so it is highly
dangerous and so its concentration should be NIL for public usage. There is a
danger that it can be converted to nitrate in human digestive system.
The amount of nitrites or nitrates can be measured by colour matching methods.
Kjedahl Nitrogen = Ammonia nitrogen + Organic Nitrogen
Total Inorganic Nitrogen = Ammonia + Nitrite + Nitrite Nitrogen
Total Oxidized Nitrogen = Nitrite + Nitrate Nitrogen
Total Nitrogen = Ammonia + Organic + Nitrite + Nitrate.
Excess presence of any form of Nitrogen is Harmful.
Metals and other substances: Metals should be limited to permissible values to
avoid harmful effects on human life.
Metal Max Harmful effects
Conc as
per IS
10500:
1991 in
ppm
Arsenic 0.01 May cause cancer, diabetes, iver damage.
Cadmium 0.01 Highly toxic to humans and livestock
Mercury 0.001 Toxic to all forms of life
Fluoride 1.0 Less than 1.0 ppm – dental
Over 1.5 ppm – skeletal
Silver -- Discoloration of Skin (Argyria)
Dissolved Gases:
Methane Gas: Has explsive tendency
Hydrogen Sulphide gas: may give bad taste and odour
Carbon Dioxide: May give bad taste & odour, also cause corrosion.
NOTE: Oxygen gas is generally absorbed by water from the atmosphere. In
such case, the deoxidized Organic matter consumes this absorbed oxygen if
the oxygen present in water is less than the saturation level.
The amount of oxygen consumed by the organic matter for oxidation is known as
Biochemical Oxygen Demand (BOD). This indicates the amount of organic
matter in water and should be NIL for treated waters.
Polluted waters continue to absorb oxygen for many months till the oxidation gets
completed. So it is not practicable to determine ultimate oxygen demand. Hence
BOD of water during first 5 days at 20oC is taken as standard demand.
Generally the BOD test is carried out by determining the dissolved oxygen on the
wastewater or a diluted mixture at the beginning of the test period, incubating the
waste water mixture at 20°C, and determining the dissolved oxygen at the end of
5 days. The difference in dissolved oxygen between the initial measurement and
the fifth day measurement represents the biochemical oxygen demand.
