Water Supply Engineering
INTRODUCTION
- Next to the air, the other important requirement for human life to exist
is water.
- Water is available in various forms such as rivers, lake, streams etc.
- The earliest civilizations organized on the banks of major river systems
and required water for drinking, bathing, cooking etc.
- With the advancement of civilization the utility of water enormously
increased and now such a stage has come that without well organized
public water supply scheme, it is impossible to run the present civic life
and develop towns.
- The importance of water from quantity viewpoint was recognized from
the earliest days but the importance of quality came to be recognized
gradually in the later days.
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WATER SUPPLY ENGINEERING
Need for Protected Water Supply
- Protected water supply means the supply of water that is treated to
remove the impurities and made safe to public health.
- Water may be polluted by physical and bacterial agents.
- Water is also good carrier of disease causing germs. The causes of
outbreak of epidemics are traced to polluted water and poor sanitation.
The objectives of the community water supply system are:-
- Provide whole some water to the consumers for drinking purpose.
- Supply adequate quantity to meet at least the minimum needs of
the individuals.
- Make adequate provisions for emergencies like fire fighting,
festivals, meeting etc
- Make provision for future demands due to increase in population,
increase in standard of living, storage and conveyance.
- Prevent pollution of water at source, storage and conveyance.
- Maintain the treatment units and distribution system in good
condition with adequate staff and materials.
- Design and maintain a system that is economical and reliable.
Whole Some Water
- Absolutely pure water is never found in nature.
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- Water found in nature contains number of impurities in varying
amounts.
- The rainwater which is originally pure, also absorbs various gases,
dust and other impurities while falling. This water when it moves on
the ground further carries silt, organic and inorganic impurities.
- Wholesome water is defined as the water which containing the
minerals in small quantities at requisite levels and free from harmful
impurities.
- Chemically pure water is also corrosive but not whole some water.
The water that is fit for drinking safe and agreeable is called potable
water.
Requirements of wholesome water:-
- Free from bacteria
- Colourless and sparkling
- Tasty, odour free and cool
- Free from objectionable matter
- Do not corrode pipes
- Have dissolved oxygen and free from carbonic acid so that it may
remain fresh.
WATER DEMANDS
- While designing the water supply scheme for a town or city, it is
necessary to determine the total quantity of water required for various
purposes by the city.
- There is need to determine the water demand of the town and then to
find suitable water sources from where the demand can be met. There
are many factors involved in demand of water.
Types of water demands of a city or town:-
- Domestic water demand
- Industrial demand
- Institution and commercial demand
- Demand for public use
- Fire demand
- Loses and wastes
Domestic Water Demand
- The quantity of water required in the houses for drinking, bathing,
cooking, washing etc is called domestic water demand and mainly
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depends upon the habits, social status and customs of the people as
well as climatic conditions.
Industrial Demand
- The water required in the industries mainly depends on the type of
industries existing in the city.
- The water required by factories (paper mills, Cloth mills, Cotton mills,
Breweries, Sugar refineries etc.) comes under industrial use.
- The quantity of water demand for industrial purpose is around 20 to
25% of the total demand of the city.
Institution and Commercial Demand
- Universities, Institution, commercial buildings and commercial centres
including office buildings, warehouses, stores, hotels, shopping centres,
health centres, schools, temple, cinema houses, railway and bus
stations etc comes under this category.
Demand for Public Use
- Quantity of water required for public utility purposes such as for
washing and sprinkling on roads, cleaning of sewers, watering of public
parks, gardens, public fountains etc comes under public demand.
- To meet the water demand for public use, provision of 5% of the total
consumption is made when designing the water works for a city.
Fire Demand
- Fire may take place due to faulty electric wires by short circuiting, fire
catching materials, explosions, bad intension of criminal people or any
other unforeseen mishappenings.
- If fires are not properly controlled and extinguished in shortest possible
time, they lead to serious damage.
- All the big cities have a fire-fighting department. Large quantity of
water is required to extinguish fire and therefore provision is made in
the water works to supply sufficient quantity of water or keep as
reserve in the water mains for this purpose.
- Fire hydrants are provided on the water mains for fire demand.
Losses and Wastes
- All the water, which goes in the distribution pipes does not reach the
consumers.
- The following are the reasons
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- Losses due to defective pipe joints, cracked and broken pipes, faulty
valves and fittings.
- Losses due to consumers keeping open their taps of public taps
even when they are not using the water and allow the continuous
wastage of water
- Losses due to unauthorised and illegal connections.
- While estimating the total quantity of water of a town; allowance of
15% of total quantity of water is made to compensate for losses,
thefts and wastage of water
PER CAPITA DEMAND
- If ‘Q’ is the total quantity of water required by various purposes by a
town per year and ‘p’ is population of town, then per capita demand
will be
Q
Per capita demand = litres/day
Px 365
- Per capita demand of the town depends on various factors like
standard of living, no. and type of commercial places in a town etc.
FACTORS AFFECTING PER CAPITA DEMAND
Climatic conditions
- The quantity of water required in hotter and dry places is more than
that of cold countries because of the use of air coolers, air conditioners,
sprinkling of water in lawns, gardens, courtyards, washing of rooms,
more washing of clothes and bathing etc.
- But in very cold countries sometimes the quantity of water required
may be more due to wastage, because at such places the people often
keep their taps open and water continuously flows for fear of freezing
of water in the taps and use of hot water for keeping the rooms warm.
Size of community
- Water demand is more with increase of size of town because more
water is required in street washing, running of sewers, maintenance of
parks and gardens.
Living standard of the people
- The per capita demand of the town increases with the standard of
living of the people because of the use of air conditioners, room
coolers, maintenance of lawns, use of flush, latrines and automatic
home appliances etc.
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Industrial and commercial activities
- The quantity of water required in certain industries is much more than
domestic demand. thus their presence in the town will enormously
increase per capita demand of the town.
- The water required by the industries has no direct link with the
population of the town.
Pressure in the distribution system
- The rate of water consumption increase with pressure in the
distribution system which in turn has effect of increasing water
demand.
System of sanitation
- Per capita demand of the towns having water carriage system (sewer)
will be more than the town where this system is not being used.
Cost of water
- The cost of water directly affects its demand. If the cost of water is
more, less quantity of water will be used by the people as compared
when the cost is low.
SOURCES OF WATER SUPPLY
- Water is the most abundant compound in nature. It covers 75% of the
earth surface.
- About 97.3% of water is contained in the great oceans that are saline
and 2.14% is held in icecaps glaciers in the poles, which are also not
useful. Barely the remaining 0.56% found on earth is in useful form for
general livelihood.
HYDROLOGICAL CYCLE
- Hydrology is the science, which deals with the increment of the water
on the ground, under the ground, evaporation from the land and water
surface and transportation from the vegetation and going back into
atmosphere where it precipitates.
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Sun
Clouds
Precipitation
on
Precipitati Evapotranspiration snow
on Percolation
on Oceans Water Table
Evaporation River
Ground
water flow
Ocean
Water Flow
- The above figure illustrates the hydrological cycle of water.
- Due to sun’s heat, water from the earth’s surfaces, lakes, rivers, seas
etc evaporates and rises upwards.
- At high altitude due to reduction in the atmosphere pressure these
water vapours expand by absorbing energy from the surrounding air,
which cools down.
- When it falls below the dew point it cannot retain the excessive
moisture, which starts falling in the form of rain, hails, dew, sleet, frost
or precipitation.
- Various factors such as temperature, atmospheric pressure, velocity
of wind, height of mountains in a region, presence of forests, position
of land and water areas etc and their complex relation are responsible
for the precipitation.
- This precipitation and evaporation processes continue forever and
balance is maintained between the two by nature.
Precipitation
- The evaporated water from the surfaces of streams, rivers, sea, ponds,
wet surfaces, trees and plants etc again returned to the earth surface
by the condensation in the form of rain, hails, dew, sleet etc is known
as precipitation.
- The major part of the precipitation occurs in the form of rain and other
forms quantities are very small. The water of precipitation further goes
off in the following ways.
Run-off
- After precipitation a portion of its water flows over the ground in the
form of rivers and streams and some water flows towards lakes and
ponds and collected there.
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Infiltration
- A portion of precipitation percolates in the ground and it is stored in
the form of sub-soil or ground water.
Evaporation
- Some portion of the precipitation is also evaporated from the lakes,
rivers, reservoirs and wet surfaces in the form of vapour due to sun’s
heat is known as evaporation
Evapo-transpiration
- The roots of the trees sucks water from the ground and some portion of
it evaporates in the atmosphere through leaves in the form of
transpiration.
SURFACE SOURCES
- Surface sources of water can be broadly divided into streams, rivers,
ponds, lakes and impounding reservoirs
Natural ponds and lakes
- In mountains, natural basins are formed with impervious bed by
springs and streams to form ponds or lakes.
- The quality of water in the natural ponds and lakes depends upon the
basin’s capacity, catchment area, annual rainfall, porosity of ground
etc.
- But lakes and ponds situated at higher altitudes contain almost pure
water which can be used without any treatment.
- Ponds formed due to construction of houses, road, and railways
contains large amount of impurities and therefore cannot be used for
water supply purposes.
Streams and rivers
- Rivers and streams are the main sources of surface water.
- In dry season the quality of river water is better than in rainy season
which receive run-off water which carries with it clay, sand, silt etc
which make the water turbid.
- So river and stream water require special treatments.
- Some rivers are snow fed and perennial and have water throughout the
year and therefore they do not require any arrangements to hold the
water.
- Some rivers dry up wholly or partially in summer and they require
special arrangements to meet the water demand during hot weather.
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- Cities situated near rivers discharge their used water into these rivers;
therefore much care should be taken while drawing water from these
rivers.
Impounding reservoirs
- In some rivers, the flow becomes very small and cannot meet the
requirements during the hot weather.
- In such cases, the water can be stored by constructing a weir or a dam
across the river at such places where minimum area of land is
submerged in the water and maximum quantity of water can be stored.
- In lakes and reservoirs, suspended impurities settle down in the
bottom, but in their beds algae, weeds, vegetable and organic growth
takes place which produce bad smell, taste and colour in water.
- Therefore this water should be used after purification. When water is
stored for long time in reservoirs it should be aerated and chlorinated
to kill the microscopic organisms.
Sub surface sources
- This include Infiltration galleries, Infiltration wells, Springs etc
Infiltration galleries
- These are horizontal tunnels which are constructed through water
bearing strata for tapping underground water near rivers, lakes or
streams.
- An infiltration gallery is a source of water at the subsurface level used
as a water source.
- It is a horizontal drain structure made of open jointed pipe or
perforated pipe to collect water from subsurface water levels.
- It can collect groundwater as well as collect water from subsurface of
rivers.
Infiltration wells
- In order to obtain large quantity of water, the infiltration wells are sunk
in series in the banks of river.
- The wells are closed at top and open at bottom. They are constructed
by brick masonry with open joints.
- For the purpose of inspection of well, the manholes are provided in the
top cover.
- The water filtrates through the bottom of such wells and as it has to
pass through sand bed, it gets purified to some extent.
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- The infiltration well in turn is connected by porous pipes to collecting
sump called jackwell and their water is pumped to purification plant for
treatment.
Springs
- A spring is a natural exit point at which groundwater emerges out of
the aquifer and flows onto the top of the Earth's crust to become
surface water.
- Springs are formed when groundwater flows onto the surface. This
typically happens when the water table reaches above the surface
level, or if the terrain depresses sharply.
QUALITY OF WATER
General introduction
- Absolutely pure water is never found in nature and contains number of
impurities in varying amounts.
- The rainwater which is originally pure, also absorbs various gases, dust
and other impurities while falling.
- This water when it moves on the ground further carries salt, organic
and inorganic impurities.
- So this water before supplying to the public should be treated and
purified for the safety of public health, economy and protection of
various industrial process. It is most essential to thoroughly analyse
and do the treatment of the raw water obtained before its distribution.
- The water supplied to the public should be strictly according to the
standards laid down from time to time.
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CHARACTERISTICS OF WATER
- For the purpose of classification, the impurities present in water may
be divided into the following three categories.
PHYSICAL CHARACTERISTICS
- The following are the physical characteristics
1. Turbidity
2. Colour and temperature
3. Taste and odour
Turbidity
- Turbidity is caused due to presence of suspended and colloidal matter
in the water.
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- The character and amount of turbidity depends upon the type of soil
over which the water has moved through.
- Ground waters are less turbid than the surface water.
- Turbidity is a measure of resistance of water to the passage of light
through it.
- Turbidity is expressed as NTU (Nephelometric Turbidity Units) or PPM
(parts per million) or Milligrams per litre (mg/l).
- Turbidity is measured by 1) Turbidity rod or Tape 2) Jacksons
Turbidimeter 3) Bali’s Turbidimeter.
- The Sample to be tested is poured into a test tube and placed in the
meter and units of turbidity are read directly on the scale by a needle
or by digital display.
- Drinking water should not have turbidity more than 10 N.T.U.
- This test is useful in determining the distension time in settling for raw
water and to dosage of coagulants required to remove turbidity.
Colour and temperature
- Colour in water is usually due to organic matter in colloidal condition
but sometimes it is also due to mineral and dissolved organic
impurities.
- The colour produced by one milligram of platinum in a litre of water has
been fixed as the unit of colour.
- The permissible colour for domestic water is 20ppm on platinum cobalt
scale.
- The colour in water is not harmful but objectionable.
- Temperature of water is measured by means of ordinary
thermometers.
- The temperature of surface water is generally at atmospheric
temperature, while that of ground water may be more or less than
atmospheric temperature.
- The most desirable temperature for public supply between 4.4°C to
10°C. The temperature above 35°C is unfit for public supply, because it
is not palatable.
Taste and odour
- Taste and odour in water may be due to presence of dead or live micro-
organisms, dissolved gases such as hydrogen sulphide, methane,
carbon dioxide or oxygen combined with organic matter, mineral
substances such as sodium chloride, iron compounds and carbonates
and sulphates of other substances.
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- The tests of these are done by sense of smell and taste because these
are present in such small proportions that it is difficult to detect them
by chemical analysis.
- The water having bad smell and odour is objectionable and should not
be supplied to the public.
- The intensities of the odours are measured in terms of threshold
number.
- This number indicates how many dilutions it takes to produce odour-
free water.
CHEMICAL CHARACTERISTICS
- In the chemical analysis of water, these tests are done that will reveal
the sanitary quality of the water.
- Chemical tests involve the determination of total solids, PH value,
Hardness of water, Chloride content etc.
Total solids and suspended solids
- Total solids include the solids in suspension, colloidal and in dissolved
form.
- The quantity of suspended solids is determined by filtering the sample
of water through fine filter, drying and weighing.
- The quantity of dissolved and colloidal solids is determined by
evaporating the filtered water obtained from the suspended solid test
and weighing the residue.
- The total solids in a water sample can be directly determined by
evaporating the filtered water obtained from the suspended solid test
and weighing the residue.
PH value of water
- PH value denotes the concentration of hydrogen ions in the water and it
is a measure of acidity or alkalinity of a substance.
- PH = - log 10[H+] or 1 / log 10[H+]
- Depending upon the nature of dissolved salts and minerals, the P H
value ranges from 0 to 14. For pure water, P H value is 7 and 0 to 7
acidic and 7 to 14 alkaline ranges.
- For public water supply PH value may be 6.5 to 8.5. The lower value
may cause tubercolation and corrosion; whereas high value may
produce incrustation, sediment deposits and other bad effects.
- Tubercolation is the formation of corrosion products (rust) on the inside
of iron pipe. Overtime, this build up grows thicker and disrupts the
pipe’s flow.
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- Incrustation is the formation of hard coating on the inside of pipes. This
also reduces the diameter of pipes (calcium carbonate, calcium
silicates)
- PH value of water is generally determined by P H papers or by using PH
meter. PH can be read directly on scale or by digital display using PH
meter.
Hardness of water
- It is a property of water, which prevents the lathering of the soap.
- Hardness is of two types.
i. Temporary hardness: It is caused due to the presence of
bicarbonates of calcium and magnesium. It is removed by boiling.
ii. Permanent hardness: It is caused due to the presence sulphates
and chlorides of calcium and magnesium. It is removed by zeolite
method the lime soda process
- Hardness is usually expressed in gm/litre or p.p.m. of calcium
carbonate in water.
- Hardness leads to the following effects.
i. Large soap consumption in washing and bathing
ii. Fabrics when washed become rough and strained with precipitates.
iii. Hard water is not fit for industrial use like textiles, paper making,
dye and ice cream manufacture.
iv. It forms scales in the boilers tubes and reduces their efficiency and
cause incrustations
Methods of removal of hardness
1. Boiling
2. Lime addition
3. Lime soda process
4. Caustic soda process
5. Zeolite process
Boiling
- Removes temporary hardness
heat
Ca (HCO3)2 -----------> CaCO3↓ + CO2↑ +H2O
heat
Mg (HCO3)2 -----------> MgCO3↓ + CO2↑ +H2O
Addition of lime
This removes temporary hardness
Ca (HCO3)2 + Ca (OH)2 -----------> 2CaCO3↓ + 2H2O
Mg (HCO3)2 + Ca (OH)2 -----------> CaCO3 + MgCO3 + 2H2O
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Removal of permanent Hardness:
Lime soda process:
- In this method, lime and sodium carbonate (soda) are used to
remove temporary and permanent hardness from water. The
chemical reactions involved in this process are as follows.
CO2 + Ca (OH) 2 -----------> CaCO3 + H2O (removal of CO2)
Ca (HCO3) + Ca (OH) 2 -----------> 2CaCO3 + 2H3O (removal of temporary
hardness)
Mg (HCO3) + Ca (OH) 2 -----------> CaCO3 + Mg (CO3) + 2H2O
MgSO4 + Ca (OH) 2 ----------> Mg (OH) 2 + CaSO4 {conversion of MgSO4 to
CaSO4}
CaSO4 + Na2CO3 ----------> CaCO3 + Na2SO4 {removal of sulphates}
CaCl2 + Ca (OH) 2 --------------> Ca (OH) 2 + CaCl2
MgCl2 + Ca (OH) 2 --------------> Mg (OH) 2 + CaCl2 {removal of chlorides}
CaCl2 + Na2CO3 --------------> CaCO3 + 2NaCl
MgCl2 + Na2CO3 --------------> Mg CO3 + 2NaCl {removal of chlorides}
Advantages of lime soda process
i. The PH value of water treated by this process go down to 9 and
which results in decrease in corrosion of the distribution system.
ii. Less quantity of coagulant will be required, if this process is adopted
iii. Removal of iron and manganese to some extent
iv. Reduction of total mineral content of water
v. The process is economical
vi. This process is most suitable for turbid and acidic waters
Disadvantages
i. Large quantity of sludge formed during this process to be disposed
off by some suitable method
ii. This process requires skilled supervision for its successful working
iii. If recarbonation is omitted, a thick layer of calcium carbonate will be
deposited in the filtering media, distribution pipes etc.
Zeolite process
- This is also known as the base-exchange or Ion exchange process.
- The hardness may be completely removed by this process.
Principle
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- Zeolites are compounds (silicates of aluminium and sodium) which
replace sodium Ions with calcium and magnesium ions when hard
water is passes through a bed of Zeolites.
- The Zeolite can be regenerated by passing a concentrated solution of
sodium chloride through the bed. The chemical reactions involved are;-
2SiO2 Al2O3 Na2O + Ca (HCO3)2 ----------> 2SiO2 Al2O3 CaO + 2NaHCO3
(Zeolite)
2SiO2 Al2O3 Na2O + CaSO4 ----------> 2SiO2 Al2O3 CaO + Na2SO4
2SiO2 Al2O3 Na2O + CaC12 ----------> 2SiO2 Al2O3 CaO + 2NaCl
Regeneration
2SiO2 Al2O3 Na2O + 2NaCl ----------> 2SiO2 Al2O3 Na2O + CaCl2
2SiO2 Al2O3 MgO + 2NaCl ----------> 2SiO2 Al2O3 Na2O + MgCl2
Advantages
i. In this process, the sludge is not formed hence problem of sludge
disposal does not arise
ii. It can be operated easily and no skilled supervision required
iii. The hardness of water reduces to zero and hence used for boiler
and textile industries
iv. The process is economical where salt is cheaply available
v. The load on Zeolite can be reduced by combining it with lime or
aeration process
Disadvantages
i. The Zeolite process cannot be used for turbid or acidic water
ii. The Zeolite process is unsuitable for water containing Iron and
Manganese
iii. The Zeolite should be operated carefully to avoid injury or damage
to the equipment
BIO-CHEMICAL OXYGEN DEMAND
- If the water is contaminated with sewage, the demand of oxygen by
organic matter in sewage is known as biochemical oxygen demand.
- As the microorganisms’ biodegrade the organic matter in sewage, they
deplete the oxygen in water.
- As the oxygen is exhausted, anaerobic action begins due to which foul
smell starts coming.
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- The aerobic decomposition of organic matters is done in two stages.
The carbonaceous matters are first oxidized and the oxidation of
nitrogenous matters takes place in the latter stage.
BACTERIAL AND MICROSCOPICAL CHARACTERISTICS
- The examination of water for the presence of bacteria is important for
the water supply engineer from the viewpoint of public health.
- The bacteria may be harmless to mankind or harmful to mankind. The
former category is known as non-pathogenic bacteria and the latter
category is known as pathogenic bacteria.
- Many of the bacteria found in water are derived from air, soil and
vegetation.
- Some of these are able to multiply and continue their existence while
the remaining die out in due course of time.
- The selective medium that promotes the growth of particular bacteria
and inhibits the growth of other organisms is used in the lab to detect
the presence of the required bacteria, usually coli form bacteria. For
bacteriological analysis, the coli form test is used to determine
contamination of water by microorganisms.
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