Notes - Water Supply Engineering

Water Supply Engineering Third Year/First Part

Shuvanjan Dahal (o68/BCE/147) Page 1

CHAPTER I

INTRODUCTION

1.1 Importance of Water

Man and animals not only consume water, but they also consume vegetation for their

food. Vegetation, in turn, cannot grow without water.

Growth of vegetation also depends upon bacterial action, while bacteria need water in

order to thrive.

Good sanitation cannot be maintained without adequate water supply system.

Man needs water for drinking, cooking, cleaning and washing.

Water maintains an ecological balance balance in the relationship between living

things and environment in which they live.

1.2 Definition of Types of Water

1.2.1 Pure and Impure Water

Pure water contains only 2 atoms of hydrogen and 1 atom of oxygen. It is not good for

health as pure water does not contain essential minerals required for human health.

Impure water, besides 2 atoms of hydrogen and 1 atom of oxygen, contains other

elements.

1.2.2 Potable and Wholesome Water

Potable water is water safe enough to be consumed by humans or used with low risk of

immediate or long-term harm.

Water that is not harmful for human beings is called wholesome water. It is neither chemically

pure nor contains harmful matters to human health. Requirements of wholesome water:

i. It should be free from radioactive substance, microorganism, disease causing bacteria,

objectionable dissolved gases, harmful salts, objectionable minerals and other

poisonous metals.

ii. It should be colourless, and sparkling which may be accepted by public.

iii. It should be tasty, odour-free, soft, cool and cheap in cost.

iv. It shouldnt corrode pipes.

v. It should have dissolved oxygen and free from carbonic acid so that it remains fresh.

1.2.3 Polluted and Contaminated Water

Contamination means containing harmful matter. It is always polluted and harmful

for use. Water consisting of microorganisms, chemicals, industrial or other wastes,

large numbers of pathogens that cause diseases is called contaminated water.

Pollution is synonymous to contamination but is the result of contamination. Polluted

water contains substances unfit or undesirable for public health or domestic purpose.



Two broad categories of water pollution: a) Point Source b) Non-point Source

a) Point Source: occurs when harmful substances are emitted directly into a body of

water. E.g. pipe from an industrial facility emitted directly into a body of water.

b) Nonpoint Source: delivers pollutants through transport or environmental charge. E.g.

fertilizer from a farm field carried into a stream by rain.

1.3 Historical Development of Water Supply System

What is Water Supply System?

Water Supply System is a network of pipelines of various sizes with control valves for carrying

water to all streets and supplying water to the consumers.

Historical Development

Most of the historical community settlements throughout the world were made near

springs, lakes and rivers from where water for drinking and irrigation purposes was

obtained.

In the ninth century, few important water supply structures were constructed by the

Moors in Spain. In the 12th century, small aqueduct was constructed in Paris. In

London, spring water was brought by means of lead pipes and masonry conduits in the

thirteenth century.

During the first phase of the Industrial Revolution, large impounding reservoirs were

developed due to the necessity of feeding canals.

The first water filter was constructed in 1804 by John Gibb at Paisley in Scotland.

The first permanent use of chlorination originated under the direction of Sir Alexander

Houston at Lincoln in 1905.

1.4 Objectives of Water Supply System

The quintessential objective of water supply system is to supply water equitably to the

consumers with sufficient pressure so as to discharge the water at the desired location within

the premises.

Water Supply System

Continuous

- Water is available 24 hours a day and seven

days a week.

Intermittent

- Water is supplied for few hours every day or

alternate days.



1.5 Schematic Diagram of Typical Water Supply System

1. City/General

2. Hilly Area/Rural Area



3. Terai Area

1.6 Components of Water Supply System and their Functions

The components of a water supply system can be divided into two major parts:

1. Transmission Line or Transmission Main: Pipeline from intake to reservoir tank.

2. Distribution Line: Pipeline from reservoir tank to tap stand.

There are three systems of supply as:

i. Gravity Flow System

ii. Pumping System

iii. Dual System

(Details will be studied in chapters to come later.)



CHAPTER II

SOURCES OF WATER

2.1 Classification of Sources of Water

Main source of water is precipitation.

2.2 Surface Sources

Surface sources have water on the surface of the earth such as in stream, river, lake, wetland or

ocean.

2.2.1 Rivers

Natural channel

Main source: either natural precipitation or snow-fed

Perennial and non-perennial rivers

Vast catchment area; hence, amount of water is large

Contaminated source

2.2.2 Streams

Natural drainage

Less catchment area

Source: Melting snow or precipitation

Found in hilly, mountain areas

Low quantity of water

Potable water

Sources of Water

Surface Source

River, Stream, Lake, Pond, Impounded

Reservoir

Sub-Surface/Under

ground/Ground Source

Spring, Well, Infiltration

Gallery, Infiltration Well



2.2.3 Lakes

Natural depression filled with water

Found in mountain and hilly areas

Quantity of water depends on: depression, catchment area and soil type

Quality varies

2.2.4 Ponds

Natural/Artificial depression found in plain areas

Bad quality of water

Not used as water supply source

Less quantity of water

Can be used for animal bathing and irrigation purposes.

2.2.5 Impounded Reservoirs

An impounding reservoir is a basin constructed in the valley of a stream or river for the

purpose of holding stream flow so that the stored water may be used when water supply is

insufficient. E.g. Sundarijal Dam

The dam is constructed across the river in such places where minimum area of land is

submerged, where river width is less and the reservoir basin remains cup shaped having

maximum possible depth of water. Hence, it is defined as an artificial lake created by the

construction of a dam across the valley containing a watercourse.

Two functions: i) To impound water for beneficial use

ii) To retard flood

The location of impounded reservoir depends upon the quality and quantity of water

available, existence of suitable dam site, distance and elevation of reservoir, density and

distribution of population, geological conditions, etc.

The water quality is the same as in streams and rivers.

2.2.6 Numerical on Capacity Determination of Impounded Reservoirs

The flow in the river during the various months of the year (in m3/s) is as follows:

January 2.97

February 1.99

March 1

April 0

May 0.51

June 1

July 2

August 3

September 4

October 5

November 4

December 2.8

The river supplies water to a community having a constant demand of 6202 million litres/month.

Determine the capacity of impounded reservoir.



I. ANALYTICAL METHOD

( )

Where, n = number of days in the month

Months Flow (in

m3/s)

Inflow (ML)

Demand (ML)

Cumulative Inflow (ML)

Cumulative Demand

(ML)

Surplus (ML)

Deficit (ML)

January 2.97 7954.848 6202 7954.848 6202 1752.85

February 1.99 4814.208 6202 12769.056 12404 365.056

March 1 2678.4 6202 15447.456 18606 3158.54

April 0 0 6202 15447.456 24808 9360.54

May 0.51 1365.984 6202 16813.44 31010 14196.6

June 1 2592 6202 19405.44 37212 17806.6

July 2 5356.8 6202 24762.24 43414 18651.8

August 3 8035.2 6202 32797.44 49616 16818.6

September 4 10368 6202 43165.44 55818 12652.6

October 5 13392 6202 56557.44 62020 5462.56

November 4 10368 6202 66925.44 68222 1296.56

December 2.8 7499.52 6202 74424.96 74424 0.96

Total 74424.96 74424

II. GRAPHICAL METHOD

The largest possible positive difference (perpendicular distance between the two

graphs) gives the value of maximum surplus.

The largest possible negative difference (cumulative demand more) gives the value

of maximum deficit.

The difference between the ends of the curves gives the value of the required

capacity of impounded reservoir.



2.3 Ground Sources

When water seeps into the ground, it moves downward due to gravity through the pore spaces

between soil particles and cracks in rocks. Eventually, the water reaches a depth where the soil

and rock are saturated with water. Water which is found in the saturated part of the ground

underneath the land surface is called ground water.

2.3.1 Confined and Unconfined Aquifers

0

10000

20000

30000

40000

50000

60000

70000

80000

0 2 4 6 8 10 12

Infl

ow

an

d D

eman

d (

Cu

mu

lati

ve)

in

ML

Months

Determination of Capacity of Impounded Reservoir

Cumulative Demand (ML)

Cumulative Inflow (ML)



2.3.2 Springs

A spring is the natural outflow of ground water appearing at the earths surface as a current of

stream of flowing water under the suitable geological conditions. Most favourable conditions

for spring formation occur in Nepal and may be suitable for water supply schemes in village

areas in hilly region of Nepal.

Springs are capable of supplying small quantity of water so it cant be used as a source of water

to big towns but a well developed or combinations of the various springs can be used for water

supply especially villages near hills or bases of hills. The quality of water in spring is generally

good and may contain sulphur in certain springs which discharge hot water which can be used

only for taking dips for the cure of certain skin diseases. It may be less costly because it may

not need treatment plant. Springs may be classified into the following two types:

a. Gravity Springs

b. Non Gravity Springs



1. Gravity Springs

These springs result from water flowing under hydrostatic pressure and they are of the

following three types:

i. Depression Spring

These springs are formed due to the overflowing of the water table, where the ground surface

intersects the water table. The flow from such spring is variable with the rise or fall of water

table and hence in order to meet with such fluctuations, a deep trench may be constructed

near such spring. The deeper the trench, the greater is the certainty of continuous flow

because the saturated ground above the elevation of the trench bottom will act as a storage

reservoir to compensate for the fluctuations of the water table.

ii. Surface Spring or Contact Spring

These are created by a permeable water bearing formation overlying a less permeable or

impermeable formation that intersects the ground surface. However, in such springs, because

of the relatively small amount of underground storage available above the elevation of the

overflow crest, the flow from them is uncertain and likely to cease after a drought. Such

springs can also be developed by the construction of a cutoff trench or a cutoff wall.

iii. Artesian Spring

These springs result from release of water under pressure from confined aquifers either at an

outcrop of the aquifer or through an opening in the confining bed. The amount of water

available in an artesian spring may be large if the catchment area is large. The flow may be

slightly increased by removal of obstructions from the mouth of the spring.

2. Non Gravity Springs

Non gravity springs include volcanic spring (associated with volcanic rocks) and fissure spring

(results from fractures extending to the great depths in the earths crust). These are also called

hot springs and contain high minerals as well as sulphur also.

2.3.3 Wells

A well is a hole or shaft, usually vertical and excavated in the ground for bringing groundwater

to the surface. Wells are classified as follows:

1. Open or Dug or Draw or Percolation Well

They are of large diameters (1 to 10 m), low yields and not very deep (2 to 20 m). These are

constructed by digging hence also called dug wells. The walls may be of brick, stone masonry

or precast rings and thickness varies from 0.5 to 0.75 m depending upon the depth of the well.

It is also further classified as following two types:

i. Shallow Open Well

ii. Deep Open Well



2. Driven Well or Percussion Well

The shallow well constructed by driving a casing pipe of 2.5 cm to 15 cm in diameter and up to

12 m deep is called driven well. The casing pipe is driven first in the ground by hammering or

by water jet and the pipes are inserted. The lower portion of the pipe, which is driven in the

water bearing strata, is perforated and the pointed bottom is called drive point or well point.

The perforated portion of pipe is covered with fine wire gauge to prevent passage of sand and

soil particle. The discharge in this well is very small and can be obtained using hand or electric

pump and can be used for domestic purposes. E.g. Rower Pump used in the Kathmandu valley.

3. Tube Well

It is the well made of small diameter pipe installed after boring and inserted deep to trap

water from different aquifers. A tube well is a long pipe sunk to the ground intercepting one or

more water bearing strata. E.g. in Terai regions of Nepal.

As compared to open wells, the diameter of tube wells is much less. Tube wells may be

classified as shallow tube well (depth up to 30 m) and deep tube well (maximum depth up to

600 m). Quality may be better but may have various impurities, which should be treated and

quantity is larger so it can be used as water supply. Tube wells may be further classified into

the following:

i. Strainer type Tube Well

ii. Cavity type Tube Well

iii. Slotted type Tube Well

iv. Perforated type Tube Well

4. Artesian Well

It is the well from where water flows automatically under pressure. Mostly they are found in

the valley portion of the hills where aquifers on the both sides are inclined towards valley. The

HGL (Hydraulic Gradient Line) passes much above the mouth of well, which causes flow

under pressure. The water flows out in the form of fountain upto a height of 2.5 m depending

upon hydrostatic pressure. Some wells, which flow continuously throughout the year and can

be stored in reservoir and taken for water supply. The quality of water in artesian wells may be

good but sometimes it contains minerals and can be used after certain treatment.

2.3.4 Infiltration Galleries and Wells

Infiltration Gallery

Infiltration Gallery is a horizontal or nearly horizontal tunnel, usually rectangular (arched

also) in cross section and having permeable boundaries so that ground water can infiltrate

into it. Hence, it is also called horizontal well. It is generally located near a perennial recharge

source such as the bank or under bed of a river and 3 to 10 meters below the ground. It is also

used to collect ground water near marshy land or water bodies and stored in storage tank and

then used for water supply.



The quantity and quality depends upon the location and area of coverage. It is constructed by

the cut and covers method and made up with dry brick masonry wall or porous concrete

blocks with weep holes and R.C.C. slab roof or an arch roof. Manholes are provided at suitable

points for inspection. The perforations are covered by the graded gravel to prevent the entry of

fine particles in the gallery. Series of galleries may be laid in the proper slope and collected at

certain reservoir then it can be used as the water supply after certain treatment.

Infiltration Wells

Shallow wells constructed in series along the banks and sometimes under the bed of rivers to

collect water seeping through the walls of the wells are called infiltration wells. These wells are

constructed of brick masonry with open joints. For purpose of inspection, manhole is provided

in the top cover of the well.

The water infiltrates through the walls and bottom of these wells and has to pass through sand

bed and gets purified to some extent. Various infiltration wells are connected by porous pipes

and collected to the collecting sump well called Jack from where it can be conveyed for water

supply. The water quality is better in such well because the bed soil acts as a filter and lesser

treatment may be required.

2.4 Selection of Water Source

The selection of the sources of water depends upon the following factors:

a. Location

It should be near to the consumers area or town as far as possible.

They may be either surface or ground sources and the selection of the source depends

upon other factors. If there is no river, stream or reservoir in the area, the ultimate

source is ground source.

Location may be at higher elevation such that required pressure may be obtained and

water can be supplied by gravity flow.

b. Quantity of Water

It should have sufficient quantity of water to meet the demand for that design period

in the wet and dry seasons also. Two or more sources can be joined for required

quantity.

If possible, there should be sufficient supply for future extension of project.

c. Quality of Water

The water should be safe and free from pathogenic bacteria, germs and pollution and

so good that water can be cheaply treated.

The water quality should be such that it has less quantity of impurity, which further

needs less treatment.

d. Cost

It should be able to supply water of good quality and quantity at the less cost.

Gravity system of flow is generally cheaper than pumping.

Lesser the impurities, lesser the treatment and cost is reduced.

Cost analysis is necessary for various options and suitable one is selected.



e. Sustainable and Safe

f. Reliable

g. Non conflict among water users

(For pictures, refer any standard book.)



CHAPTER III

QUANTITY OF WATER

3.1 Per Capita Demand of Water

It is the average quantity of water required by a person in a day. The unit is lpcd (litres per

capita demand of water).

The unit of total water demand is litres/day.

3.2 Design and Base Periods

i. Survey Year: It is the year in which survey is carried out.

ii. Base Period: It is the period between survey year and base year during which the

works of survey, design and construction are completed. Base Period is generally taken

as 2 to 3 years.

iii. Base Year: It is the year in which implementation is done.

Base Year = Survey Year + Base Period

iv. Design Period: Any water supply project is planned to meet the present requirements

of community as well as the requirement for a reasonable future period (up to service

year). This period between Base Year and Service or Design Year is taken as Design

Period. It is generally 15 to 20 years. This period is taken 15 years in communities where

the population growth rate is higher and 20 years in communities where population

growth rate is comparatively lower.

v. Design/Service Year: It is the year up to which water demand is to be fulfilled.

Service Year = Survey Year + Base Period + Design Period

= Base Year + Design Period

3.2.2 Selection Basis

Design Period is selected based on the following:

Useful lives of the component considering obsolescence, wear, tear, etc.



Expandability aspect.

Anticipated rate of growth of population including industrial, commercial

developments and migration-immigration.

Available resources.

Performance of the system during initial period.

Suppose, r = growth rate of population

If r 2, design period is 15 years and if r < 2, design period is 20 years.

3.3 Types of Water Demand

3.3.1 Domestic Demand

Water demand required for domestic purposes.

Required for both urban and rural areas.

Depends upon the habit, social status, climatic conditions, living standard, etc.

S.N. Types of Consumption Water Demand (lpcd)

1 Private Connection and Fully Plumbed System 112

2 Private Connection and Partly Plumbed System 65

3 Public Stand Post 45 (can come down to 25)

3.3.2 Livestock Demand

Quantity of water required for domestic animals and livestock including birds.

Generally considered in rural water supply.

Livestock demand should not be greater than 20% of domestic demand.

S.N. Types of Consumption Water Demand

(lpcd)

1 Big animals >> cow, buffalo 45

2 Medium animals >> goat, dog 20

3 Small animals >> birds 0.2

3.3.3 Commercial/Institutional Demand

Quantity of water required for commercial institutions like schools, colleges, hospitals,

offices, etc.

For commercial and institutional purpose, 45 lpcd can be taken.



Institutions Demand

Urban Area Rural Area

a. Hospitals/Health Posts/Clinics

i. With Bed 500 l/bed/day 325-500 l/bed/day

ii. Without Bed 2,500 l/day 1600-2500 l/hospital/day

b. Schools

i. Boarders 65 lpcd 42-60 lpcd

ii. Day Scholars 10 lpcd 6.5-10 lpcd

c. Hotels

i. With Bed 200 l/bed/day 200 l/bed/day

ii. Without Bed 500-1000 l/day 500-1000 l/day

d. Restaurants/Tea Stall 500-1000 l/day 200-500 l/day

e. Offices

i. Unclassified 500-1000 l/day 325-1000 l/office/day

ii. Resident 65 lpcd 65 lpcd

iii. Non resident 10 lpcd 10 lpcd

3.3.4 Public/Municipal Demand

Considered only in urban areas for municipal purposes e.g. cleaning roads, for public

parks.

We adopt criteria by Indian Government.

i. Street Washing = 1 to 1.5 l/m2 of surface area of road/day

ii. Public Parks = 1.4 l/m2/day

iii. Sewer Cleaning = 4.5 l/person/day

3.3.5 Industrial Demand

Normally considered in urban areas.

Quantity of Water required for various industries and factories.

Generally taken as 20 to 25% of total demand.

3.3.6 Fire Fighting Demand

Authority Formula (P in '000, Q in l/min)

1. National Board of Fire Underwriters Formula Q = 4637 P (1 - o.01 P)

2. Freeman's Formula Q = 1136 (P/5 + 10)

3. Kuichling's Formula Q = 3182 P

4. Buston's Formula Q = 5663 P

5. Indian Water Supply Manual Formula Q = 100 P, Q in cubic meter/day



3.3.7 Loss and Wastage

15% of total demand is considered to be loss and wastage.

Loss or wastage of water can occur due to defective pipe joints, cracked and broken

pipes, faulty valves and fittings, unauthorized connection (theft), allowance for

keeping tap open, etc.

Loss and wastage is about 40% in Kathmandu Valley.

Considered only for urban areas.

3.3.8 Total Demand

Total Demand = Domestic Demand + Livestock Demand + Commercial Demand +

Municipal Demand + Industrial Demand + Fire Fighting Demand + Loss and Wastage

3.4 Variation in Demand of Water

If this average demand is supplied at all the times, it will not be sufficient to meet all the

fluctuations. There are three types of variations in demand of water.

Seasonal Variation: The demand peaks during summer. Fire breaks out generally more

in summer, increasing demand. So, there is seasonal variation. Maximum seasonal

consumption is 140% and minimum seasonal consumption is 80% of average daily per

capita demand.

Daily Variation: Daily variation is due to the variation in activities. People draw out

more water on holidays and festival days, thus increasing demand on these days. Daily

variation may also occur due to climatic condition (rainy day or dry day) and the

character of the city (industrial, commercial or residential). Maximum daily

consumption is 180% of average daily per capita demand.

Hourly Variation: Hourly variations are very important as they have a wide range.

During active household working hours i.e. from six to ten in the morning and four to

eight in the evening, the bulk of the daily requirement is taken. During other hours,

the variation in requirement is negligible. The maximum hourly consumption is 150% of

average daily per capita demand.



3.5 Peak Factor

Maximum demands at all these variations are expressed in terms of percentage of average

annual daily consumption (AADC) or Qav.

AADC or Qav = P x q, where P is the population and q is per capita demand.

Peak Demand is the maximum hourly demand on the day of maximum demand of the season

of maximum demand.

Peak Demand = PFH x PFD x PFS of AADC

Where, PFH = Peak Factor of Hourly Variation

PFD = Peak Factor of Daily Variation

PFS = Peak Factor of Seasonal Variation

Hence, Peak Demand = 1.5 x 1.8 x 1.4 x AADC = 3.93 x AADC

Generalizing, Peak Demand = Peak Factor x AADC

Peak Factor is normally taken 3 in Nepal.

3.6 Factors affecting Demand of Water

i. Size of the City: Per capita demand for big cities is generally large as compared to that

for smaller towns as big cities have mostly private connection in every house with fully

plumbed system.

ii. Presence of Industries

iii. Climatic Conditions: If a community is located in hot climate, water use will be

increased by bathing, lawn sprinkling and use in parks and recreation fields. In

extreme cold climates, water may be wasted at the faucets to prevent freezing of pipes,

resulting in increased consumption.



iv. Standard of Living: The higher the standard of living is, the higher the demand and

greater the variation in demand.

v. Quality of Water: If water is aesthetically and medically safe, the consumption will

increase as people will not resort to private wells, etc.

vi. Pressure in the Distribution System: Higher pressure results in increased use while

lower pressure results in decreased use.

vii. Efficiency of water works administration: Leaks in water mains and services and

unauthorized use of water can be kept to a minimum by surveys.

viii. Cost of Water

ix. Policy of metering and charging method: Water tax is charge in two different ways: on

the basis of meter reading and on the basis of certain fixed monthly rate.

3.7 Population Forecasting Necessity and Methods

A particular method is to be adopted for a particular case or for a particular city. The selection

is left to the discretion and intelligence of the designer.

Sample Problem:

Year Population Increase in Population

% increase in

Population

Incremental increase in Population

Decrease in % increase of Population

1981 8000 - - - -

1991 12000 4000 50 - -

2001 17000 5000 41.67 1000 8.33

2011 22500 5500 32.35 500 9.32

Total 14500 124.02 1500 17.65

Average A = 4833 G = 41.34 I = 750 D = 8.82

Present Population, P = 22500

A = average increase per decade = 4833

G = average % increase in population per decade = 41.34%

I = average incremental increase per decade = 750

D = average decrease in % increase of population = 8.82

3.7.1 Arithmetical Increase Method

Assumption: The increase in population from decade to decade is assumed constant.

This method is suitable for larger and old cities which have practically reached their

maximum development (i.e. cities which have reached their saturation population).

Pn = future population at the end of n decades



n = number of decades

P = present population

From above example,

P2021 = 22500 + 1 x 4833 = 27333

P2027 = 22500 + 1.6 x 4833 = 30233

3.7.2 Geometrical Increase Method or Uniform Percentage Growth Method

Assumption: The percentage increase in population from decade to decade is constant.

This method is suitable when the city is young and rapidly increasing.

This is the most common method used in Nepal.

(

)

Pn = population after n decades

G = average % increase per decade

Gives high result than arithmetical increase method so, much safer result.

(

)

(

)

3.7.3 Incremental Increase Method

This method combines both the above two methods gives value between the above

two methods.

( )

3.7.4 Decreased Rate of Growth Method

Year % increase

2011 2021 32.35 8.82 = 23.53

2021 2031 23.53 8.82 = 14.71

2031 2041 14.71 8.82 = 5.89

2041 2051 -ve (so zero constant)



(

)

(

)

(

)

(

)

(

)

The survey data collected for a water supply scheme in a village of Nepal is given below:

Survey Year 2013

Base Year 3 years

Design Period 20 years

Population 500

Number of cows 20

Number of goats 560

Number of chickens 2200

Annual population growth

rate 1%

Number of health posts 1

Number of day scholars in

school 125

Number of boarders in

school 20

Number of tea shops 2

VDC Office 1

Calculate Design Year Total Water Demand.

At 2036,

(

)

(

)

(

)

1. Domestic Demand = 45 x 629 = 28305 l/d

2. Livestock Demand

i. Big animals = 45 x 20 = 900

ii. Medium animals = 20 x 560 = 11200

iii. Small animals = 0.2 x 2200 = 440

Total = 12540 l/d

Check: Livestock Demand = 20% of Domestic Demand = 0.2 x 28305 = 5661 l/d

Hence, actual livestock demand = 5661 l/d

3. Commercial Demand

a. Day Scholars = 10 x 157 = 1570

b. Boarders = 65 x 25 = 1625

c. Health Post = 2500



d. Tea Shop = 2 x 1000 = 2000

e. VDC Office = 500

Total = 8195 l/d

Hence, Total Water Demand = 12540 + 28305 + 5661 + 8195 = 54701 l/d



CHAPTER IV

QUALITY OF WATER

4.1 Impurities in water, their classification and effects

4.1.1 Suspended Impurities

E.g. sand, silt, algae, virus

Characteristics:

They develop colour.

They make turbidity high. Suspended impurity is measured in terms of turbidity.

They develop taste.

They invite diseases.

They are macroscopic or can be microscopic.

Removed by: Sedimentation or Chemical Treatment

4.1.2 Colloidal Impurities

Microscopic. Their size is between 10-3 mm to 10-6 mm.

Not removed by sedimentation

Develop charges (anions)

Cause colour in water and these impurities cause epidemics.

Have much less weight

They come in motion due to repulsion.

Removed by: +ve charge for neutralization and settlement

4.1.3 Dissolved Impurities

Dissolved impurities make bad taste, hardness and alkalinity. The concentration is measured

in PPM (parts per million) or mg/l and obtained by weighing the residue after evaporation of

the water sample from a filtered sample.

a. Salts of Ca and Mg

b. Minerals

c. Gases

Constituents Effects

a. Calcium and Magnesium i. Bicarbonate

ii. Carbonate iii. Sulphate iv. Chloride

Alkalinity Alkalinity and hardness Hardness Hardness, corrosion

b. Metals and Compounds i. Lead

Cumulative poisoning



ii. Arsenic iii. Iron Oxide iv. Manganese v. Barium

vi. Cadmium vii. Cyanide

viii. Boron ix. Selenium x. Silver

xi. Nitrates

Toxicity, poisoning Taste, red colour, corrosiveness, hardness Black or brown colour Toxic effect on heart, nerves Toxic, illness Fatal Affects central nervous system Highly toxic to animals and fish Discoloration of skin, eyes Blue baby condition, infant poisoning, colour and acidity

c. Gases i. Oxygen

ii. Carbon iii. Hydrogen Sulphide

Corrosive to metals Acidity, corrosiveness Odour, acidity and corrosiveness

4.2 Hardness and Alkalinity

Water is said to be hard when it contains relatively large amounts of bicarbonates,

carbonates, sulphates and chlorides of calcium and magnesium dissolved in it. It is the

property that prevents lathering of soap.

4.2.1 Types of Hardness

Permanent hardness is due to the presence of sulphates, chlorides and nitrates of calcium and

magnesium and is also known as non-carbonate hardness (NCH). Permanent hardness cant

be removed by simple boiling but requires special treatment of softening.

Temporary hardness is known as carbonate hardness (CH) and due to the presence of

carbonates and bicarbonates of calcium and magnesium. It can be removed by boiling or by

adding lime. On boiling, CO2 escapes and insoluble CaCO3 gets precipitated. So, temporary

hardness causes deposition of Ca scales in boilers.

Total Hardness (TH) = CH + NCH

Types of Hardness

Permanent Hardness

Temporary Hardness



Where, ion = Mg, Ca, Sr (Strontium)

Eq. wt. of Mg = 12.2, Eq. wt. of Ca = 20, Eq. wt. of Sr = 43.8, Eq. wt. of CaCO3 =

50

Effects of Hardness:

1. Wasteful consumption of soap while washing and bathing.

2. Modifies colour if used in dyeing work and washing clothes.

3. Produces scale in steam boiler and its pipe which reduces heat transfer and finally

causes leak.

4. Causes corrosion and incrustation of pipelines and fittings.

5. Scale formation further causes corrosion, caustic brittleness, decreases efficiency and

danger of burst of pipe line and boiler.

6. Makes food tasteless, more fuel consumption and causes bad effects to our digestive

system.

Measurement of Hardness in Water:

Hardness of water is measured in ppm or mg/l of calcium carbonate present in water.

Range (mg/l) 0 50 50 100 100 150 150 250 > 250

Hardness Level Soft Moderately Soft

Slightly Hard Moderately Hard

Hard

The hardness of water is also expressed as the degree of hardness. It may be Clark Scale,

French Scale or American Scale.

Clarks Scale: 1 Cl = Power of soap destroying is equivalent to the effect of 14.254 mg of

calcium carbonate present in one litre of water which causes wastage of about 0.6 gm of soap

in 1 litre of water (i.e. 14.254 ppm).

French Scale: 1 Fr = Power of soap destroying is equivalent to the effect of 10 mg of calcium

carbonate present in one litre of water.

American Scale: 1 Am = Power of soap destroying is equivalent to the effect of 17.15 mg of

calcium carbonate present in one litre of water.

4.2.2 Types of Alkalinity

Alkalinity is a measure of the acid-neutralizing capacity of water. It is an aggregate of the sum

of all titratable bases in the sample. When pH of water is > 7, it is said to be alkaline. Alkalinity

in most natural waters is due to the presence of carbonate (CO3--

), bicarbonate (HCO3-), and

hydroxyl (OH-) anions.



[

]

[

]

Alkalinity caused by hydroxides is called hydroxide alkalinity or caustic alkalinity, caused by

carbonate is carbonate alkalinity and caused by bicarbonate is called bicarbonate alkalinity.

4.2.3 Relation between Hardness and Alkalinity

1. When Total Hardness > Total Alkalinity

CH = Total Alkalinity

NCH = TH CH

2. When Total Hardness Total Alkalinity

CH = TH

NCH = 0

Problem:

The analysis of water from a well shows the following results in mg/l.

Ca++

= 65, Mg++

= 51, Na+

= 100, K+

= 25, HCO3-

= 248, SO4--

= 220, Cl- = 18, CO3

-- = 240

Find Total Hardness (TH), Carbonate Hardness (CH) and Non-Carbonate Hardness (NCH).

Solution:

Here, TA > TH

Hence, Carbonate Hardness (CH) = Total Hardness (TH) = 371.52 mg/l

Alkalinity

Alkalinity due to Bicarbonate

Alkalinity due to Carbonate



Non-Carbonate Hardness (NCH) = 0

1. The analysis of a water sample shows the following results in mg/l. Ca++

= 7, Mg++

=

12, Na+

= 20, K+

= 25, HCO3-

= 68, SO4--

= 7, Cl- = 40. The concentration of Sr is equal

to hardness of 2.52 mg/l and the carbonate alkalinity in water is zero. Calculate TH,

CH and NCH.

2. Total hardness obtained from the analysis of water is found to be 117 mg/l. The analysis

further showed that the concentrations of all the three principle cations causing

hardness are numerically same. If the value of CH = 57 mg/l, calculate:

i. NCH.

ii. The concentration of principle cation (Ca, Mg, Sr)

iii. Total Alkalinity (TA)

4.3 Living Organisms in Water

a. Algae

b. Bacteria

c. Virus

d. Helminthes or Worms

(Refer descriptions in any book.)

4.4 Water Related Diseases

4.4.1 Water borne Diseases

Water borne diseases are caused due to drinking water contaminated with pathogenic

microorganisms. Some of the most common water borne diseases are typhoid fever, dysentery

(amoebic and bacillary), gastro-enteritis, infectious hepatitis, schistosomiasis, etc.

a. BACTERIAL DISEASES: Botulism, Cholera, E. coli infection, Dysentery, Typhoid fever

b. PROTOZOAL DISEASES: Amoebiasis, Giardiasis

c. VIRUS DISEASES: SARS (Severe Acute Respiratory Syndrome), Hepatitis A,

Poliomyelitis

d. HELMINTHIC DISEASES: Schistosomiasis, Swimmers itch

Water borne diseases

Bacterial diseases

Protozoal diseases

Virus diseases Helminthic

(worm) diseases



4.4.2 Water washed/hygiene Diseases

Water washed diseases are caused by poor personal hygiene and skin or eye contact with

contaminated water. Examples of water washed diseases include scabies, trachoma and flea,

lice and tick-borne diseases.

4.4.3 Water based Diseases

Water-based diseases are caused by parasites found in intermediate organisms living in

contaminated water. Examples include dracunculiasis, schistosomiasis and other helminthes.

These diseases are usually passed to humans when they drink contaminated water or use it for

washing.

** Schistosomiasis is a water-based disease which is considered the second most important

parasitic infection after malaria in terms of public health and economic impact.

4.4.4 Water vector Diseases

Due to vector like mosquitoes

E.g. malaria (mosquito injects protozoa), filariasis (elephantiasis) mosquito carrier, no

circulation of blood in joints, swelling of body parts

4.4.5 Transmission Routes

Transmission routes refer to the ways in which a healthy person gets attacked by diseases.

a. Faecal-oral route

b. Penetration of skin

c. Due to vector



4.4.6 Preventive Measures

Improve the quality of drinking water at source, at the tap, or in the storage vessel

Interrupting the routes of transmission

Protecting food from flies interrupts the faeces-flies-food route (at a household level).

Chlorination of water interrupts the faeces-fluids-food and drinking water route (at

the community level).

Increase the quantity of water available. This allows better hygiene and can thus

prevent disease transmission from contaminated hands, food or household utensils.

Changing hygiene behaviour.

Care in disposing of faeces. Safe and protective measures should be adopted to avoid

contamination and to destroy infectious organisms while handling and disposing of

infant and toddler faeces.

Proper use and maintenance of water supply and sanitation systems.

Good food hygiene.

4.5 Examination of Water

4.5.1 Physical Examination of Water (tests for temperature, colour and turbidity)

i. Test for temperature

The temperature of water to be supplied should be between 10C to 20C.

Temperature higher than 25C is considered objectionable.

Temperature of water can be measured with ordinary thermometers graduated in 0.1C,

range from 0 to 50C.

At depths greater than 15m, a thermocouple may be used.

ii. Test for colour

Colour can be measured against various standards or scales such as Hazen or Platinic

Chloride Scale, Burgess Scale or Cobalt Scale using a tintometer.

In older days, test for colour of water was performed solely through visual inspection.

Test for Colour by Tintometer:

1. First, the apparent colour of water due to turbidity is removed by centrifuging.

2. A tintometer has an eye-piece with two holes.

3. A slide of the standard coloured water is seen through one hole, while the slide of the

water to be tested is seen through the other hole.

4. A number of slides of standard colour in water are kept ready for comparison.

5. The intensity of colour in water is measured in terms of arbitrary unit of colour on the

cobalt scale.

iii. Test for Turbidity

Turbidity is a measure of resistance of passing of light through water. It is imparted by the

colloidal matter present in water. Units of turbidity in older days:



i. ppm in silica scale

ii. JTU (Jackson Turbidity Unit)

iii. FTU (Formagen Turbidity Unit)

iv. NTU (Nephelometric Turbidity Unit)

Equipment: Turbidity Meter

4.5.2 Chemical Examination of Water (tests for pH, suspended, dissolved and total solids)

1. Test for pH

The hydrogen-ion concentration or pH value of water is a measure of degree of acidity or

alkalinity of water. For water at 21C,

(H+) x (OH

-) = 10

-14

Water becomes acidic when concentration of H ions is increased and alkaline when

concentration of H ions is decreased.

( ) (

)

For pure water, pH = 7.

For water with maximum acidity, pH value is zero, while for water with maximum alkalinity,

pH value is 14.

For potable waters, the pH value should between 6 and 9, and preferable between 7 and 8.5.

2. Tests for Solids in Water

Total Solids - all solids in water. Total solids are measured by evaporating all of the water out

of a sample and weighing the solids which remain.

Dissolved Solids - solids which are dissolved in the water and would pass through a

filter. Dissolved solids are measured by passing the sample though a filter, they drying

the water which passes through. The solids remaining after the filtered water is dried

are the dissolved solids.

Suspended Solids - solids which are suspended in the water and would be caught by a

filter. Suspended solids are measured by passing sample water through a filter. The

solids caught by the filter, once dried, are the suspended solids.

Settleable solids - suspended solids which would settle out of the water if

given enough time. Settleable solids are measured by allowing the sample

water to settle for fifteen minutes, then by recording the volume of solids

which have settled to the bottom of the sample.

Nonsettleable solids - suspended solids which are too small and light to settle

out of the water, also known as colloidal solids. Nonsettleable solids are

measured by subtracting the amount of settleable solids from the amount of

suspended solids.



The amount of total solids should preferable be less than 500 ppm.

4.5.3 Biological Examination of Water (multiple tube and membrane fermentation method),

most probable number

MULTIPLE TUBE FERMENTATION TECHNIQUE

The coliform group of bacteria is defined as all aerobic and facultative anaerobic, gram-

negative, rod-shaped bacteria that ferment lactose with gas and acid formation within 48

hours at 35C.

1. Presumptive Phase

This test is based on the ability of coliform group (E-coli) to ferment the lactose broth and

producing gas.

Procedure:

i. Definite amount of diluted samples of water are taken in multiples of ten, such as 0.1

ml, 1.0 ml, 10 ml etc. Then, the samples are placed in standard fermentation tubes

containing lactose broth and then kept in the incubator at a temperature of 37C for a

period of 48 hours.

ii. If gas formed is seen in the tubes, it is the indication of presence of E. coli group and

result is +ve. If no gas is formed, the result is _ve.

iii. ve result in presumptive test indicates the water is fit for drinking.

2. Confirmed Phase

The other bacteria than E. coli present also may ferment in presumptive test so the confirmed

test to indicate E. coli is necessary. This test consists of growing cultures of coliforms on media

which suppress the growth of other organisms.

Procedure:

i. Small amount of incubated sample showing gas in presumptive test is carefully

transferred to another fermentation medium containing brilliant green lactose bile

broth and placed in the incubator at 37C for a period of 48 hours. If the gas is formed,

there is presence of E. coli and then step 2 is followed.

ii. Again the small portion of incubated material showing gas in presumptive test is

marked as streaks on the plates containing Endo or Eosin-methylene blue agar and the

plates are kept in the incubator at 37C for a period of 24 hours. If colonies of bacteria

are seen after this period, it indicates the presence of E. coli and completed test is

necessary.

3. Completed Phase

This test is based on the ability of the culture grown in the confirmed test to again ferment the

lactose broth.



Procedure:

i. The bacterial colonies or cultures grown in the confirmed test are kept into lactose

broth fermentation tubes and agar tubes.

ii. The tubes are then kept in the incubator at 37C for a period of 24 to 48 hours. If gases

are seen in tubes after this period, it indicates the presence of E. coli and the test is +ve

and it contains the pathogens, then detailed tests are necessary for pathogens.

iii. If result is ve, it indicates the absence of E. coli and hence absence of pathogens.

Example:

If we take 10 test tubes out of which 3 test tubes are positive after third test and in each test

tube, 1 ml of sample is kept,

No. of positive tubes = 3

ml of sample in negative tubes = 7

ml of sample in all tubes = 10

MEMBRANE FILTRATION TECHNIQUE

The coliform group may be defined as comprising all aerobic and many facultative anaerobic,

gram -ve, rod-shaped bacteria that develop a red colony with a metallic sheen within 24 hours

at 35C on an Endo-type medium containing lactose.

Take 50 ml sample of water and a filter paper.

The water is filtered through the filter paper.

Filter paper is kept in petidions glass plate along with M. Endo medium.

Incubate at 35C for 20 hours.

We can observe colonies of coliform.

where, x = sample



Problems:

In water treatment plant, the pH values of incoming and outgoing waters are 7.3 and

8.5 respectively. Assuming a linear variation of pH with time, determine the average

pH value of time.

There are two samples A and B of water, having pH values of 4.4 and 6.4 respectively.

Calculate how many times sample A is acidic than sample B.

Find out the pH of a mixture formed by mixing the following two solutions.

Vol. 300 ml - pH = 7, Vol. 700 ml - pH = 5.

4.6 Water Quality Standard for Drinking Purpose

(refer from any book)



CHAPTER - V

INTAKES

5.1 Definition

Intakes are the structures used for safely withdrawing water from the source over

predetermined pool levels and then to discharge this water into the withdrawal conduit,

through which it flows up to water treatment plant.

5.2 Site selection of an intake

Factors governing location of intake:

1. As far as possible, the site should be near the treatment plant so that the cost of

conveying water to the city is less.

2. The intake must be located in the purer zone of the source to draw best quality water

from the source, thereby reducing load on the treatment plant.

3. The intake must never be located at the downstream or in the vicinity of the point of

disposal of wastewater.

4. The site should be such as to permit greater withdrawal of water, if required at a future

date.

5. The intake must be located at a place from where it can draw water even during the

driest period of the year.

6. The intake site should remain easily accessible during floods and should not get

flooded. Moreover, the flood water should not be concentrated in the vicinity of the

intake.

5.3 Classification of Intake

1. According to source types

2. According to its position

3. According to water available in the chamber

1. a. River Intake

An intake tower constructed at the bank or inside of the river to withdraw water is called river

intake.

These intakes consist of circular or rectangular, masonry or RCC intake tower from where

water can be withdrawn even in the dry period. Several inlets called penstocks for drawing

water are provided at the different levels to permit the withdrawal of water when the water

level drops. All inlet ends are provided with a screen (to prevent the entry of floating matters)

with valves to control the flow of water operation from the control room.

The penstock discharges the water into the intake tower (intake well) from where it is pumped

or flow under gravity.

In dry river intake, there will be no water inside if the tower inlet valves are closed.



In wet river intake, there is water inside the tower even if the inlet valves of the tower are

closed. Since, these types of intakes remain wet, inspection cannot be done easily.

b. Reservoir Intake

There is a large variation in the discharge of river during monsoon and summer. When there

is no sufficient water in the dry period, the water in monsoon is collected in impounded

reservoir by constructing weirs or dams across the river. The intake tower used in such cases is

called reservoir intakes. Two types of reservoir intakes are commonly used to suit the type of



dam constructed. One type is at the slope of earthen dams and other type is within the dam

itself in case of RCC dams.

In case of earthen dam, the intake may consist of an intake tower constructed on the upstream

toe at dam from where intake can draw sufficient quantity of water even in the driest period.

The water is withdrawn through intake pipes located at different levels with a common

vertical pipe so as to draw water in the driest period. The vertical pipe is connected at the

bottom to an intake conduit which is taken out through the body of dam. Each inlet of intake

pipe is covered with a hemispherical shaped screen to enter relatively clear water. The intake

is provided with valves to control flow from control room. Since there is no water inside the

tower (only in inlet pipes), this intake is called dry intake tower.

In case of RCC masonry dams, dry intake is constructed inside the dam itself and only porters

or intake pipes are provided at various levels with control valves.

c. Lake Intake

It is a submersible intake normally constructed at the central portion of the bed of lake for

withdrawal of water because maximum depth of water is available at the central portion of

natural lake. It consists of an intake conduit laid on the bed of lake with its inlet end placed in

the middle of the lake projecting above the bed. The inlet end is then covered by protective



timber or concrete crib. The water enters in the pipe through bell mouth (may be with screen)

and flows under the gravity to treatment plant directly or to the sump well from where it can

be pumped to treatment plant. More than one intake conduit can be used as per requirement.

Since Lake Intake is submersible, there is no obstruction to the navigation, no danger from

floating bodies and no trouble due to ice and cheap in construction. It can draw small quantity

of water and hence can be sued in small water supply schemes whereas it is not easily

accessible for maintenance.

d. Canal Intake

When intake is constructed on canal for water supply purpose, the intake is called canal

intake. It consists of simple structure constructed on the bank and not necessary to provide

porters at various levels because water level in the canal remains more or less constant. It

consists of a pipe placed in a brick masonry or RCC chamber constructed partly in the canal

bank. On one side of the chamber, an opening is provided with coarse screen to enter water. A

bell-mouth with hemispherical fine screen in the inlet end of the inlet pipe inside is provided

and the outlet pipe is brought through the canal bank and taken to the treatment plant. One

sluice valve operated by a wheel from the top of masonry chamber is provided to control flow

in the inlet pipe.

e. Spring Intake

An intake constructed at the spring source to withdraw water is called spring intake. It is

generally constructed in small rural water supply scheme in Nepal. Spring intake should be

impervious and provided around the source to prevent the source contamination and physical

damage by runoff water. Simply one or more springs can be joined for greater discharge and

all sources should be protected from animals, exposure, runoff and bathing etc. Protection

work is done by fencing, digging catch drain, bioengineering works, etc.



2. a. Submerged Intake: Constructed entirely under water. It is commonly used to obtain

supply from a lake.

b. Exposed Intake: It is in the form of a well or tower constructed near the bank of a river, or in

some cases even away from the river banks.

3. a. Wet Intake: The water level is practically the same as the water level of the sources of

supply. Sometimes known as a jack well and most commonly used.

b. Dry Intake: There is no water in the water tower. Water enters through entry port directly

into the conveying pipes. The dry tower is simply used for the operation of valves.



CHAPTER VI

WATER TREATMENT

The available raw water must be treated and purified before they can be supplied to the public

for their domestic, industrial or any other uses.

The layout of conventional water treatment is as follows:

6.1 Objectives of Water Treatment

a. To remove the colour, odour (taste causing substances)

b. To remove the turbidity present in water

c. To remove pathogenic organisms

d. To remove hardness

e. To make water potable

f. To prevent the spread of diseases

6.2 Treatment Processes and Impurity Removal

1. SCREENING: Bulky and floating suspended matters are removed by the process of

screening.

2. PLAIN SEDIMENTATION: Heavy and coarse suspended matters are removed by the

process of plain sedimentation.

3. SEDIMENTATION WITH COAGULATION: This process helps to remove fine

suspended and colloidal matters.

4. FILTRATION: It is the most important stage in the purification process of water. It

removes very fine suspended impurities and micro-organisms.

5. DISINFECTION: It is carried out to eliminate or reduce pathogenic micro-organisms

that have remained after the process of filtration.

6. SOFTENING: Removes hardness of water.

7. AERATION: Aeration removes odour and tastes due to volatile gases like hydrogen

sulphide and due to algae and related organisms. Aeration also oxidize iron and

manganese, increases dissolved oxygen content in water, removes CO2 and reduces

corrosion and removes methane and other flammable gases.

8. Removal of Fe and Mn.

9. Removal of other harmful constituents.



6.3 Screening

6.3.1 Purpose

The function of screening is to remove large floating, suspended and settleable solids. The

treatment devices for the purpose of screening include bar racks and screens of various

description.

6.3.2 Coarse, Medium and Fine Screens

COARSE SCREENS:

Coarse screens are called racks, are usually bar screens, composed of vertical or inclined bars

spaced at equal intervals across a channel through which water flows. Bar screens with

relatively large openings of 75 to 150 mm are provided ahead of pumps, while those ahead of

sedimentation tanks have smaller opening of 50 mm.

Bar screens are usually hand cleaned and sometimes provided with mechanical devices. These

cleaning devices are rakes which periodically sweep the entire screen removing the solids for

further processing or disposal. Hand cleaned racks are set usually at an angle of 45 to the

horizontal to increase the effective cleaning surface and also facilitate the raking operations.

Mechanically cleaned racks are generally erected almost vertically.



MEDIUM SCREENS:

Medium screens have clear opening of 20 to 50 mm. Bar are usually 10 mm thick on the

upstream side and taper slightly to the downstream side. The bars used for screens are

rectangular in cross section usually about 10 x 50 mm, placed with larger dimension parallel to

flow.

FINE SCREENS:

Fine screens are mechanically cleaned devices using perforated plates, woven wire cloth or

very closely spaced bars with clear openings of less than 20 mm. They are used to remove

smaller suspended impurities at the surface or ground water intakes, sometimes alone or

sometimes following a bar screen.

In case of surface intakes, fine screens are usually arranged with rotary drum perforated with

holes and are called rotary drum strainer. Micro strainer also can be used for this purpose

where some device is set up to clean continuously so that fine screens do not get clogged up.

Fine screens normally get clogged and are to be cleaned frequently. So they are avoided

nowadays for surface intakes and fine particles are separated in sedimentation.

HEAD LOSS:

The head loss created by a clean screen may be calculated by considering the flow and the

effective areas of screen openings, the latter being the sum of the vertical projections of the

openings. The head loss through clean flat bar screens is calculated from the following

formula:

h = 0.0729 (V2 - v2)

where, h = head loss in m

V = velocity through the screen in m/s

v = velocity before the screen in m/s

Another formula often used to determine the head loss through a bar rack is Kirschmer's

equation:

h = b (W/b) 4/3 hv sin q

where h = head loss, m

b = bar shape factor (2.42 for sharp edge rectangular bar, 1.83 for rectangular bar with semicircle upstream, 1.79 for circular bar and 1.67 for rectangular bar with both u/s and d/s face as semi-circular).

W = maximum width of bar u/s of flow, m



b = minimum clear spacing between bars, m

hv = velocity head of flow approaching rack, m = v2/2g

q = angle of inclination of rack with horizontal

The head loss through fine screen is given by

h = (1/2g) (Q/CA)

where, h = head loss, m Q = discharge, m3/s C = coefficient of discharge (typical value 0.6) A = effective submerged open area, m2

6.4 Plain Sedimentation

When the impurities are separated from suspending fluid by action of natural forces alone i.e.

by gravitation and natural aggregation of the settling particles, the operation is called plain

sedimentation.

6.4.1 Purpose

The main purpose of plain sedimentation is to remove large amounts of suspended solids present in raw water. It is done after screening and before sedimentation with coagulation and located near the filter unites and in case of variation of demand it can be used as the storage reservoir.

6.4.2 Theory of Settlement

Principle of Sedimentation:

Suspended solids present in water having specific gravity greater than that of water tend to settle down by gravity as soon as the turbulence is retarded by offering storage, thereby making easy to remove the sediments (called sludge) and floating matters (called scum).

Basin in which the flow is retarded is called settling tank or sedimentation tank or settling basin or sedimentation basin.

Theoretical average time for which the water is detained in the settling tank is called the detention period/time or retention period/time.

The sedimentation is affected by:

i. Velocity of flowing water ii. Size, shape and specific gravity of particles

iii. Viscosity of water iv. Detention time v. Effective depth and length of settling zone



vi. Inlet and outlet arrangements

Types of Settling

Type I: Discrete Particle Settling: Particles settle individually without interaction with neighbouring particles.

Type II: Flocculent Particles: Flocculation causes the particles to increase in mass and settle at a faster rate.

Type III: Hindered or Zone Settling: The mass of particles tends to settle as a unit with individual particles remaining in fixed positions with respect to each other.

Type IV: Compression: The concentration of particles is so high that sedimentation can only occur through compaction of the structure.

6.4.2.1 Derivation of Stokes Law

In Discrete Particle Settling, particles settle individually without interaction with neighbouring particles. Size, shape and specific gravity of the particles do not change with time. Settling velocity remains constant.

If a particle is suspended in water, it initially has two forces acting upon it.

If the density of the particle differs from that of the water, a net force is exerted and the particles are accelerated in the direction of the force:

( )

This net force becomes the driving force.

Once the motion has been initiated, a third force is created due to viscous friction. This force, called the drag force, is quantified by:

Because the drag force acts in the opposite direction to the driving force and increases as the square of the velocity, acceleration occurs at a decreasing rate until a steady state velocity is reached at a point where the drag force equals the driving force:



( )

For spherical particles,

Thus,

(

( )

)

Also, we have,

( )

Hence,

(

( )

)

The above equation is called Hazens Equation and applicable for particles having diameter greater than 0.1 and less than 1 mm and Reynolds Number Re greater than 1 and less than 1000. The nature of settling is neither laminar nor turbulent and so the settling is called transition settling.

Expressions for CD change with characteristics of different flow regimes.

( )

( )

( )

( )

Temperature T (C) 0 5 10 15 20 25 30

-kinematic viscosity (mm2/s or centistokes)

1.792 1.519 1.308 1.141 1.007 0.897 0.804

Hazen further indicated that for particles having diameter d 0.1 mm and Reynolds number Re 1, Stokes Law is applicable. Mathematically,



Now,

Putting

, we get,

Thus,

( )

This is Stokes Equation.

6.4.2.2 Temperature Effect on Settlement

Since kinematic viscosity of water depends on temperature; the settlement process also depends on temperature.

Alternatively, if temperature T is introduced in place of in above formula, it can be expressed as:

( )

These equations are valid for d 0.1 mm and Re 1. In this range, settling of particles is laminar and so it is termed as laminar settling of particles.

If the nature of settling of particles is turbulent (i.e. 1000 < Re 10000) and d > 1 mm, the value of CD = 0.4. Then, Hazens equation becomes:

( )

( )

( )

This equation is called Newtons Equation.

6.4.3 Ideal Sedimentation Tank

Sedimentation tanks may function either intermittently or continuously. The intermittent tanks also called quiescent type tanks are those which store water for a certain period and keep it in complete rest. In a continuous flow type tank, the flow velocity is only reduced and the water is not brought to complete rest as is done in an intermittent type.



Settling basins may be either long rectangular or circular in plan. Long narrow rectangular tanks with horizontal flow are generally preferred to the circular tanks with radial or spiral flow.

In practice, settling occurs in flowing water. An ideal horizontal flow settling tank has the following characteristics:

At the inlet, the suspension has a uniform composition over the cross-section of the tank.

The horizontal velocity vo is the same in all parts of the tank. A particle that reaches the bottom is definitively removed from the process.

6.4.4 Types of Sedimentation Tank

Sedimentation tanks are generally made of RCC and may be rectangular or circular in shape. According to the method of function or operation, they are classified into:

i. Quiescent or fill and draw type ii. Continuous flow type

Quiescent or Fill and Draw Type

This tank is normally rectangular in plan. The water is first filled and then allowed for some retention period of 30 to 60 hours (normally 24 hours) for sedimentation of particles. The clear water is drawn from outlet and the tank is then emptied and cleaning of sediments is done. After cleaning, again the filling and emptying process is similarly repeated. These tanks need more detention period, more labour and supervision. More than one tank is required and head loss is high; hence, these tanks are not used nowadays.

Continuous Flow Type

Raw water is admitted continuously through inlet and allowed to flow slowly in the tank for continuous settlement, cleaning and clear water continuously flows out through outlet. These tanks work under the principle that by reducing the velocity of flow of water, large amounts of particles present in water can be made to settle down. The velocity of flow of water in these tanks is reduced by providing sufficient length of travel for water in the tank. Further, the velocity of flow of water in these tanks is so adjusted that the time taken by particles of water to move from inlet to outlet is slightly more than that required for settling of suspended particles in water.

Continuous flow type sedimentation tanks may be rectangular, circular or square in shape.

a. Horizontal Flow Type b. Vertical Flow Type



Long Rectangular Settling Basin



Long rectangular basins are hydraulically more stable and flow control for large volumes is easier with this configuration.

A typical long rectangular tank has length ranging from 2 to 4 times its width. The bottom is slightly sloped to facilitate sludge scraping. A slow moving mechanical sludge scraper continuously pulls the settled material into a sludge hopper from where it is pumped out periodically.

A long rectangular settling tank can be divided into four different functional zones:

Inlet Zone: Region in which the flow is uniformly distributed over the cross section such that the flow through settling zone follows horizontal path.

Settling Zone: Settling occurs under quiescent conditions.

Outlet Zone: Clarified effluent is collected and discharged through outlet weir.

Sludge Zone: For collection of sludge below settling zone.

Inlet and Outlet Arrangements

Inlet Devices: Inlets shall be designed to distribute the water equally and at uniform velocities. A baffle should be constructed across the basin close to the inlet and should project several feet below the water surface to dissipate inlet velocities and provide uniform flow.

Outlet Devices: Outlet weirs or submerged orifices shall be designed to maintain velocities suitable for settling in the basin and to minimize short-circuiting. Weirs shall be adjustable, and at least equivalent in length to the perimeter of the tank. However, peripheral weirs are not acceptable as they tend to cause excessive short-circuiting.



Circular Basins

Circular settling basins have the same functional zones as the long rectangular basin, but the flow regime is different. When the flow enters at the centre and is baffled to flow radially towards the perimeter, the horizontal velocity of the water is continuously decreasing as the distance from the centre increases. Thus, the particle path in a circular basin is a parabola as opposed to the straight line path in the long rectangular tank.

Sludge removal mechanisms in circular tanks are simpler and require less maintenance.



Vertical Flow Type Sedimentation Tank

These tanks may be square or circular in shape at the top and have hopper bottom. So it is also called hopper bottom tank. The flow of water in this tank is vertical. Water enters into the tank through centrally placed pipe and by the action of deflector box, it travels vertically downwards. The sludge is collected at the bottom and removed from the sludge pipe with pump. The clear water flows out through a circumferential weir discharging into the draw off channel.

6.4.5 Design of Sedimentation Tank

Design of sedimentation tank needs the following:

a. Inlet Zone with appropriate Inlet Structure: Suitable inlet structure should be

designed. It is kept at the halfway between the surface and the floor of the tank and



mid of the width of the water depth. The length of the inlet zone is taken as 0.2 to 1 m

according to velocity.

b. Outlet Zone with appropriate Outlet Structure: Suitable outlet structure should be

designed. The length of the outlet zone is taken as 0.2 to 1 m according to velocity.

c. Sludge Zone: The zone in the bottom of the tank in which sludge is retained before

being removed is called sludge zone. The depth of the sludge zone depends upon the

quantity of sediments in the raw water and the de-sludging period. Depth of sludge

zone is taken as 0.5 to 1.5 m (generally 1 m).

d. Free Board: The free space left on the top of the water level on the tank is called free

board (FB) and in design FB is taken as 0.1 to 1 m (generally 0.3 to 0.5 m).

e. Others such as Baffles, Washout/Drain and Overflow etc.: Baffle walls are

provided to improve L/B ratio without increasing tank size. Washout is provided at the

bottom of the sloped portion for drain at cleaning. Overflow is provided just below

from the inlet in suitable side for overflow.

f. Settling Zone or Effective Zone: Actual settlement occurs in this zone. Hence,

effective dimensions [effective length (l), width (b) and effective depth (d)] of this zone

is very important for design.

Settling Operations

Particles falling through the settling basin have two components of velocity:

1. ( )

2.



The path of the particle is given by the vector sum of horizontal velocity (vh) and vertical settling velocity (vt).

Assume that a settling column is suspended in the flow of the settling zone and that the column travels with the flow across the settling zone. Consider the particle in the batch analysis for type-1 settling which was initially at the surface and settled through the depth of the column Zo, in the time to. If to also corresponds to the time required for the column to be carried horizontally across the settling zone, then the particle will fall into the sludge zone and be removed rom the suspension at the point at which the column reaches the end of the settling zone.

All particles with vt > vo will be removed from suspension at some point along the settling zone.

Now consider the particle with settling velocity < vo. If the initial depth of this particle was such that Zp/vt = to, this particle will also be removed. Therefore, the removal of suspended particles passing through the settling zone will be in proportion to the ratio of the individual settling velocities to the settling velocity vo.

The time to corresponds to the retention time in the settling zone.

Thus, the depth of the basin is not a factor in determining the size of particles that can be removed completely in the settling zone. The determining factor is the quantity Q/As, which has the units of velocity and is referred to as the overflow rate (SOR Surface Overflow Rate or Surface Loading Rate) qo. This overflow rate is the design factor for settling basins and corresponds to the terminal settling velocity of the particle that is 100% removed. As = effective surface area of tank

Removal Efficiency of Sedimentation Tank

Let, is the settling velocity of smaller particles less than SOR (i.e.

(

)) and if out of xo

particles, x particles settle down and are removed, the ratio of removal of these particles (x/xo) is called removal efficiency of sedimentation tank for discrete particles of same size and is given by,

(

)



Where,

is SOR and represents the settling velocity of the slowest particles, which are 100%

removed.

Design Criteria of Sedimentation Tank/Design Details

1. Detention Period: For plain sedimentation: 3 to 4 hours, and for coagulate sedimentation: 2 to 2.5 hours

2. Velocity of Flow: Not greater than 30 cm/min (horizontal flow) 3. Tank Dimensions: L:B = 3 to 5:1. Generally L = 30 m (common); maximum 100 m.

Breadth = 6 to 10 m. Circular: Diameter not greater than 60 m. Generally 20 to 40 m.

4. Depth 2.5 to 5.0 m (3 m). 5. SOR: For plain sedimentation: 12000 to 18000 L/d/m2 tank area; for thoroughly

flocculated water: 24000 to 30000 L/d/m2 tank area. 6. Slopes: Rectangular 1% towards inlet and circular 8%.

6.5 Sedimentation with Coagulation/Clarification

General Properties of Colloids

1. Colloidal particles are so small that their surface area in relation to mass is very large. 2. Electrical Properties: All colloidal particles are electrically charged. If electrodes from a

D.C. source are placed in a colloidal dispersion, the particles migrate towards the pole of opposite charge.

3. Colloidal particles are in constant motion because of bombardment by molecules of dispersion medium. This motion is called Brownian motion (named after Robert Brown who first noticed it).

4. Tyndall Effect: The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension.

5. Adsorption: Colloids have high surface area and hence have a lot of active surface for adsorption to occur. The stability of colloids is mainly due to preferential adsorption of ions. There are two types of colloids:

i. Lyophobic Colloids: that is solvent hating. ii. Lyophilic Colloids: that is solvent loving.

6.5.1 Purpose

Colloidal particles are difficult to separate from water because they do not settle by gravity and are so small that they pass through the pores of filtration media.

To be removed, the individual colloids must aggregate and grow in size.

The settling down and removal of such fine suspended particles and colloidal matters can be achieved by chemically assisted sedimentation called sedimentation with coagulation or clarification. The chemicals added are called coagulants; the formed insoluble gelatinous precipitate is called floc; the process of adding coagulants to raw water and mixing it thoroughly is known as coagulation and the process of formation of floc is called flocculation.

If the content of suspended solids in raw water is greater than 50 mg/l, the sedimentation with coagulation is used to effect more complete removal of the suspended matters.



6.5.2 Coagulants (types and their chemical reactions)

The following chemicals are used as coagulants:

1. Aluminium sulphates or alum

2. Iron salts

3. Chlorinated copperas

4. Sodium aluminate

The dose of coagulants depends upon turbidity, colour, pH, temperature and the time of the

settlement.

1. Aluminium Sulphates or Alum [Al2(SO4)3.18H2O]

It is the commonly used coagulant for coagulation in water in which alum is added and for

alum water shall contain some alkalinity. If bicarbonate alkalinity is present in water, the floc

formed is given by:

( ) ( ) ( ) ( )

If raw water contains little or no alkalinity, then either lime (hydrated lime) or soda ash is

added for alkalinity. Then,

( ) ( ) ( ) ( )

( ) ( ) ( )

Amount of alum required depends upon turbidity and colour of raw water. Usual dose is 5

mg/l for relatively clear water to 30 mg/l for highly turbid water. Average dose for normal

water is 14 mg/l but amount to be added is determined by jar test.

Advantages:

i. It forms excellent floc which is better than that formed by any other coagulant.

ii. The floc formed is stable and not broken easily.

iii. It is relatively cheap and removes colour, odour and taste.

iv. It doesnt require skilled supervision and produces clear and crystal free water.

Disadvantages:

i. It requires alkalinity ranging pH from 6.5 to 8.5 in water for effective use.

ii. The product calcium sulphate may cause permanent hardness and carbon dioxide may

cause corrosion.

iii. Difficult to dewater the heavy sludge formed because it is not suitable for filling in the

low levels.

2. Iron Salts

The various iron salts used as coagulants are ferrous sulphates, ferric sulphates and ferric

chloride.



1. Ferrous Sulphates [FeSO4.7H2O]

It is also known as copperas and used as coagulant in conjunction with lime.

When ferrous sulphates is added first (with bicarbonate alkalinity)

( ) ( )

( ) ( ) ( )

When lime is added first

( ) ( )

In above equation, Fe(OH)2 is unstable and absorbs dissolved oxygen and forms the stable

floc.

( ) ( ) ( )

The effective range of pH value for coagulation with ferrous sulphates and lime is 8.5 and

above.

2. Ferric Sulphates [Fe2(SO4)3]

It is also used as a coagulant in conjunction with lime and the reaction is:

( ) ( ) ( ) ( )

The effective range of pH for coagulation with ferric sulphates is 4 to 7.

3. Ferric Chloride [FeCl3]

It is used as a coagulant in conjunction with lime or without lime.

Reactions:

When used without lime:

( )

When used with lime:

( )

Documents

Notes - Water Supply Engineering