Men Tse Geba

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1.Domestic Water Demand

It includes the quantity of water required in the houses for drinking, bathing, washing hands

and face, flushing toilets, washing cloths, floors, utensils, etc.

In developed countries the domestic water demand may be as high as 350l/cap/day. In many

cases water demands are fixed by governmental agencies. Water demand data provided by

ministry of water resources of Ethiopia are given in tables below.

Table 1 Estimation of per capita demand for piped water in l/c/d (1997) for population of

greater than 30,000(urban and rural)

Table 2Estimate of per capita demand for piped water in l/c/d (1997) for population of less

than 30,000(for urban between 2500 and 30000).

No. Activity House

Connection

Yard

Connection

Public fountains

(Stand pipes)

Rural

Schemes

1 Drinking 1.5 1.5 1.5 1.5

2 Cooking 5.5 3.5 3.5 3.5

3 Ablutions 15 10 6 5

4 Washing dishes 5 2 2 2

5 Laundry 15 8 7 36 House cleaning

7 Bath and shower 4 1

8 Toilets 20 4

70 30 20 15

No. Activity House

Connection

Yard

Connection

Public fountains

(Stand pipes)

Rural

Schemes

1 Drinking 2.5 2.5 2.5 2.5

2 Cooking 7.5 5.5 4.5 3.53 Ablutions 17 12 7 5

4 Washing dishes 5 4 4 3

5 Laundry 15 8 7 4

6 House cleaning 7 3 2 2

7 Bath and shower 20 4 3

8 Toilets 6 1

80 40 30 20


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Table 3Estimate of per capita demand for rural schemes in l/c/d (1997)

No. Activity Minimum Average Maximum

1 Drinking 1.5 1.5 3.5

2 Cooking 2.5 3.5 4.5

3 Ablutions 4 5 5

4 Washing dishes 2 2 4

5 Laundry 3 3

6 House cleaning

7 Bath and shower

8 Toilets

10 15 20

2.Commercial Water Demand

It is the water required for commercial buildings & centers include stores, hotels, shopping

centers cinema houses, restaurants, bar airport, automobile service station, railway and bus

stations, etc (table 4).

3.Institutional Water Demand

This is also known as public demand. It is the water required for public buildings and

institution such as schools, hospitals, public parks, play grounds, gardening, sprinkling on rods,

etc, (table 4).

Table 4 Commercial and institutional demand

Categories Typical rate of water use per day

Day school 5lit/pupil

Boarding school 100lit/pupil

Hospitals 100lit/bed

Church/Mosque 5lit/visitor

Cinema houses 5lit/visitor

Public paths 100lit/visitor

Abattoir 300lit/cow

Hotels 100lit/bed

Restaurant-bar 15lit/seat

Offices 5lit/person

Bus terminals 10lit/visitor

Prison 30lit/person


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4.Industrial Water Demand

The water requirements for this purpose defend up on the type and size of the industry (table 5)

Table 5Typical values of water use for various industries

Industry Range of flow (*Gal/ ton Product)

-----------------------------------------------------------------------------------------------------

Cannery

Green beans 12000-17000

Peaches & pears 3600-4800

Other fruits & vegetables 960-8400

Chemical

Ammonia 24000-72000

Carbon dioxide 14400-21600Lactose 144000-192000

Sulfur 1920-2400

Food and beverage

Beer 2400-3840

Bread 480-960

Meat packing 3600-4800

Milk products 2400-4800

Whisky 14400-19200

Pulp and paper

Pulp 60000-190000

Paper 29000-38000

Textile

Bleaching 48000-72000

Dyeing 7200-14400

1gal. = 3.7854 lit

5.Fire fighting water demand (Fire demand)

Fires generally break in thickly populated localities and in industrial area and cause serious

damages of properties and some time life of people are lost. Fire may take place sue to faulty

electric wires by short circuiting, fire catching materials, explosions, bad iterations of criminal

people or any other unforeseen happenings. If fires are not properly controlled and extinguished

in minimum possible time, they lead to serious damages and may burn the cities.


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In cities fire hydrants should be provided on the mains at a distance of 100 to 150m apart. Fire

brigade men immediately connect these fire hydrants with their engines & start throwing water

at very high rate on the fire.

Fire demand is treated as a function of population and some of the empirical formulae

commonly used for calculating demand as follows:

a) John R.Freeman s formula:

Q = 1136.50*( 105+

P)

Where Q = Quantity of water required in 1/min.

P = population in thousands

He also states that

F = 2.8* P

Where F = period of occurrence of fire in years.P = population in thousands

b) Knucklings formula

Q = 3182* P



c) National Boarded of Fire Underwriters formula (widely used in USA)

Q = 4637* P *(1 - 0.01* P )



Example 1

Calculate the fire demand for a population of 100,000 by using formulae of Freeman,

Knuckling and National Board of Fire Underwriters.

Name of Formula Formula Fire Demand in l/min

Q = 1136.50 ( )10

5+

P

34,095

F = 2.8 =100 28 year

Knuckling Q = 3182* P 31,820

National Board of Fire

Underwriter

Q =

4637* )01.01(* PP

41,733

Although the actual amount of water in a year for firefighting is smaller than the rate of use, the

Insurance Service Office (USA) uses the formula


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Q = 18*C*(A)0.5

Where Q = the required fire flow in gpm (lit/min/3.78)

C = a coefficient related to the type of construction which ranges from a max of 1.5 for

wood frameto a minimum of 0.60 forfire resistive construction.

A = total floor area ft2(m2x10.76) excluding the basement of the building

The fire flow calculated from the formula is not to exceed 30,240 lit/min in general, nor

22,680 lit/min for one story construction .The minim fire flow is not to be less than 1890

lit/min. Additional flow may be required to protect nearby buildings. The total for all

purposes for a single fire is not to exceed 45,360 lit/min nor be less than 1990 lit/min.

For groups of single and two-family residences, the following table may be used to determine

the required flow.

The fire flow must be maintained for a minimum of 4 hours as shown in table 7. Most

communities will require duration of 10 hours.

Table 6: Residential fire flows

Distance b/n adjacent units in m Required fire flow in lit/min

> 30.5

9.5 - 30.5

3.4 - 9.2

< = 3.0

1890

2835 - 3780

3780 - 5670

5670 7560*

* For continuous construction use 9450 lit/min

Table 7: Fire flow duration

Required fire flow in l/min Duration in hrs

< 3780 4

3780 4725 5

4725 5670 6

5670 6615 7

6615 7560 8

7560 8505 9

>8505 10

Example 2

In order to determine the max water demand during a fire, the fire flow must be added to the

maximum daily consumption. It is assumed that a community with a population of 22,000 has

an average consumption of 600 lit/capita/day and flow directed by a building of ordinary


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construction(C = 1) with a floor area of 1000m2

and a height of 6 stories, the calculation is as

follows:

Average domestic demand = 22,000*600 = 18.2*106lit/day

Maximum daily demand = 1.8*13.2*106

= 23.76*106lit/day

F = 18(1) (1000*10.76*6)0.5 = 17,288 lit/min = 24.89*106lit/day

Maximum rate = 23.76*106

+ 24.89*106

= 48.65*106lit/day

= 2,211 lit/capita/day for 10 hours

The total flow required during this day would be

= 23.76 + 24.89*10/24

= 34.13*106

liters = 1,551 lit/capita/day

The difference between the maximum domestic rate and the above values is frequently

provided from elevated storage tanks.

6) Unaccounted for Water

These include the quantity of water due to wastage, losses, thefts, etc, i.e.

Waste in the pipelines due to defective pipe joints, cracked and broken pipes, faulty

valves and fittings

Water that is lost when consumers keep open their taps or public taps even when they

are not using water and allow continuous wastage of water.

Water that is lost due to unauthorized and illegal connection

While estimating the total water demand of water for a town or city, allowance for these losses

and wastage should be done. Generally, 15 40% of the total quantity of water is made to

compensate for lose, thefts and wastage of water.

1.3 Per capita Demand

If Q is the total quantity of water required by various purposes by a town per year and P is

the population of town, then per capita demand will be

day

litres

P

QdemandcapitaPer

365*

=

For the purposes of estimation of total requirement the water demand is expressed in

liters/capita/day i.e. per capita demand.

The following are the main factors affecting per capita demand of the town:

(i) Climatic condition: The requirement of water in summer is more than that in

winter. The quantity of water required in hotter and dry places is more than cold


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countries because of the use of air coolers, more washing of clothes and bathing

etc.

(ii) Size of the 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.(iii)

Standard of living: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.

(iv) Industries and commercial activities:As the quantity of water required in certain

industries is much more than domestic demand, their presence in the town will

enormously increase per capita demand of the town. As a matter of the fact the

water required by the industries has no direct link with the population of the town.

(v) Quality of water: If the quality of water is good, the people will consume more

water. On the other hand, if the water has unpleasant taste or odor, the rate of

consumption will down.

(vi) System of sanitation: If a town is provided with water carriage system of

sanitation, the per capita demand increases because the people will use more

quantity of water for flushing sanitary fixtures.

(vii) Cost of water:The higher the cost, the lower will be the per capita demand and

vice versa.

(viii) Use of water meters: If metering is introduced for the purpose of charging, the

consumer will be cautious in using water and there will be less wastage of water.So per capita demand may lower down.

(ix) System of supply:The supply of water may be continuous or intermittent. In the

former case, water is supplied for 24 hour and in the latter case water is supplied

for certain duration of day only.

It is claimed that intermittent supply system will reduce per capita demand. But sometimes, the

results are proved to be disappointing, mainly for the following reasons:

During non-supply period, the water taps are kept open and hence, when the supply

starts, water flowing through open taps is unattended and this results in waste of water.

There is tendency of many people to through away water stored previously during non-

supply hours to collect fresh water. This also results in waste of water and increase per

capita demand.


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Variation in rate of consumption

The per capita daily water consumption (demand) figures discussed above have been based

upon annual and it indicates the average consumption. The annual average daily consumption,

while useful, does not tell the full story.

In practice it has been seen that this demand does not remain uniform throughout the year.

Climatic conditions, the working day, etc tends to cause wide variations in water use. The

variation may be categorized into two broad classes:

i.

Seasonal fluctuation

ii.

Daily and hourly fluctuation.

Through the week, Monday will usually have the highest consumption, and Sunday the lowest.

Some months will have an average daily consumption higher than the annual average. In most

cites the peak month will be July or august. Especially hot, dry weathers will produce a week ofmaximum consumption, and certain days will place still greater demand upon the water system.

Peak demands also occur during the day, the hours of occurrence depending upon the

characteristics of the city. There will usually be a peak in the morning as the days activities

start and a minimum about 4am. A curve showing hourly variation in consumption for a limited

area of city may show a characteristic shape. But there will be a fairly high consumption

through the working day. The night flow, excluding industries using much water at night, is a

good indication of the magnitude of the loss and waste.

Fig 1 Variation in rate of water consumption throughout the day


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Fig 2 Seasonal variation of water demand

The important of keeping complete records of water consumption of city for each day and

fluctuations of demand throughout the day cannot be overemphasized. So far as possible the

information should be obtained for specific areas. These are the basic data required for

planning of water works improvement. If obtained and analyzed, they will also indicate trends

in per capita consumptions and hourly demands for which further provision must be made.

In the absence of data it is some times necessary to estimate the maximum water consumption

during a month, weekday, or hours. The maximum daily consumption is likely to be 180 % of

the annual average and may rich 200 %. The formula suggested by R.O Goodrich is convenient

for estimating consumption and is:

p = 180t- 0.10

Where p = the percentage of the annual average consumption for the time t in days from 2/24 to

360.

The formula gives consumption for the maximum day as 180 percent of the average, the

weekly consumption 148 percent, and the monthly as 128 percent. These figures apply

particularly to smaller residential cites. Other cites will generally have smaller peaks.

The maximum hourly consumption is likely to be about 150 percent of the average for that day.Therefore, the maximum hourly consumption for a city having an annual average consumption

of 670 lit/day per capita would occur on the maximum day and would be 670*1.8*1.5 or 1809

lit/day.

The fire demand must also be added, according to the method indicated in the above articles.


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Peaks of water consumption in certain areas will affect design of the distribution system. High

peaks of hourly consumption can be expected in residential or predominantly residential

sections because of heavy use of water for lawn watering especially where under ground

system are used, air condition or in other water using appliance. Since use of such appliances is

increasing peak hourly consumptions are also increasing.The determination of this hourly variation is most necessary because on its basis the rate of

pumping will be adjusted to meet up the demand in all hours.

1.3.

Before designing and construction a water supply scheme, it is the engineers duty to assure

that the water works should have sufficient capacity to meet the future water demand of the

town for number of years. The number of years for which the designs of the water works have

been done is known as the design period.The period should neither should neither be to short or too long. Mostly water works are

designed for design period of 22 - 30 years which is fairly good period. In some specific

components of the project, the design period may be modified. Different segments of the water

treatment and distribution systems may be approximately designed for differing periods of time

using differing capacity criteria, so that expenditure far ahead of utility is avoided. Table 7

gives the design periods for various units of water supply system:

Table 7 Design periods for various units of water supply system

S. No. Name of Unit Design period in years

1

2

3

4

5

6

Storage (dam)

Electric motors & pumps

Water treatment units

Distribution (pipe line)

Pipe connection to several treatment

plants and other appurtenants

raw water and clear water conveyance pipes

50

15

15

30

30

30

In general the following points should be kept in mind while fixing the design period for anywater supply scheme.

Funds available for the completion of the project (the higher the availability of the fund

the higher will be the design period.)

Life of the pipe and other structural materials used in the water supply scheme. (Design

period in no case should have more life than the components and materials used in the


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scheme. At least the design period should be nearly equal to the materials used in water

supply works.)

Rate of interest on the loans taken to complete the project.(if the interest rate is less, it

will be good to keep design period more otherwise the design period should be small)

Anticipated expansion rate of the town.

1.4.

The data about the present population of a city under question can always be obtained from the

records of municipality or civic body. The knowledge of population forecasting is important for

design of any water supply scheme.

When the design period is fixed the next step is to determine the population of a town or city

population of a town depends upon the factors like births, deaths, migration and annexation.

The future development of the town mostly depends upon trade expansion, developmentindustries, and surrounding country, discoveries of mines, construction of railway stations etc

may produce sharp rises, slow growth and stationary conditions or even decrease the

population.

The following are the common methods by which the forecasting of population is done.

1. Arithmetic increases method

2. Geometric increase method

3. Incremental increase method

4. Decrease rate method

5. Simple graphical method

6. Master plan curve method

7. Logistic curve method

8. Ration & correlation

1. Arithmetic increase method

This method is based on the assumption that the population is increasing at a constant rate i.e.

the rate of change of population with time is constant.

kdt

dp=

Pn

dp/dt

Po

0 n

=n

o

p

p

dtKdpn

o


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Pn= Po+ nK

Where; Pn= population at n decades or years

Po= present/initial population at the base year

n = decade or year

K= arithmetic increase

This method is generally applicable to large and old cities.

Example 3:The following data has been noted from the statistics authority for certain town.

Calculate the probable population in the year 1980, 1990, 2000, and 2006.

2. Geometric increase methodThis method is based on the assumption that the percentage increase in population remains

constant.

P1 = Po+ K Po = Po(1 + K)

P2 = P1(1 + K) = Po(1 + K)(1 + K)

P3 = P2 (1 + K) = Po(1 + K) (1 + K) (1 + K)

Pn= Po (1+K)n

Where Po= initial population

Pn= population at n decades or years

n = decade or year

K = percentage (geometric) increaseThis method is mostly applicable for growing towns and cities having vast scope of expansion.

Example 4: Forecast the population of example 3 by means of geometric increase method.

Year 1940 1950 1960 1970

Population 8000 12000 17000 22500

Po

Pn

Year (decade) n

0


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3. Incremental increase method

This method is improvement over the above two methods. From the census data for the past

several decades, the actual increase in each decade is first found. Then the increment in

increase for each decade is found. From these, an average increment of the increase iscalculated. The population in the next decade is found by adding to the present population the

average increase plus the average incremental increase per decade. Thus, the future population

at the end of n decade/year is given by:

rnn

nIPPn2

)1( +++=

Where P = present population

I = average increase per decade/year

r = average incremental increase

n = number of decades/years

Example 5:Forecast the population of example 3 above using incremental increase method

4. Decrease growth rate method

In this method, the average decrease in the percentage increase is worked out and is subtracted

from the latest percentage increase for successive period. This method is applicable only in

such cases, where the rate of growth of population shown a downward trend. It assumed that

the city has some limiting saturation population and its rate of growth is a function of its

population deficit:

)('' PPKdt

dPs = Ps

K may be determined from the successive census

0

ln1

''PP

PP

nK

s

s

= P0

Where P and P0 are populations recorded n years apart.

Future population can then be estimated using Year

)1)((''

00

tK

s ePPPP +=


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5. Logistic curve method

When the population of a town is with plotted with respect to time, the curve so obtained under

normal condition shall be S shaped logistic curve.

Ps

Arithmetic

Geometric

P0 Decreasing rate

According to P.F. Verhulst, the logistic curve can be represented by the equation

)(log1 1 ntemPP s

+=

Where Ps = Saturation population

P0= Population at starting point

P = Population at any time t from the starting point

0

0

P

PPm s

=

sKPn =

Taking three points from the range of census population data at equal time intervals (t 1, P1), (t2,

P2) and (t3, P3)

Where t2= t1+t

t3 = t2+t

2

231

21

2

2321 )(2

PPP

PPPPPPPs

+=

Example 6:The following data have noted form the statics Authority.

P1980= 40, 000

P 1990= 100, 000P 1990 = 130,000

Determine the saturation population and the problem population in the year 2010.

Ans. P2010= 136,291


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6. Graphical extension method

In this method the population of last few years is correctly plotted to a suitable scale on the

graph with respect to years. Then, the curve is smoothly extended to forecast the future

population.

Example 7:Solve example 3 above by using graphical extension method

Ans. P1980= 29, 400, P 1990= 36, 000, P2000= 41, 600

7. Master plan method

In the method, the master plan of the city or town is used to determine the future expected

population. The population densities for various zones (residential, commercial, industrial and

other zones) of the town are fixed and hence the future population of the city when fully

developed can easily be worked out.

8. Ration and correlation method

In this method, the rate of population growth of a town is related to the rate of population

growth of state or nation. Hence it is possible to estimate the population of a town under

consideration by considering the rate of population growth of state or nation.

Example 8:Country, P1980 = 1, 000,000 P2004 = 1,5000,000

Town, P1980 = 10,000 P2004 = 15,000


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9. Method used by Ethiopians statistic Authority (geometric increase method)kn

on epp *=

Where, Pn= population at n decades or years

Po = initial population

n = decade or year

k = growth rate in percentage

Example 9:

According to CA the population of certain town is 15,640 in the year 1994. Determine the

probable population in the year 2010 for k = 3%.


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2. A

2.1. Sources of Water Supply..18

2.1.1 Surfaces Sources...........18

2.1.2 Subsurface Sources..19

2.2 Intakes for Collecting Surface Water....24

2.2.1 Types of Intake structures....24

2.3 Water Sources Selection Criteria.....27

1. 2.1

The origin of all water is rainfall. Water can be collected as it falls as rain before it reaches the

ground; or as surface water when it flows over the ground; or is pooled in lakes or ponds; or as

ground water when it percolates in to the ground and flows or collects as groundwater; from the

sea/ocean in to which it finally flows.

All the sources of water can be broadly divided into:

1.

Surfaces sources and

2. Sub surface sources

2.1.1 Surfaces Sources

The surface sources further divided intoi. Streams and rivers

ii.

Ponds and Lakes

iii. Impounding reservoirs etc.

i. Streams and Rivers

Rivers and streams are the main source of surface source of water. In summer the quality of

river water is better than monsoon because in rainy season the run-off water also carries with

clay, sand, silt etc which make the water turbid. So, river and stream water require special

treatments. Some rivers are perennial and have water throughout the year and therefore they

dont require any arrangements to hold the water. But some rivers dry up wholly or partially in

summer. So they require special arrangements to meet the water demand during hot weather.

Mostly all the cities are situated near the rivers discharge their used water of sewage in the

rivers; therefore much care should be taken while drawing water from the river.


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ii. Natural Ponds and Lakes

In mountains at some places natural basins are formed with impervious bed by springs and

streams are known as lakes. The quantity of water in the natural ponds and lakes depends

upon the basins capacity, catchment area, annual rainfall, porosity of ground etc. Lakes and

ponds situated at higher altitudes contain almost pure water which can be used without any

treatment. But ponds formed due to construction of houses, road, and railways contains large

amount of impurities and therefore cannot be used for water supply purposes.

iii.Impounding Reservoirs

In some rivers the flow becomes very small and cannot meet the requirements of hot weather.

In such cases, the water can be stored by constructing 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

to 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 color 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 which are born in water.

2.1.2 Subsurface Sources

These are further divided into

(i) Infiltration galleries

(ii)

Infiltration wells

(iii)Springs

(iv)

Well

i. Infiltration Galleries

A horizontal nearly horizontal tunnel which is constructed through water bearing strata for

tapping underground water near rivers, lakes or streams are called Infiltration galleries. The

yield from the galleries may be as much as 1.5 x 104lit/day/meter length of infiltration gallery.

For maximum yield the galleries may be placed at full depth of the aquifer. Infiltration galleries

may be constructed with masonry or concrete with weep holes of 5cm x 10cm.


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Fig 2.2 Infiltration Gallery

ii. Infiltration Wells

In order to obtain large quantity of water, the infiltration wells are sunk in series in the blanks

of river. The wells are closed at top and open at bottom. They are constructed by brick masonry

with open joints as shown in fig. 2.3

Fig 2.3 Infiltration Well Fig 2.4 Jack Well

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. The infiltration wells in turn are connected by porous pipes to collecting sump

called jack well and there water is pumped to purification plant for treatment (fig 2.4).

iii. Springs

Sometimes ground water reappears at the ground surface in the form of springs. Springs

generally supply small quantity of water and hence suitable for the hill towns. Some springs


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discharge hot water due to presence of sulphur and useful only for the curve of certain skin

disease patients.

Types of springs:

1. Gravity Springs: When the surface of the earth drops sharply the water bearing stratum is

exposed to atmosphere and gravity springs are formed as shown in fig.2.5

Fig 2.5 Gravity spring

2. Surface Spring: This is formed when an impervious stratum which is supporting the

ground water reservoir becomes out crops as shown in fig.2.6

Fig 2.6 Surface spring

3. Artesian Spring: When the ground water rises through a fissure in the upper impervious

stratum as shown in fig.2.7


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Fig 2.7 Artesian Spring

When the water-bearing stratum has too much hydraulic gradient and is closed between two

imperious stratums, the formation of artesian spring from deep seated spring.

Fig 2.8 Artesian Spring

iv. Wells

A well is defined as an artificial hole or pit made in the ground for the purpose of tapping

water.

The three factors which form the basis of theory of wells are

1.

Geological conditions of the earths surface

2.

Porosity of various layers

3. Quantity of water, which is absorbed and stored in different layers

The following are different types of wells

1.

Shallow wells

2.

Deep wells

3.

Tube wells

4.

Artesian wells

1. Shallow Wells

Shallow wells are constructed in the uppermost layer of the earths surface. The diameter of

well varies from 2 to 6m and a maximum depth of 7m. Shallow wells may be lined or unlined


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from inside. Fig. 2.9 shows a shallow well with lining (staining). These wells are also called

draw wells or gravity wells or open wells or drag wells or percolation wells.

Fig 2.9 Shallow well

Quantity of water available from shallow wells is limited as their source of supply is uppermost

layer of earth only and sometimes may even dry up in summer. Hence they are not suitable for

public water supply schemes. The quantity of water obtained from shallow wells is better than

the river water but requires purification. The shallow wells should be constructed away from

septic tanks, soak pits etc because of the contamination of effluent.

The shallow wells are used as the source of water supply for small villages, undeveloped

municipal towns, isolated buildings etc because of limited supply and bad quality of water.

2. Deep Wells

The deep wells obtain their quota of water from an aquifer below the impervious layer as

shown in fig 2.10. The theory of deep well is based on the travel of water from the outcrop to

the site of deep well. The outcrop is the place where aquifer is exposed to the atmosphere. The

rain water entered at outcrop and gets thoroughly purified when it reaches to the site of deep

well. But it dissolves certain salts and therefore become hard.

In such cases, some treatment would be necessary to remove the hardness of water.

Fig 2.10 Deep Well


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The depth of deep well should be decided in such a way that the location of out crop is not very

near to the site of well. The water available at a pressure greater atmospheric pressure, therefore

deep wells are also referred to as apressure wells.

2.2 Intakes for Collecting Surface Water

The main function of the intakes works is to collect water from the surface source and then

discharge water so collected, by means of pumps or directly to the treatment water.

Intakes are structures which essentially consist of opening, grating or strainer through which

the raw water from river, canal or reservoir enters and carried to the sump well by means of

conducts water from the sump well is pumped through the rising mains to the treatment plant.

The following points should be kept in mind while selecting a site for intake works.

1.

Where the best quality of water available so that water is purified economically in less

time.

2.

At site there should not be heavy current of water, which may damage the intakestructure.

3. The intake can draw sufficient quantity of water even in the worst condition, when the

discharge of the source is minimum.

4.

The site of the work should be easily approachable without any obstruction

5. The site should not be located in navigation channels

6. As per as possible the intake should be near the treatment plant so that conveyance cost is

reduced from source to the water works

7.

As per as possible the intake should not be located in the vicinity of the point of sewage

disposal for avoiding the pollution of water.8.

At the site sufficient quantity should be available for the future expansion of the water-

works.

2.2.1 Types of Intake structures

Depending upon the source of water the intake works are classified as following

1. Lake Intake

2.

Reservoir Intake

3. River Intake

4.

Canal Intake

1. Lake Intake

For obtaining water from lakes mostly submersible intakes are used. These intakes are

constructed in the bed of the lake below the water level; so as to draw water in dry season also.

These intakes have so many advantages such as no obstruction to the navigation, no danger

from the floating bodies and no trouble due to ice. As these intakes draw small quantity of


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water, these are not used in big water supply schemes or on rivers or reservoirs. The main

reason is that they are not easily approachable for maintenance.

Fig 2.11 Lake intake

2.

River Intake

Water from the rivers is always drawn from the upstream side, because it is free from the

contamination caused by the disposal of sewage in it. It is circular masonry tower of 4 to 7 m in

diameter constructed along the bank of the river at such place from where required quantity of

water can be obtained even in the dry period. The water enters in the lower portion of the intake

known as sump well from penstocks.

Fig. 2.12 River intake


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Fig. 2.14 Canal intake

The entry of water in the intake chamber takes through coarse screen and the top of outlet pipe

is provided with fine screen. The inlet to outlet pipe is of bell-mouth shape with perforations ofthe fine screen on its surface. The outlet valve is operated from the top and it controls the entry

of water into the outlet pipe from where it is taken to the treatment plant.

2. 2.3

The choice of water supply to a town or city depends on the following:

1. Location: The sources of water should be as near as to the town as possible.

2. Quantity of water:the source of water should have sufficient quantity of water to meet up

all the water demand through out the design period.

3. Quality of water: The quality of water should be good which can be easily and cheaply

treated.

4. Cost: The cost of the units of the water supply schemes should be minimum.

The selection of the source of supply is done on the above points and the source, which will

give good quality, and quantity at least cost will be selected. This economic policy may lead to

the selection of both surface and ground water sources to very big cities.

For example, the source of the Arba Minch Town water supply is springs.

Surface water sources can be developed for drinking water but special care must be taken to

ensure the quality of the water.

The choice of a method depends on many factors including the source and resources available

and community preferences.

Table 2.1 compares the various methods of developing surface water discussed in this technicalnote.


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Table 2.1 Summary of methods of developing sources of surface water

Method Quality Quantity Accessibility Reliability

Springs and

Seeps

Good quality; disinfection

recommended after

installation of springprotection.

Good with little variation

for artesian flow springs;

variable with seasonalfluctuations likely for

gravity flow springs.

Storage necessary for

community water

supply; gravity flowdelivery for easy

community access.

Good for artesian

gravity overflow;

gravity depressionmaintenance need

installation.

Ponds and

Lakes

Fair to good in large

ponds and lakes; poor to

fair in smaller water

bodies; treatment

generally necessary.

Good available quantity;

decrease during dry

season.

Very accessible using

intakes; pumping

required for delivery

system; storage

required.

Fair to good; need

good program of o

and maintenance

pumping and trea

systems.

Streams

and Rivers

Good for mountain

streams; poor for streams

in lowland regions;

treatment necessary.

Moderate: seasonal

variation likely; some

rivers and streams will

dry up in dry season.

Generally good; need

intake for both gravity

flow and piped delivery.

Maintenance requ

both type systems

higher for pumped

riverside well is areliable source.

Rain

Catchment

Fair to poor; disinfection

necessary

Moderate and variable;

supplies unavailable

during dry season; storage

necessary.

Good; cisterns located in

yards of users; fair for

ground catchments.

Must be rain; som

maintenance requ


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3. WATER QUALITY AND POLLUTION

3.1Water Quality Characteristics. .......29

3.1.1 Physical Characteristics293.1.2

Chemical Characteristics...31

3.1.3 Biological Characteristics.....37

3.2 Examination of Water......38

3.3 Water Quality Standards........ 39

3.4 Sources of Water Pollution. .......................................................41

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 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 processes, it is

most essential for the water work engineer to thoroughly check, analyze and do the treatment of

the raw water obtained the sources, before its distribution. The water supplied to the public

should be strictly according to the standards laid down from time to time.

3.1

For the purpose of classification, the impurities present in water may be divided into the

following three categories.

3.1.1 Physical Characteristics

Physical characteristics include:

Turbidity

Color

Taste and odor

Temperature, and

Foam.

1. Turbidity

Turbidity is caused due to presence of suspended and colloidal solids. The suspended solids

may be dead algae or other organisms. It is generally silt, clay rock fragments and metal oxides

from soil.


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The amount and character of turbidity will depend upon:

The type of soil over which the water has run and

The velocity of the water

When the water becomes quite, the heavier and larger suspended particles settle quickly, while

the lighter and more finely divided ones settle very slowly. Very finely divided clay mayrequire months of complete quiescence for settlement. Ground waters are normally clear

because, slow movement through the soil has filtered out the turbidity. Lake waters are clearer

than stream waters, and streams in dry weather are clearer than streams in flood because of the

smaller velocity and because dry-weather flow is mainly ground water seepage. Low inorganic

turbidity (silt and clay) may result in a relatively high organic turbidity (color). The explanation

of this is that low inorganic turbidity permits sunlight to penetrate freely into the water and

stimulates a heavier growth of algae, and further, that organics tend to be absorbed upon soil

fractions suspended in water.

Turbidity is a measure of resistance of water to the passage of light through it. Turbidity isexpressed as NTU (Nephelometric Turbidity Units) or PPM (parts per million) or Milligrams

per liter (mg/l).

Turbidity is measured by:

1) Turbidity rod or Tape 2) Jacksons Turbidimeter 3) Balis Turbidimeter

The sample to be tested is poured into a test tube and placed in the meter and a unit of turbidity

is read directly on the scale by a needle or by digital display.

Drinking water should not have turbidity more than 10 NTU. This test is useful in determining

the detention time in settling for raw water and to dosage of coagulants required to remove

turbidity. Sedimentation with or without chemical coagulation and filtration are used remove it.

2. Color

Color is caused by materials in solution or colloidal conditions and should be distinguished

from turbidity, which may cause an apparent (not true) color.

True color is caused by dyes derived from decomposing vegetation. Colored water is not only

undesirable because of consumer objections to its appearance but also it may discolor clothing

and adversely affect industrial processes.

Before testing the color of water, total suspended solids should be removed by centrifugal force

in a special apparatus. The color produced by one milligram of platinum in a liter of water hasbeen fixed as the unit of color. The permissible color for domestic water is 20ppm on platinum

cobalt scale.

3. Temperature

Temperature increase may affect the portability of water, and temperature above 150c is

objectionable to drinking water. The temperature of surface waters governs to a large extent the


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biological species present and thereof activity. Temperature has an effect on most chemical

reactions that occur in natural water systems. It also has pronounced effect on the solubility of

gases in water.

4. Foam

Foam form various industrial waste contributions and detergents is primarily objectionable

from the aesthetic standpoint.

5. Tastes and Odor

The terms taste and odor are themselves definitive of this parameter. Because the sensations of

taste and smell are closely related and often confused, a wide variety of tastes and odors may be

attributed to water by consumers. Substances that produce an odor in water will almost in

variably impart a taste as well. The converse is not true, as there are many mineral substances

that produce taste but no odor.Many substances with which water comes into contact in nature or during human use may

import perceptible taste and odor. These include minerals, metals, and salts from the soil, and

products from biological reactions, and constituents of wastewater. Inorganic substances are

more likely to produce tastes unaccompanied by odor. Alkaline material imports a bitter taste to

water, while metallic salts may give salty or bitter taste.

Organic material, on the otter hand, is likely to produce both taste and odor. a multitude of

organic chemicals may cause taste & odor problems in water with petroleum-based products

being prime offenders. Biological decomposition of organics may also result in taste-and odor-

producing liquids and gases in water. Principal among these are the reduced products of sulfurthat impart a rotten egg taste and odor. Also certain species of algae secrete an oily substance

that may result in both taste and odor.

Consumers find taste and odor aesthetically displeasing for obvious reasons. Because water is

thought of as tasteless and odorless, the consumer associates taste and odor with contamination

and may prefer to use a tasteless, odorless water that might actually pose more of a health

threat.

3.1.2 Chemical Characteristics

1. Total 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


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the suspended solid test and weighing the residue. The total solids in a water sample can be

directly determined by evaporating the water and weighing the residue of the residue of total

solids is fused in a muffle furnace the organic solids will decompose where as only inorganic

solids will remain. By weighing we can determine the inorganic solids and deducting it from

the total solids, we can calculate organic solids.2. Alkalinity

It is defined as the quantity of ions in water that will react to neutralize hydrogen ions.

Alkalinity is thus the measure of the ability of water to neutralize acids. By far the most

constituents of alkalinity in natural waters are carbonate (CO32-

), bicarbonate (HCO3-) and

hydroxide (OH-). These compounds result from the dissolution of mineral substances in the soil

atmosphere.

Effects:

i)

Non pleasant taste

ii)

Reaction between alkaline constituent and cation (positive ion) produces precipitation inpipe.

3. pH

pH is a measure of the concentration of free hydrogen ion in water. It expresses the moral

concentration of the hydrogen ion as its negative logarithm. Water and other chemicals in

solution therein, will ionize to a greater or lesser degree. Pure water is only weakly ionized.

The ionization reaction of water may be written:

HOH H++OH-

The reaction has an equilibrium defined by the equation:

[H][OH]/ [HOH] = Kw

In which HOH, H, OH is the chemical activities of the water hydrogen and hydroxyl ion

respectively. Since water is solvent, its activity is defined as being unity. In dilute solution,

molar concentrations are frequently substituted for activities yielding

[H][OH) = Kw (10-14 at 20oC)

Taking negative logs of both sides, Log [H] + Log [OH] = -14

- Log [H] - Log [OH] = 14

Defining Log = p; pH + pOH = 14

In neutral solutions at equilibrium (OH) = (H), hence pH = pOH = 7.

Mathematically it is expressed as; pH = -log [H+] = 7][

1log =

+H

Increasing acidity leads to higher values of (H), thus to lower values of pH. Low pH is

associated with high acidity, high pH with caustic alkalinity.

pH is important in the control of a number of water treatment and waste treatment processes

and in control of corrosion. It may be readily measured potentially by use of a pH meter.


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4. Dissolved Oxygen (DO)

Dissolved oxygen is present in variable quantities in water. Its content in surface waters is

dependent upon the amount and character of the unstable organic matter in the water. Clean

surface waters are normally saturated with DO.

The amount of oxygen that water can hold is small and affected by the temperature. The higherthe temperature, the smaller will be the DO. Gases are less soluble in warmer water.

Temperature ( C) 0 10 20 30

DO (mg/1) 14.6 11.3 9.1 7.6

Oxygen saturated waters have pleasant taste and waters lacking in DO have an insipid tastes.

Drinking water is thus aerated if necessary to ensure maximum DO. The presence of oxygen in

the water in dissolved form keeps it fresh and sparkling. But more quantity of oxygen causes

corrosion to the pipes material.Observing a heated pot of water, one can observe that bubbles form on the walls of the pot

prior to reaching the boiling point. These cannot be filled with only water vapor because liquid

water will not begin to vaporize until it has reached its boiling point. One can surmise that this

gas is oxygen, or at least a mixture of gases from the air, because bubbles of this sort form in

water from virtually every source: what other gas mixture besides air is in constant contact with

water? When these bubbles form, they eventually grow to a sufficient size to leave the surface

of the pot and escape to the air: the dissolved gas in the liquid has decreased. This seems to

support the hypothesis that dissolved oxygen will decrease when temperature is increased.

5. Oxygen Demand

Organic compounds are generally unstable be oxidized biologically or chemically to stable,

relatively inner end produce such as CO2, H2O & NO3. Indicators used for estimation of the

oxygen demanding substance in water are Biological Oxygen Demand (BOD), Chemical

Oxygen Demand (COD), Total Oxygen Demand (TOD) and Total Organic Carbon (TOC).

An indication of the organic content of water can be by measuring the amount of oxygen

required for stabilization.

BOD is the quality of oxygen required for the biochemical oxidation of the decomposable

matter at specified temperature within specified time. (20oC and 5 day)

It depends on temperature and time t.

6. Nitrogen

The forms most important to water quality engineering include;

a) Organic nitrogen: in the form of proton, amino acids and urea.


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Generally a hardness of 100 to 150 mg/liter is desirable. Excess of hardness leads to the

following effects:

1. Large soap consumption in washing and bathing

2.

Fabrics when washed become rough and strained with precipitates.

3.

Hard water is not fit for industrial use like textiles, paper making, dye and ice cream

manufactures.

4.

The precipitates clog the pores on the skin and makes the skin rough

5. Precipitates can choke pipe lines and values

6. It forms scales in the boilers tubes and reduces their efficiency

7.

Very hard water is not palatable

When softening is practices when hardness exceeds 300mg/lit. Water hardness more than

600mg/lit have to rejected for drinking purpose.

Methods of removal of hardness

1. Boiling

2. Lime addition

3.

Lime soda process

4. Caustic soda process

5.

Zeolite process

Methods 1 and 2 are suitable for removal of temporary hardness and 3 to 5 for both temporary

and permanent hardness.

Boiling

Lime soda processIn this method, the lime and is sodium carbonate or soda as have used to remove permanent

hardness from water. The chemical reactions involved in this process are as follows.


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Zeolite process

This is also known as the base-exchange or Ion exchange process. The hardness may becompletely removed by this process.

Zeolites are compounds (silicates of aluminum 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:

8. Chloride

The natural waters near the mines and sea dissolve sodium chloride and also presence of

chlorides may be due to mixing of saline water and sewage in the water. Excess of chlorides is

dangerous and unfit for use. The chlorides can be reduced by diluting the water. Chloride may

demonstrate an adverse physiological effect when present in concentration greater than

250mg/l and with people who are acclimated. However, a local population that is acclimated to

the chloride content may not exhibit adverse effect from excessive chloride concentration.Because of high chloride content of urine, chlorides have sometimes been used as an indication

of pollution.


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9. Fluoride

It is generally associated with a few types of sedimentary or igneous rocks; fluoride is seldom

found in surface waters and appears in ground water in only few geographical regions. Fluoride

is toxic to humans and other animals in large quantities, while small concentrations can

beneficial.

Concentrations of approximately 1.0mg/1 in drinking water help to prevent dental cavities in

children. During formation of permanent teeth, fluoride combines chemically with tooth

enamel, resulting in harder, stronger teeth that are more resistant to decay. Fluoride is often

added to drinking water supplies if quantities for good dental formation are not naturally

present.

Excessive intakes of fluoride can result in discoloration of teeth. Noticeable discoloration,

called mottling, is relatively common when fluoride concentrations in drinking water exceed

2.0mg/1, but is rare when concentration is less that 1.5mg/1.

Adult tooth are not affected by fluoride, although both the benefits and liabilities of fluoride

during teeth formation years carry over into adulthood.

Excessive concentrations of greater than 5mg/1 in drinking water can also result in bone

fluorisis and other skeletal abnormalities.

10.Metals and other chemical substances

Water contains various minerals or metal substances such as iron, manganese, copper, lead,

barium, cadmium, selenium, fluoride, arsenic etc.

The concentration of iron and manganese should not allow more than 0.3ppm. Excess will

cause discoloration of clothes during washing and incrustation in water mains due to deposition

of ferric hydroxide and manganese oxide. Lead and barium are very toxic, low p.p.m of these

are allowed. Arsenic, Selenium are poisonous, therefore they must be removed totally. Human

beings are affected by presence of high quantity of copper in the water.

3.1.3 Biological Characteristics

A feature of most natural water is that they contain a wide variety of micro organisms

forming a balance ecological system. The types and numbers of the various groups of micro

organisms present are related to water quality and other environmental factors.Microbiological indicators of water quality or pollution are therefore of particular concern

because of their relationships s to human and animal health. Water polluted by pathogenic

micro- organisms may penetrate into private and or public water supplies either before or after

treatment.

1. Bacterium


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ii. Colorimetric method (using color as the basis)

Measuring amount of color produced by mixing with reagents at fixed wavelength (using

spectrophotometer) or comparison with colored standards or discs (comparator).

The recommended determinations made by colorimetric method are: color, turbidity, iron

(Fe

++

), manganese (Mn

++

), chlorine (Cl2), flurried (F

-

), nitrate (NO3-

), nitrite (NO2), phosphate(PO4

---), ammonia (NH4

+), arsenic, phenols, etc.

iii. Gravimetric method (using weight as the basis)

Using weight of insoluble precipitates or evaporated residues in glassware or metal and

accurate analytical balance

The recommended determinations made by gravimetric methods are: sulfate (SO4), Oil and

grease, TDS, TSS, TS, etc.

iv.Electrical method

Using probes to measure electrical potential in mill volts against standard cell voltage.

The recommended determinations made by electrical methods are: pH, Fluoride (F-), DO,

nitrate (NO3), etc.

v. Flame spectra (emission & absorption) method

At fixed wave length characteristics to ions being determined measuring intensity of emission

or absorption of light produced by ions exited in flame or heated sources.

The recommended determinations made by flame spectra methods are: sodium (Na+),

potassium (K+), lithium (Li

+), etc.

3.4. Water Quality StandardsPublic water supplies are obliged to provide a supply of wholesome water which is suitable and

safe for drinking purposes.

Potable water is water which is satisfactory for drinking, culinary and domestic purposes.

Water quality standards may be set regional, national, or international bodies. Guidelines for

drinking water quality have established by the World Health Organization (WHO) as shown in

table below.


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Table WHO Guideline for drinking water quality

Parameter Unit Guideline value

Microbial quality

Fecal coli forms

Coli form organisms

Arsenic

Cadmium

Chromium

Cyanide

Fluoride

Lead

Mercury

Nitrate

Selenium

Aluminum

Chloride

Color

Copper

Hardness

IronManganese

pH

Sodium

Total dissolved solids

Sulfate

Taste and odor

Turbidity

Zinc

Number/ 100 ml

Number /100 ml

mg/1

mg/1

mg/1

mg/1

mg/1

mg/1

mg/1

mg/1

mg/1

mg/1

mg/1

True color unit(TCU)

mg/1

mg/1(as CaCO3)

mg/1mg/1

mg/1

mg/1

mg/1

NTU

mg/1

Zero*

Zero*

0.05

0.005

0.05

0.1

0.5 - 1.5(3)

0.05

0.001

10

0.01

0.2

250

5(15)

1.0

500

0.3(3)0.3

6.5 to 8.5

200

1000

400

Non objectionable

5(10)

5.0

* Treated water entering the distribution system


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3.5. Sources of Water Pollution

Following are the main sources of water pollution.

1. Domestic Sewage

If domestic sewage is not properly after it is produced or if the effluent received at the end ofsewage treatment is not of adequate standard, there are chances of water pollution.

The indiscriminate way of hading domestic sewage may lead to the pollution of under ground

sources of water supply such, as wells. Similarly if sewage or partly treated sewage is directly

discharged into surface waters such as rivers, the waters of such rivers get contained.

2. Industrial Wastes

If industrial wastes are thrown into water bodies without proper treatments, they are likely to

pollute the watercourses. The industrial wastes may carry harmful substances such as grease,

oil, explosives, highly odorous substances, etc.

3. Catchment Area

Depending upon the characteristics of catchment area, water passing such area will be

accordingly contained. The advances made in agricultural activities and extensive use of

fertilizers and insecticides are main factors, which may cause serious pollution of surface

waters.

4. Distribution System

The water is delivered to the consumers through a distribution of pipes which are laid

underground. If there are cracks in pipes or if joints are leaky, the following water gets

contaminated by the surrounding substances around the pipes.

5. Oily Wastes

The discharge of oily wastes from ships and tankers using oil as fuel may lead to pollution.

6. Radioactive Wastes

The discharge of radioactive wastes from industries dealing with radioactive substance may

seriously pollute the waters. It may be noted that radioactive substances may not have color,

odour, turbidity or taste. They can only be detected by and measured by the use of special

precise instruments.

7. Travel of Water

Depending upon the properties of ground through which water travels to reach the source of

water supply; it is charged with the impurities. For instance, ground water passing through

peaty land possesses brown color.


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Fig. 2 Tray aerator

ii. Spray aerators: - spray droplets of water into the air from stationary or moving orifices or

nozzles. Water is pumped through pressure nozzles to spray in the open air as in fountain to

a height of about 2.5m.

Fig. 3 Spray aerator

iii.Air diffuser

In diffused aeration systems, water is contained in basins. Compressed air is forced into this

system through the diffusers. This air bubbles up through the water, mixing water and air and

introducing oxygen into the water.

Fig. 4 Air diffusion aerator


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s = the mass density of the particle (M/L3)

= the mass density of the fluid (M/L3)

g = the acceleration due to gravity (L/T2)

d = the diameter of the particle (L)

Cd= is dimensionless drag coefficient defined byThe values of drag coefficient depend on the density of water (), relative velocity (u), particle

diameter (d), and viscosity of water (), which gives the Reynolds number Re.

The value of Cd decreases as the Reynolds number increases. For Re less than 2 or 1, Cd is

related to R by the linear expression as follows:

....(2)

Substitute eq.(2) into eq. (1)

18

)( 2dgV ss

= ...(3)

This expression is known as the Stokes equationfor laminar flow conditions.

In the region of higher Reynolds numbers (2 < Re< 500 - 1000), Cd becomes

...(4)

The value of Vs is solved by iteration. First, guess Cd, compute Vs and Re and with the

computed Recompute Cduntil the values of Vsconverges.

In the region of turbulent flow (500 - 1000 < R e < 200,000), the Cd remains approximately

constant at 0.44.


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Example 1: Estimate the terminal settling velocity in water at a temperature of 15c ( =

0.00113Ns/m2) of spherical silicon particles with specific gravity 2.40 and average diameter of

(a) 0.05mm and (b) 1.0mm

Design Aspects of Sedimentation Tanks

In practice, settling of the particles is governed by the resultant of horizontal velocity of water

and the vertical downward velocity of the particle. The path of the settling particle is as shown

in Fig 6.

The critical particle in the settling zone of an ideal rectangular sedimentation tank, for design

purposes, will be one that enters at the top of the settling zone and settles with a velocity just

sufficient to reach the sludge zone at the outlet end of the tank.

In an ideal sedimentation tank with horizontal or radial flow pattern, particles with settling

velocity less than Vs can still be removed partially.

Fig. 6 Settling of particles

The design aspects of sedimentary tanks are:

1. Velocity of flow

2. Capacity of tank

3.

Inlet and outlet arrangements

4.

Settling and sludge zones

5. Shapes of tanks

6.

Miscellaneous considerations.

(1) Velocity of flow: The velocity of flow of water in sedimentation tanks should be sufficient

enough to cause the hydraulic subsidence of suspended impurities. It should remain

uniform throughout the tank and it is generally not allowed to exceed 150mm to 300mm

per minute.


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(2) Capacity of tank: Capacity of tank is calculated by

i)

Detention period

ii)

Overflow rate

i) Detention period: The theoretical time taken by a particle of water to pass between entry

and exit of a settling tank is known as the known as the detention period. The capacity of tankis calculated by:

C = Q x T where CCapacity of tank

QDischarge or rate of flow

TDetention period in hours

The detention period depends on the quality of suspended impurities present in water. For plain

sedimentation tanks, the detention period is found to vary from 4 to 8 hours.

ii) Overflow Rate: In this method it is assumed that the settlement of a particle at the bottom

of the tank does not depend on the depth of tank and depends upon the surface area of the tank.

Settling time =s

sV

Ht = ..(5)

Detention time =U

LtR = . (6)

HW

QU= (7)

To get the desired settling with most efficient tank size, tR= ts which occurs when Vo= Vs.

po A

Q

LW

Q

HW

Q

L

H

L

HUV ====

..(8)

Where, LLength of tank

WWidth of tank

ApPlan area of tank

CCapacity of tank

TDetention period

QDischarge or rate of flow

VsVelocity of descend of a particle to the bottom of tank

Vooverflow rate or surface loading rate = Vs

(3) Inlet and Outlet Arrangements

Inlet zone

The inlet is a device, which is provided to distribute the water inside a tank. The two primary

purposes of the inlet zone of a sedimentation basin are to distribute the water and to control the

water's velocity as it enters the basin. In addition, inlet devices act to prevent turbulence of the


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The following are the parameters for satisfactory performance.

1. Detention period .. 3 to 4 hours for plain settling

2 to 2.5 hours for coagulant settling

1 to 1.5 hours for vertical flow type

2. Overflow rate 15 - 30 m3/m2/day for plain settling

30 - 40m3/m

2/day for horizontal flow

40 - 50m3/m2/day for vertical flow

3. Velocity of flow.. 0.5 to 1.0 cm/sec

4. Weir loading... 300m3/m/day

5. L:W .. 3:1 to 5:1

Breadth of tank.. (10 to 12m) to 30 to 50m

6. Depth of tank. 2.5 to 5m (with a preferred value of 3m)

7. Diameter of circular tank. up to 60m8. Solids removal efficiency.. 50%

9. Turbidity of water after sedimentation 15 to 20 NTU.

10. Inlet and Outlet zones. 0.75 to 1.0m

11. Free board 0.5m

12. Sludge Zone. 0.5m

Example 2: A water treatment plant has four clarifiers treating 0.175 m3/s of water. Each

clarifier is 4.88m wide, 24.4m long and 4.57m deep. Determine: (a) the detention time, (b)

overflow rate, (c) horizontal velocity, and (d) weir loading rate assuming the weir length is 2.5times the basin width.

4.2.4 Coagulation (Coagulation Aided with Sedimentation)

The hydraulic settling values of small size particles in water are very small and therefore, they

require longer time to settle in plain sedimentation tanks. For example, a slit particle of size

0.05mm will require about 11hrs to settle down through a depth of 3m and clay particle of size

0.002mm will require about 4 days time to settle the same height of 3m at normal temperature

of about 25c. Moreover, water may be containing colloidal impurities which are even finer

than 0.0001mm and which also carry electrical charge on them. Due to electrical charge theyremain continuously in motion and never settle down by gravity in water. Therefore, when

water is turbid due to presence of such fine size and colloidal impurities, plain sedimentation is

of no use.

For dealing water with such impurities a chemical process was evolved. This process removes

all these impurities within reasonable period of 2 3hrs. This chemical process is called


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coagulation and the chemical used in the process is called coagulant. The objective of

coagulation is to unit several colloidal particles together to form bigger sized settable flocs

which may settle down in the tank.

The principle of coagulation can be explained from the following two conditions:

1. Floc formation

When coagulants (chemicals) are dissolved in water and thoroughly mixed with it, they

produce a think gelatinous precipitate. This precipitate is known as flocand this floc has got the

property of arresting suspended impurities in water during downward travel towards the bottom

of tank. The gelatinous precipitate has therefore, the property of removing fine and colloidal

particles quickly.

2. Electric charges

Most particles dissolved in water have a negative charge, so they tend to repel each other. As aresult, they stay dispersed and dissolved or colloidal in the water.

The purpose of most coagulant chemicals is to neutralize the negative charges on the turbidity

particles to prevent those particles from repelling each other. The amount of coagulant which

should be added to the water will depend on the zeta potential, a measurement of the

magnitude of electrical charge surrounding the colloidal particles. You can think of the zeta

potential as the amount of repulsive force which keeps the particles in the water. If the zeta

potential is large, then more coagulants will be needed.

Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the

negative particles in the water, as shown below.

.Negatively charged particles repel Positively charged coagulants attract to negatively

each other due to electricity charged particles due to electricity.

The combination of positive and negative charge results in a neutral. As a result, the particles

no longer repel each other.


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The next force which will affect the particles is known as van der Waal's forces. Van der

Waal's forces refer to the tendency of particles in nature to attract each other weakly if they

have no charge.

Neutrally charged particles attract due to van der Waal's forces.

Once the particles in water are not repelling each other, van der Waal's forces make the

particles drift toward each other and join together into a group. When enough particles have

joined together, they become floc and will settle out of the water.

Particles and coagulants join

together into floc.

Factors affecting coagulation:

1.

Type of coagulant

2. Dose of coagulant

3.

Characteristic of water

1.

Type and quantity of suspended matter

2. Temperature of water

3. pH of water

4.

Time and method of mixing

Common Coagulants

Coagulant chemicals come in two main types - primary coagulants and coagulant aids.

Primary coagulantsneutralize the electrical charges of particles in the water which causes the

particles to clump together. Coagulant aids add density to slow-settling flocs and add

toughness to the flocs so that they will not break up during the mixing and settling processes.


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In water treatment plants, the following are the coagulants most commonly used:

i. Aluminum sulfate [Al 2(SO4)3.18H2O].

It is also called Alum. It is the most widely used chemical coagulant in water purification work.

Alum reacts with water only in the presence of alkalinity. If natural alkalinity is not present,

lime may be added to develop alkalinity. It reacts with alkaline water to form aluminum

hydroxide (floc), calcium sulphate and carbon dioxide. Due to the formation of calcium

sulphate, hardness and corrosiveness of water is slightly increased. .

Chemical Reaction Taking Place

i) Al 2(SO4) 3.18H2O + 3Ca (HCO3)2 2Al(OH)3+ 3CaSO4 + 6CO2 +18 H2O

ii)

Al (SO4)3.18H2O+ 3Ca(OH)2 2Al(OH)3+ 3CaSO4 + 18H2O

iii) Al2(SO4)318H2O+3Na2CO3 2Al(OH)3+ 3Na2SO4 + 3CO2 + 18H2O

The chemical is found to be most effective between pH range of 6.5 to 8.5. Its dose may varyfrom 5 to 30mg/lit, for normal water usually dose being 14mg/l. actually, dose of coagulant

depends on various factors such as turbidity, colour, taste, pH value, temperature etc.

Due to the following reason, Alum is the most widely used chemical coagulant.

1.

It is very cheap

2. It removes taste and color in addition to turbidity

3. It is very efficient

4.

Flocs formed are more stable and heavy

5.

It is not harmful to health

6.

It is simple in working, doesnt require skilled supervision for dosing

ii. Sodium aluminates (Na2Al2O4)

In the process of coagulation, it can remove carbonate and non-carbonate hardness. It reacts

with calcium and magnesium salts to form flocculent aluminates of these elements.

Chemical reactions:

i)

Na2Al2O4 + Ca (HCO3)2 CaAl2O4+ Na2CO3 + CO2+ H2O

ii) Na2Al2O4 + CaSO4 CaAL2O4+ Na2SO4

iii)

NaAl2O4 + CaCl2 CaAl2O4+ 2NaCl

The pH should be within the range of 6 and 8.5.

iii. Chlorinated Copperas

Combination of Ferric sulphate and Ferric chloride

When solution of Ferrous Sulphate is mixed with chlorine, both Ferric sulphate and Ferric

chloride are produced.


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6FeSO4.7H2O + 3Cl2 2Fe3(SO4)2+ 2FeCl3+ 42H2O

Ferric sulphate and Ferric chloride each is an effective floc and so also their combination.

Both Ferric sulphate and Ferric chloride can be used independently with lime as a coagulant

If alkalinity is insufficient, lime is added.

Chemical reaction taking place2FeCl3 + 3Ca(OH)2 2Fe(OH)3+ CaCl2

Fe2(SO4)3 + 3Ca(OH)2 2Fe(OH)3+ 3CaSO4Ferric chloride effective pH range 3.5 6.5 or above 8.5 and Ferric sulphate is effective with

pH range of 4 7 or above 9.

iv. Polyelectrolytes

They are special types of polymers. They may be anionic, cationic, and non-ionic depending

upon the charge they carry. Out of these only cationic polyelectrolytes can be used

independently.

Example 3:

Find out the quantity of alum required to treat 18million liters of water per day. The dosage of

alum is 14mg/lit. Also work out the amount of CO2released per liter of treated water.

Feeding of coagulant

In order to feed chemicals to the water regularly and accurately, some type of feeding

equipment must be used.

Coagulants may be put in raw water either in powder form or in solution form.

1. Dry-feedTypeDry powder of coagulant is filled in the conical hopper. The hoppers are fitted with agitating

plates which prevent the chemical from being stabilized. Agitating plates are used to prevent

arching of chemicals. Feeding is regulated by the speed of toothed wheel or helical spring (fig

5). Activated carbon and lime are added to raw water in powder form.

Fig 5 Dry feeding devices


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2. Wet feeding type

First, solution of required strength of coagulant is prepared. The solution is filled in the tank

and allowed to mix in the mixing channel in required proportion to the quantity of water. It can

be easily controlled with automatic devices.

Mixing devices

The process of floc formation greatly depends upon the effective mixing (rapid mixing) of

coagulant with the raw water.

Rapid mixing of the mixture of coagulant and raw water is used to:

-

Disperse chemicals uniformly throughout the mixing basin

- Allow adequate contact between the coagulant and particles

- Formation of microflocs

The mixing is done by mixing device.

1.

Hydraulic jump - flume with considerable slope is developed

2. Pump method - centrifugal pump is used to raise raw water

3.

Compressed air method compressed air is diffused from bottom of the mixing tank

4.

Mixing channels

Mixing of raw water and coagulant is made to pass through the channel in which flume

has been done. Vertical baffles are also fixed at the end of the flumed part on both

sides of the channel (fig 6).

5. Mixing basin with baffle wall

6.

Mechanical mixing basins

Mechanical means are used to agitate the mixture to achieve the objective of thorough

mixing. Flash mixers and deflector plate mixers are used.

Fig 6 mixing channel


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4.2.5 Flocculation

After adding the coagulant to the raw water, rapid agitation is developed in the mixture to

obtain a thorough mixing. Next to rapid mixing, mixture is kept slowly agitated for about 30 to

60min. Slow mixing process in which particles are brought into contact in order to promote

their agglomeration is called flocculation. The tank or basin in which flocculation process is

carried out is calledflocculation chamber. The velocity of flow in the chamber is kept between

12 18cm/sec. Activated carbon in powder form can be used to speed up the flocculation

The rate of agglomeration or flocculation is dependent upon

-

Type and concentration of turbidity

- Type of coagulant and its dose

- Temporal mean velocity gradient G in the basin

The mean velocity gradient is the rate of change of velocity per unit distance normal to the

section - (meter per second per meter) (T-1

). The value of G can be computed in terms of powerinput by the following equation

Where P power dissipated (watt)

- absolute viscosity (Ns/m2)

V - the volume to which P is applied (m3)

G - temporal mean velocity gradient (s-1

)

The flocculation technique most commonly used involves mechanical agitation with rotatingpaddle wheels or vertical mounted turbines (fig 9).

The design criteria of a horizontal continuous flow rectangular basin flocculator:


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Example 4:

Design a settling tank (coagulationsedimentation) with continuous flow for treating water for

a population of 48,000 persons with an average daily consumption of 135lit/head. Take

detention period of 3hrs and maximum day factor of 1.8.

4.2.7 Filtration

The effluent obtained after coagulation does not satisfy the drinking water standard and is not

safe. So it requires further treatments. Filtration is one of the water purification process in

which water is allowed to pass through a porous medium to remove remaining flocs or

suspended solids from the previous treatment processes.

Filtration process assist significantly by reducing the load on the disinfections process,

increasing disinfection efficiency.

Theory of Filtration

Filtration consists of passing water through a thick layer of sand. During the passage of water

through sand, the following effects take place.

i) Suspended matter and colloidal matter are removed

ii) Chemical characteristic of water get changed

iii)

Number of bacteria considerably reduced.

These phenomena can be explained on the basis of the following mechanisms of filtration.

1. Mechanical straining Mechanical straining of suspended particles in the sand pores.

2. Sedimentation and Adsorption-

The interstices between the sand grains act as sedimentation basins in which the

suspended particles smaller than the voids in the filter-bed settle upon the sides of the

sand grains.

-

The particles stick on the grains because of the physical attraction between the two

particles of matter and the presence of the gelatinous coating formed on the sand grains

by the previously deposited bacteria and colloidal matter.

3. Electrolytic action

Due to the friction between medium and suspended solids, certain amount of dissolved and

suspended matter is ionized. Suspended matter in water is ionized, carries charge of one

polarity and the particles of sand in filter which are also ionized, possess electrical charges

of opposite polarity. These neutralize each other; change the chemical character of water.

4. Biological Action

The growth and life process of the living cells, biological metabolism. The surface layer

gets coated with a film in which the bacterial activities are the highest and which feed on


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Fig 11 Slow sand filter

Operation

The water from sedimentation tanks enters the slow sand filter through a submersible inlet as

shown in fig 11. This water is uniformly spread over a sand bed without causing any

disturbances. The water passes through the filter media at an average rate of 2.4 to

3.6m3/m

2/day. This rate of filtration is continued until the difference between the water level on

the filter and in the inlet chamber is slightly less than the depth of water above the sand. The

difference of water above the sand bed and in the outlet chamber is called the loss of head.

During filtration as the filter media gets clogged due to the impurities, which stay in the pores,

the resistance to the passage of water and loss of head also increases. When the loss of head

reaches 60cm, filtration is stopped and about 2 to 3cm from the top of bed is scrapped and

replaced with clean sand before putting back into service to the filter.

The scrapped sand is washed with the water, dried and stored for return to the filter at the time

of the next washing. The filter can run for 6 to 8 weeks before it becomes necessary to replace

the sand layer.

Uses

The slow sand filters are effective in removal of 98 to 99% of bacteria of raw water and

completely all suspended impurities and turbidity is reduced to 1 N.T.U. Slow sand filters also

removes odours, tastes and colours from the water but not pathogenic bacteria which requires

disinfection to safeguard against water-borne diseases. The slow sand filter requires large area

for their construction and high initial cost for establishment. The rate of filtration is also very

slow.


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Maintenance

The algae growth on the overflow weir should be stopped. Rate of filtration should be

maintained constant and free from fluctuation. Filter head indicator should be in good working

condition. Trees around the plant should be controlled to avoid bird droppings on the filter bed,

No coagulant should be used before slow sand filtration since the floc will clog the bed quickly.

ii. Rapid Sand Filter

The rapid sand filter differs from the slow sand filter in a variety of ways, the most important of

which are the much greater filtration rate ranging from 100 to 150m3/m

2/day, the ability to

clean automatically using backwashing and require small filter area. The mechanism of

particle removal also differs in the two types of filters - rapid sand filters do not use biological

filtration and depend primarily on adsorption and some straining.

The main features of rapid sand filter are as follows

Effective size of sand - 0.45 to 0.70mm

Uniformity coefficient of sand - 1.2 to 1.7

Depth of sand - 60 to 75cm

Filter gravel - 2 to 50mm size

(Increase size towards bottom)

Depth of gravel - 45c

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