Notes - Water Supply Engineering

Water Supply Engineering Third Year/First Part

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. It should be colourless, and sparkling which may be accepted by public. It should be tasty, odour-free, soft, cool and cheap in cost. It shouldn’t corrode pipes. It should have dissolved oxygen and free from carbonic acid so that it remains fresh.

ii. iii. iv. v.

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.

Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 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.

Water Supply System

Continuous

Intermittent

- Water is available 24 hours a day and seven days a week.

- Water is supplied for few hours every day or alternate days.

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. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 1.5 Schematic Diagram of Typical Water Supply System 1. City/General

2. Hilly Area/Rural Area


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Water Supply Engineering Third Year/First Part 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. ii. iii.

Gravity Flow System Pumping System Dual System

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


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CHAPTER – II SOURCES OF WATER 2.1 Classification of Sources of Water

Sources of Water

Surface Source

SubSurface/Under ground/Ground Source

River, Stream, Lake, Pond, Impounded Reservoir

Spring, Well, Infiltration Gallery, Infiltration Well

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


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Water Supply Engineering Third Year/First Part 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

May – 0.51

September – 4

February – 1.99

June – 1

October – 5

March – 1

July – 2

November – 4

April – 0

August – 3

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.


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Water Supply Engineering Third Year/First Part I.

ANALYTICAL METHOD (

)

Where, n = number of days in the month Flow Inflow Months (in (ML) m3/s) January 2.97 7954.848 February 1.99 4814.208 March 1 2678.4 April 0 0 May 0.51 1365.984 June 1 2592 July 2 5356.8 August 3 8035.2 September 4 10368 October 5 13392 November 4 10368 December 2.8 7499.52 Total 74424.96

II.

Demand (ML) 6202 6202 6202 6202 6202 6202 6202 6202 6202 6202 6202 6202 74424

Cumulative Cumulative Surplus Deficit Inflow Demand (ML) (ML) (ML) (ML) 7954.848 6202 1752.85 12769.056 12404 365.056 15447.456 18606 3158.54 15447.456 24808 9360.54 16813.44 31010 14196.6 19405.44 37212 17806.6 24762.24 43414 18651.8 32797.44 49616 16818.6 43165.44 55818 12652.6 56557.44 62020 5462.56 66925.44 68222 1296.56 74424.96 74424 0.96

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.


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Inflow and Demand (Cumulative) in ML

Determination of Capacity of Impounded Reservoir 80000 70000 60000 50000 40000

Cumulative Demand (ML)

30000

Cumulative Inflow (ML)

20000 10000 0 0

2

4

6

8

10

12

Months

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


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2.3.2 Springs A spring is the natural outflow of ground water appearing at the earth’s 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 can’t 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


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Water Supply Engineering Third Year/First Part 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 earth’s 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. ii.

Shallow Open Well Deep Open Well


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Water Supply Engineering Third Year/First Part 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. ii. iii. iv.

Strainer type Tube Well Cavity type Tube Well Slotted type Tube Well 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. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 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 consumer’s 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. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part e. Sustainable and Safe f. Reliable g. Non conflict among water users (For pictures, refer any standard book.)


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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.


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Water Supply Engineering Third Year/First Part 

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 2 3

Private Connection and Fully Plumbed System Private Connection and Partly Plumbed System Public Stand Post

112 65 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 2 3

Big animals >> cow, buffalo Medium animals >> goat, dog Small animals >> birds

45 20 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.


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Institutions a. Hospitals/Health Posts/Clinics i. With Bed ii. Without Bed b. Schools i. Boarders ii. Day Scholars c. Hotels i. With Bed ii. Without Bed d. Restaurants/Tea Stall e. Offices i. Unclassified ii. Resident iii. Non resident

Demand Urban Area

Rural Area

500 l/bed/day 2,500 l/day

325-500 l/bed/day 1600-2500 l/hospital/day

65 lpcd 10 lpcd

42-60 lpcd 6.5-10 lpcd

200 l/bed/day 500-1000 l/day 500-1000 l/day

200 l/bed/day 500-1000 l/day 200-500 l/day

500-1000 l/day 65 lpcd 10 lpcd

325-1000 l/office/day 65 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 1. National Board of Fire Underwriters Formula 2. Freeman's Formula 3. Kuichling's Formula 4. Buston's Formula 5. Indian Water Supply Manual Formula


Formula (P in '000, Q in l/min) Q = 4637 √P (1 - o.01 √P) Q = 1136 (P/5 + 10) Q = 3182 √P Q = 5663 √P Q = 100 √P, Q in cubic meter/day

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Water Supply Engineering Third Year/First Part 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.


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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.


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Water Supply Engineering Third Year/First Part iv.

Standard of Living: The higher the standard of living is, the higher the demand and greater the variation in demand. Quality of Water: If water is aesthetically and medically safe, the consumption will increase as people will not resort to private wells, etc. Pressure in the Distribution System: Higher pressure results in increased use while lower pressure results in decreased use. Efficiency of water works administration: Leaks in water mains and services and unauthorized use of water can be kept to a minimum by surveys. Cost of Water 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.

v. vi. vii. viii. ix.

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 1991 2001 2011

8000 12000 17000 22500 Total Average

4000 5000 5500 14500 A = 4833

50 41.67 32.35 124.02 G = 41.34

1000 500 1500 I = 750

8.33 9.32 17.65 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 Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 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 2011 – 2021 2021 – 2031 2031 – 2041 2041 – 2051 Shuvanjan Dahal (o68/BCE/147)

% increase 32.35 – 8.82 = 23.53 23.53 – 8.82 = 14.71 14.71 – 8.82 = 5.89 -ve (so zero – constant) Page 20

Water Supply Engineering Third Year/First Part (

)

(

) (

)

(

) (

)

The survey data collected for a water supply scheme in a village of Nepal is given below: Survey Year – 2013

Number of goats – 560

Base Year – 3 years

Number of chickens – 2200

Design Period – 20 years

Annual population growth rate – 1%

Population – 500 Number of cows – 20

Number of health posts – 1

Number of day scholars in school – 125 Number of school – 20

boarders

in

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 Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 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


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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 a. Calcium and Magnesium i. Bicarbonate ii. Carbonate iii. Sulphate iv. Chloride b. Metals and Compounds i. Lead Shuvanjan Dahal (o68/BCE/147)

Effects Alkalinity Alkalinity and hardness Hardness Hardness, corrosion Cumulative poisoning Page 23

Water Supply Engineering Third Year/First Part ii. iii. iv. v. vi. vii. viii. ix. x. xi.

Arsenic Iron Oxide Manganese Barium Cadmium Cyanide Boron Selenium Silver 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

Types of Hardness

Permanent Hardness

Temporary 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 can’t 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


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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 CaCO 3 = 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) Hardness Level

0 – 50 Soft

50 – 100 100 – 150 150 – 250 > 250 Moderately Slightly Hard Moderately Hard Soft Hard The hardness of water is also expressed as the degree of hardness. It may be Clark Scale, French Scale or American Scale. Clark’s 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.


Page 25


Alkalinity

Alkalinity due to Bicarbonate

Alkalinity due to Carbonate [ [

] ]

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 Shuvanjan Dahal (o68/BCE/147)

Page 26

Water Supply Engineering Third Year/First Part 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. b. c. d.

Algae Bacteria Virus 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.

Water borne diseases

Bacterial diseases

Protozoal diseases

Virus diseases

Helminthic (worm) diseases

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, Swimmer’s itch Shuvanjan Dahal (o68/BCE/147)

Page 27

Water Supply Engineering Third Year/First Part 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


Page 28

Water Supply Engineering Third Year/First Part 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 10°C to 20°C. Temperature higher than 25°C is considered objectionable. Temperature of water can be measured with ordinary thermometers graduated in 0.1°C, range from 0 to 50°C. 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: Shuvanjan Dahal (o68/BCE/147)

Page 29

Water Supply Engineering Third Year/First Part i. ii. iii. iv.

ppm in silica scale JTU (Jackson Turbidity Unit) FTU (Formagen Turbidity Unit) 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 21°C, +

-

(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. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 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, gramnegative, rod-shaped bacteria that ferment lactose with gas and acid formation within 48 hours at 35°C. 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.

ii. iii.

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 37°C for a period of 48 hours. 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. –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.

ii.

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 37°C for a period of 48 hours. If the gas is formed, there is presence of E. coli and then step 2 is followed. 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 37°C 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. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part Procedure: i. ii.

iii.

The bacterial colonies or cultures grown in the confirmed test are kept into lactose broth fermentation tubes and agar tubes. The tubes are then kept in the incubator at 37°C 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. 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 35°C 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 35°C for 20 hours. We can observe colonies of coliform.

where, x = sample


Page 32

Water Supply Engineering Third Year/First Part 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)


Page 33



Page 34


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. Shuvanjan Dahal (o68/BCE/147)

Page 35


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


Page 36

Water Supply Engineering Third Year/First Part 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 Shuvanjan Dahal (o68/BCE/147)

Page 37

Water Supply Engineering Third Year/First Part 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.


Page 38

Water Supply Engineering Third Year/First Part 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.


Page 39


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. b. c. d. e. f.

To remove the colour, odour (taste causing substances) To remove the turbidity present in water To remove pathogenic organisms To remove hardness To make water potable To prevent the spread of diseases

6.2 Treatment Processes and Impurity Removal 1. 2. 3. 4. 5. 6. 7.

8. 9.

SCREENING: Bulky and floating suspended matters are removed by the process of screening. PLAIN SEDIMENTATION: Heavy and coarse suspended matters are removed by the process of plain sedimentation. SEDIMENTATION WITH COAGULATION: This process helps to remove fine suspended and colloidal matters. FILTRATION: It is the most important stage in the purification process of water. It removes very fine suspended impurities and micro-organisms. DISINFECTION: It is carried out to eliminate or reduce pathogenic micro-organisms that have remained after the process of filtration. SOFTENING: Removes hardness of water. 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. Removal of Fe and Mn. Removal of other harmful constituents.


Page 40

Water Supply Engineering Third Year/First Part 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.


Page 41

Water Supply Engineering Third Year/First Part 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 Shuvanjan Dahal (o68/BCE/147)

Page 42

Water Supply Engineering Third Year/First Part 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. ii. iii. iv. v.

Velocity of flowing water Size, shape and specific gravity of particles Viscosity of water Detention time Effective depth and length of settling zone


Page 43

Water Supply Engineering Third Year/First Part 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 Stoke’s 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:


Page 44

Water Supply Engineering Third Year/First Part (

)

For spherical particles,

Thus, (

√(

)

)

Also, we have, ( ) Hence, (

√(

)

)

The above equation is called Hazen’s Equation and applicable for particles having diameter greater than 0.1 and less than 1 mm and Reynold’s 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) ‘ ’-kinematic viscosity (mm2/s or centistokes)

0 1.792

5 1.519

10 1.308

) 15 1.141

20 25 1.007 0.897

30 0.804

Hazen further indicated that for particles having diameter ‘d’ ≤ 0.1 mm and Reynold’s number ‘Re’ ≤ 1, Stoke’s Law is applicable. Mathematically, Shuvanjan Dahal (o68/BCE/147)

Page 45


Now,

Putting

, we get,

Thus, (

)

This is Stoke’s 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, Hazen’s equation becomes: ( ) (

)

√

(

)

This equation is called Newton’s 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.


Page 46


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. ii.

Quiescent or fill and draw type 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


Page 47


Long Rectangular Settling Basin


Page 48

Water Supply Engineering Third Year/First Part  

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.


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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.


Page 50


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


Page 51


b. c.

d. e.

f.

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. 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. 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). 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). 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. 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.


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Water Supply Engineering Third Year/First Part 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,

(


)

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Water Supply Engineering Third Year/First Part Where, is SOR and represents the settling velocity of the slowest particles, which are 100% removed. Design Criteria of Sedimentation Tank/Design Details 1. 2. 3.

4. 5. 6.

Detention Period: For plain sedimentation: 3 to 4 hours, and for coagulate sedimentation: 2 to 2.5 hours Velocity of Flow: Not greater than 30 cm/min (horizontal flow) 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. Depth 2.5 to 5.0 m (3 m). SOR: For plain sedimentation: 12000 to 18000 L/d/m2 tank area; for thoroughly flocculated water: 24000 to 30000 L/d/m2 tank area. 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. ii.

Lyophobic Colloids: that is solvent hating. 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. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 6.5.2 Coagulants (types and their chemical reactions) The following chemicals are used as coagulants: 1. 2. 3. 4.

Aluminium sulphates or alum Iron salts Chlorinated copperas 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. ii. iii. iv.

It forms excellent floc which is better than that formed by any other coagulant. The floc formed is stable and not broken easily. It is relatively cheap and removes colour, odour and taste. It doesn’t require skilled supervision and produces clear and crystal free water.

Disadvantages: i. ii. iii.

It requires alkalinity ranging pH from 6.5 to 8.5 in water for effective use. The product calcium sulphate may cause permanent hardness and carbon dioxide may cause corrosion. 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. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part 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: (

)

(

)

The effective range of pH for coagulation with ferric chloride is 3.5 to 6.5. Advantages of Iron Salts: 1.

Produces fast forming, denser, quick settling and less breakable floc than alum at low temperature. 2. Can be used in the wider range of pH and ferric chlorides and/or ferric sulphates may remove manganese at > 9 pH. 3. Ferric chloride is effective in removing H2S, taste and colour. Shuvanjan Dahal (o68/BCE/147)

Page 56

Water Supply Engineering Third Year/First Part Disadvantages of Iron Salts: 1.

Additional lime increases the treatment cost and iron salts impart more corrosiveness to water than alum. 2. Iron salts are difficult to handle due to corrosiveness and require skilled supervision on application. 3. Promotes the growth of bacteria in distribution system. 4. It is suitable for sewage treatment.

3. Chlorinated copperas [FeCl3.Fe2(SO4)3] The mixture of ferric chloride and ferric sulphate is called chlorinated copperas and prepared by adding 1 part chlorine to 7.8 part ferrous sulphate. [

]

[

(

) ]

This chlorinated copperas when added to water forms a tough floc which is removed in sedimentation. The effective range of chlorinated copperas is from pH of 6 to 8. It is effective to remove colour but very corrosive and common in sewage treatment. 4. Sodium Aluminate [Na2Al2O4] It is sometimes used as coagulant. This chemical when dissolved and mixed with water reacts with salts of calcium and magnesium and forms the precipitate of calcium and magnesium aluminate. (

)

Similar chemical reactions take place in case of Mg salts. The effective range of pH for coagulation with sodium aluminate is 6 to 8.5. This removes temporary and permanent hardness but very costly and not used in public water supplies and used to treat boiler water. 6.5.3 Mixing Devices (Purpose and Types) Following operations are involved in sedimentation with coagulation: 1. 2. 3. 4.

Feeding the coagulant Mixing of coagulant Flocculation Sedimentation


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Water Supply Engineering Third Year/First Part 1. Feeding the Coagulant The coagulant may be fed to raw water either in powder form (called dry feeding) or in solution form (called wet feeding). Coagulant feeding is done using dry feeding or wet feeding devices. The choice between wet and dry feeding depends on: a. Characteristics of coagulant and convenience of its application. b. Dosages of coagulants: High dose-dry feed and vice versa. c. Size of the treatment plant: Large size-wet feeder is used and vice versa. 2. Mixing of Coagulant After the addition of coagulants to raw water, they are thoroughly and vigorously mixed so that the coagulants get fully dispersed into the entire mass of water. Various mixing devices are as follows but the first two types are common: I. II. III. IV. V. VI.

Mixing basin with baffle walls Mixing basin with mechanical means Mixing channels Hydraulic jump method Compressed air method Centrifugal pumping method

a. Mixing Basin with Baffle Walls


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Water Supply Engineering Third Year/First Part These are the rectangular basins or tanks which are provided with baffle walls. The disturbance created by the presence of baffle walls in the path of following water cause vigorous agitation of water which resolves in through mixing of water with coagulant. Such basins are of two types: i. Horizontal or Round End Type The mixture and coagulants after entering the basin through an inlet provided at one end of the basin, flows horizontally for short distance and due to the presence of baffle walls; it takes turn and moves further as shown by the arrows and comes out through an outlet provided at the other end of the basin to the flocculator. ii. Vertical or Over and Under Type The mixture of water and coagulant after entering the basin through an inlet provided at one end of the basin, flows up and down as shown by the arrow due to the presence of vertical walls projecting alternatively from the roof and the floor of the basin. Ultimately it flows out through outlet at the other end of the basin to the flocculator. Mixing basin should be properly designed to get the desired effect. The various considerations for design are: i. ii. iii. iv.

Velocity of flow in the channel should be between 0.15 m/s to 0.45 m/s (between baffle walls). Detention period kept is 20 to 50 minutes. Distance between successive baffle walls should be at least 0.45 m. End opening between end baffle walls and basin walls should be about 1.5 times the distance between the successive baffle walls subjected to minimum 0.6 m.

b. Mixing Basin with Mechanical Means


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Water Supply Engineering Third Year/First Part Mixing basin with baffle walls are used only for small treatment plant but for large plant mixing basins with mechanically driven paddles is used. It is also called flash mixture and consists of a deep circular or square tank which is provided with a propeller type impeller fixed at the lower end of a vertical shaft which is driven by electric motor. Diameter of impeller provided is 0.2 to 0.4 times the tank diameter and impeller rotation speed is of 100 rpm. 3. Flocculation (6.5.4 Flocculation tanks)

Longitudinal Flow Flocculator

Vertical Flow Flocculator Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part From the mixing basin, water is taken to the flocculator for flocculation. In a flocculator, slow stirring of water is brought about to permit build up of the floc particles. There are various types of flocculators but the mechanical flocculators are most commonly used. Mechanical flocculator consists of a tank provided with paddles for stirring of water; hence, it is called paddle flocculator. Depending upon the direction of flow of water in the tank, the mechanical flocculator may be longitudinal or vertical flow flocculator. A longitudinal flow flocculator consists of a rectangular tank provided with paddles revolving on a horizontal shaft as shown in figure. A vertical flow flocculator consists of a circular tank provided with paddles revolving on a vertical shaft as shown in second figure above. The paddles are moved by electric motor. The water coming from mixing basins enters the flocculator through an inlet and leaves through outlet to sedimentation tank. In longitudinal flow flocculator, inlet and outlet are provided near the top of tank in opposite end but in vertical type, inlet is provided at the bottom and outlet is provided near the top in opposite end. Design Criteria: A. B. C. D. E.

Depth of tank = 3 to 4.5 m Detention period = 10 to 45 minutes (30 min common) Velocity of flow = 0.2 to 0.8 m/min (0.4 m/min common) Total area of paddles = 10 to 25 % (15% common) of X-section of area of tank Outflow velocity = 0.15 to 0.25 m/s to prevent settling or breaking of floc.

4. Sedimentation (6.5.5 Clarifier) The water from the flocculator is taken to the sedimentation tank also called the sedimentation tank or clarifier. It consists of floc chamber and sedimentation tank. The detention period for floc chamber is about 15 to 40 minutes and that for sedimentation tank is about 3 to 4 hours. The surface overflow rate is from 20 to 40 meter cube/meter square/day. The depth of floc chamber is usually kept about half of the depth of the sedimentation tank. The cleaning of this tank is usually carried out at an interval of 3 to 6 months. 6.5.6 Jar Test The jar test is a common laboratory procedure used to determine the optimum operating conditions for water or wastewater treatment. This method allows adjustments in pH, variations in coagulant or polymer dose, alternating mixing speeds, or testing of different coagulant or polymer types, on a small scale in order to predict the functioning of a large scale treatment operation. Jar Testing Apparatus The jar testing apparatus consists of six paddles which stir the contents of six 1 litre containers. One container acts as a control while the operating conditions can be varied among the


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Water Supply Engineering Third Year/First Part remaining five containers. An rpm gage at the top centre of the device allows for the uniform control of the mixing speed in all of the containers.

The jar test procedures involve the following steps: 

  



Fill the jar testing apparatus containers with sample water. One container will be used as a control while the other 5 containers can be adjusted depending on what conditions are being tested. For example, the pH of the jars can be adjusted or variations of coagulant dosages can be added to determine optimum operating conditions. Add the coagulant to each container and stir at approximately 100 rpm for 1 minute. The rapid mix stage helps to disperse the coagulant throughout each container. Turn off the mixers and allow the containers to settle for 30 to 45 minutes. Then measure the final turbidity in each container. Reduce the stirring speed to 25 to 35 rpm and continue mixing for 15 to 20 minutes. This slower mixing speed helps promote floc formation by enhancing particle collisions which lead to larger flocs. Residual turbidity vs. coagulant dose is then plotted and optimal conditions are determined. The values that are obtained through the experiment are correlated and adjusted in order to account for the actual treatment system.

6.6 Filtration 6.6.1 Purpose The resultant water after sedimentation will not be pure and may contain some very fine suspended particles and bacteria in it. To remove or to reduce the remaining impurities still further, the water is filtered through the beds of fine granular materials such as sand, etc. The process of passing the water through the beds of such granular materials is known as filtration. The main purpose of filtration is to remove colour, odour, taste, bacteria and colloidal impurities.


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Water Supply Engineering Third Year/First Part 6.6.2 Theory of Filtration There are four basic filtration mechanisms: a. Mechanical Straining: Suspended matters larger than the size of interstices (voids between sand grains) cannot pass through them and are trapped or arrested and then removed which is called mechanical straining. Being smaller in size, colloidal matters or bacteria may not be strained. b. Sedimentation and Adsorption: The interstices between the sand grains act as very small sedimentation tanks where the suspended matters settle. Due to physical attraction between the suspended particles and sand grains and presence of gelatinous coating forded due to these matters, other suspended particles, colloidal matters and bacteria adhere there and are removed. c. Biological Metabolism: Organic matters such as algae, plankton also caught by voids between sand grains and these matters are used by bacteria for survival and convert them into harmless compounds from biological metabolism. These harmless compounds formed are deposited at the surface of sand and contains a zoological jelly called ‘dirty skin’ which further enhances in adsorbing and straining activities. Bacteria not only break organic impurities into harmless compounds but they destroy each other and make a balanced life in the filter. d. Electrolytic Action: As per ionic theory, when two substances of opposite charges come into contact, the charge is neutralized and in doing so, new chemical substances are formed. Sand particles in filter media also have charges of some polarity which attracts the suspended, colloidal and dissolved matters of opposing polarity in neutralizing and changes the chemical characteristics of water. After a long use, charges in the sand grains get exhausted and it becomes necessary to clean filter for regeneration of charges. 6.6.3 Types of Filter A tank or device with filter media used for the filtration is called filter. Sand, either fine or coarse, is generally used as filter media. Based on the filtration rate and driving force to overcome friction by water flowing to filter, filters are classified as: a) Slow Sand Filter (SSF) b) Rapid Sand Filter (RSF) c) Pressure Filter (PF) 6.6.3.1 Slow Sand Filter SSF consists of fine sand, supported by gravel. This earliest type of gravity filter has a slow rate of filtration (≤ 1/20th of that of RSF or PF). It captures particles near the surface of the bed and is usually cleaned by scraping away the top layer of sand that contains the particles. Pretreatment of water is not needed for < 20 NTU turbidity and can be directly fed to filter but if water has high turbidity, plain sedimentation or sometimes sedimentation with coagulation is done before feeding into SSF. Shuvanjan Dahal (o68/BCE/147)

Page 63


SSF consists of following: I.

II.

III.

IV.

V.

Enclosure Tank: It is open and water tight rectangular tank made of masonry or concrete. The depth is 2.5 to 3.5 m, surface area is 10 to 20 m2 or more (depends upon filtration rate), filtration rate is 100 to 200 lph/m2. The floor is provided at cross slope of 1 in 100 to 1 in 200 towards central drain. Filter Media: Consists of 90 to 110 cm thick sand layer with effective size (D10) of sand 0.25 to 0.35 mm (0.3 mm is common) and coefficient of uniformity (C u) of 3-5. Finer the sand better will be the removal of turbidity and bacterial removal efficiency but lowers the filtration rate. The sand should not contain >2% of Ca and Mg. Base Material: The sand layer is supported on base material of 30 to 75 cm thick gravel bed. The gravel beds are provided four layers of each about 15 cm thicknesses with size 3 to 6 mm, 6 to 20 mm, 20 to 40 mm and 40 to 65 mm from the top. Mid layers between top layer and bottom layer are called intermediate layers. Under Drainage System: It consists of central main drain and lateral drains 2 to 3 m apart and starts from 50 to 80 cm distance of walls of tank. Lateral drain consists of earthenware or perforated pipes laid with open joint or patented drain in slope. Appurtenances: For efficient working, vertical air pipes, depth controlling device, head loss measuring device, rate maintaining devices etc. are installed.

Principles of Slow Sand Filtration  

In a slow sand filter, impurities in the water are removed by a combination of processes: sedimentation, straining, adsorption, and chemical and bacteriological action. During the first few days, water is purified mainly by mechanical and physical-chemical processes. The resulting accumulation of sediment and organic matter forms a thin layer on the sand surface, which remains permeable and retains particles even smaller than the spaces between the sand grains.


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As this layer (referred to as “Schmutzdecke”) develops, it becomes living quarters of vast numbers of micro-organisms which break down organic material retained from the water, converting it into water, carbon dioxide and other oxides. Most impurities, including bacteria and viruses, are removed from the raw water as it passes through the filter skin and the layer of filter bed sand just below. The purification mechanisms extend from the filter skin to approx. 0.3-0.4 m below the surface of the filter bed, gradually decreasing in activity at lower levels as the water becomes purified and contains less organic material. When the micro-organisms become well established, the filter will work efficiently and produce high quality effluent which is virtually free of disease carrying organisms and biodegradable organic matter.





They are suitable for treating waters with low colors, low turbidities and low bacterial contents. 6.6.3.2 Rapid Sand Filter

RSFs are the most commonly used gravity filters in large water supply system. They consist of larger sand grains supported by gravel and capture particles throughout the bed. They are cleaned by backwashing water through the bed to ‘lift out’ the particles. RSF consists of following: I.

Enclosure Tank: It is open and water tight rectangular tank made of masonry or concrete. The depth is 2.5 to 3.5 m, surface area is 10 to 50 m2, and filtration rate is 3000 to 6000 lph/m2. Various number of filter units in series may be provided. Minimum


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II.

III.

IV.

V.

four units should be provided for large water supplies and minimum two for small supplies. The length to width ratio is 1.25 – 1.35. Filter Media: It should be free from dirt and clay. Consists of 60 to 90 cm thick sand layer with effective size (D10) of sand 0.35 to 0.60 mm and coefficient of uniformity (Cu) of 1.3-1.7. Base Material: The sand layer is supported on base material of 45 to 60 cm thick gravel bed. The gravel beds are provided four layers of each about 15 cm thicknesses with size 2 to 6 mm, 6 to 12 mm, 12 to 20 mm and 20 to 50 mm from the top. Under Drainage System: The under drainage system is provided to collect the filtered material and to provide uniform distribution for back water wash. Commonly used drainage systems are (i) Perforated Pipe System and (ii) Pipe and Strainer System. Appurtenances: Wash water trough, air compressor, rate control device, head loss indicator meters, valves, etc. are used.

(Consult a book for working of RSF.)

Backwashing of Rapid Sand Filter 

 

For a filter to operate efficiently, it must be cleaned before the next filter run. If the water applied to a filter is of very good quality, the filter runs can be very long. Some filters can operate longer than one week before needing to be backwashed. However, this is not recommended as long filter runs can cause the filter media to pack down so that it is difficult to expand the bed during the backwash. Treated water from storage is used for the backwash cycle. This treated water is generally taken from elevated storage tanks or pumped in from the clear well. The filter backwash rate has to be great enough to expand and agitate the filter media and suspend the floc in the water for removal. However, if the filter backwash rate is too high, media will be washed from the filter into the troughs and out of the filter.

When is Backwashing Needed? The filter should be backwashed when the following conditions have been met:   

The head loss is so high that the filter no longer produces water at the desired rate; and/or Floc starts to break through the filter and the turbidity in the filter effluent increases; and/or A filter run reaches a given hour of operation.

Operational Troubles in Rapid Gravity Filters Air Binding: 



When the filter is newly commissioned, the loss of head of water percolating through the filter is generally very small. However, the loss of head goes on increasing as more and more impurities get trapped into it. A stage is finally reached when the frictional resistance offered by the filter media exceeds the static head of water above the bed. Most of this resistance is offered by the


Page 66






top 10 to 15 cm sand layer. The bottom sand acts like a vacuum, and water is sucked through the filter media rather than getting filtered through it. The negative pressure so developed, tends to release the dissolved air and other gases present in water. The formation of bubbles takes place which stick to the sand grains. This phenomenon is known as Air Binding as the air binds the filter and stops its functioning. To avoid such troubles, the filters are cleaned as soon as the head loss exceeds the optimum allowable value.

Formation of Mud Balls: 

The mud from the atmosphere usually accumulates on the sand surface to form a dense mat. During inadequate washing this mud may sink down into the sand bed and stick to the sand grains and other arrested impurities, thereby forming mud balls.

Cracking of Filters: 

The fine sand contained in the top layers of the filter bed shrinks and causes the development of shrinkage cracks in the sand bed. With the use of filter, the loss of head and, therefore, pressure on the sand bed goes on increasing, which further goes on widening these cracks.

Remedial Measures to Prevent Cracking of Filters and Formation of Mud Balls   

Breaking the top fine mud layer with rakes and washing off the particles. Washing the filter with a solution of caustic soda. Removing, cleaning and replacing the damaged filter sand.

Sand Filters vs. Rapid Sand Filters 



   



Base material: In SSF it varies from 3 to 65 mm in size and 30 to 75 cm in depth while in RSF it varies from 3 to 40 mm in size and its depth is slightly more, i.e. about 60 to 90 cm. Filter sand: In SSF the effective size ranges between 0.2 to 0.4 mm and uniformity coefficient between 1.8 to 2.5 or 3.0. In RSF the effective size ranges between 0.35 to 0.55 and uniformity coefficient between 1.2 to 1.8. Rate of filtration: In SSF it is small, such as 100 to 200 L/h/sq.m. of filter area while in RSF it is large, such as 3000 to 6000 L/h/sq.m. of filter area. Flexibility: SSF are not flexible for meeting variation in demand whereas RSF are quite flexible for meeting reasonable variations in demand. Post treatment required: Almost pure water is obtained from SSF. However, water may be disinfected slightly to make it completely safe. Disinfection is a must after RSF. Method of cleaning: Scrapping and removing of the top 1.5 to 3 cm thick layer is done to clean SSF. To clean RSF, sand is agitated and backwashed with or without compressed air. Loss of head: In case of SSF approx. 10 cm is the initial loss, and 0.8 to 1.2 m is the final limit when cleaning is required. For RSF 0.3 m is the initial loss, and 2.5 to 3.5 m is the final limit when cleaning is required.


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Water Supply Engineering Third Year/First Part 6.6.3.3 Pressure Filter

(Consult any standard textbook for details.) 6.7 Disinfection 6.7.1 Purpose The filtered water may normally contain some harmful disease producing bacteria in it. These bacteria must be killed in order to make the water safe for drinking. The process of killing the pathogenic bacteria using chemicals called disinfectants is known as disinfection. Disinfection Kinetics: When a single unit of microorganisms is exposed to a single unit of disinfectant, the reduction in microorganisms follows a first order reaction.

This equation is known as Chick’s Law. N = number of microorganisms (No is initial number) K = disinfection constant T = contact time


Page 68

Water Supply Engineering Third Year/First Part 6.7.2 Methods of Disinfection (introduction only) 1.

2.

3.

4.

5. 6. 7.

8. 9.

Chlorination: The germicidal action of chlorine is explained by the recent theory of enzymatic hypothesis, according to which the chlorine enters the cell walls of bacteria and kill the enzymes which are essential for the metabolic processes of living organisms. Boiling: The bacteria present in water can be destroyed by boiling it for a long time. However, it is not practically possible to boil huge amounts of water. Moreover, it cannot take care of future possible contaminations. Treatment with Excess Lime: Lime is used in water treatment plant for softening. But if excess lime is added to the water, it can, in addition, kill the bacteria also. Lime when added raises the pH value of water making it extremely alkaline. This extreme alkalinity has been found detrimental to the survival of bacteria. This method needs the removal of excess lime from the water before it can be supplied to the general public. Treatment like recarbonation for lime removal should be used after disinfection. Ozone Treatment: Ozone readily breaks down into normal oxygen and releases nascent oxygen. This nascent oxygen is a powerful oxidizing agent and removes the organic matter as well as the bacteria from the water. Iodine Treatment Bromine Treatment Potassium Permanganate Treatment: This is a common method of disinfection in rural areas, where mostly the water supplies are from wells which contain lesser amount of bacteria. Potassium permanganate is dissolved in a bucket of well water and the bucket full of this water is mixed with the well water thoroughly. Potassium permanganate not only kills the bacteria but it also helps in oxidizing the taste producing organic matter. Silver Treatment: Silver when immersed in water has been observed to exert an inhibiting action on bacterial life. UV Ray Treatment: Light is effective in killing both the active bacteria as well as spores.

6.7.3 Chlorination Chlorine is added to the water supply in two ways. It is most often added as a gas, Cl 2 (g). However, it also can be added as a salt, such as sodium hypochlorite (NaOCl) or bleach. Chlorine gas dissolves in water following Henry’s Law. ( )

(

)

Once dissolved, the following reaction occurs forming hypochlorous acid (HOCl): (

)

Hypochlorous acid is a weak acid that dissociates to form hypochlorite ion (OCl-).

All forms of chlorine are measured as mg/L of Cl2 (MW = 2 x 35.45 = 70.9 g/mol).


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Water Supply Engineering Third Year/First Part Hypochlorous acid and hypochlorite ion compose what is called the free chlorine residual. These free chlorine compounds can react with many organic and inorganic compounds to form chlorinated compounds. If the products of these reactions posses oxidizing potential, they are considered the combined chlorine residual. A common compound in drinking water systems that reacts with chlorine to form combined residual is ammonia. Reactions between ammonia and chlorine form chloramines, which is mainly monochloramine (NH 2Cl), although some dichloramine (NHCl2) and trichloramine (NCl3) also can form. Many drinking water utilities use monochloramine as a disinfectant. If excess free chlorine exits once all ammonia nitrogen has been converted to monochloramine, chloramine species are oxidized through what is termed the breakpoint reactions. The overall reactions of free chlorine and nitrogen can be represented by two simplified reactions as follows: Monochloramine Formation Reaction: This reaction occurs rapidly when ammonia nitrogen is combined with free chlorine up to a molar ration of 1:1.

Breakpoint Reaction: When excess free chlorine is added beyond the 1:1 initial molar ratio, monochloramine is removed as follows: ( )

The formation of chloramines and the breakpoint reaction create a unique relationship between chlorine dose and the amount and form of chlorine as illustrated below:


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Water Supply Engineering Third Year/First Part Chlorine Demand Free chlorine and chloramines readily react with a variety of compounds, including organic substances, and inorganic substances like iron and manganese. The stoichiometry of chlorine reactions with organics can be represented as shown below: HOCl:

OCl-:

NH2Cl:

The amount of free chlorine consumed in such oxidation is called chlorine demand of water. Chlorine Demand = Total amount of chlorine added – Amount of residual chlorine required after a specified contact period

Chlorine Dose The quantity of chlorine required to be added to water to leave 0.2 mg/l or ppm of freely available residual chlorine after 10 minutes of contact period is called optimum dose of chlorine. This is generally of about 1 ppm. 6.7.4 Types of Chlorine a. Bleaching Powder Bleaching powder of calcium hypochlorite Ca (OCl2) is a chlorinated lime and when mixed with water it dissociates and reacts with hydrogen ions in water.

This process of chlorination is called hypochloration. Bleaching powder contains 30 – 35 % of available chlorine and it is unstable and goes on losing when exposed to atmosphere. So it requires careful storing. Dose: For normal water, 0.5 to 2.5 kg/million litres of water. It is not adopted for large public water supply but can be used in small colonies and swimming pools. Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part b. Chloramines Chloramines are compounds formed by reaction between ammonia and chlorine. Ammonia, which is quickly soluble in water, is added just before chlorine (1 part ammonia + 4.5 part Cl).

(

) ( (

) )

The compounds are quite stable in water and remains in water as residual for a sufficient time so are more effective than chlorine alone. The reaction is slow hence water treated can only be supplied to consumer after 20 minutes to 1 hour of application. It does not cause bad taste and odour when left at residual but is weaker disinfectant than free chlorine disinfectant. c. Chlorine Gas or Liquid Gas or Free Chlorine Chlorine gas is fed directly to the point of application to the water supply in a pressure of 7 kg/cm2 or first dissolved in small flow of water than free chlorine disinfectant. Application of gas chlorine is less expensive but less satisfactory because of poor diffusion of chlorine and at lower temperature crystalline hydrates of Cl are formed and possibility of choking of pipes. Undissolved Cl may corrode pipes and valves. Hence it is not generally used. Chlorine gas or liquid chlorine dissolved in small quantities of water is normally used. d. Chlorine-dioxide In this method, chlorine dioxide gas if produced by passing chlorine gas through sodium chloride solution. The Cl is unstable and reacts as follows:

ClO2 has oxidizing capacity of 2.5 times than that of chlorine and most effective for removal of taste and odour. 6.7.5 Forms of Chlorination A. Plain Chlorination: Application of chlorine to plain or raw water is called plain chlorination. Dose of plain chlorination is 0.5 to 1 ppm. B. Pre Chlorination: Application of chlorine to water before the treatment is called prechlorination. Especially, we use before filtration. Dose adjusted for residual chlorine is 0.1 to 0.5 ppm. Advantages of pre-chlorination are as follows: i. ii. iii. iv.

Reduces the quantity of coagulants required. Reduces the bacterial load in the filters. Helps in maintaining and running filters longer. Controls the algae and planktons in the filter.


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Water Supply Engineering Third Year/First Part v.

Eliminates taste and odour. C. Post Chlorination: Application of chlorine after the treatment is called post chlorination. It is applied after filtration and before entering into the distribution pipe. Doses are adjusted so that it leaves residual chlorine of 0.1 to 0.2 ppm. D. Double or Multiple Chlorination: When raw water contains large number of bacteria, chlorine is applied at two or more points in purification process; it is called double or multiple chlorination. It consists of pre-chlorination just before raw water enters the sedimentation tank and post-chlorination as water leaves the filter and before entering the distribution system. E. Breakpoint Chlorination: (Already described) Breakpoint chlorination has the following effects: i. ii. iii. iv.

It removes taste, colour and manganese. It has adequate bacterial effect. It has the desired residual chlorine. It completes the oxidation of ammonia and other compounds.

F. Super Chlorination: Application of chlorine beyond the breakpoint chlorination is called super chlorination. Generally 2 to 3 ppm beyond the breakpoint is applied for super chlorination. Super chlorination is done during epidemics in a certain locality due to water borne diseases. G. De-chlorination: The process of removing excessive chlorine from water before distribution to the consumers to avoid chlorine taste is known as de-chlorination. It is done either by aeration or adding sodium thiosulphate, sodium metabisulphate, sodium sulphite, sodium bisulphate, ammonia and sulphur dioxide. 6.7.6 Factors affecting efficiency of Chlorination The killing efficiency of bacteria due to chlorination is called bacterial efficiency of chlorine, which depends upon the following factors: 1. 2. 3. 4. 5. 6. 7.

Turbidity: If turbidity is present in water, bacterial efficiency is decreased. Hence, for effective chlorination, water should be turbidity free. Presence of metallic compound: More chlorine is utilized to oxidize metallic ions; therefore, bacterial efficiency is decreased. Ammonia compound: Efficiency is decreased due to formation of combined available chlorine but it lasts for longer time. pH value of water: If pH is high in water, efficiency is low because HOCl is formed at pH of 5 to 7. Temperature: If temperature decreases, amount of free available chlorine is decreased so the efficiency of chlorine is decreased. Time of contact: For effective chlorination, time of contact should be at least 30 minutes. Type, condition and concentration of micro-organism: For bacteria, efficiency is high for viruses, more concentration is required and efficiency is low. Efficiency becomes low if the favourable condition for bacteria is available and concentration of bacteria is high.


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Water Supply Engineering Third Year/First Part 6.8 Softening 6.8.1 Purpose The main purpose of softening is to remove hardness in water. 6.8.2 Removal of Temporary Hardness 6.8.2.1 Boiling Method It is costlier and not used in public water supply. (

)

(

)

The insoluble precipitates

and

are removed by sedimentation.

6.8.2.2 Lime Treatment Method ( (

) )

The insoluble precipitates

( (

)

)

and

are removed by sedimentation.

6.8.3 Removal of Permanent Hardness 6.8.3.1 Lime Soda Method In this process, lime and sodium carbonate is added to water either separately or together and allow them for 30 – 60 minutes for reaction. It removes permanent as well as temporary hardness. ( ( (

) )


) (

(

)

(

)

)

(

)

(

)

(

)

(

)

(

)

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( ) are insoluble and removed by sedimentation. Other products are soluble and and do not impart hardness. This process is economical but a large quantity of sludge is formed and requires skilled supervisor. 6.8.3.2 Zeolite Method It is also called Base Exchange or Ion Exchange process. It also removes temporary hardness and is a commonly used process. Zeolite is a natural or artificial granular substance. Natural zeolite is green in colour and artificial is white and commonly used is also called Permutit. The commonly used Permutit is sodium aluminium silicate ( ) and Permutit is manufactured from feldspar, kaolin and soda.

If we denote Permutit as

(

)

(

)

(Z is anionic component of the exchanger), then, ( {


) (

)

{

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Water Supply Engineering Third Year/First Part Here, the calcium and magnesium are replaced by sodium and water and the sodium salts formed are soluble in water but do not impart hardness and water is softened. The product and remains in zeolite. Regeneration of Zeolite: Due to continuous use of zeolite, it exhausts and zeolite doesn’t work. It can be checked by soap solution test. If it gets exhausted, it needs regeneration by passing NaCl in it.

It can be written as: (

)

(

)

Advantages: 1. 2. 3. 4. 5.

No sludge is formed. Compact and small space is required. Because of simplicity in operation, no skilled persons are required. Hardness may reduce to zero. No other chemicals are required.

Disadvantages: 1. Not suitable for turbid water and water containing Fe and Mn. 2. Growth of bacteria occurs in zeolite if water contains organic matter. 6.8.3.3 Ionization Process/Demineralization Process It is a costlier process and used in industries. It is very similar to zeolite process but metallic ions are exchanged for hydrogen ions and substances like zeolite is used. The substances may be: 1. Acidic Resin (removes alkali substances) e.g. zero karb, organolite, catex etc. 2. Base Resin (removes acidic substances) If we denote these resins as H2R (H means hydrogen and R means organic part of the substance), then,

(

(

)

)

(

)

(

)

In above process, Ca and Mg are replaced by H and water gets softened.


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Water Supply Engineering Third Year/First Part Disadvantages: The disadvantage of this process is that acids are formed. If the quantity of acid is high, it is removed by: i. ii. iii.

Diluting treated water with raw water. Neutralizing treated water with alkaline substance. Absorbing excessive acids by de-acidic substance ‘D’ regenerated with sodium or caustic soda. ( (

)

) ( (

)

)

6.9 Miscellaneous Treatments 6.9.1 Aeration 6.9.1.1 Purpose   

Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due to algae and related organisms. Aeration also oxidizes iron and manganese, increases dissolved oxygen content in water, removes carbon dioxide and reduces corrosion and removes methane and other flammable gases. Principle of treatment underlines on the fact that volatile gases in water escape into atmosphere from the air-water interface and atmospheric oxygen takes their place in water provided the water body can expose itself over a vast surface to the atmosphere. This process continues until an equilibrium is reached depending on the partial pressure of each specific gas in the atmosphere.

6.9.1.2 Types of Aeration I. II. III. IV.

Free Fall or Gravity Aerators Spray Aerators Diffused Aerators Mechanical Aerators

Gravity Aerators In gravity aerators, water is allowed to fall by gravity such that a large area of water is exposed to atmosphere, sometimes aided by turbulence.


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Fountain Aerators/Spray Aerators These are also known as spray aerators with special nozzles to produce a fine spray. Each nozzle is 2.5 to 4 cm diameter discharging about 18 to 36 l/h. Nozzle spacing should be such that each m3 of water has aerator area of 0.03 to 0.09 m2 for one hour.

Injection or Diffused Aerators It consists of a tank with perforated pipes, tubes or diffuser plates, fixed at the bottom to release fine air bubbles from compressor unit. The tank depth is kept as 3 to 4 m and tank width is within 1.5 times its depth. If depth is more, the diffusers must be placed at 3 to 4 m depth below water surface. Time of aeration is 10 to 30 min and 0.2 to 0.4 litres of air is required for 1 litre of water.

Mechanical Aerators Mixing paddles as in flocculation are used. Paddles may be either submerged or at the surface. 6.9.2 Removal of Iron and Manganese Iron and manganese mix into water through soils either in suspension, hydrated oxides, soluble or insoluble form. Ferric hydroxide is insoluble and ferrous bicarbonate is soluble in water. When iron and manganese > 0.3 ppm, they become objectionable. Insoluble ferric hydroxide is removed in sedimentation. Effects: i. ii.

Produces taste, odour and brown red colour. Stains on clothes, corrosion and clogging of pipes by accumulation of precipitates.


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Water Supply Engineering Third Year/First Part iii.

Causes difficulty in various industrial processes.

Methods: (a) By aeration: In this case, aeration is done before sedimentation. Fe: (

)

(

)

Fe (HCO3)2: (

)

Mn:

(b) By adding lime (c) Passing over manganese zeolite 6.9.3 Removal of Colour, Odour and Taste Colour, odour and taste are due to organic and vegetable matters, industrial waste, domestic sewage, dissolved gases and minerals, microorganisms, etc. Methods: The colour, odour and taste are removed to some extent in sedimentation with coagulation followed by filtration, pre chlorination, etc. For effective removal of these, following methods are used: a. Aeration b. Activated carbon treatment: It is the commonly used process for removal of colour, odour and taste. Activated carbon is manufactured by heating saw dust, paper mill waste etc. at 500°C in a closed vessel in controlled condition of burning at 800°C. It is readily available in market in powder or granular form. The powder is mixed with water before filtration and granular is used as filter materials. It absorbs organic matters and removes colour, odour and taste. c. Using Copper Sulphate: It is available in powder or crystal form and easily soluble in water. It is applied in the distribution system or reservoirs to 0.3 to 0.65 ppm. Is application is common in swimming pools.


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CHAPTER – VII RESERVOIRS AND DISTRIBUTION SYSTEM 7.1 System of Supply Water may be supplied to the consumers by the following two systems: i. ii.

Continuous System Intermittent System

7.1.1 Continuous System If water is supplied to the consumers for all 24 hours a day from a system of supply, it is called the continuous system. It is the best system and has following advantages and disadvantages: Advantages: 1. Water is available whenever needed; hence, there is no need of private storage tank. 2. No stagnant in the pipe at any instant; hence, fresh water is always available. 3. Adequate quantity of water is available at any time for fire fighting. Disadvantages: 1.

More wastage of water if the people do not possess any civic sense and do not understand the importance of water. 2. If there is leakage in the system, large volume of water is wasted because of long duration of flow. 3. On repairing, supply may be interrupted during supply hours. 7.1.2 Intermittent System If water is supplied to the consumers only during fixed hours of a day from a system of supply, it is called the intermittent system. It is the most common system adopted in Nepal. The timings are fixed normally in the morning or evening. Timing may be changed to suit climatic and seasonal conditions. Advantages: 1.

Useful when either sufficient pressure or quantity of water is not available at the source to meet the demand. 2. At various distribution zones of the city, water can be supplied by turn. 3. Repairing works can be done in non-supply hours. 4. Leakage in the system causes less wastage of water because of small durations of flow. Disadvantages: 1.

Inconvenience to customers because they have to remain alert to collect the water during supply periods.


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Water Supply Engineering Third Year/First Part 2. Requires domestic storage in small tanks in each house to use water for non-supply period. Consumers may not have sufficient storage, which may cause insanitary condition. 3. No water is available for fire fighting in non-supply hours and before the system is on, fire may cause huge damage. 4. During the non-supply time, taps left open unknowingly or due to negligence, which leads to more wastage of water. 5. Greater diameter of pipes is required because full day supply should be done in a short period. 6. During non-supply time, pressure in the line may fall below atmospheric pressure, which may induce suction of external maters and soil through leak joints. 7.2 Clear Water Reservoirs According to use, reservoirs may be classified into clear water reservoirs and service reservoirs or distribution reservoirs. Clear water reservoir is used to store the filtered water until it is pumped or conveyed into the service reservoirs for distribution. The minimum capacity must be 14 to 16 hours average daily flow and it should be divided into two or more compartments to enable repairing or cleaning. The reservoirs are generally built under ground or half below ground level and half above the ground level depending on site conditions and constructed with masonry or RCC. Hence, construction is similar to masonry or RCC reservoir. 7.3 Service Reservoirs 7.3.1 Purpose and Construction It is used to store the filtered water from clear water reservoir and constructed before distribution system. It is constructed with masonry and RCC. Elevated types are also popular. These service reservoirs should be designed for balancing storage, breakdown storage and fire storage. Purpose: i. ii. iii.

To absorb the hourly variations in demand. To maintain constant pressure in the distribution mains. Water stored can be supplied during emergencies.

Location and Height of Distribution Reservoirs:  

Should be located as close as possible to the centre of demand. Water level in the reservoir must be at a sufficient elevation to permit gravity flow at an adequate pressure.

Types of Reservoirs: i. ii. iii. iv.

Underground reservoirs Small ground level reservoirs Large ground level reservoirs Overhead tanks


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Water Supply Engineering Third Year/First Part Storage Capacity of Distribution Reservoirs The total storage capacity of a distribution reservoir is the summation of: i.

Balancing Storage: The quantity of water required to be stored in the reservoir for equalising or balancing fluctuating demand against constant supply is known as the balancing storage (or equalising or operating storage). The balance storage can be worked out by mass curve method or analytical method.

Mass Curve Method: A mass diagram is the plot of accumulated inflow (i.e. supply) or outflow (i.e. demand) versus time. The mass curve of supply (i.e. supply line) is, therefore, first drawn and is superimposed by the demand curve. The procedure to construct such diagram is as follows:    

From the past records, determine the hourly demand for all 24 hours for typical days (maximum, average and minimum). Calculate and plot the cumulative demand against time, and thus plot the mass curve of demand. Read the storage required as the sum of the two maximum ordinates between demand and supply line as shown in fig. Repeat the procedure for all the typical days (maximum, average and minimum), and determine the maximum storage required for the worst day.

Analytical Method:    

Calculate the cumulative hourly demand and cumulative hourly supply for 24 hours in tabular form. Find the hourly excess of demand (deficit), excess of supply (surplus), total demand (TD) and total supply (TS). Then note the maximum cumulative surplus (MCS) and maximum cumulative deficit (MCD). Then the capacity of balancing reservoir (CBR) is given by: If TS > TD, CBR = MCS + MCD – TS + TD and If TS ≤ TD, CBR = MCS + MCD

ii.

iii.

Breakdown Storage: The breakdown storage or often called emergency storage is the storage preserved in order to tide over the emergencies posed by the failure of pumps, electricity, or any other mechanism driving the pumps. A value of about 25% of the total storage capacity of reservoirs, or 1.5 to 2 times of the average hourly supply, may be considered as enough provision for accounting this storage. Fire Storage: The third component of the total reservoir storage is the fire storage. This provision takes care of the requirements of water for extinguishing fires. A provision of 1 to 4 per person per day is sufficient to meet the requirement.

The total reservoir storage can finally be worked out by adding all the three storages.


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Water Supply Engineering Third Year/First Part 7.3.2 Types of Service Reservoirs i. ii. iii.

Surface Reservoirs Elevated Reservoirs Stand Pipes

Surface reservoirs are made mostly of masonry or concrete. Common practice is to line surface reservoirs with concrete, gunite, asphalt or asphaltic membrane to check leakage of water. Sometimes, these reservoirs may be built underground, especially when they are of large size, and a park may be constructed on its top. Surface reservoirs should be located at high points in the distribution system, so that gravity supply can be done directly. In some cases however, pumps are used to pump water, from the clear water storage surface reservoir to the elevated distribution reservoir. Elevated reservoirs are constructed at an elevation from the ground level and made of RCC or steel. These are also called overhead tanks and the shapes may be circular, rectangular, egg shaped, spherical, elliptical, etc. Any elevated reservoir consists of inlet, outlet, overflow for water; ladder for accessibility, manhole for inspection, ventilator for air circulation, a water level indicator and a lightening rod. Standpipe is also an elevated reservoir usually constructed of steel (sometimes RCC), circular in plan and up to 15 to 30 metres high. The main function of standpipe is to increase pressure in the distribution system by creating extra storage in the tank above the elevation required to give the necessary pressure for distribution. The diameter of these tanks varies from 10 to 15 m. The volume of water stored in the tank above the entrance of the outlet pipe can be used and hence it is the useful storage of standpipe. 7.5 Layout of Distribution System The purpose of distribution system is to deliver water to consumer with appropriate quality, quantity and pressure. Distribution system is used to describe collectively the facilities used to supply water from its source to the point of usage. Requirements of Good Distribution System 1. Water quality should not get deteriorated in the distribution pipes. 2. It should be capable of supplying water at all the intended places with sufficient pressure head. 3. It should be capable of supplying the requisite amount of water during fire fighting. 4. The layout should be such that no consumer would be without water supply, during the repair of any section of the system. 5. All the distribution pipes should be preferably laid one metre away or above the sewer lines. 6. It should be fairly water-tight as to keep losses due to leakage to the minimum. Layouts of Distribution Network The distribution pipes are generally laid below the road pavements, and as such their layouts generally follow the layouts of roads. There are, in general, four different types of pipe networks; any one of which either singly or in combinations, can be used for a particular place. Shuvanjan Dahal (o68/BCE/147)

Page 83

Water Supply Engineering Third Year/First Part 7.5.1 Tree System/Dead End System

In this system, one main pipeline through the centre of the area to be served and from both sides of the main, the sub-mains takes off. The sub-mains are further divided into several branches from which service connections are given to the consumers. It is suitable for old towns and cities having no definite pattern of roads. Advantages: a. b. c. d.

Relatively cheap. Determination of discharges and pressure easier due to less number of valves. Pipe lying is very simple. Pipes are designed only for population likely to be served by them.

Disadvantages: a. Due to many dead ends, stagnation of water occurs in pipes. b. The water available for fire fighting is low because supply can neither be increased not be diverted. c. Many scour valves are required and less successful in maintaining satisfactory pressure in the far areas. 7.5.2 Grid Iron System One main pipeline through the centre of the are to be served and from both sides of the main, the sub-mains are take off in perpendicular direction; then, branch lines inter connect all submains so that water can be circulated through the entire distribution system. It is suitable for cities with rectangular layout, where the water mains and branches are laid in rectangles.


Page 84


Advantages: a. Water is kept in good circulation due to the absence of dead ends. b. In the cases of a breakdown in some section, water is available from some other direction. c. Fire fighting water can be made easily available by diverting water from the other sections to the affected area using valves. Disadvantages: a. Exact calculation of sizes of pipes is not possible due to provision of valves on all branches. b. More number of cutoff valves and longer length of pipers are required. c. Overall cost is high. 7.5.3 Ring System


Page 85

Water Supply Engineering Third Year/First Part The supply main is laid along the peripheral roads and sub mains branch out from the mains. Thus, this system also follows the grid iron system with the flow pattern similar in character to that of dead end system. So, determination of the size of pipes is easy. Advantage:   

Water can be supplied to any point from at least two directions. Suitable for cities having well planned roads and roads in circular or rectangular pattern. The length of main pipe is larger and hence, large quantity of water is available for fire fighting.

Other advantages and disadvantages are as same as in grid iron system. 7.5.4 Radial System

In this system, the area is divided into different zones. The water is pumped into the distribution reservoir kept in the middle of each zone and the supply pipes are laid radially ending towards the periphery. Advantages: 1. It gives quick service. 2. Calculation of pipe sizes is easy. 3. High pressure of distribution. Disadvantage: 

The major disadvantage of this system is that it requires more reservoirs. All other advantages and disadvantages are same as in grid iron system.


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Water Supply Engineering Third Year/First Part 7.6 Design of Distribution System It consists of the design of the pipeline and its network adopted in the system. For design, the following points are considered: a. b. c. d.

Type of flow (continuous or intermittent) Method of distribution (gravity or pumping) Probable future demand Life of pipes etc.

7.6.1 Pipe Hydraulics Hydraulic design of pipes is required to determine the size of the pipes between inlet and exit just to carry sufficient amount of water. For design of pipes, the following two basic equations of hydraulics are used: a. Continuity Equation b. Bernoulli’s Equation (Read descriptions on your own) HEAD LOSS IN PIPES: a. Major Loss – Darcy Weisbach Formula, Manning’s Formula, Hazen William Formula b. Minor Losses – due to sudden enlargement, sudden contraction, at the entrance, at the exit, due to gradual enlargement or gradual contraction, at the bend, due to various pipe fittings (Read descriptions on your own) 7.6.2 Design Criteria 1.

Discharge: Discharge should be sufficient to meet the future demand. Pipelines are designed for 2.5 to 3 times the average demand. Generally peak factor is taken as 3 to 4 in Nepal. 2. Pressure: Pipelines are designed for sufficient residual pressure so that it reaches to the desired height. The residual head for single storey is taken as 7 m, two storeys is 12 m and three storeys is 17 m and it shouldn’t be greater than 22 m above the ground level. In rural areas of Nepal, the minimum available head should be 5 m above the public tap level. 3. Minimum size of pipes: The lower the diameter the head loss is greater. For population less than 20,000, minimum diameter of distribution pipe is 10 cm and for greater than 20,000, it is 15 cm. For dead end pipes, it is 10 cm, for distribution and service pipe 10 cm and 20 cm for house connections but for grid pipes and dead end pipes less than 10 cm may be used. 4. Velocity: If velocity is low, larger diameter pipes are required and problem of silting may occur. If velocity is high, cost becomes high in pumping and cost of pipes and fittings will increase to bear extra pressure. On the other hand, higher the velocity, smaller the diameter which leads to loss of energy. Hence, it shouldn’t be too low and


Page 87

Water Supply Engineering Third Year/First Part not too high. It is taken as 0.6 to 3 m/s in pumping and 0.6 to 1.5 m/s for gravity supply. 5. Gradient: No need of gradient in pressure flow pipes but pipes should be laid below the HGL. The gradient of HGL shows the residual head available at any selected point hence high slop of HGL means head loss is high. 7.6.3 Design Steps 1.

Survey and preparation of contour maps and plans  Land between treatment plant and distribution area is surveyed and contour maps and plans showing the position of the roads, streets, lawns, parks, position of underground service lines are prepared and then studied. 2. Tentative layout  The tentative layout of various zones are marked (includes tentative mains, sub mains, branches, valves, service reservoirs, etc.). 3. Calculation of discharge  Based on population and type of distribution zone and fire demand. 4. Computation of pipe diameters  Hazen-Williams Formula is common.  Pipelines are designed for discharge of 2.25 to 3 times the average rate of demand. 5. Computation of available residual pressure head  If the available residual head is lesser or too high, pipe size should be revised. 7.6.4 Hardy Cross Method Analysis of water distribution system includes determining quantities of flow and head losses in the various pipe lines, and resulting residual pressures. In any pipe network, the following two conditions must be satisfied: 1.

The algebraic sum of pressure drops around a closed loop must be zero, i.e. there can be no discontinuity in pressure. 2. The flow entering a junction must be equal to the flow leaving that junction; i.e. the law of continuity must be satisfied. Based on these two basic principles, the pipe networks are generally solved by the methods of successive approximation. The widely used method of pipe network analysis is the HardyCross method. Hardy-Cross Method This method consists of assuming a distribution of flow in the network in such a way that the principle of continuity is satisfied at each junction. A correction to these assumed flows is then computed successively for each pipe loop in the network, until the correction is reduced to an acceptable magnitude. If Qa is the assumed flow and Q is the actual flow in the pipe, then the correction d is given by d=Q-Qa; or Q=Qa+d Now, expressing the head loss (HL) as Shuvanjan Dahal (o68/BCE/147)

Page 88

Water Supply Engineering Third Year/First Part HL=K.Qx We have the head loss in a pipe =K. (Qa+d) x =K. [Qax + x.Qax-1d + .........negligible terms] =K. [Qax + x.Qax-1d] Now, around a closed loop, the summation of head losses must be zero. SK. [Qax + x.Qax-1d] = 0 Or, SK.Qax = -SKx Qax-1d Since, d is the same for all the pipes of the considered loop; it can be taken out of the summation. a

x

= -d. SKx Qax-1

Or, d=-SK.Qax/ Sx.KQax-1 Since d is given the same sign (direction) in all pipes of the loop, the denominator of the above equation is taken as the absolute sum of the individual items in the summation. Hence, Or, d=-SK.Qax/ S l x.KQax-1 l Or, d=-SHL / x.S lHL/Qal Where HL is the head loss for assumed flow Qa. The numerator in the above equation is the algebraic sum of the head losses in the various pipes of the closed loop computed with assumed flow. Since the direction and magnitude of flow in these pipes is already assumed, their respective head losses with due regard to sign can be easily calculated after assuming their diameters. The absolute sum of respective KQax-1 or HL/Qa is then calculated. Finally the value of d is found out for each loop, and the assumed flows are corrected. Repeated adjustments are made until the desired accuracy is obtained. The value of x in Hardy- Cross method is assumed to be constant (i.e. 1.85 for Hazen-William's formula, and 2 for Darcy-Weisbach formula).


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CHAPTER – VIII CONVEYANCE OF WATER 8.1 Pipe Materials Pipe is a circular closed conduit through which the water may flow either under gravity or under pressure. When pipes do not run full, they run under gravity, such as in sewer lines. However, in supply, pipes mostly run under pressure. Pipe may be made of the following materials: a. b. c. d. e. f. g. h. i. j.

Cast iron Wrought iron Steel Galvanised iron Cement concrete Asbestos cement Plastic Lead Copper Wood

8.1.1 Requirements of Good Material


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Water Supply Engineering Third Year/First Part 8.1.2 Types of Pipe Material In the past, many types of materials have been used in conveying water from one point to another. Masonry and wood were probably the first materials used. Plastics are the newest, and are now being used quite extensively.

1. CAST IRON PIPES These pipes were earlier cast vertically but this type has been largely superseded by spun iron pipes which are manufactured by spinning or centrifugal action and are now universally used. The spun iron C.I. pipes are comparatively lighter in weight, longer in length and have improved metal qualities. These pipes are generally upto 1000 mm diameter and 6 m long and are classified on the basis of thickness of the pipe barrel as class A and B, each differing from the other by 10 percent increase in thickness. These pipes can withstand hydraulic test Shuvanjan Dahal (o68/BCE/147)

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Water Supply Engineering Third Year/First Part pressure of 24 kg/cm2. They are strong, durable (life upto 100 years), easy to join and most commonly used. Disadvantages: a) Difficulty in transportation of large sizes b) Decrease in their carrying capacities with age and with the quality of water transported. 2. WROUGHT IRON AND STEEL PIPES These are stronger than cast iron, can withstand much higher pressures but are of lighter sections and hence easier to transport. They are, however, less durable (life upto 50 years), more liable to corrosion and there is also the difficulty of easy availability of pipe specials viz., joints, bends, etc. Carrying capacity is also a little less for equal diameter of pipe. 3. CONCRETE PIPES Concrete pipes are very durable, heavier and can be had in sizes upto 1800 mm. Transportation costs are much reduced if the pipes are cast in situ. Concrete pipes have low maintenance costs, are resistant to corrosion and particularly suitable to soft and aggressive (acidic) waters. They, however, cannot withstand high pressures unless reinforced. 4. PVC PIPES Today more PVC pipes are being used than any other pipe product. PVC is very corrosion resistant. It is not a conductor and will not have an electrochemical reaction with acids and bases that it comes in contact with. PVC also has a high chemical resistance. While it will react with some chemicals, there are a large number of chemicals it will not react with, making it an excellent product for industrial applications. Because PVC is mostly a 'rigid' pipe product, PVC pipe is an excellent pipe choice for just about any application that does not require a 'flexible' solution. The most common uses for PVC are: 

Water Distribution Underground Fire Main Distribution Gravity Sanitary Sewer Collection Forcemain Sewage Transmission Irrigation Mains Reclaimed Water Distribution Electrical & Communications Conduit Numerous Industrial Applications

8.2 Pipe Joints 8.2.1 Purpose


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Water Supply Engineering Third Year/First Part The pipe joints are required to join together pipes which are available in smaller lengths say 3.6 to 6 m only. The requisites if a jointing material are: (a) (b) (c) (d) (e) (f) (g) (h)

Imperviousness Elasticity Strength Durability Adhesiveness Availability Workability Economy

8.2.2 Types A. SOCKET AND SPIGOT

This joint is commonly used in case of cast iron pipes. The spigot of one pipe is centred into the socket of the preceding pipe; tarred gasket or hemp yarn is then wrapped around the spigot, leaving unfilled the required depth of socket for lead. The gasket or hemp yarn is caulked tightly home with a yarning tool. A jointing ring or a kneaded-clay ring is then placed around the barrel and against the face of the socket. Molten pig lead is poured into the remainder of the socket. Lead is now solidly caulked with suitable caulking tools or hammers of 2 kg weight around the joint, to make up for the shrinkage of molten metal on cooling. B. FLANGED JOINT

A gasket of rubber, canvas or lead is introduced between the two flanges of Cast Iron (C.I.) pipes, which are then tightened with bolts and nuts. Flanged joints are strong and rigid and are easy to disjoint; as such used where the pipe joints have to be occasionally opened out for carrying out repair work, as in pumping chambers.


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Water Supply Engineering Third Year/First Part C. EXPANSION JOINT

Expansion joints allow the pipes to expand or contract freely under the changing temperature conditions. The space between the socket and spigot is filled with an elastic gasket. D. COLLAR JOINT

E. SCREWED SOCKET JOINTS

This is a simple type of joint used for jointing screwed wrought iron or galvanized iron pipes. The two ends of the pipes are threaded on the outside and on them a suitable jointing compound with a grummet of few strands of fine yarn are used before screwing a socket having corresponding threads from inside. 8.3 Laying of Pipes Shuvanjan Dahal (o68/BCE/147)

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Notes - Water Supply Engineering

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