Shuvanjan Dahal (o68/BCE/147) Page 1
CHAPTER – I
INTRODUCTION
1.1 Importance of Water Man and animals not only consume water, but they also consume vegetation for their food. Vegetation, in turn, cannot grow without water.
Growth of vegetation also depends upon bacterial action, while bacteria need water in order to thrive.
Good sanitation cannot be maintained without adequate water supply system. Man needs water for drinking, cooking, cleaning and washing.
Water maintains an ecological balance – balance in the relationship between living things and environment in which they live.
1.2 Definition of Types of Water
1.2.1 Pure and Impure Water
Pure water contains only 2 atoms of hydrogen and 1 atom of oxygen. It is not good for health as pure water does not contain essential minerals required for human health. Impure water, besides 2 atoms of hydrogen and 1 atom of oxygen, contains other
elements.
1.2.2 Potable and Wholesome Water
Potable water is water safe enough to be consumed by humans or used with low risk of immediate or long-term harm.
Water that is not harmful for human beings is called wholesome water. It is neither chemically pure nor contains harmful matters to human health. Requirements of wholesome water:
i. It should be free from radioactive substance, microorganism, disease causing bacteria, objectionable dissolved gases, harmful salts, objectionable minerals and other poisonous metals.
ii. It should be colourless, and sparkling which may be accepted by public. iii. It should be tasty, odour-free, soft, cool and cheap in cost.
iv. It shouldn’t corrode pipes.
v. It should have dissolved oxygen and free from carbonic acid so that it remains fresh. 1.2.3 Polluted and Contaminated Water
Contamination means containing harmful matter. It is always polluted and harmful for use. Water consisting of microorganisms, chemicals, industrial or other wastes, large numbers of pathogens that cause diseases is called contaminated water.
Pollution is synonymous to contamination but is the result of contamination. Polluted water contains substances unfit or undesirable for public health or domestic purpose.
Shuvanjan Dahal (o68/BCE/147) Page 2 Two broad categories of water pollution: a) Point Source b) Non-point Source
a) Point Source: occurs when harmful substances are emitted directly into a body of water. E.g. pipe from an industrial facility emitted directly into a body of water.
b) Nonpoint Source: delivers pollutants through transport or environmental charge. E.g. fertilizer from a farm field carried into a stream by rain.
1.3 Historical Development of Water Supply System
What is Water Supply System?
Water Supply System is a network of pipelines of various sizes with control valves for carrying water to all streets and supplying water to the consumers.
Historical Development
Most of the historical community settlements throughout the world were made near springs, lakes and rivers from where water for drinking and irrigation purposes was obtained.
In the ninth century, few important water supply structures were constructed by the Moors in Spain. In the 12th century, small aqueduct was constructed in Paris. In London, spring water was brought by means of lead pipes and masonry conduits in the thirteenth century.
During the first phase of the Industrial Revolution, large impounding reservoirs were developed due to the necessity of feeding canals.
The first water filter was constructed in 1804 by John Gibb at Paisley in Scotland. The first permanent use of chlorination originated under the direction of Sir Alexander
Houston at Lincoln in 1905.
1.4 Objectives of Water Supply System
The quintessential objective of water supply system is to supply water equitably to the consumers with sufficient pressure so as to discharge the water at the desired location within the premises.
Water Supply System
Continuous - Water is available 24 hours a day and seven
days a week.
Intermittent - Water is supplied for few hours every day or
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1.5 Schematic Diagram of Typical Water Supply System
1. City/General
Shuvanjan Dahal (o68/BCE/147) Page 4 3. Terai Area
1.6 Components of Water Supply System and their Functions
The components of a water supply system can be divided into two major parts:
1. Transmission Line or Transmission Main: Pipeline from intake to reservoir tank. 2. Distribution Line: Pipeline from reservoir tank to tap stand.
There are three systems of supply as: i. Gravity Flow System ii. Pumping System iii. Dual System
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CHAPTER – II
SOURCES OF WATER
2.1 Classification of Sources of WaterMain source of water is precipitation.
2.2 Surface Sources
Surface sources have water on the surface of the earth such as in stream, river, lake, wetland or ocean.
2.2.1 Rivers
Natural channel
Main source: either natural precipitation or snow-fed Perennial and non-perennial rivers
Vast catchment area; hence, amount of water is large Contaminated source
2.2.2 Streams
Natural drainage Less catchment area
Source: Melting snow or precipitation Found in hilly, mountain areas Low quantity of water
Potable water Sources of Water Surface Source River, Stream, Lake, Pond, Impounded Reservoir Sub-Surface/Under ground/Ground Source Spring, Well, Infiltration Gallery, Infiltration Well
Shuvanjan Dahal (o68/BCE/147) Page 6 2.2.3 Lakes
Natural depression filled with water Found in mountain and hilly areas
Quantity of water depends on: depression, catchment area and soil type Quality varies
2.2.4 Ponds
Natural/Artificial depression found in plain areas Bad quality of water
Not used as water supply source Less quantity of water
Can be used for animal bathing and irrigation purposes. 2.2.5 Impounded Reservoirs
An impounding reservoir is a basin constructed in the valley of a stream or river for the purpose of holding stream flow so that the stored water may be used when water supply is insufficient. E.g. Sundarijal Dam
The dam is constructed across the river in such places where minimum area of land is submerged, where river width is less and the reservoir basin remains cup shaped having maximum possible depth of water. Hence, it is defined as an artificial lake created by the construction of a dam across the valley containing a watercourse.
Two functions: i) To impound water for beneficial use ii) To retard flood
The location of impounded reservoir depends upon the quality and quantity of water available, existence of suitable dam site, distance and elevation of reservoir, density and distribution of population, geological conditions, etc.
The water quality is the same as in streams and rivers.
2.2.6 Numerical on Capacity Determination of Impounded Reservoirs
The flow in the river during the various months of the year (in m3/s) is as follows: January – 2.97 February – 1.99 March – 1 April – 0 May – 0.51 June – 1 July – 2 August – 3 September – 4 October – 5 November – 4 December – 2.8
The river supplies water to a community having a constant demand of 6202 million litres/month. Determine the capacity of impounded reservoir.
Shuvanjan Dahal (o68/BCE/147) Page 7 I. ANALYTICAL METHOD
( )
Where, n = number of days in the month
Months Flow (in m3/s) Inflow (ML) Demand (ML) Cumulative Inflow (ML) Cumulative Demand (ML) Surplus (ML) Deficit (ML) January 2.97 7954.848 6202 7954.848 6202 1752.85 February 1.99 4814.208 6202 12769.056 12404 365.056 March 1 2678.4 6202 15447.456 18606 3158.54 April 0 0 6202 15447.456 24808 9360.54 May 0.51 1365.984 6202 16813.44 31010 14196.6 June 1 2592 6202 19405.44 37212 17806.6 July 2 5356.8 6202 24762.24 43414 18651.8 August 3 8035.2 6202 32797.44 49616 16818.6 September 4 10368 6202 43165.44 55818 12652.6 October 5 13392 6202 56557.44 62020 5462.56 November 4 10368 6202 66925.44 68222 1296.56 December 2.8 7499.52 6202 74424.96 74424 0.96 Total 74424.96 74424 II. GRAPHICAL METHOD
The largest possible positive difference (perpendicular distance between the two graphs) gives the value of maximum surplus.
The largest possible negative difference (cumulative demand more) gives the value of maximum deficit.
The difference between the ends of the curves gives the value of the required capacity of impounded reservoir.
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2.3 Ground Sources
When water seeps into the ground, it moves downward due to gravity through the pore spaces between soil particles and cracks in rocks. Eventually, the water reaches a depth where the soil and rock are saturated with water. Water which is found in the saturated part of the ground underneath the land surface is called ground water.
2.3.1 Confined and Unconfined Aquifers
0 10000 20000 30000 40000 50000 60000 70000 80000 0 2 4 6 8 10 12 Inf low a nd D em and (Cum ul at iv e) in M L Months
Determination of Capacity of Impounded Reservoir
Cumulative Demand (ML) Cumulative Inflow (ML)
Shuvanjan Dahal (o68/BCE/147) Page 9 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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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. Shallow Open Well ii. Deep Open Well
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2. Driven Well or Percussion Well
The shallow well constructed by driving a casing pipe of 2.5 cm to 15 cm in diameter and up to 12 m deep is called driven well. The casing pipe is driven first in the ground by hammering or by water jet and the pipes are inserted. The lower portion of the pipe, which is driven in the water bearing strata, is perforated and the pointed bottom is called drive point or well point. The perforated portion of pipe is covered with fine wire gauge to prevent passage of sand and soil particle. The discharge in this well is very small and can be obtained using hand or electric pump and can be used for domestic purposes. E.g. Rower Pump used in the Kathmandu valley.
3. Tube Well
It is the well made of small diameter pipe installed after boring and inserted deep to trap water from different aquifers. A tube well is a long pipe sunk to the ground intercepting one or more water bearing strata. E.g. in Terai regions of Nepal.
As compared to open wells, the diameter of tube wells is much less. Tube wells may be classified as shallow tube well (depth up to 30 m) and deep tube well (maximum depth up to 600 m). Quality may be better but may have various impurities, which should be treated and quantity is larger so it can be used as water supply. Tube wells may be further classified into the following:
i. Strainer type Tube Well ii. Cavity type Tube Well iii. Slotted type Tube Well iv. Perforated type Tube Well
4. Artesian Well
It is the well from where water flows automatically under pressure. Mostly they are found in the valley portion of the hills where aquifers on the both sides are inclined towards valley. The HGL (Hydraulic Gradient Line) passes much above the mouth of well, which causes flow under pressure. The water flows out in the form of fountain upto a height of 2.5 m depending upon hydrostatic pressure. Some wells, which flow continuously throughout the year and can be stored in reservoir and taken for water supply. The quality of water in artesian wells may be good but sometimes it contains minerals and can be used after certain treatment.
2.3.4 Infiltration Galleries and Wells
Infiltration Gallery
Infiltration Gallery is a horizontal or nearly horizontal tunnel, usually rectangular (arched also) in cross section and having permeable boundaries so that ground water can infiltrate into it. Hence, it is also called horizontal well. It is generally located near a perennial recharge source such as the bank or under bed of a river and 3 to 10 meters below the ground. It is also used to collect ground water near marshy land or water bodies and stored in storage tank and then used for water supply.
Shuvanjan Dahal (o68/BCE/147) Page 12 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.
Shuvanjan Dahal (o68/BCE/147) Page 13 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 WaterIt 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:
Shuvanjan Dahal (o68/BCE/147) Page 15 Expandability aspect.
Anticipated rate of growth of population including industrial, commercial developments and migration-immigration.
Available resources.
Performance of the system during initial period.
Suppose, r = growth rate of population
If r ≥ 2, design period is 15 years and if r < 2, design period is 20 years.
3.3 Types of Water Demand
3.3.1 Domestic Demand
Water demand required for domestic purposes. Required for both urban and rural areas.
Depends upon the habit, social status, climatic conditions, living standard, etc.
S.N. Types of Consumption Water Demand (lpcd)
1 Private Connection and Fully Plumbed System 112 2 Private Connection and Partly Plumbed System 65
3 Public Stand Post 45 (can come down to 25)
3.3.2 Livestock Demand
Quantity of water required for domestic animals and livestock including birds. Generally considered in rural water supply.
Livestock demand should not be greater than 20% of domestic demand.
S.N. Types of Consumption Water Demand
(lpcd)
1 Big animals >> cow, buffalo 45
2 Medium animals >> goat, dog 20
3 Small animals >> birds 0.2
3.3.3 Commercial/Institutional Demand
Quantity of water required for commercial institutions like schools, colleges, hospitals, offices, etc.
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Institutions Demand
Urban Area Rural Area a. Hospitals/Health Posts/Clinics
i. With Bed 500 l/bed/day 325-500 l/bed/day
ii. Without Bed 2,500 l/day 1600-2500 l/hospital/day
b. Schools
i. Boarders 65 lpcd 42-60 lpcd
ii. Day Scholars 10 lpcd 6.5-10 lpcd
c. Hotels
i. With Bed 200 l/bed/day 200 l/bed/day
ii. Without Bed 500-1000 l/day 500-1000 l/day d. Restaurants/Tea Stall 500-1000 l/day 200-500 l/day
e. Offices
i. Unclassified 500-1000 l/day 325-1000 l/office/day
ii. Resident 65 lpcd 65 lpcd
iii. Non resident 10 lpcd 10 lpcd
3.3.4 Public/Municipal Demand
Considered only in urban areas for municipal purposes e.g. cleaning roads, for public parks.
We adopt criteria by Indian Government.
i. Street Washing = 1 to 1.5 l/m2 of surface area of road/day ii. Public Parks = 1.4 l/m2/day
iii. Sewer Cleaning = 4.5 l/person/day 3.3.5 Industrial Demand
Normally considered in urban areas.
Quantity of Water required for various industries and factories. Generally taken as 20 to 25% of total demand.
3.3.6 Fire Fighting Demand
Authority Formula (P in '000, Q in l/min) 1. National Board of Fire Underwriters Formula Q = 4637 √P (1 - o.01 √P)
2. Freeman's Formula Q = 1136 (P/5 + 10)
3. Kuichling's Formula Q = 3182 √P
4. Buston's Formula Q = 5663 √P
Shuvanjan Dahal (o68/BCE/147) Page 17 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.
Shuvanjan Dahal (o68/BCE/147) Page 19 iv. Standard of Living: The higher the standard of living is, the higher the demand and
greater the variation in demand.
v. Quality of Water: If water is aesthetically and medically safe, the consumption will increase as people will not resort to private wells, etc.
vi. Pressure in the Distribution System: Higher pressure results in increased use while lower pressure results in decreased use.
vii. Efficiency of water works administration: Leaks in water mains and services and unauthorized use of water can be kept to a minimum by surveys.
viii. Cost of Water
ix. Policy of metering and charging method: Water tax is charge in two different ways: on the basis of meter reading and on the basis of certain fixed monthly rate.
3.7 Population Forecasting – Necessity and Methods
A particular method is to be adopted for a particular case or for a particular city. The selection is left to the discretion and intelligence of the designer.
Sample Problem:
Year Population Increase in Population % increase in Population Incremental increase in Population Decrease in % increase of Population 1981 8000 - - - - 1991 12000 4000 50 - - 2001 17000 5000 41.67 1000 8.33 2011 22500 5500 32.35 500 9.32 Total 14500 124.02 1500 17.65 Average A = 4833 G = 41.34 I = 750 D = 8.82 Present Population, P = 22500
A = average increase per decade = 4833
G = average % increase in population per decade = 41.34% I = average incremental increase per decade = 750
D = average decrease in % increase of population = 8.82 3.7.1 Arithmetical Increase Method
Assumption: The increase in population from decade to decade is assumed constant. This method is suitable for larger and old cities which have practically reached their
maximum development (i.e. cities which have reached their saturation population).
Shuvanjan Dahal (o68/BCE/147) Page 20 n = number of decades
P = present population From above example,
P2021 = 22500 + 1 x 4833 = 27333 P2027 = 22500 + 1.6 x 4833 = 30233
3.7.2 Geometrical Increase Method or Uniform Percentage Growth Method
Assumption: The percentage increase in population from decade to decade is constant. This method is suitable when the city is young and rapidly increasing.
This is the most common method used in Nepal.
( ) Pn = population after ‘n’ decades
G = average % increase per decade
Gives high result than arithmetical increase method – so, much safer result.
( ) ( )
3.7.3 Incremental Increase Method
This method combines both the above two methods – gives value between the above two methods. ( )
3.7.4 Decreased Rate of Growth Method
Year % increase
2011 – 2021 32.35 – 8.82 = 23.53
2021 – 2031 23.53 – 8.82 = 14.71
2031 – 2041 14.71 – 8.82 = 5.89
Shuvanjan Dahal (o68/BCE/147) Page 21 ( ) ( ) ( ) ( ) ( )
The survey data collected for a water supply scheme in a village of Nepal is given below: Survey Year – 2013
Base Year – 3 years Design Period – 20 years Population – 500
Number of cows – 20
Number of goats – 560 Number of chickens – 2200 Annual population growth rate – 1%
Number of health posts – 1
Number of day scholars in school – 125
Number of boarders in school – 20
Number of tea shops – 2 VDC Office – 1
Calculate Design Year Total Water Demand. At 2036, ( ) ( ) ( ) 1. Domestic Demand = 45 x 629 = 28305 l/d 2. Livestock Demand i. Big animals = 45 x 20 = 900
ii. Medium animals = 20 x 560 = 11200 iii. Small animals = 0.2 x 2200 = 440
Total = 12540 l/d
Check: Livestock Demand = 20% of Domestic Demand = 0.2 x 28305 = 5661 l/d
Hence, actual livestock demand = 5661 l/d 3. Commercial Demand
a. Day Scholars = 10 x 157 = 1570 b. Boarders = 65 x 25 = 1625 c. Health Post = 2500
Shuvanjan Dahal (o68/BCE/147) Page 22 d. Tea Shop = 2 x 1000 = 2000
e. VDC Office = 500 Total = 8195 l/d
Shuvanjan Dahal (o68/BCE/147) Page 23
CHAPTER – IV
QUALITY OF WATER
4.1 Impurities in water, their classification and effects4.1.1 Suspended Impurities E.g. sand, silt, algae, virus Characteristics:
They develop colour.
They make turbidity high. Suspended impurity is measured in terms of turbidity. They develop taste.
They invite diseases.
They are macroscopic or can be microscopic. Removed by: Sedimentation or Chemical Treatment 4.1.2 Colloidal Impurities
Microscopic. Their size is between 10-3 mm to 10-6 mm. Not removed by sedimentation
Develop charges (anions)
Cause colour in water and these impurities cause epidemics. Have much less weight
They come in motion due to repulsion.
Removed by: +ve charge for neutralization and settlement 4.1.3 Dissolved Impurities
Dissolved impurities make bad taste, hardness and alkalinity. The concentration is measured in PPM (parts per million) or mg/l and obtained by weighing the residue after evaporation of the water sample from a filtered sample.
a. Salts of Ca and Mg b. Minerals
c. Gases
Constituents Effects
a. Calcium and Magnesium i. Bicarbonate
ii. Carbonate iii. Sulphate
iv. Chloride
Alkalinity
Alkalinity and hardness Hardness
Hardness, corrosion b. Metals and Compounds
Shuvanjan Dahal (o68/BCE/147) Page 24 ii. Arsenic
iii. Iron Oxide iv. Manganese v. Barium vi. Cadmium vii. Cyanide viii. Boron ix. Selenium x. Silver xi. Nitrates Toxicity, poisoning
Taste, red colour, corrosiveness, hardness Black or brown colour
Toxic effect on heart, nerves Toxic, illness
Fatal
Affects central nervous system Highly toxic to animals and fish Discoloration of skin, eyes
Blue baby condition, infant poisoning, colour and acidity
c. Gases i. Oxygen ii. Carbon
iii. Hydrogen Sulphide
Corrosive to metals Acidity, corrosiveness
Odour, acidity and corrosiveness
4.2 Hardness and Alkalinity
Water is said to be ‘hard’ when it contains relatively large amounts of bicarbonates, carbonates, sulphates and chlorides of calcium and magnesium dissolved in it. It is the property that prevents lathering of soap.
4.2.1 Types of Hardness
Permanent hardness is due to the presence of sulphates, chlorides and nitrates of calcium and magnesium and is also known as ‘non-carbonate hardness’ (NCH). Permanent hardness 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
Types of
Hardness
Permanent
Hardness
Temporary
Hardness
Shuvanjan Dahal (o68/BCE/147) Page 25
Where, ion = Mg, Ca, Sr (Strontium)
Eq. wt. of Mg = 12.2, Eq. wt. of Ca = 20, Eq. wt. of Sr = 43.8, Eq. wt. of CaCO3 = 50
Effects of Hardness:
1. Wasteful consumption of soap while washing and bathing. 2. Modifies colour if used in dyeing work and washing clothes.
3. Produces scale in steam boiler and its pipe which reduces heat transfer and finally causes leak.
4. Causes corrosion and incrustation of pipelines and fittings.
5. Scale formation further causes corrosion, caustic brittleness, decreases efficiency and danger of burst of pipe line and boiler.
6. Makes food tasteless, more fuel consumption and causes bad effects to our digestive system.
Measurement of Hardness in Water:
Hardness of water is measured in ppm or mg/l of calcium carbonate present in water. Range (mg/l) 0 – 50 50 – 100 100 – 150 150 – 250 > 250 Hardness Level Soft Moderately
Soft
Slightly Hard Moderately Hard
Hard The hardness of water is also expressed as the degree of hardness. It may be Clark Scale, French Scale or American Scale.
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.
Shuvanjan Dahal (o68/BCE/147) Page 26 [ ] [ ]
Alkalinity caused by hydroxides is called hydroxide alkalinity or caustic alkalinity, caused by carbonate is carbonate alkalinity and caused by bicarbonate is called bicarbonate alkalinity. 4.2.3 Relation between Hardness and Alkalinity
1. When Total Hardness > Total Alkalinity CH = Total Alkalinity
NCH = TH – CH
2. When Total Hardness ≤ Total Alkalinity CH = TH
NCH = 0 Problem:
The analysis of water from a well shows the following results in mg/l. Ca++ = 65, Mg++ = 51, Na+ = 100, K+ = 25, HCO3 - = 248, SO4 -- = 220, Cl- = 18, CO3 -- = 240 Find Total Hardness (TH), Carbonate Hardness (CH) and Non-Carbonate Hardness (NCH).
Solution: Here, TA > TH
Hence, Carbonate Hardness (CH) = Total Hardness (TH) = 371.52 mg/l
Alkalinity
Alkalinity due
to Bicarbonate
Alkalinity due
to Carbonate
Shuvanjan Dahal (o68/BCE/147) Page 27 Non-Carbonate Hardness (NCH) = 0
1. The analysis of a water sample shows the following results in mg/l. Ca++ = 7, Mg++ = 12, Na+ = 20, K+ = 25, HCO3- = 68, SO4-- = 7, Cl- = 40. The concentration of Sr is equal to hardness of 2.52 mg/l and the carbonate alkalinity in water is zero. Calculate TH, CH and NCH.
2. Total hardness obtained from the analysis of water is found to be 117 mg/l. The analysis further showed that the concentrations of all the three principle cations causing hardness are numerically same. If the value of CH = 57 mg/l, calculate:
i. NCH.
ii. The concentration of principle cation (Ca, Mg, Sr) iii. Total Alkalinity (TA)
4.3 Living Organisms in Water
a. Algae b. Bacteria c. Virus
d. Helminthes or Worms (Refer descriptions in any book.)
4.4 Water Related Diseases
4.4.1 Water borne Diseases
Water borne diseases are caused due to drinking water contaminated with pathogenic microorganisms. Some of the most common water borne diseases are typhoid fever, dysentery (amoebic and bacillary), gastro-enteritis, infectious hepatitis, schistosomiasis, etc.
a. BACTERIAL DISEASES: Botulism, Cholera, E. coli infection, Dysentery, Typhoid fever b. PROTOZOAL DISEASES: Amoebiasis, Giardiasis
c. VIRUS DISEASES: SARS (Severe Acute Respiratory Syndrome), Hepatitis A, Poliomyelitis
d. HELMINTHIC DISEASES: Schistosomiasis, Swimmer’s itch
Water borne
diseases
Bacterial
diseases
Protozoal
diseases
Virus diseases
Helminthic
(worm) diseases
Shuvanjan Dahal (o68/BCE/147) Page 28 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
Shuvanjan Dahal (o68/BCE/147) Page 29 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 30 i. ppm in silica scale
ii. JTU (Jackson Turbidity Unit) iii. FTU (Formagen Turbidity Unit) iv. NTU (Nephelometric Turbidity Unit) Equipment: Turbidity Meter
4.5.2 Chemical Examination of Water (tests for pH, suspended, dissolved and total solids) 1. Test for pH
The hydrogen-ion concentration or pH value of water is a measure of degree of acidity or alkalinity of water. For water at 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) Page 31 The amount of total solids should preferable be less than 500 ppm.
4.5.3 Biological Examination of Water (multiple tube and membrane fermentation method), most probable number
MULTIPLE TUBE FERMENTATION TECHNIQUE
The coliform group of bacteria is defined as all aerobic and facultative anaerobic, gram-negative, rod-shaped bacteria that ferment lactose with gas and acid formation within 48 hours at 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. 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.
ii. If gas formed is seen in the tubes, it is the indication of presence of E. coli group and result is +ve. If no gas is formed, the result is _ve.
iii. –ve result in presumptive test indicates the water is fit for drinking.
2. Confirmed Phase
The other bacteria than E. coli present also may ferment in presumptive test so the confirmed test to indicate E. coli is necessary. This test consists of growing cultures of coliforms on media which suppress the growth of other organisms.
Procedure:
i. Small amount of incubated sample showing gas in presumptive test is carefully transferred to another fermentation medium containing brilliant green lactose bile broth and placed in the incubator at 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.
ii. Again the small portion of incubated material showing gas in presumptive test is marked as streaks on the plates containing Endo or Eosin-methylene blue agar and the plates are kept in the incubator at 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) Page 32 Procedure:
i. The bacterial colonies or cultures grown in the confirmed test are kept into lactose broth fermentation tubes and agar tubes.
ii. The tubes are then kept in the incubator at 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.
iii. If result is –ve, it indicates the absence of E. coli and hence absence of pathogens.
Example:
If we take 10 test tubes out of which 3 test tubes are positive after third test and in each test tube, 1 ml of sample is kept,
No. of positive tubes = 3
ml of sample in negative tubes = 7 ml of sample in all tubes = 10
√
MEMBRANE FILTRATION TECHNIQUE
The coliform group may be defined as comprising all aerobic and many facultative anaerobic, gram -ve, rod-shaped bacteria that develop a red colony with a metallic sheen within 24 hours at 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.
Shuvanjan Dahal (o68/BCE/147) Page 33 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
Shuvanjan Dahal (o68/BCE/147) Page 35
CHAPTER - V
INTAKES
5.1 DefinitionIntakes 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.
Shuvanjan Dahal (o68/BCE/147) Page 36 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
Shuvanjan Dahal (o68/BCE/147) Page 37 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 38 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.
Shuvanjan Dahal (o68/BCE/147) Page 39 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.
Shuvanjan Dahal (o68/BCE/147) Page 40
CHAPTER – VI
WATER TREATMENT
The available raw water must be treated and purified before they can be supplied to the public for their domestic, industrial or any other uses.
The layout of conventional water treatment is as follows:
6.1 Objectives of Water Treatment
a. To remove the colour, odour (taste causing substances) b. To remove the turbidity present in water
c. To remove pathogenic organisms d. To remove hardness
e. To make water potable
f. To prevent the spread of diseases
6.2 Treatment Processes and Impurity Removal
1. SCREENING: Bulky and floating suspended matters are removed by the process of screening.
2. PLAIN SEDIMENTATION: Heavy and coarse suspended matters are removed by the process of plain sedimentation.
3. SEDIMENTATION WITH COAGULATION: This process helps to remove fine suspended and colloidal matters.
4. FILTRATION: It is the most important stage in the purification process of water. It removes very fine suspended impurities and micro-organisms.
5. DISINFECTION: It is carried out to eliminate or reduce pathogenic micro-organisms that have remained after the process of filtration.
6. SOFTENING: Removes hardness of water.
7. AERATION: Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due to algae and related organisms. Aeration also oxidize iron and manganese, increases dissolved oxygen content in water, removes CO2 and reduces corrosion and removes methane and other flammable gases.
8. Removal of Fe and Mn.
Shuvanjan Dahal (o68/BCE/147) Page 41
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.
Shuvanjan Dahal (o68/BCE/147) Page 42
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 (V
2- v
2)
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)
4/3h
vsin 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).
Shuvanjan Dahal (o68/BCE/147) Page 43 b = minimum clear spacing between bars, m
hv = velocity head of flow approaching rack, m = v2/2g q = angle of inclination of rack with horizontal
The head loss through fine screen is given by
h = (1/2g) (Q/CA) where, h = head loss, m
Q = discharge, m3/s
C = coefficient of discharge (typical value 0.6) A = effective submerged open area, m2
6.4 Plain Sedimentation
When the impurities are separated from suspending fluid by action of natural forces alone i.e. by gravitation and natural aggregation of the settling particles, the operation is called plain sedimentation.
6.4.1 Purpose
The main purpose of plain sedimentation is to remove large amounts of suspended solids present in raw water. It is done after screening and before sedimentation with coagulation and located near the filter unites and in case of variation of demand it can be used as the storage reservoir.
6.4.2 Theory of Settlement Principle of Sedimentation:
Suspended solids present in water having specific gravity greater than that of water tend to settle down by gravity as soon as the turbulence is retarded by offering storage, thereby making easy to remove the sediments (called sludge) and floating matters (called scum).
Basin in which the flow is retarded is called settling tank or sedimentation tank or settling basin or sedimentation basin.
Theoretical average time for which the water is detained in the settling tank is called the detention period/time or retention period/time.
The sedimentation is affected by: i. Velocity of flowing water
ii. Size, shape and specific gravity of particles iii. Viscosity of water
iv. Detention time
Shuvanjan Dahal (o68/BCE/147) Page 44 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:
Shuvanjan Dahal (o68/BCE/147) Page 45
( )
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) 0 5 10 15 20 25 30 ‘ ’-kinematic viscosity (mm2/s or centistokes) 1.792 1.519 1.308 1.141 1.007 0.897 0.804
Hazen further indicated that for particles having diameter ‘d’ ≤ 0.1 mm and Reynold’s number ‘Re’ ≤ 1, Stoke’s Law is applicable. Mathematically,
Shuvanjan Dahal (o68/BCE/147) Page 46 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.
