Environmental Engineering
Raw Water Source
The various sources of water can be classified into two categories:
1. Surface sources, such as
a. Ponds and lakes;
b. Streams and rivers;
c. Storage reservoirs; and
d. Oceans, generally not used for water supplies, at present.
2. Sub-surface sources or underground sources, such as
a. Springs;
b. Infiltration wells ; and
c. Wells and Tube-wells.
Water Quantity Estimation
The quantity of water required for municipal uses for which the water supply scheme has to be designed requires following data:
1. Water consumption rate (Per Capita Demand in litres per day per head)
2. Population to be served.
Quantity= Per capita demand x Population
Water Consumption Rate
It is very difficult to precisely assess the quantity of water demanded by the public, since there are many variable factors affecting water consumption. The various types of water demands, which a city may have, may be broken into following classes:
Water Consumption for Various Purposes:
Fire Fighting Demand:
The per capita fire demand is very less on an average basis but the rate at which the water is required is very large. The rate of fire demand is sometimes traeted as a function of population and is worked out from following empirical formulae:
Factors affecting per capita demand:
a. Size of the city: Per capita demand for big cities is generally large as compared to that for smaller towns as big cities have sewered houses.
b. Presence of industries.
c. Climatic conditions.
d. Habits of people and their economic status.
e. Quality of water: If water is aesthetically & medically safe, the consumption will increase as people will not resort to private wells, etc.
f. Pressure in the distribution system.
g. Efficiency of water works administration: Leaks in water mains and services; and unauthorised use of water can be kept to a minimum by surveys.
h. Cost of water.
i. Policy of metering and charging method: Water tax is charged in two different ways: on the basis of meter reading and on the basis of certain fixed monthly rate.
Fluctuations in Rate of Demand
Average Daily Per Capita Demand = Quantity Required in 12 Months/ (365 x Population)
If this average demand is supplied at all the times, it will not be sufficient to meet the fluctuations.
· Seasonal variation: The demand peaks during summer. Firebreak outs are generally more in summer, increasing demand. So, there is seasonal variation .
· Daily variation depends on the activity. People draw out more water on Sundays and Festival days, thus increasing demand on these days.
· 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 requirement is negligible. Moreover, if a fire breaks out, a huge quantity of water is required to be supplied during short duration, necessitating the need for a maximum rate of hourly supply.
So, an adequate quantity of water must be available to meet the peak demand. To meet all the fluctuations, the supply pipes, service reservoirs and distribution pipes must be properly proportioned. The water is supplied by pumping directly and the pumps and distribution system must be designed to meet the peak demand. The effect of monthly variation influences the design of storage reservoirs and the hourly variations influences the design of pumps and service reservoirs. As the population decreases, the fluctuation rate increases.
Maximum daily demand = 1.8 x average daily demand
Maximum hourly demand of maximum day i.e. Peak demand
= 1.5 x average hourly demand
= 1.5 x Maximum daily demand/24
= 1.5 x (1.8 x average daily demand)/24
= 2.7 x average daily demand/24
= 2.7 x annual average hourly demand
Design Periods & Population Forecast
This quantity should be worked out with due provision for the estimated requirements of the future . The future period for which a provision is made in the water supply scheme is known as the design period.
Design period is estimated based on the following:
· Useful life of the component, considering obsolescence, wear, tear, etc.
· Expandability aspect.
· Anticipated rate of growth of population, including industrial, commercial developments & migration-immigration.
· Available resources.
· Performance of the system during initial period.
Population Forecasting Methods
The various methods adopted for estimating future populations are given below. The particular method to be adopted for a particular case or for a particular city depends largely on the factors discussed in the methods, and the selection is left to the discrection and intelligence of the designer.
1. Arithmetic Increase Method
2. Geometric Increase Method
3. Incremental Increase Method
4. Decreasing Rate of Growth Method
5. Simple Graphical Method
6. Comparative Graphical Method
7. Ratio Method
8. Logistic Curve Method
Intake Structure
The basic function of the intake structure is to help in safely withdrawing water from the source over predetermined pool levels and then to discharge this water into the withdrawal conduit (normally called intake conduit), through which it flows up to water treatment plant.
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 noy get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.
Design Considerations
1. sufficient factor of safety against external forces such as heavy currents, floating materials, submerged bodies, ice pressure, etc.
2. should have sufficient self weight so that it does not float by upthrust of water.
Types of Intake
Depending on the source of water, the intake works are classified as follows:
Pumping
A pump is a device, which converts mechanical energy into hydraulic energy. It lifts water from a lower to a higher level and delivers it at high pressure. Pumps are employed in water supply projects at various stages for following purposes:
1. To lift raw water from wells.
2. To deliver treated water to the consumer at desired pressure.
3. To supply pressured water for fire hydrants.
4. To boost up pressure in water mains.
5. To fill elevated overhead water tanks.
6. To backwash filters.
7. To pump chemical solutions, needed for water treatment.
Classification of Pumps
Based on principle of operation, pumps may be classified as follows:
1. Displacement pumps (reciprocating, rotary)
2. Velocity pumps (centrifugal, turbine and jet pumps)
3. Buoyancy pumps (air lift pumps)
4. Impulse pumps (hydraulic rams)
Capacity of Pumps
Work done by the pump,
H.P.=wQH/75
where, w= specific weight of water kg/m3, Q= discharge of pump, m3/s; and H= total head gainst which pump has to work.
H= Hs + Hd + Hf + (losses due to exit, entrance, bends, valves, and so on)
where, Hs=suction head, Hd = delivery head, and Hf = friction loss.
Efficiency of pump (E) = wQH/Brake H.P.
Total brake horse power required = wQH/E
Provide even number of motors say 2,4,with their total capacity being equal to the total BHP and provide half of the motors required as stand-by.
Conveyance
There are two stages in the transportation of water:
1. Conveyance of water from the source to the treatment plant.
2. Conveyance of treated water from treatment plant to the distribution system.
In the first stage water is transported by gravity or by pumping or by the combined action of both, depending upon the relative elevations of the treatment plant and the source of supply. In the second stage water transmission may be either by pumping into an overhead tank and then supplying by gravity or by pumping directly into the water-main for distribution.
Free Flow System
In this system, the surface of water in the conveying section flows freely due to gravity. In such a conduit the hydraulic gradient line coincide with the water surface and is parallel to the bed of the conduit. It is often necessary to construct very long conveying sections, to suit the slope of the existing ground. The sections used for free-flow are: Canals, flumes, grade aqueducts and grade tunnels.
Pressure System
In pressure conduits, which are closed conduits, the water flows under pressure above the atmospheric pressure. The bed or invert of the conduit in pressure flows is thus independant of the grade of the hydraulic gradient line and can, therefore, follow the natural available ground surface thus requiring lesser length of conduit. The pressure aqueducts may be in the form of closed pipes or closed aqueducts and tunnels called pressure aqueducts or pressure tunnels designed for the pressure likely to come on them. Due to their circular shapes, every pressure conduit is generally termed as a pressure pipe. When a pressure pipe drops beneath a valley, stream, or some other depression, it is called a depressed pipe or an inverted siphon. Depending upon the construction material, the pressure pipes are of following types: Cast iron, steel, R.C.C, hume steel, vitrified clay, asbestos cement, wrought iron, copper, brass and lead, plastic, and glass reinforced plastic pipes.
Hydraulic Design
The design of water supply conduits depends on the resistance to flow, available pressure or head, and allowable velocities of flow. Generally, Hazen-William's formula for pressure conduits and Manning's formula for freeflow conduits are used.
Hazen-William's formula
U=0.85 C rH0.63S0.54
Manning's formula
U=1/n rH2/3S1/2
where, U= velocity, m/s; rH= hydraulic radius,m; S= slope, C= Hazen-William's coefficient, and n = Manning's coefficient.
Darcy-Weisbach formula
hL=(fLU2)/(2gd)
The available raw waters must be treated and purified before they can be supplied to the public for their domestic, industrial or any other uses. The extent of treatment required to be given to the particular water depends upon the characteristics and quality of the available water, and also upon the quality requirements for the intended use..
The layout of conventional water treatment plant is as follows:
Depending upon the magnitude of treatment required, proper unit operations are selected and arranged in the proper sequential order for the purpose of modifying the quality of raw water to meet the desired standards. Indian Standards for drinking water are given in the table below.
Water Distribution Systems
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. They are:
Dead End System
Grid Iron System
Ring System
Radial System
Distribution Reservoirs
Distribution reservoirs, also called service reservoirs, are the storage reservoirs, which store the treated water for supplying water during emergencies (such as during fires, repairs, etc.) and also to help in absorbing the hourly fluctuations in the normal water demand.
Functions of Distribution Reservoirs:
· 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 center of demand.
· water level in the reservoir must be at a sufficient elevation to permit gravity flow at an adequate pressure.
Types of Reservoirs
1. Underground reservoirs.
2. Small ground level reservoirs.
3. Large ground level reservoirs.
4. Overhead tanks.
Storage Capacity of Distribution Reservoirs
The total storage capacity of a distribution reservoir is the summation of:
1. 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.
2. 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 othe 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.
3. 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.
Pipe Network Analysis
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 Hardy-Cross 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
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.
SK.Qax = - 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)
Bacteriological Characteristics:
Bacterial examination of water is very important, since it indicates the degree of pollution. Water polluted by sewage contain one or more species of disease producing pathogenic bacteria. Pathogenic organisms cause water borne diseases, and many non pathogenic bacteria such as E.Coli, a member of coliform group, also live in the intestinal tract of human beings. Coliform itself is not a harmful group but it has more resistance to adverse condition than any other group. So, if it is ensured to minimize the number of coliforms, the harmful species will be very less. So, coliform group serves as indicator of contamination of water with sewage and presence of pathogens.
The methods to estimate the bacterial quality of water are:
Standard Plate Count Test
Most Probable Number
Membrane Filter Technique
Indian Standards for drinking water
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