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Domestic Water Treatment
Transcript of Domestic Water Treatment
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,
many non pathogenic bacteria such as E.Coli, a member of coliform group, also live in the intestinal tract of human beings.
So, coliform group serves as indicator of contamination of water with sewage and presence of pathogens
Municipal Water Supply: Sources and Quality
Raw Water Source
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.
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
Fluctuations in Rate of Demand
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 is estimated based on the following:
Useful life of the component.
Growth of population, including industrial, commercial developments & migration-immigration.
Performance of the system during initial period.
Design Periods & Population Forecast
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 discretion and intelligence of the designer.
Arithmetic Increase Method
dP/dt=constant=k; Pt= P0+kt.
Geometric Increase Method
dP/dt=kP; lnP= lnP0+kt.
Incremental Increase Method
Decreasing Rate of Growth Method
Simple Graphical Method
Comparative Graphical Method
Logistic Curve Method
The three factors responsible for changes in population are :
(i) Births, (ii) Deaths and (iii) Migrations.
Population Forecasting Methods
Domestic Water Treatment (1)
The quantity of water required for municipal uses for which the water supply scheme has to be designed requires following data:
Water consumption rate (Per Capita Demand in litres per day per head).
Population to be served.
Per capita demand x Population
Water Quantity Estimation
Water Consumption Rate
Factors affecting per capita demand:
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.
Presence of industries.
Habits of people and their economic status.
Quality of water: If water is aesthetically $ medically safe, the consumption will increase as people will not resort to private wells, etc.
Pressure in the distribution system.
Efficiency of water works administration: Leaks in water mains and services; and unauthorised use of water can be kept to a minimum by surveys.
Cost of water.
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.
Types of Intake:
Depending on the source of water, the intake works are classified as follows:
To lift raw water from wells.
To deliver treated water to the consumer at desired pressure.
To supply pressured water for fire hydrants.
To boost up pressure in water mains.
To fill elevated overhead water tanks.
To back-wash filters.
To pump chemical solutions, needed for water treatment.
There are two stages in the transportation of water:
Conveyance of water from the source to the treatment plant.
Conveyance of treated water from treatment plant to the distribution system.
Water Treatment Philosophy
Preliminary Treatment: Silt Excluder Design( by aeration)
Sedimentation Tank Design
Coagulation - Flocculation Theory
Rapid Mixing, Coagulation - Flocculation
Coagulation - Flocculation
Rapid Sand Filtration
Unit Processes in Municipal Water Treatment
Population Forecast by Different Methods
Predict the population for the years 1981, 1991, 1994, and 2001 from the following census figures of a town by different methods.
Factors Governing Location of Intake
As far as possible, the site should be near the treatment plant so that the cost of conveying water to the city is less.
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.
The intake must never be located at the downstream of the point of disposal of wastewater.
The intake site should remain easily accessible during floods and should not get flooded. Moreover, the flood waters should not be concentrated in the nearest of the intake.
Intake, Pumping and Conveyance
The various sources of water can be classified into two categories:
Surface sources, such as
Ponds and lakes;
Streams and rivers;
Storage reservoirs; and
Oceans, generally not used for water supplies, at present.
Sub-surface sources or underground sources, such as
Infiltration wells ; and
Wells and Tube-wells
The raw or treated water is analysed by testing their physical, chemical and bacteriological characteristics:
Taste and Odour
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 treated as a function of population and is worked out from following empirical formula:
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.
v=0.85 C rH0.63S0.54
Sedimentation tank design