Environmental Engineering-II
7th Semester
Engr Aneel Manan
Lecture : 03
Swedish College of Engineering
and Technology
Estimation of Sewage Quantities
1. Domestic Sewage
The flow of sanitary sewage alone in the absence of storms
in dry season is known as dry weather flow (DWF).
Quantity = Per capita sewage contributed per day x
Population
Sanitary sewage is mostly the spent water of the community
draining into the sewer system. It has been observed that a
small portion of spent water is lost in evaporation, seepage
in ground, leakage, etc. Usually 80% of the water supply may
be expected to reach the sewers.
Domestic Sewage
Three characteristics of a population are geographic distribution, density, and
growth rate. The density refers to the number of people per unit area. The
growth rate is the ability of a population to increase.
Statistical data shows that population of a city changes over time and follows
some trends. There are numerous reasons of change of a population, however
some main causes include
i. New Births
ii. Migration of people due to favorable socio economic conditions
iii. City expansions
Domestic Sewage
1.1 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. It is suggested that
the construction of sewage treatment plant may be carried out in phases with
an initial design period ranging from 5 to 10 years excluding the construction
period.
Design period is estimated based on the following:
i. Useful life of the component, wear, tear, etc.
ii. Expandability aspect.
iii. Anticipated rate of growth of population, including industrial, commercial
developments & migration-immigration.
iv. Anticipated trend change in wastewater generation
v. Available resources.
vi. Performance of the system during initial period.
Population Forecast
1.1.1 Arithmetic Increase Method
This method is based on the assumption that the population increases at a
constant rate; i.e.
dP/dt=constant=k;
Pt = P0+kt.
t = time in years after base year
P0 = Population at base year
Pt = Population after t years
k = population increment per year = dP/dt
Example 1
1.1.2 Geometric Increase Method
This method is based on the assumption that percentage growth rate
is constant i.e
t
Pt = P (1+i/100)
1.1.3 Incremental Increase Method
Growth rate is assumed to be progressively increasing or decreasing, depending
upon whether the average of the incremental increases in the past is positive or
negative. The population for a future decade is worked out by adding the mean
arithmetic increase to the last known population as in the arithmetic increase
method, and to this is added the average of incremental increases, once for first
decade, twice for second and so on.
Class Problem
Arithmetic Increase Method
1.2 Variations and rates of flows
Wastewater does not generate at constant rate and variation in wastewater
generation in different hours of a day (hourly variation), different days of week
or month (daily variation), and different months of year (monthly or seasonal
variation) can be observed. The following figure shows a typical diagram of
different kinds of variations observed.
2. Industrial Sewage
Industrial sewage is used water from manufacturing or chemical processes or
discharges from different industries
Following Points should be Consider for the calculation of industrial sewage:
• Amount of water supply to industry.
• Comparison the amount of sewage discharged from identical
industry.
• Private source of water supply
3. Storm Sewage / Storm water
Storm sewage or storm water is runoff from precipitation that is collected in a
system
Following Points should be Consider for the calculation of Storm sewage:
• Intensity of rainfall.
• Runoff.
• Contours of the area.
4. Self Cleaning Velocity
The flow velocity in the sewers should be such that the suspended materials in
sewage do not get silted up; i.e. the velocity should be such as to cause automatic
Self cleaning effect.
The generation of such a minimum self cleaning velocity in the sewer, at least once
a day is important because if certain deposition take place and is not removed, it
will obstruct free flow causing further deposition and finally leading to the complete
blocking of the sewer.
The sewage contain particles in suspension, the heavier of which may settle down at
the bottom of the sewers as and when the flow velocity reduces, resulting in the
clogging of sewers.
To avoid silting of sewers, it is necessary that the sewer pipes be laid at such a
gradient, as to generate self cleaning velocities at different possible discharges.
4. Self Cleaning Velocity
Self Cleaning Velocity according to solid Matters in the sewage
S.No Nature of Material Self Cleaning Velocity
1 Angular stones 100 cm/sec 3 ft/sec
2 Round pebbles 60 3
3 Coarse sand 30 1
4 Fine sand 20 0.75
5 Fine sand and clay 15 0.5
4. Self Cleaning Velocity
Self Cleaning Velocity according to Size of Sewer
Sr.NO Diameter of Sewer Self Cleaning Velocities
cm in. cm/sec ft/sec
1 15-30 6-12 100 3
2 30-60 12-24 75 2.5
3 Above 60 Above 24 60 2
5. Non Scouring Velocity
• Maximum velocity or non scouring velocity the interior surface of the
sewer pipe get scored due to the continuous abrasion caused by
suspended solids present in sewage. The scoring is pronounced at higher
velocity than what can be tolerated by the pipe materials.
• It is, therefore necessary to limit the maximum velocity in the sewer
pipe. This limiting or non-scouring velocity will mainly depend upon the
material of the sewer.
No. Material of Pipe Non Scouring Velocities
cm/sec ft/sec
1 Earthen channel 60-120 2-4
2 Brick lined sewer 150-240 5-8
3 Cement concrete sewer 240-300 8-10
4 Stone ware sewer 300-450 10-15
Thank you
