about the quantity of sewage

1. SANITARY ENGINEERING
B. E. IN CIVIL ENGINEERING (III/II)
LECTURE SLIDES
Unit 2
Quantity of wastewater
Kaji RamKarki
1

2. 2
Quantity of Wastewater
2.1 Dry Weather Flow (DWF) and Wet Weather Flow (WWF)
Essential to know the total quantity of sewage or wastewater
Sewer must be estimated as correctly as possible.
Effect on design due to Underestimation and Overestimation
Sewage consists of the following two components
Dry weather
flow (DWF)
Wet weather
flow (WWF)
2.2 Sources of sanitary sewage
1. Water supplied to the public by local authority
2. Water supplied to the by local authority.
3. Water drawn by individual houses.
4. Water drawn by industries.
5. Infiltration of ground water into sewers.
6. Unauthorized entrance of rainwater in sewer line.

3. 3
2.3 Factors affecting quantity of sanitary sewage
 Rate of water supply
 Population
 Types of area served
 Infiltration and Exfiltration (Unauthorized connection and infiltration)
The quantity of infiltration water depend on –Height of underground water
table, Permeability of soil, Types of joints and Workmanship, Material of
sewer.
2.4 Determination of quantity of sanitary sewage, peak factor, peak flow
Quantity of sanitary sewage = Total quantity of water supplied +
(addition due to industries) * CF (Conversion factor)
Average quantity of Sanitary sewage = 70-80% of (Population * per
capita rate of water supply)
Peak factor, Peak flow – Due to Variation in quantity of sanitary sewage-
The quantity of sewage as calculated by per capita demand will be purely
hypothetical and cannot be used directly for design purposes. The design
should be done for maximum possible flow which would ever pass through it
in worst possible condition.

4. 4
Maximum or peak quantity of sanitary sewage = Average quantity of
sanitary sewage * peak factor.
The peak factor of 2-4 is generally adopted in Nepal.
Generally,
Maximum flow of sewage/average flow of sewage =1.5-2.5
Average/Minimum =1.2
Maximum flow/Minimum flow = 2-4 in case of main sewer
= 4-6 in branch sewer.
The flow in sewers during night is minimum even less than 30% of average
flow.

5. 5
2.5 Determination of quantity of storm water
Storm sewage & factors affecting storm sewage:-
Factors affecting the storm sewage
1.Catchments area
2.Rainfall intensity and duration
3.Nature of soil and its degree of porosity. Or nature of surface over which
rainfall occurs.
4.Slope and shape of the catchments area.
5.Obstruction in the flow of water e.g. Trees, fields, garden etc.
6.Initial states of the catchments area with respect to wetness.
7.Atmospheric temperature, wind and humidity.
8.Nature & size of ditches present in the areas.
Storm sewage: The quantity of liquid waste which includes surface runoff
(storm and flood) developed during and immediately after rainfall over the
concerned area.

6. 6
Estimation of Storm water flow
Rational method and its limitation
The quantity of storm water is estimated from two methods.
 Rational Method
 Empirical method.
Empirical method is useful for the estimation of storm water in large areas.
In both cases,
Quantity of storm water = f (intensity of rainfall)
= f (coeff. of runoff)
= f (area in hectares)
Rational formula:
Most commonly used for design of storm drains and is
Q= CIA/360
This formula is used for urban areas, generally not exceeding 5 Km2 areas.
For larger area other techniques may be more suitable.

7. 7
Catchment area (A)
Runoff coefficient
Runoff coefficient also depends on – Characteristics of ground surface as
porosity, wetness, ground cover etc. Its value varies from-----
0.01(forest) - 0.95 (watertight) for various types of surfaces.
 Runoff coefficient for various types of localities varies from 0.35 (Thin
populated) to 0.85(highly congested areas)

8. 8
Rainfall intensity (I)
 For determining intensity of rainfall special types of rain gauges are
required. (Simpson’s rain gauge is widely used in sewer design)
 It can be obtained directly using Empirical formulae; British ministry of
health suggested the following formula-
I (mm/hr) = 760/t+10 (For storm duration of 5-20 minutes)
I = 1010/t+20 (For storm duration from 20-100 minutes)

9. 9
Time of concentration (Tc)
It is the time required for storm water to run from farthest point
of area to the maximum flow into sewer.
Time of concentration = Time of entry + Time of flow = Te +Tf
Tf = Length of drain/ velocity of flow.
Time of entry or inlet time or time of equilibrium
Time of flow or Channel flow time or gutter flow time.

10. 10
Time area Graph: