Waste water estimation

ESTIMATION OF WASTEWATER QUANTITIES
FINAL TERM COURSE
Lecture No 1
Prepared By
Engr. Umair Afzal
(*MSc Water Resources Engg., UET Lahore)

BASIC TERMS
• Sewage: It is the Liquid Waste or Wastewater
produced as a result of water use.
• Sewer: It is a pipe or conduit for carrying
sewage. It is generally closed and flow takes
place under gravity .

• Sewerage: Sewerage is the system of collection of
wastewater and conveying it to the point of disposal
with or without treatment.
Sources of Wastewater
1.Dometic: It is wastewater from houses offices,
other buildings, hotels and institutions
2.Industrial: It is the liquid waste from industrial
process
3.Storm-water: It includes surface run-off
generated by rainfall and the street wash

COMPONENTS OF WASTEWATER
ENGINEERING
1. Collection System  Network of Sewer pipes
2. Disposal  Sewage Pumping Stations and
Outfalls
3.Treatment Works  Wastewater treatment
Plants

Fig: Components of Wastewater Engineering

TYPES OF SEWER SYSTEMS
• It is the system and infrastructure of collecting,
treating and disposal of sewage.
There are three sewerage systems types:
• 1. Separate System
• 2. Combined System
• 3. Partially Separated System

1. Separate System
If storm water is carried separately from domestic
and industrial wastewater the system is called as
separate system.
• In this system the sanitary sewage and storm
water are carried separately in two sets of
sewers.
• The sewage is conveyed to waste water
treatment plant (WWTP) and the storm water is
discharges into rivers without treatment.

2. Combined System
It is the system in which the sewers carry both
sanitary and storm water, combined system is
favored when;
(i) Combined sewage can be disposed off without
treatment
(ii) Both sanitary and storm water need treatment
(iii) Streets are narrow and two separate sewer
cannot be laid

3. Partially Combined System
If some portion of storm or surface run-off is
allowed to be carried along with sanitary
sewage the system is known as partially
combined system.
(In Urban area of developing countries, mostly
partially combined system is employed as it is
economical)
In Pakistan we use this system

3. Partially Combined System
This system is the compromise between
separate and combine system taking the
advantages of both systems.
In this system the sewage and storm water
of buildings are carried by one set of sewers
while the storm water from roads, streets,
pavements etc are carried by other system
of sewers usually open drains.

QUANTITY ESTIMATION OF SEWAGE
 Before designing the sewer, it is necessary to know the
discharge i.e., quantity of sewage, which will flow in it
after completion of the project.
 Accurate estimation of sewage discharge is necessary for
hydraulic design of the sewers.
 Far lower estimation than reality will soon lead to
inadequate sewer size after commissioning of the scheme
or the sewers may not remain adequate for the entire
design period.

QUANTITY ESTIMATION OF SEWAGE
 Very high discharge estimated will lead to larger sewer
size affecting economy of the sewerage scheme.
 Lower discharge actually flowing in the sewer may not
meet the criteria of the self cleansing velocity and
hence leading to deposition in the sewers.

DRY WEATHER FLOW
Dry weather flow is the flow that occurs in sewers in
separate sewerage system or the flow that
occurs during dry seasons in combined system.
This flow indicates the flow of sanitary sewage.
This depends upon the following:
 rate of water supply,
 type of area served,
 economic conditions of the people,
 weather conditions and
 infiltration of groundwater in the sewers, if sewers
are laid below groundwater table.

EVALUATION OF SEWAGE DISCHARGE
Apart from accounted water supplied by water
authority that will be converted to wastewater,
following quantities are considered while estimating
the sewage quantity
a. Addition due to unaccounted private water
supplies
b. Addition due to infiltration
Storm water drainage may also infiltrate into
sewers. This inflow is difficult to calculate. This
extra quantity can be taken care of by extra empty
space left at the top in the sewers, which are
designed for running ¾ full at maximum design
discharge.

EVALUATION OF SEWAGE DISCHARGE
c. Subtraction due to water losses
d. Subtraction due to water not entering the
sewerage system
Net quantity of sewage:
Generally, 75 to 80% of accounted water supplied is
considered as quantity of sewage produced

DESIGN DISCHARGE OF SANITARY SEWAGE
 The max. quantity of sewage generated per day is
estimated as product of forecasted population at the
end of design period considering per capita sewage
generation and appropriate peak factor.
 The per capita sewage generation can be considered
as 75 to 80% of the per capita water supplied per
day.
 The increase in population also result in increase in
per capita water demand and hence, per capita
production of sewage.

DESIGN DISCHARGE OF SANITARY SEWAGE
 This increase in water demand occurs due to increase in living
standards, betterment in economical condition, changes in
habit of people, and enhanced demand for public utilities.
Variation in Sewage Flow

FACTORS AFFECTING THE QUANTITY OF
STORM WATER
The surface run-off resulting after precipitation
contributes to the storm water. The quantity of
storm water reaching to the sewers or drains is very
large as compared with sanitary sewage.
The factors affecting the quantity of storm water
flow are as below:
i. Area of the catchment
ii. Slope and shape of the catchment area
iii. Porosity of the soil
iv. Obstruction in the flow of water as trees, fields,
gardens, etc.

FACTORS AFFECTING THE QUANTITY OF
STORMWATER
v. Initial state of catchment area with respect to wetness.
vi. Intensity and duration of rainfall
vii. Atmospheric temperature and humidity
viii. Number and size of ditches present in the area etc.

MEASUREMENT OF RAINFALL
The rainfall intensity could be measured by
using rain gauges and recording the amount of
rain falling in unit time.
The rainfall intensity is usually expressed as
mm/hour or cm/hour.
The rain gauges used can be manual recording
type or automatic recording rain gauges.

Rain gauge Station

Automatic recording rain gauges.

METHODS FOR ESTIMATION OF
QUANTITY OF STORM WATER
1. Rational Method
2. Empirical formulae method
In both the above methods, the quantity of storm water is
considered as function of intensity of rainfall, coefficient of
runoff and area of catchment.
Time of Concentration: The period after which the entire
catchment area will start contributing to the runoff is called as
the time of concentration.
The rainfall with duration lesser than the time of concentration
will not produce maximum discharge.

The runoff may not be maximum even when the duration of the
rain is more than the time of concentration. This is because in
such cases the intensity of rain reduces with the increase in its
duration.
The runoff will be maximum when the duration of rainfall is
equal to the time of concentration and is called as critical
rainfall duration.
The time of concentration is equal to sum of inlet time and time
of travel.
Time of concentration = Inlet time + time of travel

Inlet Time: The time required for the rain in falling on the
most remote point of the tributary area to flow across the ground
surface along the natural drains or gutters up to inlet of sewer is
called inlet time.
The inlet time ‘Ti’ can be estimated using relationships similar
to following.
These coefficients will have different values for different
catchments.
Ti = [0.885 L3/H] 0.385

Where,
Ti = Time of inlet, minute
L = Length of overland flow from critical point to mouth of drain
(Km)
H = Total fall of level from the critical point to mouth of drain
(m)
Time of Travel: The time required by the water to flow in the
drain channel from the mouth to the point under consideration
or the point of concentration is called as time of travel.
Time of Travel (Tt) = Length of drain / velocity in drain
Time of concentration Tc = Ti + Tt

Runoff Coefficient: The total precipitation falling on any area
is dispersed as percolation, evaporation, storage in ponds or
reservoir and surface runoff.
The runoff coefficient can be defined as a fraction, which is
multiplied with the quantity of total rainfall to determine the
quantity of rain water, which will reach the sewers. The runoff
coefficient depends upon the porosity of soil cover, wetness and
ground cover.

EMPIRICAL FORMULAE FOR RAINFALL INTENSITIES
The relationships between rainfall intensity and duration are
developed based on long experience in field. intensity of rainfall
in design is
usually in the range 12 mm/h to 20 mm/h.
For T varying between 5 to 20 minutes
I
For T varying between 20 to 100 minutes
I

The overall runoff coefficient for the catchment area can be
worked out as follows:
Overall runoff coefficient
Where, A1, A2, ….An are types of areas with C1, C2, …Cn as
their coefficient of runoff respectively.

The typical runoff coefficient for the different ground cover is
provided in the below table
Runoff coefficient for various sources
Sno Type of Surface Value of C
1 Water Tight Roof surface 0.70 - 0.95
2 Asphalt Pavement 0.85 – 0.90
3 Stone, brick, wood-block
pavement with cemented joints
0.75 - 0.85
4 Stone, brick, wood-block
pavement with uncemented joints
0.50 - 0.70
5 Water bond Macadam roads 0.25 - 0.60
6 Gravel road and walks 0.15 – 0.30
7 Unpaved streets and vacant lands 0.10 – 0.30
8 Parks, Lawns, gardens, meadows
etc.,
0.05 – 0.25
9 Wooden lands 0.01 – 0.20

(1) RATIONAL METHOD
Storm water Runoff, Use any one of these units systems
Q = C.I.A
Q = Quantity of storm water, m3/hr
C = Coefficient of runoff (From Table)
I = intensity of rainfall (mm/hr) for the duration equal
to time of concentration
A = Drainage area (m2 )
Q= C.I.A *
1
36
Q = Quantity of storm water, m3/sec
C = Coefficient of runoff (From Table)
I = intensity of rainfall (cm/hr
A = Drainage area (hectares )
Note: (1 ha = 10,000 m2) , ( 1 ha-cm/hr = 1/36 𝑚3/𝑠 )

(2) DICKEN’S FORMULA
Peak Discharge in
cumecs
QP = Peak Discharge in
cumecs
M = Catchment area in
Km2
C = a constant depending
upon all those fifteen to
twenty factors which affect
the runoff (C=11.5)

(3) DICKEN’S FORMULA
QP = Peak Discharge in cumecs
M = Catchment area in Km2
C1 = a constant depending upon all those fifteen to twenty
factors which affect the runoff (C=6.8)
Location of Catchment Value of C1
Areas within 24Km from the coast 6.8
Areas within 24Km – 16Km from
the coast
8.8
Limited areas near hills 10.1

CALCULATION OF PEAK FACTOR
The peaking factor (PF) is the ratio of the maximum
flow to the average daily flow in a water system.

CALCULATION OF INFILTRATION FLOW
Infiltration Inflow:
• Q(infiltration) is taken as [24-95 m3/day/km]
or
[0.5 m3/day/diameter (cm)], take the bigger value of the two.
• Qinflow is taken as 0.2-30 [m3/ha/day]. ( hectare = 10,000 m2 )
• Qdes = Qmax + QI/I ( if found)
Where,
QI/I = Qinfil + Qinflow
• Qmax = [0.80* Qavg] * Pƒ ( 0.8 > 80% return from water
supply).

CALCULATION OF MINIMUM DISCHARGE
𝑄 𝑚𝑖𝑛= 0.2 * 𝑃
1
6 * [𝑄 𝑎𝑣𝑔]𝑤
𝑄 𝑚𝑖𝑛=
1
3
* [𝑄 𝑎𝑣𝑔]𝑤
Or

METHODS FOR ESTIMATION OF QUANTITY OF STORM
WATER
Question 1
Determine designed discharge for a combined system serving
population of 50000 with rate of water supply of 135 LPCD. The
catchment area is 100 hectares and the average coefficient of
runoff is 0.60.
Given
Population = 50,000
Rate of water supply = 135 lpcd
Catchment Area = 100 Hectares
Average coefficient of runoff =0.60
To Find
Designed Discharge for combined system

Solution
Estimation of sewage quantity
STEP 1
Assumption 1: Considering 80% of the water supplied will result
in wastewater generation
Quantity of sanitary sewage Q
[𝑄 𝑎𝑣𝑔]𝑤 = 𝑄 𝑎𝑣𝑔 x 0.8
= Population x Quantity of water supply x 0.8
= 50000 x 135 x 0.80
= 5400 m3/day = 0.0625 m3/sec
STEP 2
Assumption 2: Considering peak factor of 2.5
Design discharge for sanitary sewage = 0.0625 x 2.5
= 0.156 m3/sec

Estimation of storm water discharge
STEP 3
Intensity of rainfall,
I
Therefore, I = 100/(30 + 20) = 2 cm/hr
Storm water runoff, Q = C.I.A* 1/36
Q = 0.6 x 2 x 100/(36) = 3.33 m3/sec
Design discharge for combined sewer
= 3.33 + 0.156 = 3.49 m3/sec

WATER
Question 2
The catchment area is of 300 hectares. The surface cover in the
catchment can be classified as given below:
S # Type of Cover
Runoff Co-
efficient (C)
Percentage
of area (A)
1 Roof s 0.90 15
2 Pavements and yards 0.80 15
3 Lawns and gardens 0.15 25
4 Roads 0.40 20
5 Open Ground 0.10 15
6 Single Family dwelling 0.50 10

WATER
Calculate the runoff coefficient and quantity of storm water
runoff, if intensity of rainfall is 30 mm/h for rain with duration
equal to time of concentration. If population density in the area
is 350 persons per hectare and rate of water supply is 200
LPCD, calculate design discharge for separate system, partially
separate system, and combined system.
Given
Population density in the area = 350 persons per hectare
Catchment Area = 300 hectares
Rate of water supply = 200 lpcd
Intensity of rainfall = 30mm/h = 3cm/h
To Find
1) Average coefficient of runoff
2) Quantity of storm water runoff

METHODS FOR ESTIMATION OF QUANTITY OF STORM WATER
Solution
Estimation of storm water discharge for storm water drain of
separate system
STEP 1
Overall runoff coefficient
Where, A1, A2, ….An are types of area with C1, C2, …Cn as
their coefficient of runoff, respectively.

So we get C = 0.44
Estimation of storm water discharge
STEP 2
Storm water runoff, Q = C.I.A / 36
Q = 0.44 x 3 x 300/(36) = 11 m3/sec
Estimation of sewage discharge for separate system sanitary sewer
STEP 3
Assumption 1: Considering 80% of the water supplied will result
in wastewater generation
Quantity of sanitary sewage Qavg
Qavg = Population density x Area x Quantity of water supply x 0.8
Qavg = 350 x 300 x 200 x 0.80
Qavg = 16800 m3/day = 0.194 m3/sec

Assumption 2: Considering peak factor of 2 or can also use
formula
Design discharge for sanitary sewage, Ignoring the Q (infil)
Qmax= 0.194 x 2
= 0.388 m3/sec
Estimation of discharge for partially separate system
STEP 4
Storm water discharge falling on roofs and paved courtyards will
be added to the sanitary sewer.
For Roof For Paved Courtyard
C = 0.9 C= 0.8
Area = 0.15*300 = 45 ha Area = 0.15*300 = 45 ha

Average coefficient of runoff
Cavg = (0.90 x 45 + 0.80 x 45) / (45+45) = 0.85
Discharge = Q = C.I.A / 36
Q = 0.85 x 3 x 90 /(36) = 6.375 m3/sec
Total discharge in the sanitary sewer of partially separate
system = 6.375 + 0.388 = 6.764 m3/sec
Discharge in storm water drains = 11 – 6.375 = 4.625 m3/sec

Example
a. Calculate the average domestic WW flow:
[Qavg]w = 0.8 Qavg = 0.80 * 120 L/c/d * 50,000 capita* 10-3
= 4800 m3
/d
b. Calculate the peak factor:
Pf
P


4
14
1 = 26.2
504
14
1 


Solution

a. Calculate the maximum wastewater flow:
Qmax = [Qavg]w * Pƒ = 2.26 * 4800 = 10848 m3
/d
b. Calculate the minimum wastewater flow:
wavg
QPQ 



 *6
1
2.0
min
18424800*6
1
)50(2.0  m3
/d
c. Calculate the infiltration flow:
Qinfil = 30 *0.20 = 6 m3
/d
d. Calculate the design flow:
Qdes = Qmax + QI/I = 10848 + 6 = 10854 m3
/d

Waste water estimation

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