Environmental Engineering – II
Dr.Amir Farooq
PhD. Environment Management
MSc. Environmental Engineering
BSc. Civil Engineering
To learn basic concepts of wastewater
engineering:
Origin of wastewater, quantities,
characteristics, carriage (collection),
treatment, disposal/reuse.
To design wastewater collection systems
To understand and design various
wastewater treatment processes.
Course Objectives
“WATER SUPPLY AND SEWERAGE”
By
E.W. Steel and T.J. McGhee
6th Edition
TEXT BOOK
1. Wastewater Engineering, Treatment, Disposal,
Reuse by Metcalf and Eddy, 4th Edition.
2. Introduction to Environmental Engineering by
Davis and Cornwell, 2nd Edition.
3. Water and Wastewater Engineering by Fair &
Geyer
4. Water and Wastewater Technology By Mask J.
Hammer
REFERENCE BOOKS
Week-Wise Breakdown of Syllabus
Wks Topics Rmrks
1. Population characteristics, Population
Forecasting,
Sources of wastewater, Factors affecting
quantity of wastewater, sewage flow /
quantity, Infiltration variation in sewage
flow, Design periods and use of sewage
flow data,
2. Amount of Storm Sewage, Time of
Concentration, Rainfall intensity
3. Types of sewers, Types of Sewer
Systems ,Shapes, Size and Materials of
Sewers
Sewer Appurtenances
4. Design, Construction, laying &
maintenance of sewerage system,
separate and combined system
Assig-1
5. Sampling Techniques and Examination of
Wastewater (physical, Chemical and
Microbiological Parameters
6. BOD5, COD, Microbiology of Sewage,
Effluent Disposal Guidelines and
Standards
Wks Topics Rmarks
7. Pakistan National Environmental Quality
Standards(NEQS) and International
Standards
8. Types, Characteristics, Sources and
Quantities of Solid Wastes. Collection,
disposal and recycling
Quiz-1
9. Mid Term
10. Primary, Secondary and Tertiary Treatment
Screening, Grit Chamber, Skimming tanks &
Sedimentation tanks,
Activated Sludge Process Treatment
11. Trickling Filters, Oxidation Ponds
12. Receiving body, Assimilation capacity, Assignt-2
Sludge handling and Disposal, Effluent re-use
13. Soil Pipes, Anti-syphon pipes and wastewater
pipes, Sanitary fixtures and Traps
14. Cross connection and back syphonage
control
Introduction to Environmental legislation and
Regulation
15. Introduction to Envi. Impact Assessment
16. Revision Class
Activity Weightage
Mid Term 20 %
Final Term 40%
Laboratory Work 20%
Assignment 20%
Note:
Achievement of 50% marks in Laboratory work and others is
required
75% Class attendance is mandatory for appearing in final term
examination. Below 75% attendance will not be allowed to
appear in final term examination.
Grading Criteria
Collection system (network of sewer pipes)
Disposal Works (sewage pumping stations,
outfalls)
Treatment works (for rendering wastewater
safe for environment and life)
COMPONENTS OF WASTEWATER
ENGINEERING
FACTORS AFFECTING WATER CONSUMPTION /SEWAGE GENERATION
Size of City
Small cities are expected to have low water consumption
Limited demands
Presence of Industries may increase the sewage production
Population Characteristics
Despite fixed water supply, wide variations in water
consumptions and sewage generation have been
observed in cities due
Economic status of people (Same population - Water
usage and sewage production in posh and high valued
areas of cities will be more than suburban)
More washing and bathing etc
More watering of lawns
Industries and Commerce
Presence of industries greatly effect the water demand
and WW generation. (large & more water using industries
more WW generation)
No Direct relation (care must be taken in assessment )
Climatic Conditions
More water usage and thus wastage in hot season of the year
Wastage even increase in clod areas due to usage of water
at faucets to prevent freezing of pipes
Metering
Metered service leads to careful water usage and thus
less sewage generation
Unmetered service careless uses of water and thus more
sewage generation
POPULATION FORECASTING
Population forecasting methods:
1. Arithmetic Increase Method
method to be adopted for a particular case depends
largely on the factors discussed in the methods, and the
selection is left to the discretion and intelligence of the
designer.
Most applicable for large and established cities.
Based on the assumption that the population increases
at a constant rate.
dP/dt = ka
Pf tf
dp = Ka
dt
Pi ti
Pf - Pi = Kd(tf-ti)
Pf = Future Population
Pi = Present Population
tf = Future time at Pf
ti = Initial (present) time at Pi
Ka = Constant
Where
Also
Pf = Pi + ka (tf - ti)
Ka = (Pi-Pe) / (ti-te)
Where “Pe” and “te” are second last terms in the given data
ii) Geometric Increase Method
based on the assumption that increase in population is
proportional to population i.e. percentage growth rate is
constant i.e.
dP/dt = kgP
Pf tf
dp/P = Kg
dt
Pi ti
ln Pf - ln Pi = Kg(tf-ti)
Pf = Future Population, Pi = Present Population
tf = Future time at Pf, ti = Initial (present) time at Pi
Kg = Constant
Where
ln Pf = ln Pi + kg (tf - ti)
Kg = (ln Pi – ln Pe) / (ti-te)
Where “Pe” and “te” are second last terms in the given data
Also
A graph is plotted from the available data, between time
and population.
The curve is then smoothly extended upto the desired
year. This method gives very approximate results and
should be used along with other forecasting methods.
iii) Simple Graphical Method
iv) Comparative Graphical Method (Curvilinear Method)
This is graphical method and one city is compared with
the other cities.
This method assumes that, if the curve of population
increase is plotted for a number of past decennial (10
years) periods, it may be extended by following the
tendencies apparent from the known portions.
Plotting curves of cities (one or
more decades ago) had
reached the present population
of the city being studied.
City “A”, is being studied is
plotted upto 1990, the years in
which it population was 51000.
City “B” reached 51000 in 1950
and its curve is plotted from
1930 on. Similarly curves are
drawn for cities “C” and “D” and
“E” from the year in which they
reached city “A’s” 1990
population.
Now city “A’s” curve can be then
be continues, allowing it to be
influenced by the rates of
growth of the larger cities.
Larger cities chosen should
reflect condition as that are in
the city being studies.
vi) Incremental Increase Method
GR is assumed to be progressively increasing or
decreasing, depending upon whether the average of the
incremental increases in the past is positive or negative.
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
v) Decreasing Rate of Growth Method
In this method, the average decrease in the percentage increase is
worked out, and is then subtracted from the latest percentage
increase to get the percentage increase of next decade.
Local population and the country's population for the last four to five
decades is obtained (census records).
Ratios of the local popu. to national popu. are calculated for these
decades.
A graph is then plotted between time and these ratios, and extended
upto the design period to extrapolate the ratio corresponding to future
design year.
This ratio is then multiplied by the expected national population at the
end of the design period, so as to obtain the required city's future
population.
vii) Ratio Method
Drawbacks:
Depends on accuracy of national population estimate.
Does not consider the abnormal or special conditions of popu. Shifts
(migrants)
Find out the population for year 2000 from the following data Q1:
Year 1930 1940 1950 1960 2000
Population 13000 15000 17000 20000 ? (Pf)
Estimate population in the year 2000 a) by Arithmetic method
and b) by Geometric method Q2
Pf = ? , tf = 2000 , Pi = 310,000
Ti = 1980 te = 1970 Pe = 23700
A community has estimated population of 20 years which is equal to
35000 persons. The present population is 28000 persons and the
present average water consumption is 16x106 liters/day. The
existing water treatment plant has a design capacity of 5 million
gallons per day (MGD). Assuming an arithmetic rate of population
growth determine in what year the existing plant will reach its design
capacity. (1 Liter = 0.264 gallons)
Q3
Find out the population for year 2000 from the following data
Q1:
Pf = ? , tf = 2000 , Pi = 20,000
ti = 1960
Pe = 17000
Ka = Ka = (Pi-Pe) / (ti-te) = (20000-17000) / (1960-1950)= 300
Pf = Pi + ka(tf-ti)
= 20,000 + 300 (2000-1960) = 32000
Solution:
Year 1930 1940 1950 1960 2000
Population 13000 15000 17000 20000 ? (Pf)
Estimate population in the year 2000 a) by Arithmetic method
and b) by Geometric method
b) Geometric method
Kg = Kg = (lnPi-lnPe) / (ti-te) (ln310,000-ln237000) / (1980-1970)
= 0.0268512
lnPf = lnPi + kg(tf-ti)
lnPf = ln310000 + 0.0268512 (2000-1980) = 456000
lnPf 12.644328 + 0.5370243 = 13.181352
Pf 530382
a) Arithmetic method
Ka = Ka = (Pi-Pe) / (ti-te) (310,000-237000) / (1980-1970)=
7300
Pf = Pi + ka(tf - ti)
= 310000 + 7300 (2000 -1980) = 456000
Example
Pf = ? , tf = 2000 , Pi = 310,000
Ti = 1980 te = 1970 Pe = 23700
Sol
A community has estimated population of 20 years which is equal to
35000 persons. The present population is 28000 persons and the
present average water consumption is 16x106 liters/day. The existing
water treatment plant has a design capacity of 5 million gallons per day
(MGD). Assuming an arithmetic rate of population growth determine in
what year the existing plant will reach its design capacity. (1 Liter = 0.264
gallons)
Q3
Sol:
Estimated population = 35000 persons
Present population = 28000 persons
Present Ave. water
consumption
= 16x106 liters/day = 4.224x106 GPD
Existing capacity of
WTP
= 5 MGD = 5x106 gallons/day
Present Excess capacity = 5x106 - 4.224x104 = 0.77x106 galon/day
Present per capita
consumption
= 4.224x106/28000 = 150.86 gpcd
Population served by
the excess capacity
= 0.77x106 / 150.86 = 5143.94
= 5144
Population GR = (35000 – 28000) / (20-0) = 350 person / Y
No. years to reach the
design capacity
= 5144/350 = 14.7 Years
It refers to the collection, treatment and disposal
of wastewater.
Sewerage works or sewage works include all the
physical structures required for that collection,
treatment, and disposal.
SOME BASIC TERMS
It is the liquid waste or wastewater produced as a
result of water use.
SEWAGE
SEWER
It is a pipe or conduit which carries sewage.
It is generally closed but normally not flowing full.
SEWERAGE
It is wastewater from residential buildings, offices,
hotels and institutions etc.
SOURCES OF WASTEWATER
NOTE: Sanitary Sewage refers to the combined
sewage from domestic and industrial sources
Domestic Sewage
It includes the liquid discharges from industrial
processes.
Industrial Waste
It include surface run off generated by rainfall and the
street wash.
Storm Sewage
Sewer which carries sanitary sewage i.e.,
wastewater originating from a municipality
including domestic and industrial wastewater.
TYPES OF SEWERS
It carries storm sewage including surface run off
and street washes and any other wastes which
may be discharged into the streets or onto the
ground..
Sanitary Sewer
It carries both sanitary and storm sewage.
Storm Sewer
Combined Sewer
is a common sewer with no tributary flow except
from house sewers.
TYPES OF SEWERS (Cont…)
is a pipe conveying wastewater from an individual
structure to a common sewer or some other point
of disposal.
collects flow from several submains as well as
lateral and house sewers
collects flow from one or more laterals or house
sewers.
House Sewer
Lateral Sewer
Submain Sewer
Main/Trunk Sewer
are pressurized sewer lines which convey sewage
from a pumping station to another main or to a point
of treatment or disposal.
receives discharge from all collecting system and
convey it to the point of final disposal (e.g., a water
body etc)
Force Mains
Outfall Sewer
If storm sewage is carried separately from domestic
and industrial wastewaters, the system is called
separate system.
Separate systems are favored when:
There is an immediate need for collection of
sanitary sewage but not for storm sewage.
When sanitary sewage needs treatment but storm
sewage does not
Costly construction and requires extra maintenance
TYPES OF SEWER SYSTEMS
1. SEPARATE 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.
NOTE: In urban areas of developing countries, mostly partially
combined system is used.
3. PARTIALLY COMBINED SYSTEM:
It is the system in which sewers carry both sanitary as well as
storm sewage.
Combined system is favored when:
Combined sewage can be disposed off without treatment.
Both sanitary and storm sewage need treatment
Streets are narrow and two separate sewers cannot be laid.
Preferred in present construction except previously
constructed old combined sewers
But in Pakistan combined sewers are preferred as sewage is
disposed of in Canals, rivers and for irrigation
2. COMBINED SYSTEM:
Domestic and industrial sewage is derived from water
supply, so it has a relationship with amount of water
consumption.
It is generally reported that about 70-90% of the
total water supplied to a community becomes
wastewater.
Sometimes, illicit drains and water use from
privately owned source produce quantities of
sewage larger than public water withdrawals.
SEWAGE FLOW / QUANTITY
INFILTRATION: It is the water which enters the sewers form
ground through poor joints, cracked pipes, and the walls of the
manholes.
Infiltration is non-existent in dry weather but increases during
rainy seasons.
Water and Sanitation Agency (WASA) Lahore uses following
infiltration rates for the design of sewer system
INFLOW: Inflow is the water which enters the sewers from
surface through perforated manhole covers, roof drains
connected to the sewers, and drains from the flooded cellars
etc.
Sewer Dia (mm) Infiltration
225 – 600 5 % of Average Sewage flow
>600 10 % of Average Sewage flow
Relation between Sewage Generation and Water Consumption
Associated with water supply to the city/community.
Following relation prevails among the water supply and sewage
generation.
Around 70 – 130 % of the water consumed
Higher percentage (130%) is
As industries (wet processes) rely on private water sources
but discharge their effluent into municipal sewer system.
Infiltration form poor joints and pipes laid under GWT.
Hence,
No fixed Relation between sewage production and water supply.
However generally it ranges from 70 – 90 % of the water
consumption.
After taking into consideration the rate of infiltration, the
average rate of sewage flow equals the average rate of water
consumption.
Sewage flow rates vary by source and with time.
Since sewers must be able to accommodate the
MAXIMUM RATE OF FLOW, the variation in
sewage flow need to be studied.
VARIATIONS IN SEWAGE FLOW
• Generally, Herman’s Formula is used to estimate
the ratio of maximum to average flow.
Where;
P = Population in thousand.
M = Peak Factor
WASA Lahore considers the following relationship for sewer design
VARIATIONS IN SEWAGE FLOW (Cont….)
Generally taken as 50 % of average sewage flow.
Minimum rates of flows are used in:
– Design of sewage pumping station
– To investigate the velocities in sewers during
low flow periods.
MINIMUM RATE OF SEWAGE FLOW:
VARIATIONS IN SEWAGE FLOW (Cont….)
Period of design is “Indefinite” as the system is
designed to care for the maximum development of
the area.
Qmax is used for design of sewers.
Qmin is used to check velocities during low flows
DESIGN PERIODS & USE OF SEWAGE FLOW DATA
i. Design of Sewer Systems:
Design period is usually 10 years.
Rates of flow required are:
average daily, peak and minimum flow rates,
including infiltration.
ii Design of Sewage Pumping Station:
Design period is usually 15 - 20 years.
Rates of flow required are:
average daily, and peak flow both including infiltration.
iii. Design of Sewage Treatment Plants:
iv. Lateral and Submains
Lateral and Submains are designed on the basis of
1500 lpcd + normal infiltration
v. Main, trunk and Outfall Sewers
Main, Trunk and Outfall sewers are designed on the
basis of 950 lpcd + Normal Infiltration + Industrial Waste
(if in large amounts)
Estimate the design flow for sanitary sewer serving 9000 people in an
area of 2.4 Km2 with an average water consumption of 500 lpcd. The
sewer is to be designed for 30 years and the design population was
estimated to be 2 times the present population per Km2. Assume
infiltration rate will be 10% of the average waste water flow. Add
industrial allowance at the rate of 3734 m3/km2/day.
Q:
Population density = 9000/2.4 = 3750 persons/km2
Ultimate Popu. Density
after 30 yrs
= 2x3750 = 7500 persons/Km2
Design Population = 2.4x7500 = 18000 persons
Ave.water Consumption = 500 lpcd
Domestic Sewage flow = 500 x 0.7 (70% is factor to convert
water consumption to
sewage flow)
Ave.daily Wastewater flow = 18000x350 = 6300000 liters per day
= 2.5 CFS
Peak factor for this flow = 3.1
Peak flow = 3.1x6300000 = 19530000 l/day
Infiltration 10% = 0.1x6300000 = 630000
Industrial wastewater flow = 3734x2.4 = 8961.6 m3/day
= 8961600 l/day
Design flow = 19530000 + 630000 +8961600
= 29121600 l/day = 29121.6 m3/day
Sol.
Estimation of flow is the first step to design the Storm sewer.
Rainfall is the primary source of storm flow.
Amount of Storm Sewage
i) Rational Method
All under use are based upon the use of rainfall data.
Most widely used formula for urban areas, into 5 Km2,is
Rational formula.
The total volume which fall upon an area “A” per unit time
under a rainfall of intensity “i” is
Q = iA
A portion lost by evaporation, percolation and ponding.
The portion lost is not constant, determined for different
conditions of temperature, soil moisture, and rainfall duration.
Q = CiA
The actual amount which appears as run off may then be
calculated from
Where
C = Run off Coefficient
Run Off Coefficient
“C” for an area is not invariant, but tends to increase as the
rainfall continues.
For impervious surfaces C = 1.75 t1/3 or
C = t / (8 + t)
These depends on duration of rainfall in minutes.
Where “t” is the duration of the storm in minutes.
For improved pervious surfaces
C = 0.3 t / (20 + t)
Average values of C commonly used for various surfaces
Sr.No 5ype of Surface Value of C
1 Water tight roof 0.70 – 0.95
2 Asphalt cement streets 0.85 – 0.90
3 Portland Cement Street 0.80 – 0.95
4 Paved Driveways and Walks 0.75 – 0.85
5 Gravel Driveways and Walks 0.15 – 0.30
6 Lawns
i) Sandy Soil with Slope of
a) 2% 0.05 – 0.10
b) 2-7% 0.10 – 0.15
c) > 7% 0.15 – 0.20
ii) Heavy Soil with Slope of
a) 2% 0.13 – 0.17
b) 2-7% 0.18 – 0.22
c) > 7% 0.25 – 0.35
Some engineers use values of “C”
i. 0.7 – 0.9 for densely built areas (walled city of Lahore),
ii. 0.5 – 0.5 for well-built areas adjacent to densely built zones
(mall road),
iii. 0.25 to o.5 for residential areas with detached houses,
iv. 0.15 – 0.25 for suburban section with few buildings
Determine the run off coefficient for an area of 0.2 Km2. Out of this
3000 m2 is covered by buildings, 5000 m2 by paved driveways and
walks and 2000m2 by Portland cement streets. The remaining area is
flat, heavy soil, covered by grass lawns.
Q:
Total area = 0.2 Km2
Select values of C for each type of area/surface form
given values and calculate percentage of land area for
each type
For roof C1 = (0.7 – 0.95)x (3000/200000) Use
ave. of (0.7-0.95)
= 0.012375
For driveways
& walks
C2 = (0.75-0.85)x (5000/200000) = 0.02
For Portland
Cement Streets
C3 = (0.8 – 0.95)x(2000/200,000) = 0.00875
For flat, heavy
soil grass
lawns
C4 = (0.13-
0.17)x(190,000/200000)
= 0.1425
Cave. = C1+C2+C3+C4 = 0.1836
Sol:
ii) Time of Flow:
It is the time taken by the storm sewage to flow from I1 to I2 (Figure).
TIME OF CONCENTRATION
The time required for the maximum runoff rate to develop is known
as the time of concentration and
it is equal to a drop of water to run from the most remote point of
the drainage area to be drained to the point for which the runoff is
being estimated.
The time of concentration has two parts
i) Inlet Time:
It is the time required for the runoff to gain entrance into a sewer.
Consider two areas “A” and “B” as shown in Figure.
I1 is entrance for area “A” and
I2 is entrance for area “B” and I1 ______ I2 sewer line
The water flows from A enters the sewer at I1 and that from
B at I2.
The time of concentration at I2 is either the time of
concentration for area B or the inlet time plus the time of
flow from I1 to I2 , whichever is greater.
The inlet time is time of concentration at I1,
Time of flow is a function of the velocity in the line I1 – I2
and its length.
The time of concentration for each sewer line is calculated
in a similar fashion.
The time of Concentration will largely depends upon the
slope of the ground surface and slope of the sewer.
Nomogram is also used to calculate the time of
concentration. In nomogram flow distance, type of surface
and slope are used to calculate time of concentration. This
procedure neglects effect of rainfall intensity, but is
adequate for most urban drainage project.
15.RAINFALL INTENSITY
In determining rainfall intensity for use in the rational formula it must be
recognized that the shorter the duration, the greater the expected
average intensity will be.
The critical duration - that which produces maximum runoff
Max. run off will be that which is sufficient to produce flow from the entire
drainage area.
Shorter periods will provide lower flows since the total areas not involved and
longer periods will provide lower average intensities.
The storm sewer designer thus requires some relationship between duration
and expected intensity.
Intensities vary in different parts of the country and curves or equation are
specific for the areas for which they were developed.
Following equations of intensity – duration curves are often more
convenient.
i = A / (t +B) where
i = Intensity of rainfall (usually per hour)
t = Time of Conc. or critical duration of rainfall in minutes
A&B = Constant
A sewer line drains a single family residential area with C = 0.35.
The distance of the flow from the most remote point is 60 m over
ordinary grass with a slope of 4%. The area drained is 100,000 m2
and intensity-duration formula is i =5230 / (t +30)
Q:
Sol:
Distance of flow = 60 m
Surface is ordinary grass
and slope
= 4%
Consulting Nomogram
we get “ t”
= 15 min
i = 5230/ (15+30)
= 116.22 mm/h
Q = CiA
0.35x(116.22/1000)x100000
4067.78 m3/h