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