Water supply has been a matter of concern since the beginning of
civilization. Groundwater has been the main source of drinking water for a
long time. Urban centers also utilize water from adjacent surface water
sources. In many urban areas water is transported from outside sources.
Many ancient cities built aqueducts to bring water from distant sources to
central locations in the city (e.g. Roman aqueducts, ‘Qanats’ mostly in Iran
and the Middle East).
Figure 1: Roman aqueduct “Pont du Gard”.
1 WFM 6306: Urban Water Management
Figure 2: “Qanat” – underground canals.
Early water distribution systems supplied water through pipes made of
wood, clay or lead.
As cities grew and sources were contaminated by wastewater, water
treatment became essential before supply. Coagulants and filtration have
been used since at least 2000 B.C.
Water supply systems are designed primarily based on estimate of water
demand from existing and projected population.
Water consumption is also dependent on climate, economic level,
population density, degree of industrialization, cost, pressure, and quality
of the supply. Need multivariate water demand projection techniques.
Analysis of future demand should always begin by considering the present
use. Consumption should be broken down by specific uses (domestic,
commercial, industrial, public), area of the city, economic level of users,
season of the year, etc.
Great care should be taken when the demand is expressed as per capita
consumption (dividing the total use by the total population). Because (1)
the entire population may not be served by the municipal system, (2) there
may be large industrial users that will not change with population, and (3)
the characteristics and size of population may change with time.
2 WFM 6306: Urban Water Management
Population projection period may range from 5 to 50 years depending on
the design life of the elements/components of the system.
Knowledge of the planned development activities, characteristics of the
community, and external factors is important in assessing growth of urban
Assumes that the growth rate is constant. Validity of the assumption in a
community may be tested by checking past census data. The hypothesis may be
K (in differential form) (1)
=> P/t = K (2)
where P = population and t = time. K is a constant that represents the growth
rate, P/t. K is determined graphically or from populations in successive
censuses. Therefore, the population in future is estimated from,
Pt = Po +K t [integrating (1)] (3)
where Pt = the future population after time period t and P o = the present
Uniform Percentage Method:
Assumes a growth rate proportional to the population, expressed by,
K P (in differential form) (4)
where K’ is a constant. Integration of (4) gives,
ln P = ln Po + K’ t => P = exp[ ln Po + K’ t ] (5)
This hypothesis is valid if a straight line can be fitted to the historic population
data plotted on a semilog paper. The constant K’ = slope of the line. Correlation
coefficient indicates how well the straight line fits to the data.
3 WFM 6306: Urban Water Management
Example 1: Population estimation.
Estimate the population in 2020.
Year 1950 1960 1970 1980 1990 2000 2020
Population 80000 100000 125000 156250 195300 244100 ?
Arithmetic Method: Uniform Percentage Method:
K= 3250 per year (trial) K' = 0.0225 (trial)
Po = 80000 in 1950 Po = 80000 in 1950
Year 1950 1960 1970 1980 1990 2000 2020 Year 1950 1960 1970 1980 1990 2000 2020
t 0 10 20 30 40 50 70 t 0 10 20 30 40 50 70
Population 80000 112500 145000 177500 210000 242500 307500 Population 80000 100186 125465 157123 196768 246417 386459
R2 = 0.98722 R2 = 1.00000
Population Estimation: Arithmetic Method Population Estimation: Uniform Percentage Method
400000 ('Normal' plot)
1940 1960 1980 2000 2020 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030
Population Estimation: Uniform Percentage Method
1940 1950 1960 1970 1980 1990 2000 2010 2020 2030
4 WFM 6306: Urban Water Management
Graphical projection of the historic population growth trend. Compares the
projected growth to the recorded growth of similar but larger cities. Similarity
should be based on geographical proximity, economic base, access to similar
transportation systems, etc.
City A’s population was 51,000 in 1990. City B, C, D and E’s population are
plotted from the year in which it reached 51,000. The dashed line represents city
A’s projected population considering the recorded growth (average) of the
5 WFM 6306: Urban Water Management
A logistic curve has an ‘S’ shape – indicating a geometric growth rate at low
population and a declining growth rate as the city approaches some limiting
The hypothesis may be tested by plotting the census data on a ‘logistic paper’ on
which it will appear as a straight line.
In short term, the logistic projection can be expressed as,
1 e abt
where Psat = ‘saturation (limiting) population’, and a and b are constants. Psat, a
and b can be determined from the latest three successive census data (Po, P1
and P2; t = time interval since Po) and the equations,
2 PoP1P2 P1 Po P2
Psat , (7)
a ln , and (8)
1 Po Psat P1
b ln , (9)
n P1 Psat Po
where n = time interval between successive censuses.
Example 2: Logistic Method
Year 1985 1988 1991 1994 1997 2000 2003
Population 82000 124100 255000 420100 520500 571100 595500
P0 P1 P2
1980 1985 1990 1995 2000 2005
6 WFM 6306: Urban Water Management
Declining Growth Method:
This method assumes that the city has some limiting saturation population, and
the growth rate is a function of the population deficit. The growth rate is,
K Psat P
The saturation population may be estimated based on available land and existing
population density. The constant K” may be estimated from successive census
data and the equation:
1 Psat P
K ln (11)
n Psat Po
where P and Po are populations recorded n years apart. The future populations
can be estimated from,
P Po Psat Po 1 eK t (12)
Relies on judgment based on experience of past census data and population
projections. Assumes that the ratio of the population of the city being studied to
that of a larger group (regional cities) will continue to change in the same manner
as occurred in the past.
The ratio (of population between the city being studied and the population of the
regional cities) is calculated for a series of censuses, the future ratio is found
from the projected trend line, and the projected population is found by multiplying
the forecast regional population (more reliable) by the projected ratio.
Selection of an appropriate population estimation technique relies heavily
on judgment and experience. However, inapplicable techniques can be
tested and eliminated based on past census data.
The declining growth method or the logistic method may be appropriate for
cities having limited land area for future expansion.
7 WFM 6306: Urban Water Management
The population growth of cities having abundant resources of land, power
and water, and good transportation may be best predicted by the
geometric or uniform percentage method.
In almost all cases a comparison with the recorded growth pattern of
similar cities is useful.
An underestimation of the population will result in early requirement of
redesign, reconstruction and refinancing of the system, while an
overestimation will result in excess capacity being financed by a smaller
group at higher unit cost.
8 WFM 6306: Urban Water Management
Water Use for Different Purposes
Major Classifications of Municipal Water Demand:
(1) Domestic: for houses, hotels, etc., for sanitary, culinary and other
purposes. Use varies with economic level range being 75 to 380 L (20 to
100 gal) per capita per day (includes air-conditioning and lawn-watering).
Domestic consumption is about 50 percent of the total.
(2) Commercial and Industrial: for factories, offices, stores, etc. Importance
varies locally depending on whether there are large industries and
whether the industries obtain water from the municipal system. Usually
industrial water demand is much larger than the municipal demand.
Commercial and industrial water demand can be related to such factors as
units produced, number of persons employed, or floor area of the
establishment. These factors, if used, should be checked with recorded
consumption or derived locally.
(3) Public Use: for public buildings (e.g. city buildings, schools) and for public
services (e.g. flushing streets, container supplies).
(4) Loss and Waste: ‘unaccounted for’ or not assigned to a specific user
(about 25% in Dhaka!). For example, errors in meter readings,
unauthorized connections, and leaks in the distribution system. Can be
reduced significantly by careful maintenance of the system.
Table 1: Approximate water usage by different classes in a developed city.
Use Percent of total
Loss and Waste 8
9 WFM 6306: Urban Water Management
Table 2: Urban water consumption (WHO, 1987).
Sri Lanka 200
Table 3: Estimated water consumption in Dhaka city (1991).
Income group Consumption
Table 4: Other water demands in Bangladesh.
Demand category Level Demand
Industrial High density 25
Medium density 18
Low density 10
Urban service High density 20
Medium density 15
Low density 10
10 WFM 6306: Urban Water Management
Factors Affecting Water Use:
(1) Size of the city:
Small communities tend to have more limited uses for water. Also, small
communities contain areas that are not served by water and sewerage systems.
Presence of water-using industries may increase the per capita use. Availability
of sewerage system tends to increase water use.
(2) Industry and commerce:
Industrial uses do not have a direct correlation to population. In estimating water
use, one must study the existing industries, their actual water use, and assess
the probability of establishment of more industrial facilities. Many industries use
auxiliary water supplies that reduce the consumption of municipal supply.
Commercial supply is largely dependent on the number of employees in the
businesses, rather than the number of residents in the area. Estimates are
sometimes made on the basis of floor area in highly developed business areas.
(3) Characteristics of the population:
Economic level causes significant variation in use. Water use is generally higher
in high-value areas. Water use is the lowest in low-value areas where sewerage
is not provided and water supply is inadequate.
Metering substantially reduces individual use. In the absence of meters,
consumers do not have any incentive to conserve, and waste of water is much
more common. Metering also provides data on use pattern and actual usage of
water, which is useful in planning expansion of facilities and assessing loss in the
(5) Other factors:
Include climate, quality, pressure, system maintenance, and conservation
programs. Usage increases in hot, dry weather. Improvements in the quality of
11 WFM 6306: Urban Water Management
public water supplies are likely to increase consumption. Higher pressure in the
system tends to increase consumption, and leakage in the system.
A well-designed conservation program of maintenance will reduce loss and
waste in the system. These programs may be short-term (e.g. during a period of
drought) or permanent, and may include building awareness in the communities
to conserve water.
Variations in Water Use:
Water consumption varies during the day, being lower at night. Consumption
also varies during the year (each day, week and month).
Pumping records may be useful in assessing water demands. However,
demands may widely vary among different communities.
The maximum daily consumption is likely to be 180 to 200 percent of the annual
average consumption. In the absence of data, the maximum consumption rate
may be estimated by the ‘Goodrich formula’,
p 180 t 0.1 (13)
where p = percentage of the annual average rate that occurs during shorter
periods, and t = length of the period in days from 1/12 to 360. The formula
predicts the maximum daily rate to be 180 percent of the annual average rate,
the weekly maximum to be 148 percent of the annual average, and the monthly
maximum to be 128 percent of the annual average. The maximum hourly rate is
likely to be 150 percent of the average for that day.
For example, the maximum hourly rate for a community having an average
annual rate of 670 liter/capita/day could be estimated as being: 670 X 1.8 X 1.5,
or 1800 liter/capita/day.
Minimum rates are also important and depend on leakage, night industrial use,
and the portion of the peak demand that is provided from storage. Typical minima
range from 25 to 50 percent of the daily average.
Peak consumption is important in designing distribution systems. Hourly peaks
as high as 1000 percent of the annual average may occur. Residential areas
12 WFM 6306: Urban Water Management
have particularly high ratios of peak to average flow. The ratio of peak to average
flow increases with decreasing population density.
Design Periods for Water Supply Components:
The economic design period of the components of a water supply system
depends on their life, initial cost, the ease with which they can be expanded, and
the likelihood that they would be obsolete with technological advances.
Different elements of the treatment and distribution systems may be
appropriately designed for different periods, and their design may be based upon
different flow criteria.
Development of source:
Depends on nature of the source. Groundwater supplies are relatively easily
expanded by construction of additional wells; design periods may be as short as
5 years. Surface water sources require more complicated constructions and the
design periods are as high as 50 years. The design capacity of the source is
normally based upon meeting the maximum daily demand rate expected during
the design period, but not necessarily on a continuous basis.
Pipelines from the source:
Designed for a relatively long life. A design period of 25 years or more is not
unusual. The design considers provision for economical conveyance at average
daily flow at the end of the design period with suitable velocities under all
anticipated flow conditions.
Water treatment plant:
Components are commonly designed for a period of 10 to 15 years since
expansion is generally simple. Most treatment plants are designed on the basis
of average daily flow at the end of the design period. Hydraulic design should be
based on the maximum anticipated flow through the plant (not necessarily the
same as the maximum anticipated water use).
13 WFM 6306: Urban Water Management
Generally designed for a period of about 10 years since modification and
expansion are relatively easy. Pump selection and design of control system
require knowledge of maximum flow, average flow and minimum flow.
Normally consists of elevated tanks. Design of these tanks require knowledge of
average consumption, maximum demand during the hour, day, week and month,
as well as the capacities of the source and pipelines. Design life is relatively high
since they are seldom replaced.
Elements are normally installed below the streets. Their life is very long and their
replacement is very expensive. Capacity of the system depends on the maximum
anticipated development of the area to be served. Other factors to be considered
include anticipated population densities, zoning regulation (helps assess the
domestic and industrial demands), and other factors that affect the per capita
flow. Design considers adequate pressure and maximum hourly flow.
14 WFM 6306: Urban Water Management
Quality of Water Supply
The Hydrologic Cycle and Water Quality:
Availability of water supply in terms of both quantity and quality is essential
for human existence.
Early civilizations developed around water bodies that supported agriculture,
transportation and drinking water.
Water quality was judged through physical senses of sight, taste and smell.
Relatively recent developments in biological, chemical and medical sciences
allowed people to measure water quality and determine its effect on human
Over 97% of the earth’s available water volume is contained in the oceans
and other saline water bodies; not readily usable for many purposes.
Approximately 2% is inaccessible in:
- polar ice caps and glaciers
- atmospheric and soil moisture.
15 WFM 6306: Urban Water Management
Only about 0.62% is found in fresh-water lakes, rivers and groundwater.
Water in biosphere is in constant motion; forms the ‘hydrologic cycle’.
Water in nature is most nearly pure in its evaporation state.
Condensation of water around particles acquires impurities.
Additional impurities are acquired as the liquid water travels further through
the hydrologic cycle.
Human activities contribute further impurities through:
- industrial and domestic wastes
- agricultural chemicals.
Impurities in water may be in both suspended and dissolved forms.
- Suspended materials are larger than molecular size.
- Dissolved materials consist of molecules or ions that are held by the
molecular structure of water.
16 WFM 6306: Urban Water Management
- Colloids are very small suspended particles, but often exhibit many
characteristics of the dissolved substances.
Presence of impurities in water in such quantity and of such nature as to impair
the use of water for a stated purpose.
‘Pollution’ is a relative term – depends on intended use.
‘Water Quality Parameters’ qualitatively reflect the impact of impurities on
selected water uses.
Analytical procedures quantitatively measure the parameters that represent
the physical, chemical and biological characteristics of water.
Contaminants: constituents of air, water, or soil that makes the water unsuitable
for the intended use; may be chemical or biological in nature and may result from
natural actions or human activities.
Natural contaminants include viruses, bacteria and other life forms; dissolved
mineral species; soluble organic by-products of life processes; and organic and
inorganic suspended solids. Contaminants from agricultural and industrial
processes increase the overall pollutant load.
17 WFM 6306: Urban Water Management
Chemistry of Solutions:
Water is the ‘Universal Solvent’.
An atom is the smallest unit of each of the elements. Atoms form molecules of
more complex elements and compounds.
For example, two hydrogen atoms combine to form a molecule of hydrogen:
H H H2 (14)
Adding one atom of oxygen to the hydrogen molecule forms one molecule of the
H2 O H2 O (15)
The sum of the atomic mass of all elements in a molecule is the molecular mass
of the molecule. This relative mass is based on a mass of 12 for carbon. The
atomic masses of hydrogen and oxygen are 1 and 16, respectively.
What is the molecular mass of water?
A mole of an element or compound is its molecular mass expressed in common
mass units, usually grams. e.g. a mole of water is 18 gm. One mole of a
substance dissolved in sufficient water to make one liter of solution is called a
one molar solution.
Elements are bonded in a compound by electrical forces (due to
transferred/shared electrons). Dissociation of these compounds in water
produces species having opposite charges. For example, dissociation of sodium
chloride (table salt):
NaCl Na Cl (16)
The double arrows indicate reversible condition. In the presence of sufficient
mass of solids, a dynamic equilibrium exists where the rates of dissociation and
recombination are exactly equal. At this point the water is saturated with the
The charged species are called ions. Positively charged ions are called cations
and negatively charged ions are called anions. The number of positive charges
18 WFM 6306: Urban Water Management
equals the number of negative charges in a compound to maintain electrical
neutrality. Charged compounds (e.g. ammonium, NH 4 , carbonate, CO3
Communicable diseases include bacterial, viral and protozoal infections.
Bacterial diseases include typhoid, paratyphoid, salmonellosis, shigellosis,
bacillary dysentery, and cholera. Viral diseases include hepatitis, poliomyelitis,
and some varieties of gastroenteritis. Protozoas such as Giardia and
Cryptosporidium may produce gastroenteritis. Certain fungi such as Aspergillus
also produce diseases in human bodies.
Testing for all possible microorganisms is infeasible or very expensive. Possibility
of water contamination is usually assessed by determining the number of
coliform bacteria. Escherachia coli is exerted up to 4X1010 organisms per person
per day. Presence of coliform is not a proof that water is contaminated; but
absence indicates that the water is free of pathogens.
Include both suspended and dissolved materials.
Suspended and Dissolved Solids
May consist of inorganic or organic particles, or immiscible liquids:
o Inorganic solids: clay, silt or other soil constituents;
o Organic solids: plant fibers and biological solids (algal cells,
o Immiscible liquids: oils and greases.
Common constituents of surface waters.
Often result from erosive action of water over surfaces.
Very rarely found in groundwater because of the natural filtering capacity
19 WFM 6306: Urban Water Management
May also result from human use:
o Domestic wastewater usually contains large quantities of organic
o Industrial wastewater contains both inorganic and organic solids.
Generally found in surface waters; rarely found in groundwater.
Toxic to human and other animals in large quantities; small quantities may
be beneficial: approx. 1 mg/L in drinking water may help prevent dental
cavity, and form decay resistant teeth.
Excessive fluoride in drinking water causes discoloration of teeth
(mottling) and bone formation abnormalities.
All metals are soluble to some extent in water.
Metals that are harmful in relatively small amounts are considered toxic.
Metals in natural waters originate from dissolution of natural deposits, and
discharges of domestic, industrial or agricultural wastewaters.
Measurement usually made by atomic absorption spectrophotometry
Commonly found in water include sodium, iron, manganese, aluminum,
copper and zinc.
Sodium is the most common nontoxic metal and highly reactive to other
elements; excessive concentrations of sodium salts impart a bitter taste,
and are hazardous to cardiac and kidney patients; also corrosive to metal
surfaces and toxic to plants.
Iron and manganese frequently occur together in natural waters and
present no health hazard. May produce color problems and bacterial
growth causing taste and odor.
20 WFM 6306: Urban Water Management
Significant quantities of iron are usually found in systems devoid of oxygen
such as groundwater and bottom layers of stratified lakes.
Copper and zinc, if both present, may be toxic to many biological species.
Harmful to human and other organisms in small quantities.
Toxic metals that may be dissolved in water include arsenic, barium,
cadmium, chromium, lead, mercury and silver. Arsenic, cadmium, lead
and mercury are particularly hazardous.
These metals are concentrated by the food chain.
Toxic metals usually originate from natural, industrial, or agricultural
Many organics in natural water systems originate from natural sources and are
soluble in water. May consist of the decay products of organic solids, or result
from wastewater and agricultural discharges. Artificial organics may be toxic or
Organics that are used up by microorganisms for food; usually consist of
starches, fats, proteins, alcohols, aldehydes and esters.
Microbial decomposition of dissolved organics normally takes place
accompanied by oxidation (addition of oxygen or reduction of hydrogen),
or by reduction (addition of hydrogen or reduction of oxygen).
The decomposition may occur in aerobic (oxygen-present) or anaerobic
Anaerobic decomposition results in unstable and objectionable end
When oxygen utilization occurs more rapidly than oxygen is replenished,
anaerobic conditions occur that severely affect the ecology of the system.
21 WFM 6306: Urban Water Management
Some organic materials (tannic and lignic acids, cellulose, phenols, etc.)
are resistant to biological degradation.
Benzenes and molecules having strong bonds are nonbiodegradable.
Many insecticides and herbicides (chlorinated hydrocarbons) are
nonbiodegradable, and may be extremely harmful to human and other
22 WFM 6306: Urban Water Management
Clarification and Treatment
Water is treated for a variety of reasons, including removal of pathogenic
microorganisms, tastes and odors, color and turbidity, dissolved minerals, and
harmful organic materials. The purpose of treatment may be for general domestic
use, or for industrial or other purposes having different quality standards.
The treatment processes include purely physical methods such as screening and
simple sedimentation, purely chemical methods such as adsorption and ion
exchange, and physiochemical processes in which the contaminants are altered
chemically for subsequent physical removal.
Very fine colloidal particles are flocculated by using chemicals (coagulants). The
flocculates are easier to remove from the system by screening or sedimentation.
Graded sand beds are usually used as filters. Other materials such as anthracite
and granular activated charcoal are also used as filter medium. The filters are
sometimes ‘backwashed’ for cleaning. There are two types of filters.
Slow Sand Filters:
Typically applied to waters having low turbidity. The filtration rate is normally less
than 0.4 m/hr [0.16 gal/(min-ft2)].
Rapid Sand Filters:
Generally implies a process that includes coagulation, flocculation, clarification
Disinfection is the process of killing the disease-causing microorganisms. Typical
bacterial reductions in coagulation-flocculation-sedimentation processes are 60
to 70%. The filtration process increases the removal to about 99%. However,
complete bacterial sterilization is usually neither sought nor achieved.
23 WFM 6306: Urban Water Management
Chlorine, in its various forms, is the most commonly used disinfectant.
Prechlorination (addition of chlorine prior to treatment) and postchlorination
(addition of chlorine after filtration) are both practiced.
Other disinfectants used include halogens, chlorine dioxide and ozone.
Disinfection may be also achieved by ultraviolet irradiation, heat, ultrasonic
waves, and certain metals.
Other Treatment Processes:
o Algae control by copper sulfate and chlorine.
o Iron and manganese removal.
o Water softening.
o Stabilization of the ionic condition to prevent corrosion of pipes.
o Reverse osmosis
o Treatment of brackish water
o Fluoridation and defluoridation
24 WFM 6306: Urban Water Management
Collection and Distribution
Intake structures must be designed so that the required flow can be withdrawn
despite the natural fluctuations. The intake normally consists of a screened
opening and a conduit that conveys the flow to a sump from which it may be
pumped to the treatment plant.
In locating intakes, one must consider:
- Anticipated variations in water level
- Navigation requirements
- Local currents and patterns of sediment deposition and scour
- Spatial and temporal variations in water quality
- Quantity of floating debris
Recall discussion on population estimation and water demand.
The pressure in municipal distribution systems ranges from 150 to 300 kPa (20 to
40 lb/in2) in residential areas having four-or-less storied structures to 400 to 500
kPa (60 to 70 lb/in2) in commercial districts. The minimum pressure requirement
(inside mains) for six storied buildings is 350 kPa.
The Pipe System:
The distribution network normally consists of:
o The primary or arterial mains: form the basic network and carry water from
the pumping station and reservoirs to the distribution area.
o The secondary lines: form smaller loops within the primary mains and
connect one primary line to another.
25 WFM 6306: Urban Water Management
o The small distribution mains: form a grid over the entire service area and
supply water to the end users. They are connected to primary, secondary
or smaller mains at both ends, and are valved so that they can be shut
down if required.
Maintenance of Distribution Systems:
o Cleaning: Occasional cleaning of the accumulation of sediment, rust, and
o Servicing of the hydrants and valves: Leaks of outlets should be repaired.
Inaccessible, inoperable, or closed valves may be identified by regular
o Leak surveys: Surveys should be conducted when it appears from
comparison of pumping logs and meter readings that excessive loss is
o Disinfection: Disinfection of an exiting pipe section is necessary after
repairs or modification by cutting. Normally the procedure involves
chlorination of the affected section.
Protection of Water Quality:
The quality of treated water pumped from the treatment plant may deteriorate as
it passes through the distribution system. The common contaminants include
organic and inorganic substances and bacteria.
The greatest potential hazard is the cross-connection to nonpotable water.
Cross-connections may frequently occur at hospitals, metal plating and chemical
plants, laundries and dye works.
26 WFM 6306: Urban Water Management
Sewage and Sewerage
Sewage: Liquid wastes produced in residences, commercial establishments,
public buildings and industrial units, and any subsurface, surface or storm water
that enters the sewers.
Sewer: is a pipe or conduit, generally closed, but normally not flowing full, which
carries the sewage.
Sewerage: System of collection, treatment and disposal of liquid waste;
sewerage works include all physical structures required for this purpose.
Sources of Sewage and Relation to Water Use:
Based on the type of sewage conveyed, sewers fall in three broad categories:
Sanitary or domestic sewers carry domestic sewage, industrial waste; and
ground, surface and storm water leaking into the sewer through joints,
manhole covers and defects in the system.
Storm sewers carry the surface and storm water passing through or
generated in the area of service.
Combined sewers carry all types of sewage in the same conduit.
Based on the hierarchical location, sewers are called:
House sewers carrying sewage from an individual structure.
The subsequent larger sewers are: Lateral sewers > Submain sewers >
Force mains are sewers under pressure carrying sewage from a pumping station.
Relief sewers are additional lines to increase carrying capacity.
Outfall sewers carry sewage to a disposal point.
27 WFM 6306: Urban Water Management
Sanitary sewage and industrial wastewaters are mainly derived from municipal
water supply. The proportion of water supply that will eventually reach the sewers
depends largely on the local conditions. For example,
Very little water may reach the sewers from steam generation and air-
conditioning uses. On the other hand, industries may have their own
sources of water supply, but may discharge wastewater to the municipal
The Sanitary sewage flow may vary from 70 to 130 percent of the water
consumed. Like water demand, sewage flow varies with time of the day, day of
the week, and season of the year.
Approximately one-third of the water is used for flushing toilets; one-third for
personal use via wash basin, bath and shower; and one third for food preparation
and drinking, laundry and other cleaning. A relatively small percentage of the
supplied water is used for drinking. Table shows water consumption by different
28 WFM 6306: Urban Water Management
On a daily basis, the relation between water usage and wastewater disposal may
be expressed as,
G x G (17)
where G = water consumption per person (liter/person/day), G’ = wastewater
generated per person (liter/person/day), and x = return factor (%). x varies from
60 to 95% depending on geographical location and culture; not very strongly tied
A strong diurnal pattern is observed in wastewater discharge. Figure shows a
typical variation in sewage flow during a 24 hour period.
The first peak generally occurs in the morning. The second peak occurs in the
early evening. A third peak can be also distinguished sometimes.
29 WFM 6306: Urban Water Management
Sewerage - General Considerations:
Sewage is normally treated in some manner before being discharged to the
environment depending on the water quality standard of the receiving water
The required degree of treatment may normally be achieved in a variety of ways
by combining different processes. Selection of the optimum combination requires
analyses of technical feasibility and economic viability.
Modern sewer systems are typically separate systems. In most cases sanitary
sewers are separated from storm sewers to prevent mixing with storm water.
However, storm water, particularly at the beginning of a runoff event, may be as
polluted as the wastewater. Considering the much higher flow rate and quicker
discharge, storm water is usually disposed of untreated.
The treatment of wastewaters may be divided in three steps:
1. Preliminary treatment
2. Primary treatment
3. Secondary treatment
Preliminary treatment includes measurement and regulation of incoming flow,
and removal of large floating solids, grit and grease. The devices normally used
include Flumes, meters, Racks and coarse screen, and aerators.
Primary treatment includes removal of suspended solids by simple sedimentation
process. Screens are chemicals are sometimes used to assist in the process.
Secondary treatment includes removal of soluble and colloidal organic matter,
mainly by biological processes. These processes ensure that a very large
number of microorganisms be available in a relatively small container. The basic
processes are called Attached (film) growth or Suspended growth processes.
30 WFM 6306: Urban Water Management
The liquid wastes from industrial and domestic sources are eventually disposed
of by: (1) reuse; (2) discharge to surface waters; (3) injection or percolation to
groundwaters; (4) evaporation to the atmosphere.
There is a considerable interdependence among various life forms in natural
streams. Organic matters that enter the streams is metabolized by bacteria and
converted into ammonia, nitrates, sulfates, carbon dioxide, etc., which are used
in turn by plants and algae to produce carbohydrates and oxygen. Insects and
other life forms are dependent directly or indirectly on these activities.
Introduction of excessive pollutants may upset this natural balance. As the
concentration of pollutants is reduced by dilution, precipitation, aeration, bacterial
oxidation, or other natural processes, the normal cycle and distribution of life
forms will tend to be reestablished.
Lake and Ocean Discharge:
Self purification phenomena in lakes, estuaries and oceans are similar to those in
streams. The pollutants tend to move with the seasonal mixing (turn over) of the
layers of water.
Usually preferred by coastal cities. However, these are relatively expensive for
establishment and maintenance, and pumping is almost always required.
Land Disposal and Treatment:
Usually achieved by spreading of the wastewater on land surface. May be
broadly classified into: slow rate, rapid infiltration, overland flow, wetland, and
Total Retention for Evaporation:
Wastewater is contained in a pond (sometimes called ‘oxidation pond’) for
evaporation. Sometimes a little fraction of the contained water is discharged.
31 WFM 6306: Urban Water Management
Variability and Representation of Wastewater Quality:
The characteristics of wastewater quality vary significantly from one community
to another. A regional average only reflects the ‘overall’ situation. The variability
is also ‘time-dependent’ and the ‘source-dependent’ (e.g. domestic, industrial,
etc.). Representation of the water quality situation, therefore, is subject to error.
Precision: is the reproducibility of an analytical technique when it is
repeated on a homogeneous sample under controlled conditions, without
regard to whether the measured values correspond to the actual value.
Precision is expressed by standard deviation of the test results.
Accuracy: is the correspondence between the measured value and the
Relative error: is the difference between the measured value and the
actual value as a percentage of the actual value.
One method may be precise (i.e. reproducible) but inaccurate with all measured
values closely grouped about the wrong value, while another may be accurate
but imprecise with all measured values widely scattered about the correct value.
Physical and Chemical Characteristics:
Sewage is over 99.9 percent water, but the remaining material has significant
effects upon the nature of the mixture.
Fresh domestic sewage has a slightly soapy or oily odor, often contains
considerable solids. Chemical and biological phenomena change the
characteristics with time. Stale sewage has a strong odor of hydrogen sulfide,
and is dark gray in color. This change takes about 2 to 6 hours at a temperature
The concentration of organic matters depends largely on per capita water use,
infiltration, and the quantity of industrial wastes that enter the system.
32 WFM 6306: Urban Water Management
The techniques of determining the chemical characteristics (organics) of
wastewater include: (1) Biochemical Oxygen Demand (BOD), (2) Chemical
Oxygen Demand (COD), and (3) Total Organic Carbon (TOC).
The amount of oxygen used by the microorganisms in the process of utilizing the
organic materials as the food source is the BOD. The BOD test requires a
relatively long (5 days) retention time, and is more applicable to wastewater with
lighter pollutant load. The COD test involves an acid oxidation of the waste by
potassium dichromate, and requires a relatively short retention time (on the order
of hours). TOC tests involve acidification of the sample to convert all inorganic
carbon to CO2, and are rapid and accurate, but expensive.
COD values are typically higher than BOD values. However, no definite
correlation exists between the BOD and COD values. TOC values moderately
correlate with BOD values.
Typically three types of samples are collected based on the method of collection:
(1) Grab sample: a small portion of the flow is collected.
(2) Composite sample: a mixture of grab samples collected over a period of
time, the sample volume being proportional to the flow.
(3) Continuous sample: intermittent diversion of small fraction of the flow over
a period of time.
Samples should be analyzed as soon as possible after collection. In case of
delay, samples should be stored at 4oC. Freezing should be avoided.
Typical Characteristics of Wastewater:
33 WFM 6306: Urban Water Management
Pollutant Sources of Wastewater:
Wastewater quality is influenced by the contaminants discharged into it, derived
mainly from human, household and industrial activities. The quality of the
carriage water (the original drinking water) or infiltrating groundwater can also be
Human excreta contribute a large proportion of pollutant to wastewater (200-300
gm of feces and 1-3 kg of urine per day); about 60% of the organics found in
wastewater. About 94% of organic nitrogen, and 50% of phosphorous in
wastewater is derived from excreta. Most microorganisms in wastewater
originate from feces.
In addition to various solids accidentally or deliberately discharged, a number of
cleaning, disinfecting and descaling chemicals are routinely discharged.
Digested food is source of many of the excreta-related pollutants. Undigested
food is the major contributor of fats including butter, margarine and vegetable
fats. Food residues are also a source of some organic nitrogen and
Washing and Laundry:
Washing and laundry activities add soaps and detergents to the sewer.
Constituents of synthetic detergents contribute approximately 50% of the
Most industrial wastewaters (or ‘trade effluents’) are likely to contain a very high
proportion of water, and the impurities are present as suspended, colloidal and
dissolved solids. A large variety of pollutants include:
Deficiency of nutrients
Resistant organic compounds
Carriage water and groundwater:
The sulfate in wastewater is derived from mineral content of municipal water
supply or from infiltrated saline groundwater. Use of water softeners, and
infiltration of saline water may significantly increase the chloride concentration.
34 WFM 6306: Urban Water Management
Bottom Sediment Characteristics:
Sewers accumulate significant amount of sediment. Volume and characteristics
of sediment depend mainly on concentration of solids in wastewater, land use of
the service area and velocity of flow. Storm sewers accumulate sediment from
washed-off soil that flow with runoff. Figure shows difference in characteristics of
storm sewer sediment in commercial and residential areas of Dhaka city.
Source: Khan and Chowdhury, 1997.
Table shows composition of storm sewer sediment in different land use and time
of monsoon in Dhaka city.
Source: Khan and Chowdhury, 1997.
35 WFM 6306: Urban Water Management
Urban stormwater can be heavily contaminated, sometimes as contaminated as
wastewater, with a range of pollutants originating from natural organic and
inorganic substances, and a small proportion of man-made materials derived
from transport, commercial and industrial practices. Domestic wastewater
discharge to storm sewers and waste dumping through manholes often degrade
the stormwater quality. Size of the particles may widely vary between 1 m and
10 mm. Although comprising less than 5% of the material present, particulates
less than 50 m in size have most of the pollutants associated with them: 25% of
the COD, up to 50% of the nutrients and 15% of total coliforms. These particles
are most readily washed-off by rainwater. Runoff quality will depend upon a
number of factors including geographical location, road and traffic characteristics,
building and roofing type, and weather (particularly rainfall).
Pollutant Sources of Stormwater:
Stormwater quality is affected by rainfall and catchment characteristics.
Pollutants in urban atmosphere result mainly from human activities including
vehicular emission, and industry or waste incineration. The pollutants may be
carried to the drainage system after absorption to rainfall as ‘wet fallout’, or
washed off by rainfall after settling on the surface as ‘dry fallout’. Importance of
dry and wet fallout depends on the site and the pollutant, both contributing
significantly to nitrogen, phosphorous, lead, zinc, chromium and cadmium load.
Vehicle and traffic:
Vehicular exhaust includes volatile solids and polyaromatic hydrocarbons derived
from unburned fuel, exhaust gases and vapors, lead compounds from petrol
additives, and hydrocarbon losses from fuels, lubricants and hydraulic fluids. Tire
wear releases zinc and hydrocarbons. Vehicle corrosion releases iron,
chromium, lead, zinc, copper and nickel. Wear of paved surfaces release
bitumen and aromatic hydrocarbons, tar and emulsifiers, carbonates, metals and
Buildings and roads:
Erosion of construction materials produces particles of brick, concrete, asphalt
and glass, significantly contributing to the sediment load. Roofs, gutters and
exterior paint may also produce varying amount of pollutants. Roadside metallic
36 WFM 6306: Urban Water Management
structures such as fences and benches may release toxic substances from
Human and Animals:
Roadside urination and defecation by human and (other) animals are sources of
fecal coliforms and fecal streptococci contamination and high oxygen demand.
Urban surfaces contain large amounts of debris, litter and organic material such
as dead and decaying vegetation. Decomposed leaves and grass may increase
Representation of Stormwater Quality:
Event mean concentrations:
The average concentration of pollutant is determined for the entire event. An
assumption is made that the stormwater has a constant concentration of pollutant
during the event. This method is most suitable for analyses where only the total
pollutant load is important.
The quality of stormwater is statistically regressed against a number of
describing variables such as catchment characteristics and land use. The
regression equation will be less accurate on other catchments, but can often be a
reasonable first approximation.
Build-up and wash-off models:
The most common model-based approach to quality representation is by
separately predicting pollutant build-up and wash-off.
Factors affecting build-up of pollutants on impervious surfaces include land use,
population, traffic flow, effectiveness of street cleaning, season of the year,
meteorological conditions, antecedent dry period, and street surface type and
Factors affecting wash-off load include rainfall characteristics, topography, solid
particle characteristics, and street surface type and condition.
37 WFM 6306: Urban Water Management
Numerical Water Quality Models:
The basic approaches of modeling a sewer system in an urban catchment are:
Empirical models based on observation rather than theory,
Conceptual models in which the physical system is represented by a
highly simplified ‘concept’, for example representation of pipe flow by a
simplified tank system,
Gray box models in which the physical relationships are defined, but the
relationships rely on observed data,
Stochastic models which gives a range of probabilistic answers for the
same input, and
Artificial neural networks which works similar to human brain by training
itself from past data and applying the ‘knowledge’ to new cases.
Physically-based deterministic quality models that represent sewer flow have
become available for general use. However, the physical processes in the sewer
are so complex that:
they are not fully understood,
it may simply be over-ambitious or inappropriate to represent them in a
physically-based deterministic model, and
data input and monitoring/verification requirements may be too time-
consuming and expensive to be worthwhile.
The main objective of a sewer quality model is to simulate the variation of
concentration of pollutants with time at chosen points in a system. These
simulations are useful in optimizing the performance of the system. The main
quality parameters include suspended solids, BOD, COD, nitrate, ammonia, and
others depending on the purpose of modeling.
Physically-based deterministic models are based on established flow models.
The main physical processes that are modeled are Pollutant transport and
38 WFM 6306: Urban Water Management
Pollutant transport modeling:
Governing equations are:
v 0 (18)
c c c
v D (19)
t x x x
where x = distance, t = time, c = pollutant concentration, v = mean velocity of
flow, and D = longitudinal dispersion coefficient. Equation 18 represents
advection, movement of pollutants at the mean velocity of flow. Equation 19
represents dispersion, spreading out of pollutants relative to mean velocity. Most
sewer flows can be reasonably represented by advection equation only.
Completely mixed ‘tank’:
In this approach each pipe length is treated as a conceptual tank in which
pollutants are fully mixed with the flow. The governing equation is:
Qi c i Q o c o (20)
where c, ci and co = pollutant concentrations in pipe length, at inlet and at outlet,
Qi and Qo = flow at inlet and at outlet, and S = volume of liquid in pipe length.
Dissolved pollutants are transported with the flow. Pollutants associated with
solids in suspension may be affected by the flow regime. Heavier particles may
tend to settle down although they may still be transported as bed load. All sewer
quality models consider the movement of solid-associated pollutants through the
system in terms of:
Mechanics: entrainment, transport and deposition,
Sediment bed, and
39 WFM 6306: Urban Water Management
Pollutant transformation modeling:
The main transformation processes act within or between the atmosphere, the
wastewater itself, the biofilm attached to the pipe wall, and the sediment bed.
Most important processes include those associated with the biodegradation of
the organic materials. Representation of pollutant transformation includes:
Conservative pollutants: that are not affected by any chemical or
Simple decay expressions: using a first-order decay model (dc/dt = - kc),
River modeling approach: representing advection, dispersion,
sedimentation, reaeration and conversion, and
WTP (wastewater treatment plant) modeling approach: considering the
integrated kinetics of sewer flows.
Popular software packages:
SWMM (Storm Water Management Model, USA):
SWMM includes processes associated with the accumulation and washoff of
pollutants on the catchment surface. Deposition and erosion of sediment in the
pipes are represented, and transport of pollutants is modeled in a simplified form
by treating each pipe as a completely mixed tank.
HydroWorks (Wallingford, UK):
HydroWorks makes a distinction between dissolved pollutants and sediments.
On the catchment surface, sediments with attached pollutants accumulate during
dry weather at a rate determined from land use and a ‘pollution index’. At the
location of all wastewater connections, data on population, average flow rates
per person, and concentration of dissolved pollutants and sediments are used to
create diurnal variations. Advection equations are used in pipe flows.
MOUSE TRAP (Danish Hydraulic Institute):
MOUSE TRAP models similar processes to HydroWorks, but in a much greater
range of quality parameters. The dispersion equation and a wider range of
sediment transport equations are also supported. MOUSE TRAP has been
applied to a catchment in Dhaka.
40 WFM 6306: Urban Water Management