unit 4 - water supply

8/12/2019 UNIT 4 - Water Supply

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Prepared by Prashanth J. and Harish N. Page 1

UNIT 4: WATER SUPPLY AND SEWAGE SYSTEM FOR A

COMMUNITY/TOWN/CITY

4.1 INTRODUCTION

The use of water by man, plants and animals is universal. Without it, there can be no life.

Every living thing requires water. The use of water is increasing rapidly with our growing

population. Already there are acute shortages of water. Careless pollution and contamination

of water sources has greatly impaired the quality of available water. Hence, there is a need for

proper water supply system to provide the end users with usable water free from pollutions.

The primary objective of water supply system is to take water from best available source and

to treat the water to ensure good quality, free from unpleasant taste or odour and free from

micro organisms that are detrimental to health. A typical water supply system consists of the

following units:

1. Collection works.2. Transmission or intake works.3. Purification or treatment works and4. Distribution works.

Collection works: Collection works are meant for collecting water from different sources

(Section 4.2) like rivers, lakes, springs, wells, reservoirs etc.

Intake works: In many cases, the collection works may be far away from the city where

water is to be supplied. In that case, water is conveyed to the city through transmission or

intake works. These form the connecting link between the collection works and purification

works. Depending upon the topography of the area between the two sites, the transmission

works may be in the form of conduits, canals etc. The conveyance of water is discussed in

detail in Section 4.3.

Purification works:The water collected directly from source may not be safe for drinking

because of physical, chemical and biological impurities. The municipal water works must

deliver to the consumer the water that is:

a. Hygienically safe.b. Aesthetically attractive and palatable andc. Economically satisfactory for its intended use.


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Diseases like typhoid, cholera, dysentery etc. are water borne diseases. The principal aim of

purification works is to supply clean and bacteria free water. This is dealt in detail in Section

4.6.

Distribution works: The treated and purified water is finally sent to consumers through

suitable distribution system. In order that water may flow in the water supply pipes under

pressure, the purified water is normally stored in an elevated service reservoir. There are two

patterns of water distribution system:

a. Branching pattern with dead ends, andb. Grid iron pattern.

The plan, topography and location of the area with respect to the service reservoir establish

the type of distribution system and its character of flow.

Figure 4.1 shows the layout of water supply system with all the units.

Fig. 4.1: Layout of water supply system


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4.2 SOURCES OF WATER SUPPLY

The objectives of operation and maintenance of sources of water supply schemes are:

1. The water sources should be able to supply water which is safe to drink aftertreatment.

2. The water sources should be perennial and should ensure sustainable yield.3. The quality of water should not be allowed to deteriorate.4. There should be least or no disruption in water supply systems due to depletion of

water sources.

5. There should be least possible expenditure on the repair and maintenance of the watersources.

6.

Proper record of the water sources should be maintained so that their time to timeperformance could be known.

7. A methodical long-range programme of source inspection and monitoring should beintroduced to identify problems so that a regular programme of preventive

maintenance can guarantee reliability and continuity.

8. Survey maps shall be obtained or prepared for all possible sources of water like rivers,reservoirs, lakes, canals, wells, and springs etc. The maps already available should be

updated from time to time.

4.2.1 Sources of Water

The sources of water are broadly classified into following categories:

Natural sources:Rain, snow, hail and sleet are precipitated upon the surface of the earth as

meteorological water and may be considered as the original source of all the water supplied.

Water, as source of drinking water, occurs as surface water and ground water. Three aspects

should be considered in appraising water resources e.g., the quantity, the quality, and the

reliability of available water.

Surface sources: Surface water accumulates mainly as a result of direct runoff from

precipitation (rain or snow). Precipitation that does not enter the ground through infiltration

or is not returned to the atmosphere by evaporation, flows over the ground surface and is

classified as direct runoff. Direct runoff is water that drains from saturated or impermeable

surfaces, into stream channels, and then into natural or artificial storage sites (or into the

ocean in coastal areas).

The amount of available surface water depends largely upon rainfall. When rainfall is limited,

the supply of surface water will vary considerably between wet and dry years.


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Surface water supplies may be further divided into river, lake, and reservoir supplies. Dams

are constructed to create artificial storage. Canals or open channels can be constructed to

convey surface water to the project sites. The water is also conveyed through pipes by gravity

or pumping.

In general, the surface sources are characterized by soft water, turbidity, suspended solids,

some colour and microbial contamination.

Sub surface sources: Part of the precipitation that falls infiltrates the soil. This water

replenishes the soil moisture, or is used by growing plants and returned to the atmosphere by

transpiration. Water that drains downward (percolates) below the root zone finally reaches a

level at which all the openings or voids in the earth's materials are filled with water. This

zone is called the zone of saturation. The water in the zone of saturation is called the ground

water.

Ground waters are, generally, characterized by higher concentrations of dissolved solids,

lower levels of colour, higher hardness (as compared with surface water), dissolved gasses

and freedom from microbial contamination.

A well that penetrates the water table can be used to extract water from the ground basin.

Ground water that flows naturally from the ground is called a spring.

The extraction of ground water is mainly by:

1. Dug well with or without staining walls2. Dug cum bore wells3. Cavity Bore4. Radial collector wells5. Infiltration galleries6. Tubewells & bore wells.

Table 4.1 shows the comparison between ground water and surface water in terms of quality

and quantity.

Table 4.1: Comparison of quality and quantity of ground water and surface water

Sl. No. Parameters Ground water Surface water

1 Suspended solids Very high Very high

2 Dissolved solids Very high Very less

3 Pathogens Absent Present

4 Taste Not palatable Palatable

5 Treatment Not required Very much required

6Problems in distributionsystem

Causes incrustation Causes corrosion


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7 Availability Available near the point

of usage

Generally not available

near the point of usage

8 Yield Sufficient for drinking

purposes

May or may not

sufficient

9 Possibility of faecalcontamination

Chances are very less Chances are very high

10 Contamination from other

sources such as industrial,

agricultural etc.,

Chances are very less Chances are very high

11 Capital investment Less High

12 Running cost High Less

13 Effect of power failure Seriously upsets the

system

Less serious

4.3 CONVEYANCE OF WATER

Water is conveyed or transported from the source to the community through various types of

conduits. These may be either open or closed types depending upon whether the necessary

energy is to be provided by gravity or by pumping. The various types of conduits used are:

open channels, tunnels and pipe lines. Open channels and tunnels are generally used to

convey raw water from the source to the water treatment plants. However, the transported

water must be safeguarded pollution by inferior water sources. This is a special problem

when open channels or conduits operating at low pressures are used. Due to this, pipe lines

are invariably used for conveyance of water. Another advantage of using pipe lines is the

reduction of conveyance losses, such as evaporation and seepage losses.

Open channelsare designed to convey water under conditions of atmospheric pressure. The

channels may be lined or unlined depending on the velocity of flow, slope of channel and

quantity of water supply. Pressure tunnelsare constructed to cross rivers and valleys. They

must be operated under pressure or act as open channels. Pipe lines used in important

transportation systems may require gate valves, check valves, drains, manhole, pumping

stations etc.

Pipes are circular closed conduits through which the water may flow wither under gravity or

under pressure. Pipes may be made of following materials: cast iron, wrought iron,

galvanised iron, cement concrete, steel, asbestos cement, plastic, copper and wood.


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4.4 CHARACTERISTICS OF RAW WATER

For the purpose of classification, the characteristics of water may be divided into the

following three categories.

1. Physical characteristics.2. Chemical characteristics.3. Biological characteristics.

Physical characteristics:The following are the physical characteristics,

a. Turbidity: Turbidity is caused due to presence of suspended and colloidal matter inthe water. The character and amount of turbidity depends upon the type of soil over

which the water has moved. Ground waters are less turbid than the surface water.

Turbidity is a measure of resistance of water to the passage of light through it.

Turbidity is expressed as NTU (Nephelometric Turbidity Units) or PPM (parts per

million) or Milligrams per litre (mg/l).

b. Colour and temperature: Colour in water is usually due to organic matter in colloidalcondition but some times it is also due to mineral and dissolved organic impurities.

The colour produced by one milligram of platinum in a litre of water has been fixed as

the unit of colour. The colour in water is not harmful but objectionable.

Temperature of water is measured by means of ordinary thermometers. The

temperature of surface water is generally at atmospheric temperature, while that of

ground water may be more or less than atmospheric temperature. The temperature

above 35C is unfit for public supply, because it is not palatable.

c. Taste and odour: Taste and odour in water may be due to presence of dead or livemicro-organisms, dissolved gases such as hydrogen sulphide, methane, carbon

dioxide or oxygen combined with organic matter, mineral substances such as sodium

chloride, iron compounds and carbonates and sulphates of other substances. The tests

of these are done by sense of smell and taste because these are present in such small

proportions that it is difficult to detect them by chemical analysis. The water having

bad smell and odour is objectionable and should not be supplied to the public.

Chemical characteristics: Typical chemical characteristics of water can include varying

amounts of solids and/or salts, also referred to as total solids (TS), total suspended solids

(TSS). It also includes hardness, pH of water.


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Total solids and suspended solids: Total solids include the solids in suspension colloidal and

in dissolved form. The quantity of suspended solids is determined by filtering the sample of

water through fine filter, drying and weighing. The quantity of dissolved and colloidal solids

is determined by evaporating the filtered water obtained from the suspended solid test and

weighing the residue.

pH value of water: pH value denotes the concentration of hydrogen ions in the water and it is

a measure of acidity or alkanity of a substance. For pure water, PH value is 7 and 0 to 7

acidic and 7 to 14 alkaline ranges. For public water supply PH value may be 6.5 to 8.5. The

lower value may cause tuberculation and corrosion, where as high value may produce

incrustation, sediment deposits and other bad effects.

Hardness of water: It is a property of water, which prevents the lathering of the soap.

Hardness is of two types.

i. Temporary hardness: It is caused due to the presence of carbonates and sulphates ofcalcium and magnesium. It is removed by boiling.

ii. Permanent hardness: It is caused due to the presence of chlorides and nitrates ofcalcium and magnesium. It is removed by zeolite method.

Hardness is usually expressed in gm/litre or p.p.m. of calcium carbonate in water. Hardness

of water is determined by EDTA (Ethylene Diamine Tetra Acetic acid) method. For potable

water hardness ranges from 5 to 8 degrees.

Bacterial and microscopical characteristics: The examination of water for the presence of

bacteria is important for the water supply engineer from the viewpoint of public health. The

bacteria may be harmless to mankind or harmful to mankind. The former category is known

as non-pathogenic bacteria and the later category is known as pathogenic bacteria. Many of

the bacteria found in water are derived from air, soil and vegetation. Some of these are able to

multiply and continue their existence while the remaining dies out in due course of time.

Table 4.2 shows the characteristics of some natural water sources.

Table 4.2 Characteristics of water sources(Ref: Water supply Engineering, B. C. Punmia)

Sources Characteristics

Seawater Salts

Precipitation Gases, vapours, particulates, salt nuclei, radio-active fall out

Surface runoff Particulates, organic matter, nitrates, phosphates, biocides

Groundwater Carbonates, Chlorides and Sulphates of Calcium and Magnesium, Iron


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and Manganese, SO2, H2S

Lake water Algae, odours, tastes

River water Particulates, organic matter, waste waters

4.5 DRINKING WATER STANDARDS (IS: 10500 - 1991)

Sl.

No.

Parameter Desirable

Limit mg/l

Permissible limit in

the absence of

alternate sources

Undesirable effect

outside the desirable

limit

1 Colour 5 May be extended up to

25 if toxic substances

are suspected

Above 5, consumer

acceptance decreases

2 Odour Unobjectionable - --

3 Taste Agreeable - --

4 Turbidity, 10 May be relaxed up to

25 in the absence of

alternate

Above 5, consumer

acceptance decreases

5 pH 6.5-8.5 May be relaxed up to

9.2 in the absence

Beyond this range the

water will affect the

mucous membrane and / or

water supply system

6 Total

hardness as

CaCO3, Max

300 May be extended up to

600

Encrustation in water

supply structure and

adverse effects on

domestic use

7 Iron as Fe 0.3 May be extended up to1.0 Beyond this limittaste/appearance are

affected, has adverse effect

on domestic uses and

water supply

structures, and promotes

iron bacteria

8 Chlorides as

Cl


1000

Beyond this limit tast,

corrosion and palatability

are effected

9 Free residual

chlorine

0.2 Applicable only when

water is chlorinated

--

10 Total

dissolved

solids

500 2000 Beyond this palatability

decreases and may cause

gastro intentional irritation

11 Calcium as

Ca


200

Encrustation in water

supply structure and

adverse effects on

domestic use

12 Copper as

Cu

0.05 May be extended up to

1.5

Astringent taste,

discoloration and

corrosion of pipes, fitting

and utensils will be causedbeyond this


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13 Manganese

as Mn

0.1 May be extended up to

0.5

Beyond this limit

taste/appearance are

affected, has adverse effect

on domestic uses and

water supply

structures14 Sulphates as

SO4


400

Beyond this causes gastro

intentional irritation when

magnesium or sodium are

present

15 Nitrates as

NO3

45 No relaxation Beyond this

methanemoglobinemia

takes place

16 Fluoride as F 0.6 to 1.2 If the limit is below 0.6

water should be

rejected, Max. Limit is

extended to 1.5

Fluoride may be kept as

low as possible. High

fluoride may cause

fluorosis

17 Cadmium as

Cd

0.01 No relaxation Beyond this, the water

becomes toxic

18 Cyanide as

Cn


becomes toxic

19 Mercury as

Hg


becomes toxic

20 Zinc as Zn 5 15 Beyond this limit it can

cause astringent taste and

an opalescence in water

21 Arsenic asAs

0.05 No relaxation Beyond this, the waterbecomes toxic

22 Lead as Pb 0.1 No relaxation Beyond this, the water

becomes toxic

4.6 LAYOUT OF WATER TREATMENT PLANT

The available raw waters must be treated and purified before they can be supplied to the

public for their domestic, industrial or any other uses. The extent of treatment required to be

given to the particular water depends upon the characteristics and quality of the availablewater, and also upon the quality requirements for the intended use.

The layout of conventional water treatment plant is as follows:


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Fig. 4.2: Typical layout of water treatment plant

Depending upon the magnitude of treatment required, proper unit operations are selected and

arranged in the proper sequential order for the purpose of modifying the quality of raw water

to meet the desired standards.

The typical functions of each unit operations are given in the following table:

Unit treatment Function (removal)

Aeration, chemicals use Colour, Odour, Taste

Screening Floating matter

Chemical methods Iron, Manganese, etc.

Softening Hardness

Sedimentation Suspended matter

Coagulation Suspended matter, a part of colloidal matter and bacteria

Filtration Remaining colloidal dissolved matter, bacteria

Disinfection Pathogenic bacteria, Organic matter and Reducing substances


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The types of treatment required for different sources are given in the following table:

Source Treatment required

1. Ground water and spring water fairly free from

contamination

No treatment or Chlorination

2. Ground water with chemicals, minerals and

gases

Aeration, coagulation (if

necessary), filtration and

disinfection

3. Lakes, surface water reservoirs with less

amount of pollution

Disinfection

4. Other surface waters such as rivers, canals and

impounded reservoirs with a considerable amount

of pollution

Complete treatment

4.7 SEWAGE SYSTEM

Sewage indicates the liquid waste from the community. It includes sullage, discharge from

latrines, urinals, stables industrial waste and also the ground surface and storm water that may

be admitted into the sewer. It is extremely putrescible; its decomposition produces large

quantities of malodorous gases, and it may contain numerous pathogenic or disease

producing bacteria.

Sources of Sewage: Sewage includes sanitary, commercial, industrial, agricultural and

surface runoff. The wastewater from residences and institutions, carrying body wastes

(primarily faeces and urine), washing water, food preparation wastes, laundry wastes, and

other waste products of normal living, are classed as domestic or sanitary sewage. Liquid-

carried wastes from stores and service establishments serving the immediate community,

termed commercial wastes, are included in the sanitary or domestic sewage category if their

characteristics are similar to household flows.

Wastes that result from industrial processes such as the production or manufacture of goods

are classed as industrial wastewater. Their flows and strengths are usually more varied,

intense, and concentrated than those of sanitary sewage.

Surface runoff, also known as storm flow or overland flow, is that portion of precipitation

that runs rapidly over the ground surface to a defined channel. Precipitation absorbs gases

and particulates from the atmosphere, dissolves and leaches materials from vegetation and

soil, suspends matter from the land, washes spills and debris from urban streets and

highways, and carries all these pollutants as wastes in its flow to a collection point.


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Wastewater characteristics: Wastewater characteristics can be listed as follows:

1. Physical characteristicsa) ColourFresh wastewaterlight brownish gray.

With time dark gray

More time black (septic).

Sometimes pink due to algae or due to industrial colours.

b) OdourOdour is produced by gas production due to the decomposition of organicmatter or by substances added to the wastewater.

c) Temperature Temperature of wastewater is commonly higher than that of watersupply. Depending on the geographic location the mean annual temperature varies

in the range of 10 to 21o

C with an average of 16o

C.

d) Turbidity Turbidity is a measure of the light scattering ability of suspendedmatter in the water. Waste water is more turbid than raw water.

e) Solids Solid material in wastewater may be dissolved, suspended, or settleable.Total dissolved solids (TDS) are measured as the mass of residue remaining when

a measured volume of filtered water is evaporated. The mass of dried solids

remaining on the filter is called total suspended solids (TSS). Settleable solids are

measured as the visible volume accumulated at the bottom of an Imhoff cone afterwater has settled for one hour.

2. Chemical characteristicsa) Biochemical Oxygen Demand (B.O.D): If the water is contaminated with sewage,

the demand of oxygen by organic matter in sewage is known as biochemical

oxygen demand. The aerobic action continues till the oxygen is present in sewage.

As the oxygen exhausts the anerobic action begins due to which foul smell starts

coming. Therefore indirectly the decomposable matters require oxygen, which is

used by the organisms. The general range of BOD observed for raw sewage is 100

to 400 mg/L.

b)Chemical Oxygen Demand (C.O.D): The COD gives the measure of the oxygenrequired for chemical oxidation. It does not differentiate between biological

oxidisable and non-oxidisable material. However, the ratio of the COD to BOD

does not change significantly for particular waste and hence this test could be used

conveniently for interpreting performance efficiencies of the treatment units. In


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general, the COD of raw sewage at various places is reported to be in the range

200 to 700 mg/L.

c)pH: The hydrogen ion concentration expressed as pH, is a valuable parameter inthe operation of biological units. The pH of the fresh sewage is slightly more than

the water supplied to the community. However, decomposition of organic matter

may lower the pH, while the presence of industrial wastewater may produce

extreme fluctuations. Generally the pH of raw sewage is in the range 5.5 to 8.0.

d)Solids: Though sewage contains only about 0.1 percent solids, the rest beingwater, still the nuisance caused by the solids cannot be overlooked, as these solids

are highly putrescible and therefore need proper disposal.

e)Nitrogen and Phosphorous: The principal nitrogen compounds in domestic sewageare proteins, amines, amino acids, and urea. Ammonia nitrogen in sewage results

from the bacterial decomposition of these organic constituents. Nitrogen being an

essential component of biological protoplasm, its concentration is important for

proper functioning of biological treatment systems and disposal on land.

Phosphorus is contributing to domestic sewage from food residues containing

phosphorus and their breakdown products. The use of increased quantities of

synthetic detergents adds substantially to the phosphorus content of sewage.

Phosphorus is also an essential nutrient for the biological processes.

f) Chlorides: Concentration of chlorides in sewage is greater than the normalchloride content of water supply. The chloride concentration in excess than the

water supplied can be used as an index of the strength of the sewage. The daily

contribution of chlorides averages to about 8 gm per person. Based on an average

sewage flow of 150 LPCD, this would result in the chloride content of sewage

being 50 mg/L higher than that of the water supplied.

g)Organic material: Organic compound present in sewage are of particular interestfor sanitary engineering. A large variety of microorganisms (that may be present

in the sewage or in the receiving water body) interact with the organic material by

using it as an energy or material source. The utilization of the organic material by

microorganisms is called metabolism. The conversion of organic material by

microorganism to obtain energy is called catabolism and the incorporation of

organic material in the cellular material is called anabolism.

h)Toxic metals and compounds: Some heavy metals and compounds such aschromium, copper, cyanide, which are toxic may find their way into municipal


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sewage through industrial discharges. The concentration of these compounds is

important if the sewage is to treat by biological treatment methods or disposed off

in stream or on land. In general these compounds are within toxic limits in

sanitary sewage. However, with receipt of industrial discharges they may cross the

limits in municipal wastewaters.

3. Biological characteristicsa) Bacteria: Sewage consists of vast quantities of bacteria, most of which are

harmless to man. However, typhoid, dysentery, and other intestinal disorders

causing pathogenic (disease-causing) organisms may be present in wastewater.

Bacteria can also be classified according to their dissolved oxygen requirement.

Aerobic bacteria are bacteria that require dissolved oxygen to live. Anaerobic

bacteria cannot live if dissolved oxygen is present. Facultative bacteria can live

with or without dissolved oxygen.

b) Viruses: Wastewater often contains viruses that may produce diseases. Outbreaksof infectious hepatitis have been traced through water systems because of

wastewater entering the supply.

c) Parasites: There are also many species of parasites carried by wastewater.The life cycle of each is peculiar to the given parasite. Some are dangerous to manand livestock, particularly during certain stages of the life cycle. Amoebic

dysentery is a common disease caused by amoebic parasites.

Stages of purification: The wastewater treatment is a broad term that applies to any process/

operation or combination of processes and operations that can reduce the objectionable

properties of water carried waste and render it less dangerous with the following:

(i) Removal of suspended and floatable material.(ii)Treatment of biodegradable organics.(iii)Elimination of pathogenic organisms.

A typical sewage treatment process is shown in figure. The different stages are explained in

brief below.


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Fig.4.3: Sewage treatment process

Preliminary Treatment: As the wastewater enters the plant it passes by a Protective Coarse

Bar Screen and large solid items are caught before the water enters the Grit Settling Tanks.

The grit settling tanks allow sand, gravel and other heavy materials to settle out of the water.

The solid materials caught at this stage are sent to a sanitary landfill for disposal. At the

comminution stage remaining solid materials are mechanically broken down into smaller

particles prior to entering primary treatment.

Primary Settling: The screened wastewater flows into large tanks known as primary settling

tanks, where it is detained for several hours, allowing more solids to settle to the bottom of

the tanks. Large paddles skim off the oil, grease and scum. The settled sludge and the scum

are collected and pumped to large digestion tanks for further treatment.

Aeration: The wastewater then flows by gravity to aeration tanks where it is mixed with

"activated sludge" containing aerobic bacteria (bacteria which uses oxygen). Air is pumped

into the tanks to aid the bacteria which feed on the organic material remaining in the

wastewater. The water remains in the aeration tank for several hours.

Final Settling: The "activated sludge" settles out of the wastewater in final settling tanks and

is either returned to the aeration tanks to replenish the aerobic bacteria or is sent to anaerobic

digestion tanks for further treatment.

Chlorination: An application of chlorine kills harmful microorganisms before the cleaned

wastewater, now called final effluent, flows back into the lake. Ultra-violet disinfection is

used in place of chlorination at several plants.

unit 4 - water supply

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