Water Supply Book 4-5-6-7-8-FUE GOOD

7/28/2019 Water Supply Book 4-5-6-7-8-FUE GOOD

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Table of Contents

4. Water Purification Works.................................................................................................. 3

4-1 The Purpose of Water Purification ............................................................................................ 3

4-2 Methods of Water Purification ................................................................................................. 3

4-2-1 Plain sedimentation ......................................................................................................... 3

4-2-2 Coagulation ................................................................................................................... 4

4-2-3 Filtration ....................................................................................................................... 4

4-2-4 Disinfection.................................................................................................................... 4

5. Sedimentation ....................................................................................................................7

5-1 Introduction ........................................................................................................................ 7

5-2 Settling Velocity of Particles ................................................................................................... 8

5-3 Sedimentation Behavior......................................................................................................... 9

5-3-1 Fill and Draw Type Sedimentation Basin................................................................................ 9

5-4 Factors Affecting Sedimentation .............................................................................................11

5-5 Methods Of Cleaning Settling Tanks ........................................................................................12

6. Coagulation and Flocculation .........................................................................................16

6-1 Introduction .......................................................................................................................16

6-2 Colloidal Behavior...............................................................................................................17

6-3 Mechanism of Coagulation ....................................................................................................19

6-4 Coagulants19

6-4-1 Aluminum Sulphate (Alum) ...............................................................................................20

6-4-2 Others .........................................................................................................................20

6-4-3 Chemical Dosing Plant.....................................................................................................22

6-5 Mixing and Flocculation........................................................................................................22

6-5-1 Mixing.........................................................................................................................

23

6-5-2 Flocculation ..................................................................................................................24

6-6 Clariflocculator...................................................................................................................25

7. Filtration ............................................................................................................................27

7-1 Introduction .......................................................................................................................27

7-2 Theory of Filtration ..............................................................................................................27

7-3 Factors Affecting Filtration ....................................................................................................29

7-4 Slow Sand Filters (S.S.F.) ......................................................................................................29

7-4-1 The Advantages of S.S.F..................................................................................................30

7-4-2 The Disadvantages of S.S.F. as Compared with R.S.F...............................................................30


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7-4-3 Operation of S.S.F...........................................................................................................30

7-4-4 Quality of Filtered Water from S.S.F...................................................................................32

7-5 Rapid Sand Filters Gravity Type (R.S.F)................................................................................

32

7-5-1 Operation of RS.F...........................................................................................................32

7-5-2 Washing of R.S.F............................................................................................................32

7-5-3 Quality of Filtered Water..................................................................................................33

8. Water Disinfection ............................................................................................................35

8-1 The Purpose of Water Disinfection ..........................................................................................35

8-2 Chlorination .......................................................................................................................35

8-2-1 Chlorine Gas..................................................................................................................36

8-2-2 Bleaching Powder...........................................................................................................

37

8-2-3 Hypochiorites (M.T.H.).....................................................................................................37

8-3 Ozonization........................................................................................................................38

8-4 Ultra-Violet Rays:................................................................................................................38

8-5 Other Sterilizing Agents ........................................................................................................39


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4. Water Purification Works

t is obvious that none of the natural waters so available are absolutely fit for

potable use. It is therefore necessary to subject any type of water to certain

suitable processes of purification rendering it safe for human consumption,

pleasing to the senses and suitable for ordinary domestic and industrial uses.

4-1 The Purpose of Water Purification

a- To improve the physical characteristics of water by removing turbidity, color,odor and taste in order to render it more appealing and potable.

b- To destroy any contained bacteria, especially pathogenic bacteria to make thewater safe for use from the hygienic point of view.

c- Occasionally the chemical composition of water has to be changed such as:- Removal of hardness.- Removal of iron and manganese salts.- Removal of excessive amounts of gases or salts soluble in water.

4-2 Methods of Water Purification

The method of purification necessary depends on the quality of water and the

purpose for which the supply is to be used. Relationship between particle size,

classification of impurities in natural waters and treatment unit processes are

presented in Figs. (4-1 and 4-2). In general there are nine well-known processes

for water purification.

4-2-1 Plain sedimentation

Or presetting when raw water is loaded by high suspended matter content.

Chapter

4I


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4-2-2 Coagulation

To remove suspended particles and colloidal color. Some other components are

also removed via their adsorption on coagulant products or via chemical reaction,

e.g. removal of phosphate.

4-2-3 Filtration

a- Through sand beds to remove suspended matter, bacteria which escaped theprevious steps.

b- Through carbon beds to remove refractory organics, which escape the precedingsteps, via adsorption mechanisms.

4-2-4 Disinfection

To destroy remaining bacteria and viruses and to give safeguard against probable

contamination introduced along distribution systems.


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Fig.

(4-1)flowdiagrami

nslo

wsandfiltertreatmentplantforsurfacewater


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Fig.

(4-2)flowdiagrami

nslow

sandfiltertreatmentplantforsurfacewater


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5-2 Settling Velocity of Particles

When a granular particle is left alone in a liquid at rest, it is subjected to a motion

force (W), and to a resistant force (R), the fluid drag which is the resultant of the

viscous and inertial forces as shown in fig. (5-1).

Fig. (5-1) Settling Velocity of Particles

Now to find value of (R) there are two ways:

a-

(R) must be equal to the weight in liquid of particle, because particle accelerateuntil resistance equals its weight in liquid, and then the motion becomes uni form.

Accordingly:

3

6

1)( dgffR s (8-1)

b- The resistance to the motion of the particle or the drag (a) is a function of thediameter of the particle, its settling velocity, the density and the viscosity of the

fluid.

resistanceoftcoeficcienKN)N(

.nos'ynoldReNVdfbut

VdffdVR

:analysisldimensionaBy

f)V,(d,R

2-n

rr

r

22

Where n = state of flow factor


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2n

22 VdfKfdVR

(8-2)

Equating (1) and (2)

2n

s

2

VdfK

f

)f(f

V

dg

6

1

Solving for:

21

)(

K

g

6

1 3

nn

ffdV snn

(8-3)

Which gives the general equation of velocity of a settling particle in a fluid.

5-3 Sedimentation Behavior

Sedimentation behavior performed in settling basins is accomplished in variety

two techniques as follows:

5-3-1 Fill and Draw Type Sedimentation BasinWhere water is kept standing for few hours, then drawn out the tank is then

cleaned refilled. This type is not in use any more due to its intermittent supply,

and being neither economical nor efficient.

It is assumed that the sediment is uniformly dispersed in the water at the

beginning of the settling period and that each particle settles individually

maintaining its shape and size without interference. Continuous Flow Type

Sedimentation Basin

To develop the theory of what is termed an Ideal Rectangular Basin more

assumptions are to be added to the previous ones:

The direction of flow is horizontal and the velocity is the same in all parts of the

basin.

- The concentration of particles of each size is the same in the vertical planeperpendicular to the direction of flow at the basin inlet.

- A particle is removed from the settling zone when it strikes the bottom.


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The settling path of a particle is determined by the vector sum of its settling

velocity and the velocity of the liquid. Hence, in an ideal basin (Fig. 5-2), particles

having settling velocities equal to or greater than Vo will settle out. In other words

to being equal to or less than t, the removal ratio =V/L equals unity (values more

than unity have no meaning).

Fig. (5-2)

Continuous-flow type, in which water flows through the tank continuously at a

low velocity. This type is in general use now and may be circular or rectangular in

plan and mechanically cleaned.

AreaDepth Relation of Settling Basin

If A = Surface area of settling basin it m2

C = Capacity of settling basin in m3

Q = Rate of flow in m3/h

V = Settling velocity of particle in m/h

D = Depth of settling basin in m

In actual practice, the particles in most of the suspensions dealt with in watertreatment plants flocculate during settling. Thus the assumption of individual

particles, settling interference is not valid, and accordingly the removal of

suspension is not completely independent of the depth and is influenced by both

overflow rate and retention period. Also in actual rectangular basin the velocity is

not uniform over the cross-section. Because of the drag on the walls and floor, the

velocity at these boundaries is zero and is greater than average at some points

away from the boundaries. Moreover, the velocity distribution over the cross-

section of most settling basins is not stable, owing to the disturbing influences of

water of varying density due to differences in temperature and in concentration of


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settleable solids throughout the body of liquid. This results in vertical movement

of water masses, dead zones and reversals in the direction of flow. The average

time of passage of a volume of water through the basin will be less than retention

period, thus reducing the efficiency of settling, in. the basin. This behavior is

termed short-circuiting, which is mainly caused by high velocities in the inlets and

baffing arrangements that may creats dead spaces and enable the flow to go

through faster anticipated. If there are dead spaces in a tank, in which the liquid

plays little or no part in the displacement through the tank, then the effective tank

volume would be less than the true volume. Consequently, the effective retention

period will be less than the true retention period.

The following should be followed during the design and construction of these

tanks:

a- The walls and floor should be completely impermeable.b- The side walls should be vertical and not sloping.c- Inlet and outlet arrangements should be made to minimize the disturbance of

solids already settled and to prevent short circuits.

d- A special arrangement should be made for cleaning the tank. When the time ofcleaning is due, it is emptied and the sludge either pumped or manually removed

by workmen. The walls should be cleaned before the tank is reused.

e- Depth of tank is never less than 2.5 ms usually 3 - 5 ms.f- Number of units should be such that 1 or 2 tanks are cleaned or repaired without

reducing the capacity of the water filtration plant.

5-4 Factors Affecting Sedimentation

a- Retention period: The removal ratio in a sedimentation tank in creases in adiminishing rate as retention period increases. In practice retention periods of 3 - 5

hours are in common use.

b- Velocity of flow through tank: The velocity of flow should not exceed three timesthe settling velocity of the smallest particles to be removed. In any case, the

velocity should not exceed 0.3 ms per minute.

c- The flow in the tank should be laminar avoiding any turbulance, short circuiting


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or dead zones.

d- Size of settling particles: The velocity of settling increases as the size of theparticle increases.

e- Specific weight of the particles of turbidity, increasing specific weight, theremoval ratio is increased.

f- Specific weight of liquid medium, since as specific weight decreased, the removalratio is increased.

g- Viscosity of liquid medium which affected directly by temperature and it has thesame effect as the specific weight of liquid.

h- Shape of the particles of turbidity.i- Concentration of the particles of turbidity, this factor is determined according to

the nature of suspended particles and the charge on it.

j- The surface area of the settling basin and the ratio of the wide to the length of thetank.

It was proved by Hazen theoretically that the efficiency of sedimentation depends

on surface area, settling velocity of particles:

Q

VLBE (8-9)

Where:

E is the efficiency of sedimentation tank

B is the breadth of the basin

L is the length of the basin

V is the settling velocity of the particles of turbidity

Q is the discharge flowing through the basin.

5-5 Methods Of Cleaning Settling Tanks

Settling tanks are either cleaned mechanically or manually. Mechanically cleaned

primary settling tanks as a rule, designed for a detention period ranging from two

to four hours. Manually cleaned tanks may require longer detention period.


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Sludge removal by hand, requires tanks with a gently sloping bottom in the tank

until a layer of sludge has been accumulated. In a period of time varying from few

weeks to several months the tank must be cut out of services, the liquid drawn,

and the sludge pushed to pumps. This type of tank is becoming obsolete since

they have the following disadvantages:

- Each successive day the capacity of the basin decreases, due to the accumulatedsludge, thus cutting down its detention period and hence its efficiency.

- Periodic shut down reduces the time of operation.- Cleaning process are messy.- The presence of sludge in the tank for a long period of time may produce odor and

taste in the water.

Sludge removal from tanks with one or a series of hopper bottoms is

accomplished by hydrostatic pressure while in operation. These tanks may be

squared, or round (for radial flow) or rectangular (for horizontal flow).

Modern settling tanks have scrapers or ploughs for transporting sludge along the

bottom in sump. The scrapers and ploughs are attached to rotating arms, traveling

bridges or endless chains. The tanks may be circular, square or rectangular in

plan. Wide rectangular tanks with sludge hoppers at the inlet of the tanks are

provided with cross collectors to push the sludge into sumps, as shown in fig. (5-

3).

Sedimentation may be plain without using chemical coagulants in case of large

suspended particles requiring a reasonable detention time for its settling, also if

the concentration of particles is high enough to allow flocculation of particles to

the suitable size, or sedimentation proceeded by chemical coagulation where the

suspended particles are finely divided or in the colloidal from and its

concentration is low requiring chemical process to achieve the agglomeration of

particles together in order to form flocks of suitable size for sedimentation.


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Fig. (5-3) Rectangular sedimentation tank


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Fig. (5-4) Rectangular Sedimentation Tank


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6. Coagulation and Flocculation

6-1 Introduction

he suspended solids in water range in size from coarse material, which settlereadily, to very fine material, which will not settle unless the particles coalesce

naturally and precipitate or un less a coagulant is used. The precipitatory solids

formed by coagulation are finely divided unless they are agglomerated into larger

solids or well-developed flock by agitation of the water to cause the fine solids to

contact and adhere to one another and form progressively larger particles. These

larger particles of flocK will then settle in sedimentation basins or will be

removed by filtration. Flocculation, therefore, follows treatment of water by

coagulants and is essential for the preparation of the water for sedimentation and

filtration at economically high rates of flow through rapid sand filters. The

addition of a coagulant may serve also to aid in the removal of color, odor, and

taste from water. The chemicals ordinarily used as coagulants, when properly

applied, are harmless to the consumer of the water. To understanding of the

coagulation and flocculation processes requires a distinction between successive

steps in the process. First a coagulating chemical is applied to the water. In order

that the chemical may react uniformly it must be distributed promptly through out

the body of water. This requires rapid agitation or mixing of the water at the point

where the coagulant is added. Second, complex chemical and physicochemical

reactions and changes occur, leading to coagulation and the formation of

microscopic particles. Third, much more gentle agitation of the water causes the

agglomeration of the particles, in other words, the fine particles are flocculated

into settliable flock. The coagulation process are influenced by the character and

quality of the water, the type and dose of coagulant, the water temperature, the

period of time and the degree of agitation. Those factors which are of chief

Chapter

6

T


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importance and which are subject to control and supervision by an operator need

to be clearly understood to secure effective results. Most of coagulants are salts of

iron or aluminum and on mixing with water they act by a process of double

decomposition involving the mutual interchange of groups, the end product being

hydroxides in the form of gelatinous precipitates (the floc). In soft waters where

there is insufficient alkalinity to react with the coagulants it has to be added either

as lime or soda-ash. This serves to neutralize the sulphuric acid which forms,

together with hydroxide, when sulphates hydrolyse. If left in the water the acid

which would recombine with the hydroxide and revert to sulphate. Hydroxide is

the desired end product. It is insoluble, flock-forming and heavier than water, andit carries the positive electric charge necessary to neutralize the negative charge of

the colloidal particles. By producing a heavier, faster-settling flock, this allows

smaller basins to be used, and smaller doses of the main coagulants may also be

possible. The choice of the best coagulant for any particles water is determined by

experiment.

6-2 Colloidal Behavior

The effective application of coagulation requires an under standing of the

properties and behavior of colloids. The term (Colloid) is used to describe a

system in which particles of relatively small size (the disperse phase) are

dispersed in a homogenous medium (the disperse medium).

Colloidal particles are larger than atoms and small molecules but are small enough

to pass through the pores of ordinary filters. Arbitrarily, those particles ranging in

size from 1 mm (10-6mm) to 1 u (10-3 mm) are classified as colloidal. However,

particles of even larger size do exhibit certain colloidal properties.

Several different types of colloid systems are possible. The following table lists

known types with examples of each. Sols, emulsions, and aerosols are of

particular interest to sanitary engineers. Although the following discussion deals

specifically with the properties of solids dispersed in liquids (sols), some of the

properties discussed apply to emulsions and aerosols as well:


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Table (6-1) Types of ColloidDisperse Systems

Disperse medium Disperse phase Name Example

Liquid Solid Sol Clay turbidity in water

Liquid Liquid Emulsion Oil in water

Liquid Gas Foam Whipped cream

Gas Solid Aerosol Dust, smoke

Gas Liquid Aerosol Fog

Solid Solid ------ Colored glass

Colloids present in waste water can be either hydrophobic or hydrophilic. The

hydrophobic colloids (clay, etc) possess no affinity for the liquid medium and lack

stability in the presence of electrolytes. They are readily susceptible to

coagulation. Hydrophilic colloids, such as proteins, exhibit a marked affinity for

water. The absorbed water retards flocculation and frequently requires special

treatment to achieve effective coagulation.

The affinity that hydrophilic particles posses for water results from the presence

of certain polar groups such as -OH, COCH, and NH2 on the surfaces of the

particles. These groups are water soluble and, as such, attract and hold a sheath of

water firmly around the particle, The water envelope surrounding a hydrophilic

particle is referred to as the water of hydration, or bound water, a term reflecting

the magnitude of the bond between the water and the polar groups. The following

fig. is a schematic sketch of a protein particle of colloidal size showing the

particle encased in bound water. Such a particle moves as a unit, the translation

behavior of the particle bound water complex being the same as that of a discrete,

homogeneous particle.

Hydrophobic colloid particles do not have an affinity for water; thus, they are not

encased in bond water. In general, organic colloids are hydrophilic, whereas

inorganic colloids are hydrophobic.


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6-3 Mechanism of Coagulation

A colloid system is said to be stable if the colloid condition is more or less

permanent. Since the interest in water and waste treatment centers about the

separation of the disperse phase from the disperse medium, colloid stability is of

considerable importance to the sanitary engineer.

The repulsion forces in a hydrophobic system are furnished by the zeta potential.

The stability of hydrophilic depends not only upon the zeta potential, but also

upon the bound water that envelops the colloid particle. Bound water behaves as

an elastic barrier to keep the particles from coming together.

The objective in a separation process is the reduction of the repulsion forces to the

extent that the attraction forces prevail and the particles coalesce to form larger

ones that are more easily separated from the system by gravity.

The zeta potential of colloid particles can be reduced by adjusting the pH of the

system toward the isoelectric point. At the isoelectric point, the primary charge is

zero and no double layer exists to produce a zeta potential.

The zeta potential can also be reduced by adding ions or colloids of opposite

charge to the colloid system. Precipitation with colloid commonly referred to as

mutual coagulation, appears to be a reaction similar to the formation of insoluble

precipitates through a combination of solution ions. The addition of counterions

serves to increase the concentration of counterions in both the fixed and diffuse

layers. As a result, the zeta potential is reduced.

6-4 Coagulants

Coagulants may be defined as those substances which are capable of removing

colloidal impurities from water. The commonest coagulants and coagulants aids

used are aluminum sulphate (alum), ferrous sulphate, ferric chloride, ferric-

sulphate, lime and Nalco (non-ionic polymer), chlorinated copperas, sodium

aluminates, ammonia alum. These compounds are harmless to the consumer

since they do not remain in the water. As seen by the following equations, the

chemical reaction results in a precipitate of insoluble compounds which can be

removed by settling and filtration.


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6-4-1 Aluminum Sulphate (Alum)

The material is an acid salt and hence corrosive to most metals. It is readily

soluble in water and is easily applied as a solution or as dry material. Reactions

between alum arid the natural constituents of various waters are influenced by

many factors, so it is impossible to determine accurately the amount of alum that

will react with a given amount of natural alkalinity or of lime or soda ash added to

the water. The addition of these materials are required with alum for the formation

of aluminum hydroxide floc where the alkalinity of the treated water is not

changed, that is, water treated with 1.0 p.p.m. alum arid either 0.35 p.p.m.

hydrated lime or 0.48 p.p.m soda ash will have approximately the same alkalinity

as the raw water. If no alkali is added, then the acidity of 1.0 p.p.m. alum will

lower the natural alkalinity of the raw water by about

0.45 p.p.m. The alkali required for corrosion prevention would be added to the

filtered water. Many waters have bicarbonate alkalinity naturally in them, in

which case. The chemical reaction is as follows:

Al2(SO4)3 .18H2O + 3Ca(HCO3)2 2 Al (OH)3 + 3CaSO4 + 18H2O + 6CO2 (9-1)

The aluminum hydroxide is the floc AL (OH)3. It should be rioted that all the

temporary or carbonate hardness caused by calcium bicarbonate is reduced, the

permanent or non carbonate hard ness caused by calcium sulfate is increased.

Some waters have insufficient natural alkalinity to react with alum. In these cases

lime is generally added. The lime Ca unites with water to form calcium

hydroxide, Ca (OH) which reacts with the alum as follows:

Al2(SO4)3 .18H2O + 3 Ca(OH)2 2 AL (OH)3 + 3 CaSO4 + 8H2O (9-2)

6-4-2 Others

Ferrous Sulphate

Ferrous sulfate is a frequently used coagulant in water purification. It has an

advantage over alum in that it may be less expensive and the floc is heavier andsinks more rapidly. An out standing disadvantage is the need for using lime with


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it. More complete chemical control is required, and there are greater dangers from

after precipitation in the distribution system due to the re action between surplus

lime and bicarbonate alkalinity. Ferrous sulphate is unsuitable for the treatment of

soft, colored waters because they are best coagulated at a pH below 7.0. The color

appears to become set by the addition of alkali to colored waters and hence the

use of ferrous sulphate is limited to those waters in which alkalinity will not

interfere with color removal. It is best suited to use in turbid waters of high natural

alkalinity. There is usually insufficient alkalinity in natural waters to react with

ferrous sulphate, so that lime must usually be added to produce a floe and in order

to avoid soluble compounds of iron remaining in the treated water. The chemicalreactions that occur when ferrous sulphate and lime are used in coagulation

depend, in part, on the order in which the chemicals are added to the water:

FeSO4 + Ca(OH)2 Fe(OH)2 + CaSO4 (9-3)

4Fe(OH)2 + 2H2O + O2 4Fe(OH)3 (9-4)

The ferrous hydroxide, Fe (OH)2, forms a desirable, heavy gelatinous precipitate.

Ferric Chloride

Ferric chloride, FeCl3 ferric sulphate, Fe(SO4)3 and a mixture of the two, known

as chlorinated copperas , have been successfully used in a number of water

treatment plants. Among the advantages claimed for the USC of ferric coagulants

in water purification may be included

I. Coagulation is effective over a wider range of pH than with alum.II. he time required for floe formation, conditioning, and settling is, in many cases,

considerably shorter than that required for alum

III. Manganese is successfully removed at pH values above 9.0.IV. Hydrogen sulphide is removed, and taste and odors are reduced.V. Under some cc the ferric coagulants are more economical than aluminum dioxide.

When ferric chloride is added to water, the following reaction occurs:

FeCl3 + 3H2O Fe(OH)3 + 3H+ + 3 Cl-


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The ferric hydroxide is precipitated, forming a desirable coagulant, which is

heavier than aluminum hydroxide and requires a shorter retention period and less

careful adjustment of pH. Acidities that are encountered with some soft, highly

colored waters make their coagulation with alum impossible, but they may be

successfully coagulated with ferric compounds.

Chlorinated Coppers

Chlorinated copperas is a mixture of ferric chloride and ferric sulphate prepared

by adding chlorine to a solution of ferrous sulphate in the ratio of 1 part of

chlorine to 7.8 parts of copperas. Among the advantages claimed for the use of

chlorinated copperas as a coagulant are:

I. A desirable floc formation with tough particles of floc resistant to breaking up.II. Floc formation usually settles well with only a small residual going to the filters.

III. The coagulating effect has a wide range of optimum pH from 8 or 9 to 6.IV. A compact floc of hydrated ferric oxide, which does not dissolve in alkaline waters, is

formed at all pH values above 3.5, and,

V. The coagulant is particularly effective in color removal com pared with the relativeineffectiveness of ferric and ferrous hydroxide on colloids having an isoelectric point

below 7.0. Chlorinated copperas has not been widely used in practice so that

experience with it is restricted.

6-4-3 Chemical Dosing Plant

Coagulants can be added to the water either as a solution, which is much the

commonest way - solution feed - or in powder form - dry feed.

6-5 Mixing and Flocculation

When coagulants are added to water and thoroughly mixed, the reaction is almost

instantaneous. As soon as f] forms, a further gentle stirring is advantageous in

order that the floe particles may coalesce and grow bigger. There is a similarity

between the two actions in so far as the water is stirred. However, the first act ion,

preceding floc formation, must be violent, the second, following floe for motion,

should be gentle. Both these actions occur naturally to some extent in any basin. If

coagulants are fed into an inlet pipe or channel, some mixing is bound to take


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place due to turbulence. When the dosed water carrying floc finally passes into the

settling basin through inlet parts a certain rolling motion is inevitable, which can

be accentuated by baffles in a horizontal-flow basin. There are two methods of

approach, the mixing and flocculation can be carried out either by mechanical

means in specially built chambers, or in a suitably baffled channel or

interconnected chambers. The latter method requires no mechanical equipment

but lacks flexibility be cause the system can be designed for maximum efficiency

only at one rate of flow at one temperature, whereas the speed of mechanical

paddles can be adjusted to suit the variations of flow, temperature and silt

conditions. However, the cost and added complexity of mechanical equipmentintroduce additional complications, to be avoided in a developing country, and in

practice a sinuous inlet channel preceded by violent mixing generally provides a

reasonably effective solution.

6-5-1 Mixing

Rapid mixing to distribute the coagulant throughout the water being created is

frequently called flash mixing. This rapid agitation may be provided in special

basins as shown in Fig. (6-1) with capacities equivalent to about 20 - 60 seconds

of flow, in which small propellers are driven by electric motors. Sometimes the

hydraulic jump, or standing wave, is used for flash mixing, being provided by a

channel with sloping and widening sections. The coagulant is added just before

the water flows down the channel at high velocity - velocity >1m/sec - to enter a

level portion of the channel, where the energy of rapid flow is suddenly

transformed into static head of deeper water, turbulence being produced at the

wave front of the deeper water. In other instances, turbulence is provided by

aerators, weirs, or spiral flow tanks, but flow in channels used to conduct the

co3gulant treated water to flocculation basins is not sufficiently turbulent for flash

mixing unless obstructions are placed in the channels below the point where t

chemical is applied. Efficient low lift centrifugal pumps do not provide turbulent

flow and thus do not serve as flash mixers. If the mixing chambers are used,

benefit can be obtained by having more than one and making the water pass

through them all. Where this is done, inadequate mixing and short circuiting will

be eliminated.


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Fig. (6-1) Flash mixer

6-5-2 Flocculation

Flocculation basins are of various types:

The first type is the baffled mixing basins, which also there in two types the first

is the basins were fitted with a series of baffles around the end of which the

flowing water was reversed in direction, thus causing more gentle turbulence in

the channels formed between the baffles, but more violent agitation at each point

of reversed flow. This type shown in Fig. (6-2).


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Fig. (6-2) Horizontal baffle basin

6-6 Clariflocculator

After the flash mixing and flocculation processes the water passes to the basin

which called clarifier where the flocs settle to the bottom of the basin.

The clarifiers no differ from plain sedimentation basins the factors affecting in

sedimentation and the method of design no differ from plain sedimentation basins.

There are many types of clarifiers which has the same main basic for design and

differ only in some details. The clariflocculator basin consists of two tanks one

within the other as shown in Fig (6-3). The raw water enters the inner tank which

acts as a flocculated chamber. In this chamber there is a rotating paddle for

flocculation. Then the water flows radially to the outlet weir and the flocs settle to

the bottom of the outer tank which acts as a sedimentation chamber.


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Fig. (6-3) Clariflocculator


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7. Filtration

7-1 Introduction

n the very earliest filter installation a mass of sand was used as the filtering

material, and this was done by copying nature where infiltering water is purified

by passing through the ground. It was soon found that sand was, indeed the most

suitable medium for the filtration of water.

Screening and sedimentation-with or without coagulant-removes a large

proportion of suspended solids and colloidal matter while fine flocs particles,

bacteria and other colloidal may still be present in settled water. Filtration must be

used to remove these undesirable impurities from water. Basically, filtration

involves passing of water through a porous such as sand, which in effect, strains

out most of the suspended particles found in it.

7-2 Theory of Filtration

The substances suspended in water and which are there either naturally or as a

result of previous flocculation treatment, are most frequently of a gelatinous or

sticky nature.

If an attempt were made to eliminate these substances by filtration through a bed

of very slight porosity the filter would be found to clog very quickly and would no

longer allow the water to pass because of the formation of impermeable layer on

the bed surface caused by the accumulation of the impurities retained. Fig. (7-1).

Fig. (7-1) Formation the dirty skin film

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On the other hand, when filtering is effected through a mass of sand, the

impurities penetrate the bed to a greater or lesser extent. Fig. (7-2).

Fig (7-2) The Impurities Penetrate The Sand Bed

At the start, when the filter is clean, these impurities are naturally retained by the

layers which first come into contact with the water to be filtered but as soon as

these layers begin to clog the bed, the resulting head loss causes these impurities

to penetrate through the minute channels formed by the interstices between the

grains of sand, and they lodge, in practice, in spaces formed between the surfaces

of the grains. Fig. (7-3).

Fig. (7-3) Clogging The Sand Bed

It has been found by experience that by passing water through sand, suspended

and colloidal matter are partially removed, the chemical characteristics of the

water are changed, and the quantity of bacteria is materially reduced. These

phenomena are explained on the basis of the following actions:

a- Mechanical Straining which removes the particles of suspended matter that aretoo large to pass through the pores of sand grains.

b- Sedimentation Action account for the removal of colloids, small particles of


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suspended matter and bacteria. The interstices between the sand grains act as

minute sedimentation basins in which the suspended particles settle upon the sides

of sand grains.

c- Adsorption Action These particles form a gelatinous coating on the sand grains.This gelatinous mass attracts other particles and settles down more effectively.

d- Electrolytic Action a certain amount of suspended and dissolved matter in water isionized. Some of the particles of sand are also ionized in the filter and posses

electric charges of opposite polarity. When these are neutralized, the character of

water is changed. This action shares in the removal of some dissolved solids in

water like as iron and manganese.

e- Biological Action as a result of the gelatenous or stiky nature of impurities and theaggumerelated flocs, sand grains are coated with zooglind film which contains

living organisms. These organisms feed on the organic impurities and dissolved

salts in water and change them to stable state easy to be removed by washing.

This action accounts for the removal of iron and manganese also color, odor and

taste are removed by this action.

7-3 Factors Affecting Filtration

Filtration is some what complex, technique which involves a certain number of

factors each of which may have an effect on the others:

- Depth of the filter media.- The grain size of the sand.- The speed of filtration.- Depth of water over the filter media.- The maximum permissible head loss of the filter.- The process used to wash the sand.- The preparation of the water to be filtered.- Filtering material.

7-4 Slow Sand Filters (S.S.F.)These were the first type developed for water purification dating back to 1829 as


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shown in Fig. (7-4). They still exist nowadays in cities where installations were

made before the rapid sand filter was developed. They are not preferred nowadays

and rapid sand filters are replacing them but still they are economical and more

suited to warm climate where covers on the filter are not required to protect the

filters from freezing.

Slow filtration ensures the purification of surface waters without prior coagulation

or sedimentation. The colloidal matter is coagulated by the enzymes secreted by

algae and micro-organisms which are retained on the sand surface (dirty skin).

7-4-1 The Advantages of S.S.F.- There is no need for coagulation facilities.- Equipment is simple and need not be imported.- Suitable sand is readily secured.- Supervision is simple.- The effluent is less corrosive and more uniform in quality than chemically treated

water.

- They give effective bacterial removal- Amount of head consumed is little.

7-4-2 The Disadvantages of S.S.F. as Compared with R.S.F

- A large area is required, with correspondingly large structure and volume of sandand higher structural costs.

- They have less flexibility to change in load.- They are less effective in removing color.- They give poor results with waters of high turbidities. or high algal content unless

pretreatment is practiced.

7-4-3 Operation of S.S.F

In the operation of a new slow sand filter, it is filled with water to a depth of 1-1.5

ms above the surface of the sand. Water is filtered through the sand at a rate of 3-8

meter cube per square meter of filter surface per day. This rate is continued untilthe loss of head of water in filtering media is slightly less than the depth of water


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above the sand. Then the filter is thrown out of service and 3-5 cm of sand is

scraped from the top of the sand bed, and the bed is put back into service. After

cleaning the filtered w is first carried to w3stc until the schmutzdecke (dirty skin)

is formed. A normal period of operation between cleanings is 2-3 months or

longer. The cleaning of slow sand filter takes the filter out of service for 2-3

weeks.

Fig. (7-4) Slow Sand Filter

If the water supplied to slow sand filters contains relatively large amount of fine

suspended solids, or if these filters are operated at excessive rates of filtration,


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subsurface clogging may result. This should be avoided in every way possible

because such clogging necessitates the removal of most, if not all, of the sand for

cleaning, in order to restore normal operation conditions.

7-4-4 Quality of Filtered Water from S.S.F

- Bacteria removal efficiency 99%- Turbidity Limitation The raw water from lake or rivers has turbidity up to 100

p.p.m. The slow sand filter cannot handle this load and even 50 p.p.m. turbidity

gives unsatisfactory results. The pre-sedimentation is the solution of the problem.

- Color Limitation The raw colored water over 30 p.p.m. cannot be treatedsatisfactorily.

7-5 Rapid Sand Filters Gravity Type (R.S.F)

Filters designed to operate at much higher rate than slow sand fi1ters are called

rapid sand filters or mechanical filters. The first rapid sand filter was

constructed at Somerville, N.J., in 1884. Since then the construction of rapid sand

filters has almost supplanted slow sand filter construction.

- A detailed drawing for the rapid sand filter is shown in Fig. (7-5).7-5-1 Operation of RS.F

During filtration, the influent and effluent valves are open and all other valves are

closed. The pre-coagulated and settled water is allowed to flow into the filter. This

water percolates through various layers of graded sand and gravel and through

under- drains. It is collected in one main pipe and then out through the effluent

valve to the clear water reservoir. The water level over the sand bed is controlled

by regulating the rate of inflow or by the use of float valve at the influent, while

the rate of outflow is automatically regulated with the help of rate controllers in

spite of the gradual increase in the loss of head. The rate of filtration in rapid sand

filters is from 100 -125 meter cube per meter square of filter surface per day.

7-5-2 Washing of R.S.F

Filters should be washed when the loss of head reaches (2.4-3.0 in). Longer filter

runs may be obtained when washing is delayed until the rate of filtration begins to

decrease and the rate controller is fully open. There are several systems for


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washing rapid sand filters like as:

a- Mechanically revolved rakes system.b- Air-wash systemc- High velocity wash system

Only the air wash system is discussed here. Referring to Fig. (10-9),

- Close the influent and effluent valves V1, V3.- Open valve V2 and V6 to allow the water to drain down to about 15 cm above the

sand. Then close valves V and V.

- Open valve V4 (compressed air valve) for 2-5 minutes so that to agitate the sandgrains and then close valve V4.

- Open valve V5 (wash water under pressure) gradually and also valve V6 for 10minutes to allow the wash water to flow through the filter and out through the

wash-water gutters to drain. Then close valves V4 and V6 The purpose of this step

is to clear the agitated sand grains.

- Open valves V1 and V2 for 20 minutes to allow some of the water to drain towaste. The purpose of this step is to form the gelatinous film again on the sand

grains.

- Then close valve V2 and open valve V3 The filter is back into service.Total period of washing is about 30 minutes and time between washings is from

12-24 hrs according to the quality of settled water. Washing procedure can be

done manually or automatically.

7-5-3 Quality of Filtered Water

- Bacteria removal efficiency 90 -99%- Turbidity removal The rapid sand filter is very efficient in handling a high turbid

water.

- Color Limitation It is quite efficient in color removal also.


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Fig. (7-5) Details of Rapid Sand Filter


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8. Water Disinfection

8-1 The Purpose of Water Disinfection

Water is disinfected to kill bacteria and thus prevent water-bourn diseases.

Chlorination, ozonization, ultra-violet ray method, excess line process and

application of silver or iodine and bromine method are the principal methods used

for disinfection of water. Sterilization by heating is suitable only for household

use.

8-2 Chlorination

The use of chlorine has become universal in the disinfection of water supplies.

Although chlorine is used principally for the killing of bacteria, it may be applied

to water also to aid the removal of iron and manganese, for the cleaning of sand

filter, for the sterilization of water mains, and for other purposes.

Chlorine may be applied to water in the form of a gas or in one of its compounds,

the dry gas under pressure is liquefied and stored in steel cylinders. Care must be

taken to prevent the escape of chlorine, because of its highly toxic nature, except

in high dilution.

Chlorine gas may be fed directly to the water supply, or preferably the gas may be

first dissolved in a small flow of water and the solution fed to the point of

application.

The amount of chlorine used depends on several factors:

a- Turbidity of the water which protects the bacteria from the action of chlorine.b- The temperature of the water.c- Amount of dissolved oxidisable organic matter in water.d- Amount of iron and saline ammonia in water, both retard action of chlorine.

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e- Contact time.f- Adequate mixing.g- The purpose for which water is used, and its quality.h- The concentration of chlorine added.

It is necessary to leave a residual chlorine in the water to guard against future

contamination. Usually 0.1 ppm is left except in epidemics when larger residual

amounts are left.

In Cairo 0.5 to 1.0 ppm is added to the water. The residual chlorine should be

between 0.1. to 0.3 ppm.

The following are the most important methods of chlorine application:

- Chlorine gas- Bleaching powder- Hypochlorites- Cchloraminc.

8-2-1 Chlorine Gas

Chlorine gas has been formed to have more economical effect in its application

than chlorine compounds.

Chlorine gas is supplied to water purification works in cylinders under so high

pressure that it is liquefied and it is applied to the water by means of a special

apparatus called chlorinator. In this chlorinator, chlorine gas is dissolved in

certain quantity of water so as to form a strong solution, then the solution is added

to the filtered water as it enters the filtered water storage tank.

The use of liquid chlorine has the following advantages over using chlorine compounds:

- No salt is added to the water, nor a residue is formed.- Once the chlorinator is adjusted, it works automatically giving the required dosage

day and night.

However, the chlorinator needs continuous supervision for fear of leakage of

chlorine gas or going out of order.


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8-2-2 Bleaching Powder

In the early days of chlorination of public water supplies chloride of lime or so

called bleaching powder is used as the chlorinating agent. It is a product made by

passing chlorine gas over dry slaked lime. It is usually contains 33-35 per cent

available chlorine. When using bleaching powder, the amount required is

calculated on the percentage of available chlorine in the powder. The powder is

then rendered in a paste which is gradually thinned by adding water to form an

emulsion 1:100 or 1:1000.

The emulsion is left to stand for one hour, then stand and added to the filtered

water by means of a dripping apparatus. It is now rarely used in public water

supplies.

8-2-3 Hypochiorites (M.T.H.)

Modern manufacturing processes have made it possible to prepare soluble

powdered chlorine compounds which have an available chlorine of 65-70 per

cent. A solution of these compounds usually applied to the water being treated

through special equipment called hypo-chiorinators.

8-2-3-1 Amount of Chlorine Dose

It is frequently impossible to predict accurately how much chlorine must be added

to accomplish satisfactory sterilization as some waters will absorb much more

chlorine than others. A residual chlorine of 0.1-0.2 ppm after U contact Period of

30 mm. represents a factor of safety and indicates that water had absorbed all its

needs.

The presence of organic matter, sulphides, iron manganese will increase the

absorbing power of water to chlorine. The amount of chlorine absorbed is called

chlorine-demand.

8-2-3-1 How Chlorine Kills Bacteria

The effect of chlorine on organisms in water has been a subject of considerable

controversy. However, there are three different main assumptions:

I. It is believed that nascent oxygen produced by the reaction of chlorine with wateraccomplished the destruction of bacteria. Chlorine will react with water to form


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hydrochiorous acid which being unstable breaks up into nascent oxygen and

hydrochloric acid.

H2O + Cl2 HCl +HO Cl

HO Cl HCl + O

II. It is believed that chlorine units at least in part with the cell structure of the organismto form chloroproducts which act as toxic poisons to these organisms.

III. In fact the bodies of some bacteria completely disappear after chlorination, a fact thatmay lead to the assumption that it has been converted to soluble compounds.

8-3 Ozonization

A molecule of ozone contains three atoms of oxygen. It is an unstable gas which

is very active in oxidizing the organic matter and in killing bacteria present in

water exposed to it. Advantages claimed for the use of ozone include:

a- No disinfecting chemical remains in the water.b- Odor, taste and color are removed.c- The process is not expensive.

However

a- The cost is greater than for chlorination.b- The apparatus required is complicated, and needs skillful control.c- The absence of residual chemical provides no safeguard against subsequent

contamination.

8-4 Ultra-Violet Rays:

The ultra-violet rays offer an effective method for sterilization of clear water. The

ultra-violet ray machine consists of a mercury-vapor lamp enclosed in a quarts

globe, in front of which water passes.

This method is characterized by:

a- The water should flow in a thin film close to the rays.


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b- The water should be well agitated, and the exposure to the ray should hecontinuous.

c- The effective penetration of the ray in clear and colorless water is about 30 cm..It is known that the various parts of the spectrum have different sterilization

powers. Thus, the infra red rays are devoid of any germicidal effect, while the

ultra-violet rays (3000 angstrom or less) have the highest effect. These rays kill

harmful bacteria more rapidly than the harmless ones. No taste, no odor is

imparted to the treated water. This method is used in hotels, swimming pools,

ships etc.

But the treatment is expensive, only applicable where electricity is cheap. Its

penetrating power is weak and obstructed by the suspended matter in water.

Furthermore, it needs skillful control.

8-5 Other Sterilizing Agents

Permanganate

It is used for small volumes of water, by Pinking for an hour.

Acid Sodium Sulphate:

This gives sulphuric acid to the water given in dose of O.2Z for 15 minutes.

Iodine

In the form of weak tincture, or as tablets of potassium iodide and iodate which

give iodine by action of tartaric acid. The taste is removed by thiosulphate.

Water Supply Book 4-5-6-7-8-FUE GOOD

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