Environmental Engineering-1 Unit 3

Environmental Engineering-1

Prepared by, K.Dhanabal, Lec/civil, PAAVAI ENG COLLEGE

Unit III : Water treatment In the previous second units we are discussed about how to plan

a water supply scheme and how to transport water from source

to treatment unit and treatment unit to supply area.

In the unit 3 & 4 we are going to learn about how to treat the

water in essential and advanced treatment methods..

UNIT III WATER TREATMENT

Objectives -Unit operations and processes -

Principles, functions design and drawing of Flash

mixers, flocculater, sedimentation tanks and sand

filters -Disinfection- Residue Management.

Objectives:The available raw waters must be treated and purified before they can

be supplied to the general public for their domestic, industrial or any

other uses. The extent of treatment required to be given to a particular

water depends upon the characteristics and quality of the available

water, and also upon the quality requirements for the intended use.

The available water must, therefore, be made safe, good in appearance,

and attractive to human taste and tongue. Various methods which are

used to make the water safe and attractive to the consumers are

described below. However, the method or the methods adopted for

purification depend mostly upon the character of the raw water.

Methods of Purification of Water The various methods or the techniques which may be

adopted for purifying the public water supplies are :

(i) Screening

(ii) Plain sedimentation

iii) Sedimentation aided with Coagulation

(iv) Filtration

(v) Disinfection

(vi) Aeration

{vii) Softening

(viii) Miscellaneous treatments, such as fluoridation, re

carbonation

liming, desalination, etc.

Summaries of techniques in purificationof raw water.

Most of the big and visible objects, such as trees, branches, sticks,

vegetation, fish, animal life, etc., present in raw waters of surface

sources can be removed by screening.

The coarser suspended materials can then be removed by letting the

water settle in sedimentation basins. The process is called plain

sedimentation.

The effectiveness of sedimentation may however, be increased by

mixing certain chemicals with the water, so as to form flocculent

precipitate, which carries the suspended particles as it settles. The

process is called sedimentation aided with chemical coagulation.


The finer particles in suspension, which may avoid settling

in sedimentation basins even after using chemical

coagulation, may then be removed by filtering the water

through filters. The process is called filtration

The filtered water which may still contain pathogenic

bacteria, is then made bacteria-proof by adding certain

chemicals such as chlorine, etc. This process of killing of

germs is called disinfection


The resulting water, though now becomes safe, yet may not

be attractive to the tongue of the consumers. Unpleasant

tastes and odours may then, therefore, have to be removed

by adding oxygen from the atmosphere This process is called

aeration.

The resulting water may sometimes be much harder than

permissible and may, therefore, have to be softened by a

process called softening


Sometimes, the resulting water may be given further

treatment, such as

fluoridation (i.e. the addition of soluble fluoride for

controlling dental caries),

liming (i.e. addition of lime in order to control acidity and

reduce corrosive action),

Re carbonation (i.e. addition of carbon dioxide so as to

prevent deposition of calcium carbonate scale),

De salination (i.e. removal of excess salt, if at all

present), etc. etc.

All the above techniques are now discussed in details.

SCREENING Coarse and Fine Screens

Screens are generally provided in front of the pumps or the intake works, so as to

exclude the large sized particles, such as debris, animals, trees, branches, bushes,

ice, etc. Coarse screens (generally called trash racks) are sometimes placed in front

of the fine screens.

Coarse screens consist of parallel iron rods placed vertically or at a slight slope at

about 2 to 10 cm c to c. The fine screens are made up of fine wire or perforated

metal with openings less than 1 cm wide.

The coarse screens first remove the bigger floating bodies and the organic solids ;

and the fine screens then remove the fine suspended solids. The fine screens

normally get clogged, and are to be cleaned frequently. The fine screens are,

therefore, avoided these days, and the finer particles are separated in “

Sedimentation“ rather than in "Screening".

SCREENING The coarse screens are also now normally kept inclined at about 45°—60° to the

horizontal, so as to increase the opening area to reduce the flow velocity, and thus,

making the screening more effective.

While designing the screens, clear openings should have sufficient total area, so

that the velocity through them is not more than 0.8 to 1 m/sec.

The material which is collected on the upstream side of the screens is removed

either manually or mechanically. In mechanically cleaned screens, a rake traverses

the front of the screen either continuously or intermittently. In mechanically cleaned

screens, the cross bars obstruct raking and should, therefore, be avoided as far as

possible. A fixed bar type screen is shown in Fig. Moveable bar type screens also do

exist and are useful in deep pits in front of pumps. A commonly used type of such a

screen consists of three sided cage with a bottom of perforated plates

Screening:

PLAIN SEDIMENTATION (Type I Settling)

Most of the suspended impurities present in water do have a specific

gravity greater than that of water (i.e. 1.0)*. In still water, these

impurities will, therefore, tend to settle down under gravity, although in

normal raw supplies, they remain in suspension, because of the

turbulence in water. Hence, as soon as the turbulence is retarded (slow)

by offering storage to the water, these impurities tend to settle down at

the bottom of the tank, offering such storage. This is the principle

behind sedimentation.

The basin in which the flow of the water is retarded is called the

settling tank or sedimentation tank or sedimentation basin or

clarifier, and the theoretical average time for which the water is

detained (arrested) in the tank is called the detention period

Theory of Sedimentation

settlement of a particle in water brought to rest, is opposed by the

following factors :

(j) The velocity of flow which carries the particle horizontally. The

greater the flow area, the lesser is the velocity, and hence more

easily the particle will settle down.

(ii) The viscosity of water in which the particle is travelling. The

viscosity varies inversely with temperature. However, the

temperature of water cannot be controlled to any appreciable extent

in "water purification processes“ and hence this factor is ignored.

(iii) The size shape and specific gravity of the particle. The

greater is the specific gravity, more readily the particle will settle.

The size and shape of the particle also affect the settling rate.

Stokes law:

Stokes law derivation basic concept:

Stokes law derivation:

Sedimentation Tanks: The clarification of water by the process of "sedimentation" can be

effected by providing conditions under which the suspended material

present in water can settle out. Storage reservoirs may also serve as

sedimentation basins, but they cannot effect proper sedimentation,

because of factors, such as, the density currents, the turbulences

caused by winds, etc.; and hence they cannot be relied upon. Special

basins are, therefore, constructed in order to purify the surface waters

of rivers.

But of the three forces, which control the settling tendencies of the

particles (enumerated earlier), the two forces, i.e., the velocity of

flow, and the shape and size of the particles, are tried to be controlled

in these settling tanks. The third force, i.e., the viscosity of water or

the temperature of water is left uncontrolled, as the same is not

practically possible.

Sedimentation design Details: The velocity of flow can be reduced by increasing the length of travel and by

detaining the particles for a longer time in the sedimentation basin. The size

and the shape of particles can be altered by addition of certain chemicals in

water.

Sedimentation basins are generally made of reinforced concrete, and may

be rectangular or circular in plan. Long narrow rectangular tanks with

horizontal flow are generally preferred to the circular tanks with horizontal

radial or spiral flow The capacity and other dimensions of the tank should

be properly designed, so as to effect a fairly high percentage of removal of

the suspended materials. A plain sedimentation tank under normal

conditions may remove as much as 70% of the suspended impurities

present in water.

Sedimentation Diagrams:

Types of Sedimentation Tanks The sedimentation tanks can basically be divided into two types .

(1) horizontal flout tanks ; and

(2) vertical or up flow tanks.

These tanks may be rectangular or circular in plan. Both these types of tanks are briefly

discussed here.

Horizontal flow tanks.

In the design of horizontal flow tanks, the aim is to achieve, as nearly as possible, the

ideal conditions of equal velocity at all points lying on each vertical line in the settling

zone.

Rectangular tanks with longitudinal flow, They may be provided with mechanical

scrapping devices, to scrap the sludge to the sludge pit located usually towards the

influent end, from where it is continuously or periodically removed, without stopping the

working of the tank.

Types of Sedimentation Tanks Circular tanks with radial flow, with central feed, such as the

one shown in Fig. In such a tank, the water enters at the center

of the tank into a circular well provided with multiple ports, from

which it comes out to flow radially outwards in all directions

equally. The water, thus, flows horizontally, and radially from the

center towards the periphery of the circular tank. The aim here

is to provide uniform radial flow with decreasing horizontal

velocity towards the periphery, from where the water is

withdrawn from the tank through the effluent structure (overflow

weir, etc.).

Types of Sedimentation Tanks

Vertical or Up flow settling tanks. Vertical flow

tanks usually combine sedimentation with flocculation,

although they may be used for plain sedimentation.

They may be square or circular in plan, and may have

hopper bottoms. (Refer Fig.). The influent enters at the

bottom of the unit. The up flow velocity decreases with

the increased cross-sectional area of the tank. The

clarified water is withdrawn through the

circumferential or central weir.

Vertical or up flow tank:

Sedimentation design Basic Concept:

Sedimentation Design problems:

SEDIMENTATION AIDED WITH COAGULATION

As pointed out earlier, very fine suspended mud particles and the

colloidal matter present in water cannot settle down in plain

sedimentation tank of ordinary detention period. They can,

however, be removed easily by increasing their size by changing

them into flocculated particles.

For this purpose, certain chemical compounds, called coagulants,

are added to the water, which on thorough mixing, form a

gelatinous precipitate called 'floe'. The very fine colloidal

particles present in water, get attracted and absorbed in these

floes, forming the bigger sized flocculated particles.

SEDIMENTATION AIDED WITH COAGULATION

The colloidal particles do, infect, possess surface charges resulting

from preferential adsorption or from ionization of chemical groups on

the surface. Most of the colloidal particles in water or waste water are

negatively charged. The stationary charged layer on the surface is

surrounded by a bound layer of water, as shown in Fig.

In this bound layer, called the stern layer, ions of opposite charge

drawn from the bulk Solution, produce a rapid drop in potential, called

the stern potential (n). A more gradual drop, called the zeeta

potential (Q ) occurs between the shear surface of the bound water

layer and the point of electro neutrality in the solution, as shown in

Fig..

Model for colloidal particle:

Chemicals Used for Coagulation

Various chemicals, such as alum ; iron salts like ferrous sulphate, ferric chloride, ferric sulphate ; etc., are generally used as coagulants. These chemicals are most effective when water is slightly alkaline.

In the absence of such an alkalinity in raw supplies, external alkalies like sodium carbonate, or lime, etc. are added to the water, so as to make it slightly alkaline, and thus to increase the effectiveness of the coagulants.

The important coagulants and the chemical reactions associated with them are described in details of following :

1) Use of. Alum as Coagulant.

(Alum is the name given to the aluminum sulphate with its

chemical formula as A12(S0)4 .8H20. The alum when added

to raw water, reacts with the bicarbonate alkalinities, which

are generally present in raw supplies, so as to form a

gelatinous precipitate (floe) of aluminium hydroxide.

This floe attracts other fine particles and suspended matter

(colloids), and thus grows in size, and finally settles down to

the bottom of the tank. The chemical equation is :

Use of Copperas as Coagulant. Copperas is the name given to ferrous sulphate

with its chemical formula as FeSO.7H.,0. Copperas is generally added to raw water in conjunction with lime.

Lime may be added either to copperas or vice-versa.

When lime is added first, the following reaction takes place :

(3) Use of Chlorinated Copperas as Coagulant

When chlorine is added to a solution of copperas (i.e., ferrous sulphate), the two react chemically, so as to form ferric sulphate and ferric chloride.

The chemical equation is as follows :

Use of Sodium Aluminate as a Coagulant.

Besides alum and iron salts, sodium aluminate (Na.,Al204 ) is also sometimes used as a coagulant.

This chemical when dissolved and mixed with water, reacts with the salts of calcium and magnesium present in raw water, esulting in the formation of precipitates of calcium or magnesium aluminate.

The chemical reactions that are involved are :

Comparison of Alum and Iron Salts (as Coagulants).

The alum and the iron salts are having their own advantages and disadvantages, as summarised below :

(i) Iron salts produce heavy floe and can, therefore, remove much more suspended matter than the alum.

(ii) Iron salts, being good oxidizing agents, can remove hydrogen

sulphide and its corresponding tastes and odours from water.

(iii) Iron salts can be used over a wider range of pH values.

(iv) Iron salts cause staining and promote the growth of iron

bacteria in the distribution system.

Comparison of Alum and Iron Salts (as Coagulants).

iv) Iron salts impart more corrosiveness to water than that

which is imparted by alum.

(vi) The handling and storing of iron salts require more skill and

control, as they are corrosive and deliquescent. Whereas, no

such skilled supervision is required for handling alum.

The Constituents of a Coagulation Sedimentation Plant

The coagulation sedimentation plant, sometimes called

simply a coagulation plant or a clariflocculator, contains the

following four units :

(1) Feeding device ;

(2) Mixing device or mixing basin ;

(3) Flocculation tank or flocculator ;

(4) Settling or sedimentation tank.

Feeding Devices. The chemical coagulant may be fed into the raw water either in a

powdered form or in a solution form. The former is known as dry

feeding, and the latter is known as wet feeding.

Wet feeding equipment are generally costlier- than the dry feeding

equipment's, but they have the advantage that they can be easily

controlled and adjusted.

The choice between' these two types of equipment's depends upon the

following factors :

.

Feeding Devices.

Factors affecting selection of wet or dry feeding device:

The characteristics of the coagulant and the

convenience with which it can be applied.

The amount of the coagulant to be used

The cost of the coagulant and the size of the plant

Types of feeding device:

1. wet feeding device

2. dry feeding device

Dry feeding devices. The common devices which are used for dry feeding of the coagulants are

shown in Fig.

They are in the form of a tank with a hopper bottom. Agitating plates are

placed inside the tank, so as to prevent the hollowing of the coagulant.

The coagulant, in the powdered form, is filled in the tank, and is allowed to

fall in the mixing basin. Its dose is regulated by the speed of a toothed

wheel helical screw [Fig]

The speed of the toothed wheel or the helical screw is, in turn, controlled by

connecting it to a venturi Device installed in the raw water pipes bringing

water to the mixing basin.

Dry feeding devices.

Wet feeding devices. In wet feeding, the solution of required strength of coagulant is prepared

and stored in a tank, from where it is allowed to trickle down into the

mixing tank through an outlet.

The level of coagulant solution in the coagulant feeding tank is

maintained constant by means of a float controlled valve, in order to

ensure a constant rate of discharge for a certain fixed rate of raw water

flow in the mixing basin.

When the rate of inflow of raw water changes, the rate of outflow of

coagulant must also change. In order to make these two flows in

proportion to each other, 'a conical plug type arrangement' such as

shown in Fig.

Wet feeding devices.

Mixing Devices.

After the addition of the coagulant to the raw water, the mixture is

thoroughly and vigorously mixed, so that the coagulant gets fully

dispersed into the entire mass of water. This violent agitation of

water can be achieved by means of mixing devices, such as,

centrifugal pumps, compressed air, mixing basins, etc. Out of these

devices, mixing basins are most important and normally adopted.

There are two types of mixing basins, viz.

(a) mixing basins with baffle walls ; and

(b) mixing basins equipped with mechanical devices.

They are described below in details

Mixing basins with baffle walls. The baffle type mixing basins are rectangular tanks which are

divided by baffle walls. The baffles may either be provided in such a

way as the water flows horizontally around their ends (as shown in

Fig; or they may be provided as to make the water move vertically

over and under the baffles (as shown in Fig)

The interferences and the disturbances created by the provision of

baffles in the path of flow, give it sufficient agitation, as to cause

necessary mixing to develop the floe. The flocculation energy is,

thus, derived primarily from the 180° change in the direction of flow

at each baffle. The basis of design for both types of baffled basins

remain the same.

(b) Mixing basins equipped with mechanical devices.

The mechanically agitated mixing basins provide the best type of

mixing as also the flocculating devices. The chemical added to

raw water is vigorously mixed and agitated by a flash mixer for its

rapid dispersion in raw water, and the water is then transferred to

a flocculation tank provided with a slow mixer. Mixing therefore

involves high degree of turbulence and power dissipation.

A typical mixing basin provided with a flash mixer is shown n Fig.

It consists of a rectangular tank which is provided with an impeller

fixed to an impeller shaft. The impeller is driven by an electric

motor, and it revolves at a high speed inside the tank.

Flash Mixer:

The coagulant is brought by the coagulant pipe and is

discharged just under the rotating fan. The raw water is

separately brought from the inlet end, and is deflected

towards the moving impeller by a deflecting wall. The

thoroughly mixed water is taken out from the outlet end. A

drain valve is also provided to remove the sludge from the

bottom of the flash mixer. The impeller's speed is generally

kept between 100 to 120 r.p.m. (revolutions per minute),

and the usual values of detention period may vary between

1 to 2 minutes.

Flash Mixer:Power required in flash mixing may vary from 2 to 5 kW per m3per minute. Power input in mixing and flocculation is frequentlyexpressed in terms of temporal mean velocity gradient, G', expressedby the equation:

Flocculation Tank or a Flocculator. As was pointed out earlier, the best floe will form when the

mixture of water and coagulant are violently agitated

followed by a relatively slow and gentle stirring to permit

build up and agglomeration of the floc particles.

From the mixing basin, the water is, therefore, taken to a

flocculation tank called a flocculator, where it is given a slow

stirring motion. Rectangular tanks fitted with paddles

operated by electric motors can best serve this purpose,

although even plain flocculation chambers with controlled

flow velocities are also possible.

Flocculation Tank

Flocculation TankVarious patented flocculators are now-a-days available in the market. A

typical flocculator fitted with slowly moving paddles is shown in Fig.

The water coming out from the flocculator is taken to the sedimentation

tank. The paddles usually rotate at a speed of about 2 to 3 rpm. The usual

values of detention period for this tank ranges between 20 to 60 minutes

(30 minute as the normal value) and value of velocity gradient (G') ranges

between 20 to 80 s - 1 .

The clear distance between the paddles and the wall or the floor of the

tank is about 15 to 30 cm. The velocity of flow through such a flocculator is

unimportant, because the paddles provide a rolling motion which prevents

the floe from settling.

Sedimentation Tank.

The function, design and other details of this tank are the

same as those discussed under "Plain Sedimentation". This

tank is designed on the same assumptions as a plain

sedimentation tank, except that, a lower value of detention

period (say about 2 to 4 hours) is generally sufficient here.

Also higher value of surface loading (or the overflow rate)

varying between 1000—1250 litres/hr/m2 of plan area is

generally permitted.

Combined Coagulation-cum-Sedimentation Tanks

It has been possible to combine the flocculation tank along

with the sedimentation tank, as shown in Fig. 9.24. Such a

tank is known as a coagulation sedimentation tank. In such

a tank, a plain floc-chamber without any mechanical devices

is provided before the water enters the sedimentation

chamber. The detention period for the floc-chamber is kept

about 15 to 40 minutes, and that for the settling tank, at

about 2 to 4 hours. The depth in the floe chamber may be

kept about half that of in the settling chamber.


The water from the mixing basin enters this tank, and the

clarified water comes out of the outlet end. The design

principles for such a tank are the same as those applied to a

plain sedimentation tank except that these are kept deeper.

A depth varying from 3 to 6 m is generally provided. They

may be cleaned at intervals of about 6 months or so.

FILTRATIONScreening and sedimentation removes a large percentage of the suspended

solids and organic matter present in raw supplies. The percentage of

removal of the fine colloidal matter increases when coagulants are also used

before sedimentation. But however, the resultant water will not be pure, and

may contain some very fine suspended particles (discrete, or flocculated

when coagulation is used) and bacteria present in it.

To remove or to reduce the remaining impurities still further, and to produce

potable and palatable water, the water is filtered through the beds of fine

granular material, such as sands, etc. The process of passing the water

through the beds of such granular materials (called filters) is known as

filtration. Filtration may help in removing colour, odour, turbidity, and

pathogenic bacteria from the water.

FILTRATION

Two types of filters are commonly used for treating municipal water supplies. They

are

(i) The slow sand gravity filters ; and

(ii) The rapid sand gravity filters.

A third type of a rapid sand filter works under pressure and is known as a pressure

filter. This type of filters are generally used for small plants, such as for individual

industrial supplies, or for swimming pools ; and are generally not adopted for

treating large scale municipal supplies. The slow sand gravity filters often called

slow sand filters are useful in the sense that they can remove much larger

percentage of impurities and bacteria from the water, as compared to what can be

removed by rapid sand gravity filters (often called rapid gravity filters).

FILTRATION

However, slow sand filters yield a very slow rate of filtration

(about that of that given by rapid gravity filters) and require

large areas, and are costly.

With the advancement of disinfection techniques, the

necessity of too much purification and that of the maximum

removal of bacteria (as is achieved by the slow sand filters)

has decreased, and therefore, the slow sand filters are

becoming obsolete these days.

In the modern treatment plants, rapid gravity filters are now-

a-days almost universally adopted. The water from the

coagulation sedimentation plant is directly fed into the rapid

gravity filters, and the resultant supplies are disinfected for

complete killing of germs and color removal.

Theory of Filtration

The filters, in fact, purify the water under four different processes.

These processes or actions are summarized below :

(i) Mechanical straining. The suspended particles present in

water, and which are of bigger size than the size of the voids in the

sand layers of the filter, cannot pass through these voids and get

arrested in them. The resultant water will, therefore, be free from

them. Most of the particles are removed in the upper sand layers.

The arrested particles including the coagulated floes forms a mat

on the top of the bed, which further helps in straining out the

impurities.


(ii) Flocculation and sedimentation.

It has been found that the filters are able to remove

even particles of size smaller than the size of the voids

present in the filter. This fact may be explained by assuming

that the void spaces act like tiny coagulation-sedimentation

tanks. The colloidal matter arrested in these voids is a

gelatinous mass and, therefore, attract other finer particles.

These finer particles thus settle down in the voids and get

removed.


(in) Biological metabolism.

Certain micro-organisms and bacteria are generally

present in the voids of the filters. They may either reside

initially as coatings over sand grains, or they may be caught

during the initial process of filtration. Nevertheless, these

organisms require organic impurities (such as algae,

plankton, etc.) as their food for their survival. These

organisms, therefore, utilize

Slow Sand Filters :

They were widely used since then, till the last decade

of the 19th century, when the rapid gravity filters were

invented. Their use has since decreased and they are

becoming obsolete these-days. However, they may still be

preferred on smaller plants at warm places, where covers

on filters are not required to protect the filters from

freezing. Slow sand filters normally utilize effluents from the

plain sedimentation tanks, and are used for relatively

clearer waters.

Construction of Slow Sand Filters.

A typical section of a slow sand filter is shown in Fig.. The various

parts of this filter are discussed below.

i. Enclosure tank

ii. Filter media

iii. Base material

iv. Under drainage system

v. inlet and out let arrangements

vi. Other apprentices

Slow Sand Filters :

Enclosure tank

It consists of an open water-tight rectangular tank,

made of masonry or concrete. The bed slope is kept

at about 1 in 100 towards the central drain. The

depth of the tank may vary from 2.5 to 3.5 m. The

plan area of the tank may vary from 100 to 2000 sq.

m or more, depending upon the quantity of water to

be

treated.

filtering media

The filtering media consists of sand layers, about

90 to 110 cm in depth, and placed over a gravel

support. The effective size (D10) of the sand

varies from 0.2 to 0.4 mm and the uniformity

coefficient varies from 1.8 to 2.5 or 3.0. The top 15

cm layer of this sand is generally kept of finer

variety than that of the rest which is generally

kept uniform in grain size.

Under-drainage system.

The gravel support is laid on the top of an under-

drainage system. The under-drainage system

consists of a central drain and lateral drains, as

shown in Fig. The laterals are open jointed pipe

drains or some other kind of porous drains placed 3

to 5 m apart on the bottom floor and sloping

towards a main covered central drain.

Under-drainage system.

Inlet and Outlet arrangements

An inlet chamber, is constructed for admitting the

effluent from the plain sedimentation tank without

disturbing the sand layers of the filter and to

distribute it uniformly over the filter bed. A 'filtered

water well' is also constructed on the outlet side in

order to collect the filtered water coming out from

the main under-drain.

Other appurtenances.

Besides these arrangements, certain other

appurtenances are provided for the efficient

functioning of these filters. For example, vertical air

pipe passing through the layer of sand may be

provided, and may help in proper functioning of the

filtering layers. Similarly, arrangements are made in

order to control the depth of water above the sand

layer (1 to 1.5 m).

Rapid Gravity Filters

These filters employ coarser sand, with effective

size as 0.5

mm or so. On an average, these filters may yield as

high as 30 times the yield given by the slow sand

filters. Waters from the coagulation- sedimentation

tanks are used in these filters, and filtered water is

treated with disinfectants, so as to obtain potable

supplies

DISINFECTION OR STERILISATION

The filtered water which is obtained either from the slow sand

filters or rapid gravity filters, may, normally contain some

harmful disease producing bacteria in it. These bacteria must

be killed in order to make the water safe for drinking. The

chemicals used for killing these bacteria are known as

disinfectants, and the process is known as disinfection or

sterilisation

The 'disinfection' not only removes the existing bacteria from

the water at the plant, but also ensures their immediate

killing even afterwards, in the distribution system.

Minor Methods of Disinfection

The following are the minor methods of disinfection :

(1) Boiling of water ;

(2) Treatment with excess lime ;

(3) Treatment with ozone ;

4) Treatment with iodine and bromine ;

(5) Treatment with ultra-violet rays ;

(6) Treatment with potassium permanganate ; and

(7) Treatment with silver, called Electra-Katadyn process.

These methods are summarised below :

Boiling of Water.

The bacteria present in water can be destroyed by

boiling it for a long time. It is an effective method of

disinfection, but it is not practically possible to boil

huge amounts of public water supplies. Moreover, it

can only kill the existing germs but cannot take care of

the future possible contaminations. This method is

hence, not at all used for disinfecting public supplies.

However, during water borne epidemics, public is

advised to drink water only after boiling it in their

houses.

Treatment with Excess Lime.

Lime is generally used at a water purification plant

for softening* (i.e. reducing hardness) the supplies.

But it has been found that if excess lime is added to

the water, it can in addition, kill the bacteria also. So

much so, that an addition of 14 to-43 ppm of excess

lime has been found to remove the bacterial load by

about 99.3 to 100% from highly polluted waters

Treatment with Ozone.Ozone gas is a faintly blue gas of pungent odour, and is an excellent disinfectant. Ozone gas is nothing but an unstable allotropic form of oxygen, with each of its molecule containing three oxygen atoms. It can be produced by passing a high tension electric current through a stream of air in a closed chamber, under the following chemical reaction :

Treatment with Iodine and Bromine.

The addition of iodine or bromine to water can help

in killing the pathogenic bacteria, and thereby

disinfecting the same. The quantity of these

disinfectants may be limited to about 8 ppm and a

contact period of 5 minutes is generally enough.

Treatment with Ultra-Violet Rays.

Ultra-violet rays are the invisible light rays having wave lengths of

1000 to 4000 mp. They are basically found in sun light, but can

also be produced by passing electric current through mercury

enclosed in quartz bulbs. Mercury vapour lamps enclosed in quartz

bulbs can therefore, be used as a good source of such rays.

These rays are highly effective in killing all types of bacteria, thus

yielding a truly sterilised water. The water to be treated with ultra-

violet rays should, however, be less turbid and low in colour.

Treatment with Potassium Permanganate.

This is used as a popular disinfectant for disinfecting

well water supplies in villages which are generally

contaminated with lesser amounts of bacteria.

Besides killing bacteria, it also helps in oxidising the

taste producing organic matter. It is, therefore,

sometimes added in small doses (such as 0.05 to

0.10 mg/1) even to filtered and chlorinated water.

Treatment with Silver or Electro-Katadyn Process.

In this method of disinfection, metallic silver ions are

introduced into the water by passing it through a

tube containing solid silver electrodes which are

connected to a D.C. supply of about 1.5 volts. The so

introduced silver ions have a strong germicidal

action, and thus act as disinfectant. The

recommended silver dose may vary between 0.05 to

0.1 mg/1, and the required contact period may vary

between 15 minutes to 3 hours.

Chlorination:

Chlorine in its various forms is invariably and almost

universally used for disinfecting public water supplies. It is

cheap, reliable, easy to handle, easily measurable, and above

all, it is capable of providing residual disinfecting effects for

long periods, thus affording complete protection against future

recontamination of water in the distribution system.

Its only disadvantage is that when used in greater amounts, it

imparts bitter and bad taste to the water, which may not be

liked by certain sensitive-tongued consumers.

Various Forms in which Chlorine can be Applied.

Chlorine is generally applied in the following forms :

as free chlorine

(1) In the form of liquid chlorine or as chlorine gas.

as combined chlorine

(2) In the form of hypochlorite or bleaching powder.

(3) In the form of chloramines, i.e. a mixture of

ammonia and

chlorine.

(4) In the form of chlorine dioxide.

Residue management:

The plant produces residues according to the

treatment unit. For example floating solid matters

(dead leaves, logs, plastic bottles etc.,) and other

large floating debris separated from water during

the initial screening process can be disposed of at

conventional solid waste landfills. However other

treatment process produce more complex residual

waste streams that may require advanced

processing and disposal methods to product

human health and the environment

Types of residuals:

Sludge : from the process of pre sedimentation,

coagulation, filter backwashing operations , lime

softening iron and manganese removal.

Concentrate (brines) from ion exchange regeneration and

salt water conversion, membrane reject water and spent

back wash, and activated alumina waste generate

Iron exchange resins, air emissions (from air stripping,

odour control units, or ozone destruction)

Residual Disposals:

Residue source Contaminant category

Disposal method

Sedimentation basin residuals

Metals, suspended solids, organics, biological and inorganics.

Land fillingDisposal to sanitary sewer or waste water treatment plant

Filter waste Metals, suspended solids, organics, biological and inorganics.

Recycle, Surface Discharge, Disposal to sanitary sewer or waste water treatment plant

Environmental Engineering-1 Unit 3

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Transcript of Environmental Engineering-1 Unit 3