Environmental Engineering-1 Unit 3
description
Transcript of 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.
Summaries of techniques in purificationof raw water.
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
Summaries of techniques in purificationof raw water.
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
Summaries of techniques in purificationof raw water.
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:
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 Design problems:
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.
Combined Coagulation-cum-Sedimentation Tanks
Combined Coagulation-cum-Sedimentation Tanks
Combined Coagulation-cum-Sedimentation Tanks
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.
Theory of Filtration
(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.
Theory of Filtration
(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