The lecture
Water Hygiene and Hygiene of
water-supply of inhabited
places.
Author: Lototska O.V.
Water constitutes nearly two-thirds of the total weight
of the body, 79 % of blood, 80 % of brain and muscles
and 10 % even of bones
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Its main functions are that it:
Replaces loss of fluids from
tissues.
Maintains the fluidity of blood and
lymph.
Helps elimination of waste
material of the body.
Acts as a vehicle for dissolved
food.
Helps in the secretion of digestive
juices.
Regulates body temperature and
acts as a distributor of body heat.
The population should be provided not only
with enough of water, but also with
qualitative water. Water should not cause
any pathological change in the organism,
should not cause of spread of infectious
diseases, and also not to cause unpleasant
sensations.
Waters, used for drink and everyday needs,
must correspond to the demands:
good organoleptic properties: refreshing temperature,
transparence, colorless, no smell and no taste .
harmlessness of its chemical composition
the absence of pathogenic microorganisms
safety in the radiological attitude
The pollution of water sources represents the
important ecological problem. Depending on type
of pollution there are:
 chemical,
 physical (radioactive substances, hot water),
 bacterial, virus and biological.
 Industrial wastewater is characterized by
considerable quantity of components.
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Major categories of water pollutant
1. Infections agents - Bacteria, viruses
2. Organic chemical - Pesticides, plastics,
detergents, oil, and gasoline
3. Inorganic chemicals - Acids, caustics, salts,
metals
4. Radioactive materials - Uranium, thorium,
cesium, iodine, radon
Sources of water
Impure water may be purified by either of
the following methods:
A. Natural
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(a) Pounding or Storage.
(b) Oxidation and Settlement.
B. Artificial
I. Physical
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Distillation.
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Boiling.
II. Chemical
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Precipitation.
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Disinfection or Sterilization.
III. Filtration
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"Biological" or "Slow Sand" Filtration.
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"Rapid Sand" or "Mechanical" Filtration.
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Domestic Filtration.
PURIFICATION OF WATER
Purification of water is of great
importance in community medicine. It
may be considered under two headings.
 Purification of water on large scale
 Purification of water on small scale
Three main steps in purification
of water on large scale:
Storage, Filtration, Chlorination
1. Storage:
Water is drawn out from source and impounded
in natural or artificial reservoirs. Storage provides
a reserve of water from which further pollution is
excluded.
Advantages
Physical — About 90% of suspended impurities
settle down in 24 hours by gravity.
 Chemical — The aerobic bacteria oxidize the
organic matter present in water with the aid of
dissolved oxygen. As a result the content of free
ammonia is reduced and a rise in nitrates occur.
 Biological — 90 % of total bacterial count drops in
first 5 - 7 days.
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A complete system of water supply
How is water treated?
Coagulation: Alum and other
chemicals are added to water to form
tiny sticky particles called "floc" which
attract the dirt particles.
Sedimentation: The
heavy particles (floc)
settle to the bottom
and clear water
moves to filtration.
Filtration: The water
passes through filters that
help to remove smaller
particles.
Disinfection: A small
amount of chlorine is
added to kill any bacteria
or microorganisms that
may be in the water.
Storage:
Water is placed
in a closed tank
or
reservoir
where it flows
through
pipes
to homes and
businesses
in
the community.
2. Filtration
Filtration is important because 98 – 99
% of bacteria are removed by filtration, a
part from other impurities. Two types of
filters are in use, they are:
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a. Slow sand filters (biological filters)
 b.
Rapid sand filters (Mechanical
filters)
Slow sand filter
 Supernatant
(raw) water:
 Sand bed
 Vital Layer
 Under drainage system
Filter control valves:
The filter is equipped with certain valves and devices
which are incorporated in the outlet pipe system
maintaining a steady rate of filtration.
When the vital layer becomes dense and resistance to
the passage of water is increased the supernatant water is
drained off Sand bed is cleaned by scrapping of the top
portion of the sand layer to a depth of 1 - 2 cms. Scrapping is
done 20 - 30 times.
The process is known as Filter Cleaning.
b. Rapid Sand Filter
Rapid sand filters are of two types,
the gravity type and the pressure type.
Both the types are in use. The following
steps are involved in the purification of
water by rapid sand filters.
i. Coagulation:
ii. Rapid mixing:
iii. Flocculation:
iv. Sedimentation:
v. Filtration:
Filter Beds:
Back - Washing:
Rapid sand filters need frequent washing daily or
weekly. Washing is accomplished by reversing the flow of
water through the sand bed, which is called "back-washing".
Back - washing dislodges the impurities and cleans up the
sand bed.
Comparison of Rapid & Slow Sand Filters.
Slow Sand Filter
Rapid Sand Filter
Space
Occupies large space
Occupies very little area
Rate of filtration
0.1 -0.4m3/m2/h
5- 15m3/m2/h
Effective size of sand
0.15-0.35 mm
0.6 - 2.0 mm
Preliminary treatment Plain sedimentation
Chemical coagulation
Washing
By Scraping the sand By back-washing
bed
Operations
Less skilled
Highly skilled
Removal of turbidity
Good
Good
Removal of colour
fair
Good
Removal of bacteria
99.9-99.99 per cent
98 - 99 per cent
Advantages of Slow Sand
Advantages of Rapid Sand
Filter
filter
1. Simple to construct and 1.
It
deals
with
raw
operate
water
directly.
No.
preliminary storage is needed
2.
Cost of construction is 2. Occupies less space.
cheaper
3. Physical, Biological and 3. Filtration rate is high.
Chemical quality ' of filtered
water is very high
4. Washing of filter is easy
5.
More
operation.
flexibility
in
CHLORINATION
Chlorination is the process in which chlorine is
added to water for purification. Chlorination-is more
effective when pH of water is around 7.
Effects of Chlorine:
a. Chlorine kills pathogenic bacteria, it has no effect on
spores and certain viruses.
b. It has germicidal effects.
c. It oxidizes iron, manganese and Hydrogen sulphide
d. If destroys some taste and odour producing
constituents.
e. It controls algae and slim organisms
f. It aids coagulation
Action of Chlorine
When Chlorine is added to water, there is
formation of hypochlorous and hydrochloric acid.
The hydrochloric acid is neutralised by alkalinity
of the water. The hypochlorous acid ionizes to
form hydrogen ions and hypochlorite ions as
follows.
H2O + CI2
►
HCI--+HOCI
HOCI
►
H++OCI“
The disinfecting action of-chlorine is mainly due
to hypochlorous acid and to a small extent due
to hypochloriteon.
Principles of Chlorination
a.
First, water should be clear and free
from turbidity.
b.
Chlorine demand of water should be
estimated.
c.
At least one hour is essential as a
contact period of free residual chlorine for
killing bacterial and viruses.
d.
Minimum recommended concentration of
free chlorine is 0.5 mg/L for one hour.
e.
The sum of the chlorine demand of the
specific water plus the free residual chlorine
of 0.5 mg/l constitutes the correct dose of
chlorine to be applied.
Methods of Chlorination
a.
By means of chlorine gas
It is of first choice because it is cheap, quick in
action,
efficient
and
easy
to
apply.
Chlorinating equipment is required to apply
chlorine gas to water as chlorine gas is irritant
to eyes.
b.
By means of Chloramine:
Chloramines are loose compounds of chlorine
and ammonia. They have slower action than
chlorine. They give more persistent type of
residual chlorine. They have a less tendency to
produce chlorinous taste.
Chlorine Demand It is the difference between the
amount of chlorine added to the water and the
amount of residual chlorine remaining at the end
of a specific period of contact (1 hour) at a given
temperature and pH of water.
Residual Chlorine: Amount of untreated chlorine,
remaining in the water after some time as an
effective disinfecting agent i.e. 0.3 – 0.5 mg/liter
Break point chlorination:
The point at which the chlorine demand of water is
met and if further chlorine is added free chlorine
begin to appear in water
Super Chlorination:
It is addition of large doses of chlorine to the
water and removal of excess of chlorine after
disinfection.
Agents alternative to Chlorination
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a.
b.
Ozonation
U.V. radiation
The ozonization of water
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Ozone has been used in water treatment since 1903. It is
more effective against bacteria and viruses than chlorine and adds no
chemicals to the water. Ozone cannot be stored and requires an on-site
ozone generator. In general, ozonation equipment and operating costs
are higher than other treatment procedures.
Ozone contains three oxygen atoms. It is destroyed in water,
forming atomic oxygen: O3 → O2 → O. ozonization is one of the best
methods of disinfection: water is well disinfected, organic admixtures
become destroyed, organoleptic features are improved. Water
becomes blue and it is equated with spring water.
Ozone dose is 0,5 - 6 mg/l. Sometimes, higher doses are
necessary for the lighting of water and improving other organoleptic
features. The time of disinfection is 3-5 min. The remaining ozone
should make up 0,1 – 0,3 mg/l. The concentration of the remaining
ozone 0.4 mg/l provides the reliable inactivation of 99 % viruses for 5
min.
Asaka Water Purification Plant
Advanced Water Purification System
Ultraviolet Light
Ultraviolet irradiation will kill bacteria by creating
photochemical changes in its DNA. No chemicals are added
to the water by this process. Most ultraviolet water
treatment units consist of one or more ultraviolet lamps
usually enclosed in a quartz sleeve, around which the
water flows. The UV lamps are similar to fluorescent lamps
and the quartz sleeve surrounding each lamp protects the
lamp from the cooling action of water. The killing effect of
the lamp is reduced when the lamp temperature is
lowered. Ground water is usually a constant temperature
year round and so it is possible to set a flow rate that will
not lead to excess cooling.
The effectiveness of UV irradiation depends on the
intensity of the light, depth of exposure and contact time.
Water passes in a relatively thin layer around the lamp;
therefore, water flow must be regulated to ensure that all
organisms receive adequate exposure. If the water is at all
turbid, or if it contains traces of iron, the effectiveness of
UV is greatly reduced. In such cases, the water needs to
be filtered before it reaches the UV system.
The maximal bactericidal effect is achieved by the
waves 250-260 nm, which pass even through the 25
cm layer of transparent and decolorized water.
The disinfection proceeds very quickly: vegetative
forms of microorganisms die in 1-2 min. The turbidity,
colour and iron salts decelerate the disinfection,
decreasing the transparence of water. Consequently,
it is necessary to light and decolorize water before the
disinfection.
There are some advantages of UVirradiation over the chlorination:
bactericidal rays don't denaturate the water and don't
change its organoleptic features,
 they have wider biological action.
 Their bactericidal action is spread over the spores,
viruses and worm eggs, resistant to chlorine.
Many investigators consider this method the best
for the disinfection.
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RIFICATION OF WATER ON SMALL SCALE
•House hold purification of water
•Disinfection of wells
• HOUSE HOLD PURIFICATION
a.
By Boiling:
Water should be boiled for 5 -10 minutes.
It kills all bacteria, spores, cysts & ova.
It removes temporary hardness
Taste is altered but is harmless
b.
Chemical disinfection
i) Bleaching Powder (CaOCI2)
Bleaching powder is a white amorphous powder.
Produced by action of chlorine on slaked lime.
When freshly made contains 33 % of available chlorine.
It must be stored at dark, cool, dry place in a closed
container that is resistant to corrosion.
In practise one cup (250 g) of laundry bleach is mixed
with three cups (750 ml) of water to make a litre. Three
drops of this solution are added to 1 litre water for
disinfection. Contact period is 30 minutes to 60 minutes.
ii.) Chlorine Solution
Chlorine solution may be prepared from bleaching powder.
* If 4 kg of bleaching powder with 25 % available chlorine is
mixed with 20 litres of water, it will give a 5% solution of
chlorine.
* This solution should be kept in dark, cool and dry place in
closed container
iii. Chlorine tablets
Available under different trade
name e.g. Halazone
One tablet of 0.5 g is sufficient to
disinfect 20 litres of water. Used in
camps and during travel.
iv. Iodine:
Two drops of 2 % ethanol solution
of iodine is used. A contact period
of 20 - 30 minute is sufficient for 1
litre water.
v. Potassium Permanganate.
DUAL CHLORINE
TABLET CHAMBER
UNIT — CAN HOLD UP
TO 50 TABLETS
It is a powerful oxidizing agent but not
recommended as it alters colours, smell and taste of
water.
C. By Filtration
Water can be filtered and is purified. Different filters are.
The new
filtration
system uses
centrifugal
force to spin the
untreated water
above the filter
media (sand).
This helps
remove
suspended
solids that
accumulate on
the inside walls
of the tank
DISINFECTION OF
WELL
Wells are main source
of water in rural area.
The most effective
and cheapest method
of disinfecting wells is
by bleaching powder.
Disinfection of well is
required in normal
days
and
during
epidemics.
Steps:
1. Find volume of water in well.
Measure depth of water column — (h)
metres
Measure the diameter of well— (d)
metres
Substitute (h) & (d) in:
Volume (litres) = π x d2 x h x 1000
π = 3.14
4
One cubic metre - 1,000 litres of water
2. Find amount of bleaching
powder required
Measures by Horrock's apparatus.
Roughly 2.5 gm of good quality
bleaching powder would be required
to disinfect 1,000 litres of water.
3. Dissolve bleaching powder in water
The calculated amount of bleaching powder is placed in a
bucket (not more than 100 g in one bucket) and made into a thin
paste. More water added till bucket is 3/4 full. The contents are
stirred and allowed to stand for 5 - 10 minutes. When lime settles
down, the supernatant solution which is chlorine solution is
transferred to another bucket.
4. Delivery of Chlorine solution into the well.
The bucket containing the supernatant chlorine solution is
lowered some distance below surface water. The well water is
agitated by moving the bucket violently both vertically and
laterally. Note: The precipitate or lime is never entered in well
because it increases the hardness of water.
5. Contact period - 1 hour contact period is required.
6. Ortho-Tolidine test: It is done to list for residual chlorine at the
end of one hour. If "free" residual chlorine level is less than 0.5
mg/ litre, then procedure should be repeated, before water is
drawn.
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EXPRESS METHODS OF WATER QUALITY
IMPROVING
Deodorization - elimination of smack and odour of water by
aeration, usage of oxidants (ozonization, dioxide of chlorine, large
doses of chlorine, potassium permanganate), filtrating through a
layer of absorbent coal, by introduction in water to sedimentation
of absorbent coal.
Deironation is carried out by spraying water with the purpose of
aeration in graduation towers. Thus, bivalent iron is oxydated in
ferric hydroxide, which sediments in settling tank, or delays on
the filter.
Softening. By an aged method of water softening is soda
calcareous, at which calcium and magnesium settle in a settling
tank as unsolvable salts. Today is used filtrating water through
filters, which are completed by ion exchangers. Ion exchangers
are firm, unsolvable, acinose stuffs, which have property to
exchange their ions on ions of salts, which are solved in water.
Removal of hardness
a.
b.
c.
d.
a.
b.
I.Temporary Hardness
Boiling
Addition of lime
Addition of sodium carbonate
Permutit process
II. Permanent Hardness
Addition of sodium carbonate
Permutit process/ Base exchange process.
Boiling:
It removes temporary hardness by expelling carbon
dioxide and precipitating the insoluble calcium carbonate.
Ca (HCO3)2→ CaCO3 + CO2 + H2O
Addition of Lime:
It removes temporary hardness. Lime absorbs
carbondioxide and precipitates the insoluble calcium
carbonate.
Ca (OH)2 + Ca (HCO3)2 →2 CaCO3 + 2H2O
Household
water
softeners
typically use a different process,
known
as
ion
exchange.
Ion-exchange
devices
consist of a bed of plastic
(polymer) beads covalently bound
to anion groups, such as -COO-.
The negative charge of these
anions is balanced by Na+ cations
attached to them. When water
containing Ca2+ and Mg2+ is
passed
through
the
ion
exchanger, the Ca2+ and Mg2+
ions are more attracted to the
anion groups than the Na+ ions.
Hence, they replace the Na+ ions
on the beads, and so the Na+ ions
(which do not form scale) go into
the water in their place.
Addition of Sodium Carbonate,
It removes both temporary and permanent hardness.
Na2CO3 + Ca (HCO3)2 → 2NaHCO3 + CaCOs
Na2CO3 + CaSO4 → Na2SO4 + CaCO3
Base Exchange Method
In this method sodium permutit is used, which is a
combination complex of Na, Al and Si (Na2 Al2 Si2OH2O)
Sodium permutit has property of exchanging the sodium cation
for Ca++ and Mg++ ions in water.
Na2 Al Si2O + H2O = Mg++/Ca++
When hard water passed, sodium permutit exchanges
Mg/ Ca and is converted into calcium and magnesium
permutit.
With time permutit loses effectiveness, it is regenerated
by adding conc. sol of NaCI.
*
By this process hardness of water is removed to zero.
As zero hardness is corrosive, therefore a part of raw water is
mixed with softened water.
Desalting - the sequential filtrating of water through kationite,
and then through anion exchanger permits to liberate it from
solvable salts and consequently use with the purpose of
desalting. For desalting water on water pipes, sea courts
thermal method is used which bases on evaporation of water
with the following condensation of steams. Also is used electro
dialysis with usage of selective diaphragms, freezing and other
methods.
Decontamination - at coagulation, settling and filtrating of
water on waterpipes contents of radioactive substances in it is
reduced only on 70-80%. For more penetrating
decontamination water is filtrated through ionic exchanger of
resin.
Fluoridation of water - synthetic adding of fluorine bonds
with the purpose of decrease of its rate by caries of teeth.
About 97 percent of the water on earth is in the salty
oceans. People have found many ways to desalinate, the process
for removing salt from seawater and brackish water. The
desalination processes used most commonly today are
distillation, reverse osmosis, and electrodialysis. These processes
produce fresh water from salt water.
This is a water purification
plant.
Distillation is the oldest
method
of
turning
salt
water into fresh water.
Seawater can be distilled by
simply boiling it in a teapot,
and piping the steam into a
cool bottle. The salt water
turns to vapour under the
sun's heat. The vapour rises
until it hits the underside of
the dome or glass, where it
condenses.
Most modern distillation plants use a process
called multistage flash distillation. This is a type
of the age-old method of boiling and
condensation. In flash distillation, preheated
seawater flows into a large chamber in which
the pressure is low. The low pressure causes
some of the water to instantly turn into steam
The steam is condensed into salt-free water.
The seawater passes through several distillation
chambers. Each of the chambers has a lower
pressure than the previous chamber. Often, the
final water is so pure that it is tasteless, and
some salt must be tossed back in to give it
flavour
Reverse osmosis is a widely used method for
desalting seawater and brackish water. In normal
osmosis, a less concentrated liquid flows through a
membrane into a more concentrated liquid. Thus, if salt
water and fresh water are separated in a chamber by a
special semi-permeable membrane, the fresh water will
flow through the membrane into the salt water.
Electrodialysis is used chiefly to desalt brackish
ground water and water from estuaries, or river mouths.
Electrodialysis is based on the fact that when salt is
dissolved in water, it breaks up into ions, or electrically
charged particles, of sodium and chloride. Sodium ions
carry a positive charge, and chloride ions carry a negative
charge.
Other desalting processes are also being studied.
During the 1970's, several plants experimented with
freezing as a method of desalination. When seawater
freezes, the ice crystals produced are pure water in solid
form. The salt is separated and trapped between the ice
crystals.
EXAMINATION OF WATER
Before water from any source is
declared fit for human
consumption, it is essential to
carry out the following
examination.
For hygienic purposes the
examination of water is generally
done under the following heads:
1. Physical Examination.
2. Chemical Examination.
3. Microscopical Examination.
4. Bacteriological Examination.
1. Physical qualities
a.
b.
c.
d.
Turibidity
Colour
Odour
Taste
2. Chemical qualities:
•
•
•
•
•
•
Analysis is made to determine :
The amount of organic salts which determine the
hardness of water and type of hardness.
The nature and amount of organic pollution.
The percentage and amount of poisonous metals.
The chemist determines the reaction (by means of
a litmus paper or phenolphthalein),
the type and degree of hardness,
the presence of chlorides, nitrites, nitrates,
ammonia (free and albuminoid) and metals such as
lead, copper, iron, calcium etc., before giving his
final opinion.
Bacteriological Indicators:
The main object of bacteriological examination of
water is to find out whether excretal pollution is
present or not. The sewage bacteria can be divided into
three groups:
1. - E.coli and coliform group
2. - Fecal streptococci
3. - Clostridium perfringens.
4. Radiological Standards
Gross alpha activity 3 pico curie/L
Gross beta activity 30 pico curie/L
3.
Sampling of water
For physical and chemical
examination, about 2 liters
water is essential. It must be
collected in a clean glass
stoppered bottle made of
neutral glass.
Before collecting the sample rinse the bottle well
three times with the water filling it each time, about
1/3 full. For bacteriological examination about 300 ml
water is required. It must be collected in clean
sterilized bottle made of neutral glass, provided with a
ground glass stopper having an overlapping rim. If the
water to be sampled contains or is likely to contain
chlorine, a small quantity of sodium thiosulphate is
added to bottle before sterilization.
Sampling from Tap
If
sample
is
taken from a tap in
regular use, the tap
should be opened
fully and the water
run to waste at
least for 2 minutes
in order to flush the
stagnant water in
nozzle and pipe. If
sample
is
taken
from tap not in
regular use, the tap
should be sterilized
by heating it till it is
red hot. Then allow
water to run to
waste
for
one
minute and then
collect sample.
Sampling from a well:
Tie a sample bottle with a
rope. Use a stone or piece of
metal weighing about 500 gm as
"the weight and attach the tube
bottle
just
above
it.
After
removing the cap aseptically,
lower the bottle into the well into
the well to a depth of 1m. When
no more air bubbles rise to the
surface, raise the bottle out of the
well and carefully replace the cap.
C. Sampling from stream.
a.
b.
c.
d.
e.
f.
g.
Water is taken from middle of a stream, with the
mouth of the bottle facing upstream, lower the bottle into
the stream and allow filling. Tilt bottle upwards to fill
completely. The cap is carefully screwed back, taking care
not to touch the screw thread at the top of the bottle.
After taking sample following information must be
given with bottle.
Source of water supply
Date, place and time of sampling
Geological formation of soil, if available.
In case of well, its depth, diameter and how it is used.
Recent rainfall if there.
Any suspected source of pollution in vicinity
Whether any method of purification is used.
Sampling from stream.
Water is taken from middle of a stream,
with the mouth of the bottle facing upstream, lower the
bottle into the stream and allow filling. Tilt bottle
upwards to fill completely. The cap is carefully screwed
back, taking care not to touch the screw thread at the
top of the bottle.
After taking sample following information must be
given with bottle.
a.
Source of water supply
b.
Date, place and time of sampling
c.
Geological formation of soil, if available.
d.
In case of well, its depth, diameter and how it is
used.
e.
Recent rainfall if there.
f.
Any suspected source of pollution in vicinity
g.
Whether any method of purification is used.
The principle of chlorination is based on the treatment by chlorine or the chemical
compounds, containing active chlorine and able to oxidize and provoke bactericidal
action. Chlorine is subjected to hydrolysis in water:
Cl2+HOH → HOCl+HCl,
so hydrochloric and chloricious acids are formed. Chloricious acid takes the central
place in the mechanisms of bactericidal action. It was thought earlier that the latter was
destroyed in water and discharged out atomic oxygen
(HOCl → HCl+O·), which was the main bactericidal agent. Now, such explaination is
considered insufficient. Chlorine in the structure of chloricious acid and hypochloriteion (HOCl→H +OCl ) free active chlorine, which determines bactericidal action in
water. Not large molecules and electric neutrality let chloricious acid penetrate quickly
through the bacterial membrane and influence upon the cellular enzymes, important for
the metabolism and reproduction. It is assumed, that it reacts with SH-groups of
enzymes, which become oxidized.
The reliable bactericidal effect of chlorine is achieved, if about
0,3 – 0,5 mg/l of free chlorine or 0,8 – 1,2 mg/l of connected
chlorine are left in water after 30 - 60 min. of exposure.

The chlorination by the post-break doses.
By the results of some investigations the water can be
disinfected by 2 doses of chlorine: 1mg/l (before-break dose) and
5,2 mg/l (post-break doses), as the concentration of the rest
chlorine makes 0,5 mg/l in both cases.
However, by before-break dose the remaining chlorine is
determined as chloramin, and by post-break doses - as free
chlorine. The bactericidal action of such method is very effective.
At the same time we improve water organoleptic features at the
expense of oxidation of organic substances with the bad smell. It is
necessary to use this method in hot countries widely.

Double chlorination.
In many river water-pipes chlorine is given before the settling
and than after the filtration as usual. The introduction of chlorine
before the settling improves the coagulation and decolourization
of water, inhibits the development of microorganisms in the
settling tanks, increases the reliability of disinfection. However,
the possibility of chlororganic compounds formation increases
too.

The chlorination with the preammonization
(chlorammination).
First, they introduce ammoniac solution and than, in 0.5-1 min,
chlorine to the water. As a result chloramines are formed in water:
NH2Cl - monochloramin and NHCl2 - dichloramin. The last one has the
most expressed bactericidal action. The effectiveness of such method
depends on the ratio NH3:Cl. That's why they use the doses of reagents
in the following ratios: 1:3, 1:4, 1:6, 1:8. The ratio should be chosen for
certain reservoirs individually.
This method prevents bad smells, which can appear by the
chlorination of water, containing phenol and the matters from its group
(as chlorphenols are formed). Chlorphenols impart medicinal smell and
smack to the water even in the small quantities.
The speed of disinfection by this method is lower than that by
chlorine. The exposure time should be not less than 2 hours.
If the water of reservoirs contains ammonium salts, chloramines
are also formed. This fact decelerates disinfection. So, it is necessary to
define free and connected chlorine separately to determine the
reliability of disinfection. Obviously, the presence of only free chlorine is
evidence of reliable disinfection.
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Overchlorination (superchlorination)
By this method large doses of chlorine are used in to the water, for
example 10-20 mg/l and more. As a result the reliable bactericidal effect
is achieved even after the exposure for 15 min. By the 30-60 min. of
exposure even the turbid waters are disinfected reliably. Such agents,
stable for the chlorine, as Berket's rikketsia, amoebic cysts, Koch’s
bacillus, viruses become dead. But even such doses of chlorine cannont
destroy spores of anthrax and helminthes eggs. A lot of residual chlorine
remains after the overchlorination. Water is dechlorinated by the
filtration through the layers of activated coal or by the addition of
sodium hyposulphite (Na2S2O3*5H2O) in the concentration of 3.5 mg
per 1 mg of chlorine.
advantages
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we don't need to determination chlorine's need of water,
calculate the dose of chloric lime,
time of water's disinfection is decreasing to 15-20 min in summer
and to 30-60 min in winter.
disinfection of turbid water is very well too;
during this, unpleasant smell and smack are eliminated better.
Thank your!