Lecture on Purification of Lecture on Purification of Water byWater by

Dr. Fazal Mahmood

M.B.B.S(Pb), M.C.P.S(Pak)

Purification on Large scalePurification on Large scale

When sources of water are Rivers,Streams,Lakes etc. then water is Purified by

Storage Sedimentation Filtration(rapid sand & slow sand) Disinfection(chlorination)

Purification on Medium ScalePurification on Medium Scale

When sources of water are wells,Springs,Tanks etc.Then water is purified by the addition of

Bleaching powder/Chlorinated Lime as it is Cheep Easy to use Reliable and safe

Purification on Small Purification on Small Scale(Domestic Methods)Scale(Domestic Methods)

BOILING: for 5-10 minutes kills almost all organisms & removes temporary Hardness.

DISTILLATION: not commonly used due to higher cost, used in Labs. etc.

ADDITION OF CHEMICALS Bleaching Powder: 5% solution is used

Dose: 3-6 drops/L contact time of ½ hour.

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Chlorine Tablets/Halazone Tablets. one tablet/litre.

Iodine Solution: 02 drops of 2% Soln./litre KMnO4: an amount that gives just pink

coloration to the Water. Alum: used for turbid water in a dose of

0.1-0.4 grains/5 litres of Water.

Purification of water in Purification of water in Emergency/EpidemicEmergency/Epidemic

Boiling: for 5-10 minutes.High Test Hypo chlorite: used in granular

form Dose: 01gm./10 litres of water.Liquid Bleach/5% Sodium Hypo chlorite

solution: Dose: 14 ml./1000 litres of water.Bleaching Powder: used in a dose of

2.3gm./1000 litres of water.

Purification of water on Large Purification of water on Large ScaleScale

On large scale purification, water is disinfected by addition of Chlorine after filtration.

On large scale, water is purified through the following processes.

Contnd.---------Contnd.---------

A. STORAGE: Not a sole measure of purification but

valuable as a preliminary to other processes.

There is fall in ammonia & rise in nitrates due to oxidation of organic matter by aerobic bacteria.

Contnd.----------Contnd.----------

Sedimentation, may be hastened by the addition of Alum in a dose of 35 mg./ litre for a detention period of 4-6 hrs.

Sedimentation will prevent clogging of the filters.

Limitation to storage includes algae growth which can be controlled by addition of CuSo4 in a dose of 1-5 Kg. / 5000 cu.m.

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B. FILTRATION: Filtration through sand is the oldest and

universally, most accepted method through out the world.

98-99% bacteria are removed along with other impurities through filtration.

Types of FiltersTypes of Filters

Slow sand/Biological filters.

Rapid sand filters/Mechanical filters.

Slow Sand FiltrationSlow Sand Filtration

First used in 1804 in Scotland & subsequently in London.

During 19th.Century used through out the world.

Slow sand filter consists of concrete/brick work rectangular basin containing carefully selected graded sand supported on gravel and stones.

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ELEMENTS:

1. Supernatant (raw) water: Upper most layer of raw water(1-1.5M) It provides a constant head of water It provides a waiting period of 3-12 hours

Elements contnd.-----Elements contnd.-----

2. Sand Bed: It is 1 M thick supported by a layer of fine and then

coarse gravel and finally by the stones.3. Drainage System for filtered water: It consists of perforated pipes4. Filter Control Valves: Valves & Devices incorporated in the outlet-pipe

system to maintain a constant rate of filtration Venturi meter measures bed resistance or loss of

head.

Actions of slow sand filterActions of slow sand filter

Physical: Mechanical straining/sedimentation

Chemical: Oxidation of organic matter by aerobic bacteria.

Biological: Occurs through “Vital layer” Efficiency of the filter depends upon the

“Vital layer”

Vital LayerVital Layer

Slimy growth known as “Schmutzdecke”, Vital layer, zoogleal layer or biological layer

The layer is slimy & gelatinous,It consists of threadlike algae & numerous forms of life plankton, diatoms and bacteria

Ripening of the filterVital layer is the “heart” of slow sand filter

ADVANTAGESADVANTAGES

Simple to construct and operateCost of construction cheaper than rapid

sand filterProcess: physical, chemical and biologicalBacterial count reduction is 99.9% to

99.99% and E.coli reduction is 99% to 99.9%

DISADVANTAGESDISADVANTAGES

Old fashioned and outdated method of water purification (but still in use)

Initial cost is low but maintenance cost is much more than rapid sand filter

These filters need a lot of space

RAPID SAND FILTERSRAPID SAND FILTERS

In 1885, first rapid sand filters were installed in USA, since that time they are gaining considerable popularity in highly industrialized countries as well as developing world.

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TYPES:

1. Open / Gravity type (Peterson’s Filters)

2. Closed / Pressure type (Candy’s Filters)

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STEPS / PROCESSES IN RAPID SAND FILTERS

1. COAGULATION: Raw water is first treated with Alum in a

dose of 5-40 mgs./L pH has to be adjusted by addition of lime or

soda ash as Alum needs CaCo3 to form the flock.

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2. RAPID MIXING: Water is mixed rapidly with Alum for a

few minutes in mixing chamber.

3. FLOCCULATION: From mixing chamber water is moved into

the Flocculation chamber where it rests for half an hour.

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4. SEDIMENTATION: The coagulated water is now led into the

sedimentation tank where it is detained for 2-6 hours.

Flocculant precipitate together with impurities & bacteria settle down in the tank

Contnd.--------Contnd.--------

5. FILTRATION Partially clarified water is now subjected to

the rapid sand filtration. Depth of the sand bed is 1 M & each rapid

sand filter unit has a surface of about 80-90 Sq.M.

Oxidation of NH3 also takes place in these filters.

ADVANTAGESADVANTAGES

Rapid sand filters deal with raw water directly

Occupies less space Filtration is rapid about 40-50 times as

compared to slow sand filters Washing of filter is easy More flexibility in operation

DISADVANTAGESDISADVANTAGES

Preliminary treatment with Alum is required

Operation is highly skilled Removal of color material is less than slow

sand filter Reduction of bacterial count is less than

slow sand filter

Comparison of Rapid and Slow Comparison of Rapid and Slow sand filterssand filters

Rapid.S.F. Slow.S.F. 1. Space: Occupies little space-----Occupies large area

2. Rate of filtration: 5-15 cu.m./sq.m./hr.----- 0.1-0.4

3. Effective sand size: 0.4-0.7mm-----------------0.2-0.3mm

4. Preliminary treatment: Coagulation& sedimentation--plain Sediment

5. Washing: By back washing--------By scraping the sand

6. Loss of head allowed: 6-8 feet--------------------4feet

7. Removal of turbidity: Good-----------------------Good

8. Removal of color: Good------------------------Fair

9. Removal of bacteria: 98-99 %--------------------99.9-99.99

DISINFECTION OF WATERDISINFECTION OF WATER

CHEMICAL METHODS / GERMICIDES1. CHLORINE: is used for purification of water on large scale2. KMnO4: used for disinfection of water in wells especially

during epidemics3. CuSO4/CaCl2: used for the removal of algae4. Lime: used for muddy water5. Ozonization & U.V. irradiation: both methods are used for

disinfection of water in swimming pools6. Katadyn Silver: sand coated with silver used in slow sand

filters instead of plain sand7. Charcoal: used as adsorbent to remove color,odor & taste

Disinfection of Wells Disinfection of Wells

The most effective & cheapest method of disinfecting wells is by bleaching powder

STEPS IN WELL DISINFECTION :

1. Find the volume of water in a well:

a. Measure the depth of water column..(h) M

b. Measure the diameter of well………(d) M

c. Substitute h and d in:

Cotnd…….Cotnd…….

3.14 x (d)2 x h Volume(litres)=------------------------------ x 100 4

d. One cubic meter = 1,000 litres of water

Find the amount of bleaching powder required for disinfection:

Estimate the Chlorine demand of well water by HORROCK’S APPARATUS

Contnd……..Contnd……..

3. Dissolve Bleaching Powder in water

4. Delivery of Chlorine solution into the well

5. Contact period

6. Orthotolidine arsenite test

Chlorination of waterChlorination of water

It is the most widely used method for water purification, both on large as well as on small scale.

It is the most reliable and cheapest method.It is supplement but not the substitute to

sand filtration.

Properties of ChlorineProperties of Chlorine

Killing of pathogenic bacteria by inhibiting their protein synthesis.

Oxidizes Iron, Manganese & Hydrogen Sulphide. Destroys taste & odor producing constituents. It controls algae growth. It helps in coagulation and suspension of the

organic matter.

Action Of ChlorineAction Of Chlorine

When chlorine is added to water it forms HCl & HOCl.

The disinfecting action of chlorine is mainly due to Hypochlrous acid & to a small extent due to Hypochlorite Ions.

Hypochorous acid is 70-80 times more effective than Hypochlorite Ions.

Actions of Chlorine depends Actions of Chlorine depends upon the……..upon the……..

Organic matter Metals Bacterial content Temperature & pH of water. Suspended matter and, Amount of free residual chlorine in water

Contnd………Contnd………

Efficiency of chlorine increases, when it combines with Ammonia and……

It decreases, when it combines with

Sulphides, Ferrous salts & inert matter in suspension form

Principles of ChlorinationPrinciples of Chlorination

Water to be chlorinated must be free from turbidity

Chlorine demand of water should be estimated

Contact period of one hour is essential to kill the bacteria and viruses

Minimum recommended concentration for free chlorine is 0.5mg. / liter

Chlorine demand of water Chlorine demand of water

Is the difference between the amount of chlorine added to water and the amount of residual chlorine remaining at the end of a specific period of contact (usually 60 min.) at a given temperature and pH of water.

ACUAL DOSE OF CHLORINE: It is the sum of chlorine demand of water

and free residual chlorine

Methods of ChlorinationMethods of Chlorination

1. Chlorine gas

2. Chloramine

3. Perchloron (H.T.H.)

TYPES OF CHLORINATION:

a. Superchlorination

b. Break Point Chlorination

Tests for the presence of Tests for the presence of Chlorine in WaterChlorine in Water

Orthotolidine (O.T.) Test

Orthotolidine-Arsenite (O.T.A.) Test

Starch Iodide Test

Surveillance of drinking WaterSurveillance of drinking Water

1. Sanitary survey

2. Sampling

3. Bacteriological surveillance

4. Biological examination

5. Chemical surveillance

Sampling of WaterSampling of Water

Collection, Storage & Transportation of water samples from……

Taps Rivers Streams Tanks WellsA. For Physical & Chemical ExaminationB. For Bacteriological Examination PERTICULARS OF A SAMPLE

Standards of WaterStandards of Water

PHYSICAL STANDARDS Turbidity Color Odor Taste

Contnd………Contnd………

CHEMICAL STANDARDS Hardness Chlorides Ammonia Nitrites Nitrates Absorbed Oxygen Dissolved Oxygen

Contnd……..Contnd……..

Toxic substances and a specific group of chemicals causing Non-Specific diseases of water

Poly-nuclear Aromatic Hydrocarbons and Radioactive substances ( a group of chemical substances of water affecting the acceptability of water.

Bacteriological StandardsBacteriological Standards

1. BACTERIAL STANDARDS : W.H.O. recommended these standards in

1984 for Public water supplies Ideally all samples taken from the

distribution system should be free from Coliform organisms

In Practice this standard is not always attainable

Recommended Standards for Recommended Standards for Water samples collectedWater samples collected

Throughout any year 95 % samples 100 ml. each should not contain any Coliform organisms

No samples should contain E.coliNo sample should contain more than 03

ColiformAny two consecutive samples should not

contain Coliform

Contnd………Contnd………

In case of Individual / Small water supplies such as from household Tap, Well, Spring etc.

Coliform count should not exceed 10 /100 ml. and E.coli should be nil

Contnd………..Contnd………..

2. VIRAL STANDARDS : W.H.O. Standards fix the limit for viruses

such as Eneroviruses, Retroviruses & Adenoviruses in water supplies at 01 Plaque forming unit

Faecal Bacteroiphage & Enteropathogenic viruses should be completely absent

Bacteriological IndicatorsBacteriological Indicators

These are based on organisms indicative of faecal pollution, These organisms include!

1. E.coli and Coliform group as a whole2. Faecal Streptococci3. Clostridium Perfringens4. Examination for Protozoa, Parasitic

worms & Larvae is also recommended

Contnd……..Contnd……..

Reasons why Coliform organisms are chosen as indicators of Faecal Pollution

1. Constantly present in human intestine an average person excretes 200-400 billions / day of these organisms in faeces

2. They are foreign to potable water supplies3. Easily detectable by ordinary cultural methods 4. Survival time is much better than other organisms5. Have greater resistance to natural forces of

purification

Contnd……..Contnd……..

FAECAL STREPTOCOCCI :

Faecal Strepococci occur regularly in much smaller number than E.coli

Their presence is regarded as confirmatory evidence of recent faecal contamination of water in doubtful cases

Contnd………Contnd………

CLOSTRIDIUM PERFRINGENS :

They are spore bearing organisms Their presence along with Coliform indicate

recent water contamination While the presence of their spores but the

absence of Coliform confirms remote contamination of water