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CE's Circular No.192

JOINT CIRCULAR ON QUALITY CONTROL AND PURIFICATION OF DRINKINGWATER TESTING AND CHLORINATION

Dated: 08.10.99.

CEs Circular No. 192 CEs File No. CE/WSS/POLICYCMDs Circular No. CMD/Policy/99/47 CMDs File No. DRW/51/ IV

Sub: Guidelines for the quality control and chlorination of

water for drinking and domestic use.

1.0 The imperative need for purification/disinfection of water for drinking purposecannot be overemphasized. Water supply for domestic and publicconsumption should be free from pathogenic organism, clear, potable, free

from undesirable taste and odour and free from mineral which could produceundesirable physiological effect. The physical, chemical and bacteriological

standards should be as indicated in the following paragraphs.

1.1 Responsibility of Engineering and Medical Departments :

Engineering department shall be responsible for the planning, developmentand construction of water treatment plants including chlorination and supply of

ingredients required while the Medical department shall be responsible formonitoring the quality of water (physical, chemical and bacteriological) and

shall arrange the necessary apparatus required. However, the Engineering

department will also carryout checks from time to time to ensureeffectiveness of treatment being done.

2.0 Physical and Chemical standards :The physical quality of water should not exceed the following limits.

Sl.

No.

Property Acceptable Rejectionlimit

(i) Turbidity (units on JTU scale) 2.5 10

(ii) Colour (units on platinum cobalt

scale)

5.0 25

(iii) PH Value 7.0 to 8.5 6.5 to 9.2

(iv) Total dissolved solids (mg per litre) 500 1500

(v) Total Hardness (as CaCO3)(mg/litre) 200 600

(vi) Chlorides (as Cl ) (mg/l) 200 1000

(vii) Sulphates (as SO4) (mg/l) 200 400

(viii) Fluorides (as F) (mg/l) 1.0 1.5

(ix) Nitrates (as NO3) (mg/l) 45 45

(x) Calcium (as Ca) (mg/l) 75 200

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(xi) Magnesium (as Mg) (mg/l) 30

(If there are 250mg/l of sulphates, Mg

content can beincreased to a

maximum of 125mg/l with the

reduction of

sulphates at the rateof 1 unit per every

2.5 units ofsulphates)

150

(xii) Iron (as Fe) (mg/l) 0.1 1.0

(xiii) Manganese (as Mn) (mg/l) 0.05 0.5

(xiv) Copper (as Cu) (mg/l) 0.05 1.5

(xv) Zinc (as Zn) (mg/l) 5.0 15.0

(xvi) Phenolic compounds (as

phenol)(mg/l)

0.001 0.002

(xvii) Anionic detergents (MBAS) (mg/l) 0.2 0.1

(xviii) Mineral Oil (mg/l) 0.01 0.3

(xix) Arsenic (as As) (mg/l) 0.05 0.05

(xx) Cadmium (as Cd) (mg/l) 0.01 0.01

(xxi) Chromium ( as hexavalent Cr) (mg/l) 0.05 0.05

(xxii) Cyanides (as Cn) (mg/l) 0.05 0.05

(xxiii) Lead (as Pb) (mg/l) 0.1 0.1

(xxiv) Selenium (as Se) (mg/l) 0.01 0.01

(xxv) Mercury (total as Hg) (mg/l) 0.001 0.001

Note : The figures indicated above as 'Acceptable' are the limits up to which the

water is generally acceptable. However, in the absence of better sources, thelimits shown as

'Rejection limits' may be tolerated, above which the supply may have to

be rejected.

3.0 Bacteriological standards :Water supply meant for drinking and domestic use shall satisfy the following

criteria :(a) The coliform count in any sample of 100 ml water entering the distribution

system should be zero.

(b) Water in the distribution system shall satisfy all the following three criterion :(i) E.Coli count in 100 ml of any sample should be zero.(ii) Coliform organism should not be present in 95% of samples of 100 ml

collected through out the season.(iii) Coliform organism should not be detectable in 100 ml of any two

consecutive samples. If coliform organisms are found, resamplingshould be done. The repeated finding of 1 to 10 coliform organism in

100 ml should necessitate the investigation and removal of the sourceof pollution.

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4.0 Disinfection of water :

Disinfection of water can be done broadly by the following two processes :

(1) Physical methods such as by boiling and ultrasonic waves.(2) Chemical methods such as by use of Chlorine and its compounds, bromine,

iodine, potassium permanganate, ozone and metals like copper and sliver.

(3) Radiation methods.A good chemical to be used as a disinfectant should be capable of destroyingthe pathogenic organisms present in water within the contact time availableand should not make the water toxic or impart unacceptable colour or odour

during the chemical reaction. It should leave a certain amount of residuals so

as to deal with small possible recontamination.

5.0 Chlorination :

5.1 Chlorine and its propertiesChlorine is an element represented by symbol Cl . Gaseous chlorine is greenish

yellow in colour and is about 25 times heavier than air. Under pressure of 35

Kg/cm2 it becomes liquid with an amber colour and oily in natureapproximately 15 times as heavy as water. Chlorine gas is harmful to human

beings since it is a powerful irritant to lungs and eyes. Safety limit for aworking environment should not exceed 1 ppm by volume for an exposure

period of 8 hours.

5.2 Chlorine-Water reaction

Chlorine gas reacts with water to form Hypochlorous acid (HOCl) and

Hydrochloric acid (HCl). The Hypochlorous acid dissociates into Hydrogen ions(H+ ) and hypochlorite ions (OCl -). Free available chlorine is the sum total of

Chlorine existing in water as Hypochlorous acid and Hypochlorite ions.

5.3 Chlorine demandChlorine and chlorine compounds are consumed by inorganic and organicmatters present in water before any disinfection is achieved. For achieving

proper disinfection of water by killing the pathogenic organisms, it is essential

to provide sufficient time and dose of chlorine to satisfy various chemicalreactions and leave adequate amount of chlorine as residual in the form of

free or combined chlorine. The difference between the amount of chlorineadded to water and amount of residual chlorine after a specified contact time

is the chlorine demand. In other words it is the amount Cl2 that is needed todestroy bacteria and to oxidise all organic matter and ammoniacal substances

present in the water.

6.0 Chlorination Practices :There are generally two practices as explained below :-

(i) Plain chlorination :This is applied where water is relatively less polluted andsufficient contact period of at least 30 minutes between the point of

chlorination and consumer point is available.

(ii) Super chlorination : This is adopted in case of water being heavily pollutedor where available contact period is short i.e. less than 30 minutes. In case ofsuper chlorination, it may be necessary to go in for dechlorination to remove

the excess residual chlorine.

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7.0 Residual Chlorine :Satisfactory disinfection of water is obtained by pre-chlorination to maintain

free available residual chlorine to the extent of 0.2 to 0.3 mg per litre (ppm) inthe plant effluent at normal PH value. However, at higher PH value of 8 or 9,

at least 0.4 ppm of residual chlorine is required for complete bacterial kill with

10 minutes contact period. For 30 minutes contact period, the dosage may bereduced to have 0.2 to 0.3 ppm of residual chlorine.

Where there is fear of epidemic of water borne diseases or water is

infested with nematodes, the supply should be prechlorinated for six hours tomaintain a free available residual chlorine of 0.4 to 0.5 ppm.

Where the water source is from an open river, canal or pond, it is

recommended to have an amount of free available residual chlorine to theextent of 0.4 ppm to 0.5 ppm and allow a minimum of contact period of 60

minutes.

8.0 Chlorination Methods :

Generally following three methods are adopted for chlorination of water using

different chlorine producing agents.

(a) Bleaching powder solution is added to the water, the chlorine available inbleaching power is used to disinfect the water. This method is suitable only for

smaller requirement of water.

(b) Chlorine is produced by electrolysing a solution of brine or common salt andthe chlorine so produced is used for disinfection.

(c) Chlorine is obtained directly from the cylinders filled with pure chlorine ingaseous or liquid form. This method is generally adopted for larger size of

water supply plants

9.0 Detection of residual chlorine :

Detection of residual chlorine can be done by a simple apparatus known as

chloroscope using orthotolidine test (OT) and orthotolidine arsenite test (OTA).The OT is used to determine the total residual chlorine concentration and the

OTA is used to determine the free and combined residual chlorine separately.When orthotolidine reagent is added to water sample containing chlorine, a

greenish yellow colour develops, the intensity of which is proportional to theamount of residual chlorine available in the water sample. The OT test is not

as accurate as the presence of nitrates, iron and manganese also produce ayellow colour with orthotolidine. The test using OTA is more reliable.

The method of determination of residual chlorine by Chloroscope isdescribed in Appendix-I.

10. Chlorine compounds :

Various compounds are available which make the chlorine available when theycome into contact with water. These are bleaching powder, hypochlorites and

chlorine dioxide. These are briefly described below :-

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(i) Bleaching Pow der : It is a variable mixture of calcium hydroxide, Calcium

chloride and calcium hypochlorite. When it is mixed with water, the calciumhypochlorite decomposes into calcium chloride and chlorine. Bleaching powder

is characterised by the available chlorine it liberates by complete reaction with

water. Commercial brands of bleaching power have an available chlorine of20% to 30 % by weight. Grade-I bleaching powder as per ISI code : 1065

should be used for water treatment which should have a minimum of 30% of

chlorine.

The bleaching power is an unstable compound and rapidly loses its

chlorine content on exposure to air, light, moisture and storage. But when

mixed with excess of lime, it retains its strength; this is called "stabilisedbleach". The bleaching powder should be stored in a dark, cool and dry place

in a closed container. The chlorine content of bleaching powder should befrequently checked.

Bleaching powder is generally mixed in water to make a thin slurry

which contains the chlorine in solution. This mixture is mixed with water by a

suitable feeding mechanism such as a float operated gravity box. In everyinstallations, the solution may be applied through a dip feed mechanism. In

case of deep tubewell supplies, bleaching powder solution is introduced at thesuction side of the pump. An injector may be fitted on a bleed line on the pump

discharge to suck the solution of the powder in proportion to the flow of water.

To determine the quantity of bleaching powder required to be added towater so that the residual chlorine is within the limits of 0.2 ppm to 0.4 ppm,

HORROCKS APPARATUS is mainly used. This is a very simple apparatus and is

easily available in the market. The details of some of the probable sources aregiven in Appendix-II. The operation of the HORROCKS APPARATUS is

described in Appendix-III. Appendix-IV gives in a tabular form the amount ofbleaching powder required to disinfect certain quantities of water.

(ii) Hypochlorites of Sodium and Calcium : Specially fortified brands of calciumhypochlorite such as perchloron and High Test Hypo (HTH) may have 60% to70% of available chlorine. Calcium hypochlorite can be fed either in dry or

solution form. Sodium hypochlorite is fed in solution form. Usually constant

head gravity devices with adjustable orifices are used to dose hypo solution inthe tanks. These can be fed through chemical preparationing pumps and can be

injected under pressure into pressure pipe lines by venturi or orifice feeders.

(iii) Chlorine diaoxide: Chlorine diaoxide is an unstable gas. It is formed byreacting a strong solution of chlorine with sodium chlorite. Chlorine diaoxide is

unstable and subject to explosion in gaseous form but aqueous solutions of thegas are stable and safe. It is a good sporicide and a strong oxidant.

11. Purification/ disinfectionThe purification/disinfection of water on a large scale in the DivisionalHeadquarters and in such other big settlements is generally done by installing

purification plants comprising flocculation chamber, sedimentationtank/clarifier, filtration bed, shortage tank etc. Chlorine in the form of liquid

chlorine/liquid sodium hypochlorite is added to filtered water. In the recentpast, in some plants Electro-Chlorinator has been installed. In the Electro-

Chlorination process sodium chloride(common salt) is used. This equipment

produces hypochlorite solution through electrolysis of sodium chloride(brine),where Chlorine is discharged at Anodes and a small quantity of hydrogen gas is

liberated at Cathode. Electro-Chlorination system does not evolve chlorine as a

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gas, but is available in the dissolved form as sodium hypochlorite solution andhydrogen gas is vented to the atmosphere.

The sodium hypochlorite so produced in the above process is injected into

filtered water with the help of dosing pump. The hypochlorite solution when

added to water has the same biocidal and oxidising power, which is equivalentto chlorine gas and provides the same degree of disinfection.

12. Water Quality Monitoring and Surveillance.12.1 Monitoring:-12.1.1 Testing

i) Field kit testing

ii) Laboratories testing

Sampling is very important in testing of water. Various samplingmethods and guidelines are given in Annexure-V.

12.2 SurveillanceIt is to correlate the water quality with the health condition of the

population served. This surveillance requires a good liaison with curative

medical services, to check the water borne diseases and also timely action ofthese diseases, when they break out in epidemic proportions.These also

include proper cleaning and disinfection of stored water like ground level

reservoir and over head tanks. The over head tanks should be cleaned once inthree months and the ground level reservoirs once in six months.

Proper maintenance of filtration plant is required. Periodic cleaning of

filter media, pipe lines are to be checked for contamination, ( as corrosion ofthe pipes or leakages in the joints may suck-in contaminated substances, as

water does not flow all the time in the pipe lines, causing lowering of pressure

inside the pipe lines).

Sd/- Sd/-(G.HARIHARAN) (S.P.S.JAIN)

CHIEF MEDICAL DIRECTOR CHIEF ENGINEERS.E.RAILWAY,GARDEN REACH. S.E.RAILWAY,GARDEN REACH

APPENDIX-I

CHLOROSCAPE

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For determination of residual chlorine through a Chloroscope, samples of waterare required to be collected in the prescribed test tubes and a reagent called

Orthotolidine is used. Such a test is called as ORTHOTOLIDINE Test.

ORTHOTOLIDINETEST

Orthotolidine test enables both free and combined Chlorine in water to be

determined with speed. The reagent consists of analytical grade Orthotolidine,

dissolved in 10% solution of hydrochloric acid. When this reagent is added to watercontaining chlorine, it turns yellow and the intensity of the colour varies with theconcentration of the gas. The yellow colour is produced by both free and combined

chlorine residuals.

The test is carried out by adding 0.1 ml. Of the O.T reagent to 1 ml. of water.

The yellow colour produced is matched against suitable standards of coloureddiscs/tubes containing standards solution of water. It is, however, essential to take

reading within 10 seconds after the addition of the reagent to the water. Thematching tubes of coloured solution of Chloroscope are available to determine 0.1 to

2.0 ppm. of residual chlorine.

APPENDIX-II

Names of some of the Firms who supply HORROCK Water Testing Apparatus are givenbelow:-

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1. Sanjoy Jain, (Director),

J&BB TREXIM PVT.LTD.,13/11-B Swinhoe Street,

Calcutta-700019, b Ph.440-9934,440-4401.

2. TITANOR COMPONENTS LTD.,(An oronzio Donora Group Company),

Plot No.184, 185 & 189 Kundaim Industrial Estate,Kundaim, Goa-403110, Tele (0834)395340/41/42/43,FAX (0834)395268/69.

3. Murcury (International)Pvt.Ltd.,(Environmental Management Division),

2A/244-4A, Azad Nagar, Kanpur-208-002Ph:560303, 560311,

FAX (0512)560-338.

4. Sumeet Instruments & Chemicals29-B Rabindra Sarani, 4th floor, Room No.427(near Naaz Cinema), Calcutta-700073.

Ph-235-0122/235-0993FAX 033-2350993.

5. M/s. Gudimanu Enterprises (India),70, Okhla Industrial Complex,Phase-I, New Delhi-110020

6. M/s. Engineering Enterprises,14, Brindawan Extension Street,

Madras-600033.

7. M/s. Indian Refrigeration Engineers,57, Mangalwar Peth,Barne Road, Pune-1.

APPENDIX-III

HORROCKS APP ARATUS

Horrocks water testing apparatus is designed to fine out the dose of bleaching

powder required for disinfection of water.

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CONTENTS :

1. 6 white cups (200 ml capacity each)2. One black cup with a circular mark on the inside3. 2 metal spoons (each holds 2g of bleaching powder when filled level with brim)4. 7 glass stirring rods5. One special pipette6. Two droppers7. Starch-iodine indicator solution8. Instruction folder

PROCEDURE:

1. Take one level spoonful (2g) of bleaching powder in the black cup make it intoa thin paste with a little water. Add more water to the paste and make up the

volume up to the circular mark with vigorous stirring. Allow to settle. This is

the stock solution.

2. Fill the 6 white cups with water to be tested, up to about a cm. Below thebrim.

3. With the special pipette provided add one drop of the stock solution to the 1stcup, 2 drops to the 2nd cup, 3 drops to the 3rd cup, and so on.

4. Stir the water in each cup using a separate rod.5. Wait for half an hour for the action of chlorine.6. Add 3 drops of starch-iodide indicator to each of the white cups and stir again.

Development of blue colour indicates the presence of free residual chlorine.

7. Note the first cup which shows distinct blue colour. Supposing the 3rd cupshows blue colour, then 3 level spoonfuls or 6 grams of bleaching powder

would be required to disinfect 455 litres of water.

APPENDIX-IV

AMOUNTS OF CHEMICALS NEEDED TO DISINFECT WATER FOR DRI NKING:

Water (Cubic

Metre)

Bleaching Powder

(25-35%)(g)

High Strength

Calciumhypochlorite

Liquid bleach (5%

sodiumhypochlorite)(ml)

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(70%)(g)

1

1.5

22.5

34

5

67

1015

2030

4050

60

70100

150200

150300

400

500

2.33.5

56

79

12

1416

2335

5070

90

120140

160230

350470

580700

940

1170

11.5

22.5

34

5

67

1015

2030

40

5060

70100

150200

250300

400

500

1421

2835

42

5670

8498

140210

280

420560

700840

9801400

21002800

35004200

5600

7000

*Approximate dose-0.7 mg of applied Chlorine per litre of water.

APPENDIX-V

WATER-QUALITY-MONITORING

Indian Railways being the largest employer and also a commercialorganization has the responsibility to provide safe drinking water not only to the

Railway beneficiaries but also to the consumer. The provision of safe drinking water

even though is the domain of Civil Engg. Deptt, the monitoring of the quality is done

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by the Medical Deptt. in the Indian Railway context. Ideally if Public Health Engg.Deptt. exists it can look after both the functions. However, the following points are to

be observed in the water quality monitoring.

For monitoring the quality of the water there should be adequateinfrastructure, qualified/trained personnel and a well-designed schedule. Laboratories

with adequate facilities and manned by qualified personnel are essential for inspection

and evaluation of the suitability of water for public use. The ultimate aim of analysisof water is to ensure that potable water conforming to the drinking water standard issupplied to be consumers. Test carried out in the laboratory are intended to assess

the quality and classify the raw water to be treated, to determine the need and extent

of treatment, to check that water has been properly prepared for each phase oftreatment process, to ensure that the phase of treatment proceeds accordingly to

plan and to examine the finished water to ascertain that it conforms to the standard.

Types of analysis of water

i) Physical analysis: This determine the aesthetic quality and assesses theperformance of various water treatment units.

ii) Chemical analysis: This determine concentrations of chemical substance whichmay affect the quality of water and the indicative of pollution and which reflectvariations due to treatment.

iii) Bacteriological Analysis: This indicates the presence of bacteria characteristicof pollution and also determines the safety of water for human consumption.

iv) Biological Analysis: This will find application in providing information on causesof objectionable testes and orders; of clogging of filters.

Sampling:

The value of any laboratory analysis and test depends upon the method of

sampling. Failure to observe proper precautions in securing a representative sample

may result in an analysis which is of little use since it may unnecessarily condemn agood water supply or more frequently it may certify a bad water as satisfactory.

All samples of water should be properly labeled and accompanied by complete

and accurate Identifying and descriptive data. The data should include date and timeof collection, type of source of the sample and temperature of water at the time of

collection. When samples are collected from the same sampling point for differentanalysis, it is essential that the sample for bacteriological analysis be taken first. For

transport, bottles may be packed in wooden, metal, plastic or heavy fibreboard cases,with a separate compartment for each bottles. Boxes may be lined corrugated fibre

paper, felt or other resilient material or may be provided with spring loaded cornerstrips to prevent breakage.

Sampling for physical and chemical Examination of water to be tested hasbeen detailed in the IS Specification No. IS-3025 (Part-I)-1987.

Use of Appropriate container:- The choice and the preparation of a container can be

of major importance. However, it should be remembered that the container in whichthe sample is stored and the stopper should not:-

a) be a cause of contamination (for example, Borosilicate or Soda limeglass may increase the content of Silica or Sodium)

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b) absorb the constituents to be determined (for example, hydrocarbonsmay be absorbed in a polythene container, tracks of metals may be absorbed on the

surface of a glass container) and

c) react with certain constituents in the sample (for example fluridesreacting with glass).

Collection of the sample from a Tap.

When the sample is to be taken from a tap in regular use, the tap should beopened fully and the water run to waste at least for two minutes in order to flush the

interior of the nozzle and to discharge the stagnant water in the service pipe.

Sample should be collected in containers of pyrex glass or other inert material

like polythene. These bottles must be carefully cleaned before use. Glass bottles maybe rinsed with a chromic acid cleaning mixture (prepared by adding one ltr. Of

concentrated sulphuric acid slowly with stirring of 35 ml. Of saturated sodiumdichromate solution followed by oxalic acid solution). After having been cleaned,

bottle must be rinsed thoroughly with tap water and then with distilled water. About

2.5 ltrs of the sample is required for us. Prior to filling the sample, bottle should berinsed out 2 or 3 times with water to be collected. The sample should reach the place

of analysis as quickly as possible. The time elapsed between collection and analysisshould be recorded in the laboratory report.

Certain parameters like temperature, PH, dissolved gasses like Carbon di

oxide, Hydrogen sulphide, Chlorine and Oxygen may change significantly duringtransport. For this reason these tests should be carried out on the spot.

Hot samples collected under pressure should be cooled while under pressure.Samples from wells should be collected only after the well has been pumped for a

sufficient time to ensure that the sample will be representative of the ground water.

Sampling for bacteriological analysis:- Metod of sampling and microbiological

examination of water have been brought out in detail in IS specification No.IS 1622-1981 (Reaffirmed in 1987).

Sampling bottles:- Sterilised glass bottles provided with ground glass stopper

having and overlapping rim should be used. This stopper and the neck of the bottleshould be protected by ground paper. This sterilisation is carried out in an autoclave

at 1 Kg./Sq. cm. Pressure for 15 minutes or by dry heat at 160 degree Centigrade for1 hr.

Dechlorination:- It is necessary for chlorinated water samples. Sodiumthiosulphate

should be added to the cleaned dry sampling bottles before sterilisation (2 ml.10%Thiosulphate solution to be added for 250 ml. Bottles).

Sample Collection: The sample should be representative of water to be tested and

they should be collected with utmost care to ensure that no contamination occurs atthe time of collection prior to examination. The sample bottle should not be opened

till the time of filling. The stopper with cap should be removed with care to eliminatesoiling. During sampling, the stopper and the neck of the bottle should not be

touched by hand and they should be protected from contamination. The bottle should

be held near the base, filled with rinsing and the stopper to be replaced immediately.The bottle should not be filled completely but sufficient air space should be left for

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shaking before analysis. Then the brown paper wrapping should be tied to protect thesample from contamination.

Sampling from taps: The tap should be opened fully and the water allowed to run to

waste for 2 or 3 minutes to permit clearing of the service line. The flow from the tap

should be restricted to permit filling the bottle without splashing. Leaking taps whichallow water to flow over the outer surface of the bottle must be avoided at sampling

points. If it becomes necessary to collect from such points the leak should be

attended to before sampling. When a tap is not in continuous service it is advisable towipe the tap free of any grease or preferably flamed before collection of the sample.It should be ascertained whether a tap from where the sample is collected is

supplying water from a service pipe directly connected with main or with a cistern or

storage tank.

Sampling direct from a source: When the sample is to be collected directly from astream, lake, river, reservoir, spring or shallow well it should be taken from a point

which is neither too near to the bank nor too far from the point of draw off. Samplefrom these sources can be taken by holding the bottle in the hands near its base in

plunging its neck downward below the surface. The bottle should then be turned until

the neck points slightly upward, the mouth being directed against the current.

The volume of the sample should be sufficient for carrying out all the testsrequired and in no case, which should be less than 250 ml.

Reservation and storage:- Water sample should be examined immediately after

collection. However, this is seldom practical and hence it is recommended that asample should be preferably analised within 1 hr. after collection and in any case this

time should not exceed 24 hrs. During transit the temperature of the sample should

be maintained as close as possible to that of the source of sample, at the time ofsampling. The time and temperature of samples should be recorded. If the sample

cannot be analised within 24 hrs., the sample must be preserved in the ice untilanalysis. No sample is fit for bacteriological analysis after 72 hrs.

Frequency of sampling:

Population Served Maximum intervalsbetween successive

sampling

Minimum No. of samples tobe taken from entire

distribution system

Up to 20,000 One month One sample per 5,000

population per month

20,000-50,000 Two weeks One sample per 5,000population per month

50,000-1,00,000 Four Days One sample per 5,000population per month

More than 1,00,000 One Day One sample per 10,000population per month

The frequency of collection of samples for chemical analysis depends on

variability of quality of tested water, the types of treatment process used and otherlocal factors. Sample of general systematic chemical examination should be collected

at least once every three months in supplies upto 50,000 inhabitants.

It is necessary to collect samples of both raw and treated water forexamination of toxic substances at least every three months and more frequently

when sub-tolerance levels of tixic substances are known to be generally present in

the source of supply or where such potential pollution exists.

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For bacteriological sampling, the samples should be taken from different pointson each occasion to enable over all assessment. In the event of an epidemic or

immediate danger of pollution, it should be borne in mind that much more frequentbacteriological examination would be required than the recommended minimum

frequencies for routine examination.