chlorination exercises

St. John’s Medical College

Department of Community Health

Exercises on water purification & quality:

A. Chlorination exercises:

1) Determine the quantity of bleaching powder with 25% available chlorine

required to chlorinate a well measuring 4 metres in length and 3 metres in

breadth with 6 metres of water in it. Horrocks test done on a sample of water

shows blue colour in the 3rd cup. Explain the practical procedure of

disinfection of the well.

Solution:

In order to determine the quantity of bleaching powder required to

chlorinate the given well, we need to find the latter’s volume.

Volume of water in the given = l x b x h x 1000

rectangular well (in l)

where, =3.14, l=length of the well (in m), b= breadth of the well (in m),

h=height of the water column in the well (in m). (1 m3=1000 l)

Substituting the values in the above formula, we get:

Volume (V) = 4 x 3 x 6 x 1000

= 72000 l

Since Horrock’s test done on the given sample of well water shows blue

colour in 3rd cup onwards, 3 level spoonfuls of the Horrock’s apparatus,

i.e., 3 x 2g = 6g of the given bleaching powder would be required to

disinfect 455 l of the given sample of water.

455 l of well water require 6 g of the given bleaching powder.

72000 l of the well water require

Horrock’s test result itself takes into account the strength of the

bleaching powder.

Hence 949.45 kg of the given bleaching powder with 25% available chlorine is

72000 x 6455 = 949.45g g

Given data:1. l = 4 m2. b = 3 m3. h = 6 m4. Horrock’s test: 3rd cup +ve,5. Given bleaching powder has 25% available chlorine.

required to chlorinate the given well.

Practical Procedure of disinfection of the well

a. Finding the volume of water in the well:

1. Measuring the depth of water column: Tie a stone to one end of a rope and

let it sink till it just touches the bottom of the well. Pull the rope immediately &

measure the length of the rope from the stone tied end upto the upper end of

the water soaked part. Take average of several readings of the above

measurements. This gives the depth/height (h) of the water column, which is

expressed in meters. (6 m in the given case)

2. Measuring the length & breadth of the well: Stand outside the well & note

the distance of the rope placed across the top of the well along the longest

dimension. Take average of several readings of the above measurements.

This gives the length (l) of the water column, which is expressed in meters. (4

m in the given case). Similarly note the distance of the rope placed across the

top of the well along the shortest dimension, perpendicular to the length. Take

average of several readings of the above measurements. This gives the

breadth (b) of the water column, which is expressed in meters. (3 m in the

given case).

3. Calculation of the volume of well water (in litres) using the formula for

volume of a rectangular cuboid (in the given case) l x b x h x 1000

and by substitution of l,b & h, we get 72000 l

b. Finding the quantity of bleaching powder required for disinfection:

Estimation of the chlorine demand of the well

water by Horrock’s apparatus (3rd cup +ve in the given case) & calculation of

the quantity of bleaching powder required (949.45 g in the given case)

c. Dissolution of the required bleaching powder:

The required amount of bleaching powder for

disinfecting well is placed in a bucket (Not more than 100 gm at a time).

It should be made in to a thin paste.

More water is added until the bucket is nearly 3/4th

full.

The contents are stirred well and allowed to

sediment for 5 to 10 min when lime settles down.

The supernatant solution, which is Cl2 solution is

transferred to another bucket and the chalk or lime is discarded.

d. Delivery of the chlorine solution:

The bucket containing Cl2 solution is lowered into

the well some distance below water surface.

The well water is agitated by moving the bucket

violently both vertically and horizontally.

This should be done several times so that the Cl2

solution mixes intimately with water inside the well.

e. Contact period:

A contact period of 1 hr is allowed before water is drawn for use.

f. Measurement of residual chlorine:

Orthotolidine arsenite (OTA) test at the end of contact period is done by

adding 0.1ml of OTA reagent to 1ml of the chlorinated well water as a

semiquantitative measure of free residual chlorine.

0.1ml OTA reagent + 1ml of the chlorinated well water ---> Yellow colour

matched against colour standards.

Free residual chlorine level ≥0.5mg/l indicates proper disinfection. If it is

<0.5mg/l, the chlorination procedure should be repeated before any water is

drawn. Wells are best disinfected at night after the day’s draw off.

2 ) The 3rd cup in the Horrock's outfit gives a definite blue colour when the

indicator is added. Calculate the quantity of bleaching powder required to

disinfect a well 6 metres in diameter with 3 metres of water in it. Explain the

practical procedure of disinfection of the well.

Solution:



Volume of water in the given =

cylindrical well (in l)

where, =3.14, d=diameter of the well (in m),



Volume (V) =

=

= 84780 l








bleaching powder.

Hence 1.12 kg of the given bleaching powder is required to chlorinate the

given well.







3.14 x 62 x 3 x 1000

4

x d2 x h x 10004

3.14 x 36 x 3 x 1000

4

84780 x 6455 = 1117.98g

g

Given data:1. d = 6 m2. h = 3 m3. Horrock’s test: 3rd cup +ve,



2. Measuring the diameter of the well: Stand outside the well & note the

maximum distance of the rope placed across the top of the well. Note the

inner diameter of the well. Take average of several readings of the above

measurements. This gives the diameter (d) of the water column, which is



volume of a cylinder (in the given case)

and by substitution of d & h, we get 84780 l




the quantity of bleaching powder required (1118 g in the given case)






full.












x d2 x h x 10004

e. Contact period:











3 ) Determine the quantity of bleaching powder with 20% available chlorine

required to chlorinate a rectangular well measuring 4 metres in length, 2

metres in breadth with 8 metres of water. Horrocks test done on a sample of

water shows blue colour in the 4th cup. Explain the practical procedure of

disinfection of the well.

Solution:

In order to determine the quantity of bleaching powder required

To chlorinate the given well, we need to find the latter’s volume.






Volume (V) = 4 x 2 x 8 x 1000

Given data:1. l = 4 m2. b = 2 m3. h = 8 m4. Horrock’s test: 4rd cup +ve,5. Given bleaching powder has 20% available chlorine.

= 64000 l


colour in 4th cup onwards, 4 level spoonfuls of the Horrock’s apparatus,






bleaching powder.



















given case).




64000 x 8455 = 1125.74g

g



water by Horrock’s apparatus (4th cup +ve in the given case) & calculation of

the quantity of bleaching powder required (1126 g in the given case)






full.












e. Contact period:











4) Determine the quantity of bleaching powder required to disinfect a well 3

metres diameter with 5 metres of water in it given that the 4th cup in Horrock's

test gives a definite blue colour.

Solution:








Volume (V) =

=

= 35325 l








bleaching powder.

Hence 621.1g of the given bleaching powder is required to chlorinate the given well.

3.14 x 32 x 5 x 1000

4

x d2 x h x 10004

3.14 x 9 x 5 x 1000

4

35325 x 8455 = 621.09 g

Given data:1. d = 3 m2. h = 5 m3. Horrock’s test: 4th cup +ve,

5) A rectangular well measuring 2m X 3m X with 7 metres of water is to be

disinfected. Horrock's test done on a sample of water shows blue coloration in

the 4th, 5th and 6th cups. Calculate the amount of bleaching powder required

to chlorinate the water in the well and explain the practical procedure in

disinfection.

Solution:








Volume (V) = 3 x 2 x 7 x 1000

= 42000 l








bleaching powder.


given well.

42000 x 8455 = 738.46g g

Given data:1. l = 3 m2. b = 2 m3. h = 7 m4. Horrock’s test: 4th cup +ve,

















given case).













full.












e. Contact period:











6) A rectangular well measuring 2m X 3m with 9 metres of water is to be

disinfected. Horrock’s test done on a sample of water shows blue colour in the

3rd cup. Calculate the amount of bleaching powder required to chlorinate the

water in the well and explain the practical procedure.

Solution:


chlorinate the given well, we need to find the latter’s volume. Given data:1. l = 3 m2. b =2 m3. h = 9 m4. Horrock’s test: 3rd cup +ve,






Volume (V) = 3 x 2 x 9 x 1000

= 54000 l








bleaching powder.


given well.













54000 x 6455 = 712.09g g





given case).













full.












e. Contact period:











7) Determine the quantity of bleaching powder with 25% available chlorine

required to chlorinate a well 5 meters in diameter and 6 meters of water

Horrocks test done on a sample of water shows blue colour in the 5th and 6th

cup. Explain the practical procedure of disinfection of the well.

Solution:








Volume (V) =

=

3.14 x 52 x 6 x 1000

4

x d2 x h x 10004

3.14 x 25 x 6 x 1000

4

Given data:1. d = 5 m2. h = 6 m3. Horrock’s test: 5th cup +ve,4. Given bleaching powder has 25% available chlorine.

= 117750 l








bleaching powder.











2. Measuring the diameter of the well: Stand outside the well & note the

maximum distance of the rope placed across the top of the well. Note the

inner diameter of the well. Take average of several readings of the above

measurements. This gives the diameter (d) of the water column, which is



volume of a cylinder (in the given case)

and by substitution of d & h, we get 117750 l


117750 x 10455 = 2587.9 g

x d2 x h x 10004









full.












e. Contact period:











B. Water quality:

1) A sample of water when analysed, shows that the Coliform count is 15 per

100ml. Traces of Nitrites and Chlorides in excess of the normal are found.

Comment on the quality of water and give your opinion with reasons whether it

is fit or unfit for drinking.

Solution:

Constituent Given Values Expected Values

Coliform count 15/100 ml 0/100 ml

Nitrites >3 ppm ≤3 ppm

Chlorides >200 ppm 100-200 ppm

Quality of the given sample of water for drinking purpose:

I. Microbiological aspect:

Bacteriological indicator

1. Coliforms:

Coliforms are foreign to potable water and hence their presence in water is looked

upon as evidence of recent fecal contamination. Their presence in the given water

sample indicates the propable presence of intestinal pathogens. Immediate

investigative action must be taken if coliforms are detected in drinking water.

Normal range of coliform – 0/100ml of water.

II. Acceptability aspect:

Inorganic constituent

1. Chlorides :

Chlorides result from organic pollution and salts. In the given sample of water,

excess of chlorides over the normal range arouses suspicion of water contamination.

The high level of salinity affects the taste & also causes corrosion of metallic vessels.

Normal range : 100-300 mg/l

III. Chemical aspect:

Health related inorganic constituent

1. Nitrites

Nitrates originate from decomposing organic matter indicating recent pollution.

Excess of nitrates result in Methaemoglobinemia. Nitrates are converted into nitrites

which indicate past contamination. In the given sample of water, traces of nitrites

indicate past contamination.

Normal range : Nitrites ≤ 3 mg l

the given water sample is polluted probably with sewage and not fit for drinking.

2. Give a detailed report on this water sample and state whether it is potable:

Constituent Given Values

Total solids 250 ppm

Total hardness 180 ppm

Free ammonia 0.244 ppm

Albuminoid ammonia 0.81 ppm

Nitrites 0.005 ppm

Nitrates 21.0 ppm

Chlorides 45.00 ppm

Coliform count 15/100 ml

Interpretation:

Constituent Given Values Expected Values

Total solids 250 ppm < 650 ppm

Total hardness 180 ppm 50-150 ppm

Free ammonia 0.244 ppm < 0.05 ppm

Albuminoid ammonia 0.81 ppm < 0.1 ppm

Nitrites 0.005 ppm ≤ 3 ppm

Nitrates 21.0 ppm 50 ppm

Chlorides 45.00 ppm 100 – 300 ppm

Coliform count 15/100 ml 0/100 ml

Quality of the given sample of water for drinking purpose:

I. Acceptability aspect:

Inorganic constituents

1. Total solids:

Total dissolved solids (TDS) can have an important effect on the taste of

drinking water. The palatability of water with a TDS level of less than 600mg/l

is generally considered to be good. Water with extremely low concentrations

of TDS may be unacceptable because of its flat, insipid taste. The presence of

high level of TDS may also be objectionable to consumers owing to excessive

scaling in water pipes, heaters. boilers and household appliances.

2. Hardness:

Drinking water should be moderately hard.

3. Free ammonia: The given sample has value which is more than the

recommended limit for free (NH3) ans saline(NH4+) ammonia. It indicates

contamination due to sewage & industrial waste, and decomposition of

organic matter with recent pollution.

4. Albuminoid ammonia: The given sample has value which is more than the

recommended limit. It indicates undecomposed organic matter with recent

pollution.

Ammonia in the environment originates from metabolic, agricultural and

industrial processes and from disinfection with chloramine Intensive rearing of

farm animals can give rise to much higher levels in surface water. Ammonia

contamination can also arise from cement mortar pipe linings. Ammonia in

water is an indicator of possible bacterial, sewage and animal waste pollution.

Ammonia can compromise disinfection efficiency, result in nitrite formation in

distribution systems, can cause the failure of filters for the removal of

manganese, and cause taste and odour problems.

4. Chlorides :

Chlorides result from organic pollution and salts. In the given sample of water, deficit

of chlorides under the normal range indicates no contamination of water thereby not

interfering in taste & causes no corrosion of metallic vessels.

II. Chemical aspect:

Health related inorganic constituent

1. Nitrites:

Nitrates are converted into nitrites which indicate past contamination. In the

given sample of water, value of nitrites is less than the recommended value,

hence it indicates no past contamination.

Normal range : Nitrites ≤ 3 mg l

2. Nitrates:

Nitrates originate from decomposing organic matter indicating recent pollution

and bacterial activity. Excess of nitrates result in Methaemoglobinemia. In the

given sample of water, value of nitrates is less than the recommended value,

hence it indicates no recent pollution.

Normal range: Nitrates ≤ 50 mg l

Because of the possibility of simultaneous occurrence of nitrite and nitrate in

drinking water, the sum of the ratios of the concentration of each to its

guideline value should not exceed 1, i.e.

0.005 + 21 = 0.42 which is ≤ 13 50

III. Microbiological aspect:

Bacteriological indicator

1. Coliforms:

Coliforms are foreign to potable water and hence their presence in water is looked

upon as evidence of recent fecal contamination. Their presence in the given water

sample indicates the propable presence of intestinal pathogens. Immediate

investigative action must be taken if coliforms are detected in drinking water.

Normal range of coliform – 0/100ml of water.

Hence it is not potable.

chlorination exercises

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