WASH Cluster – Water in Emergencies
W4.5
Handout
W4 – Practical Chlorination
Source of this material:
The information in this handout has mainly been adapted from the following source:

OXFAM Humanitarian Department (no date) ‘Instruction manual for coagulation and disinfection
equipment, Covering upflow clarifier, chlorine and aluminium sulphate dosing options’, OXFAM
Water Supply Scheme for Emergencies.
Information has also been used from:

Davis, J & Lambert, R (2002, 2nd edition) ‘Engineering in Emergencies, A Practical Guide for
Relief Workers’, REDR & IT Publications

Buttle, M & Smith, M (1999) ‘Out in the Cold, Emergency Water Supply and Sanitation for Cold
Regions’, WEDC

Additional good practice advice from water quality / treatment experts
Contents:
1. Background to chlorination
2. Safety when using chlorine
3. Bulk chlorination and making 1% stock solutions
4. Container based chlorination as an immediate emergency response
5. Use of floating pot chlorinators
6. Chlorination when there is a pH > 8.0, a pH < 6.8, low temperatures or a turbidity > 5 NTU
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WASH Cluster – Water in Emergencies
W4.5
1. Background to chlorination
Chlorine is the chemical most widely used for disinfection of treating drinking water because of its ease
of use, ability to measure its effectiveness, availability and cheapness. Under the right conditions chlorine
will kill all viruses and bacteria, but some species of protozoa and helminths are resistant to chlorine.
WHO recommends adequate protection of the source as the most effective way of dealing with the more
resistant helminths and protozoa by preventing faecal contamination entering the water.
If the water to be disinfected contains a lot of suspended solids and/or organic matter (i.e. is highly
turbid) it will have a high chlorine demand. WHO guidelines recommend that turbidity is less than 1 NTU
for chlorination to be effective in destroying all bacteria and viruses, although 5 NTU is a more
achievable limit and will be adequate in most cases. It is, therefore, desirable to remove suspended
solids as much as possible before the chlorination process begins. This will significantly reduce the
amount of chlorine needed and improve its efficiency as a disinfectant.
The most important factor in any use of chlorine is the determination of the concentration of chlorine
required in the water that is about to be consumed by the human user. Chlorine is absorbed by all
forms of organic material which may be present in any water or on water-carrying utensils. It is vital,
therefore, that enough is added to leave a residual at the end of the distribution system.
Contamination, which is introduced along the distribution system, will thus be destroyed. There are
many potential points of water contamination, both before and after it has been collected by the
consumer, but especially when stored in the home.
A minimal residual, of 0.2 – 0.3 mg/litre, is required at the final delivery point. But, because of
the potential contamination in the distribution system, it is necessary to dose at a higher concentration
of chlorine in the storage tank. This may be as high as 2-3 mg/litre or more, but will need to be
determined by experiment and testing of chlorine levels through the delivery chain. After addition of
chlorine, the water needs a certain amount of time (called contact time) to kill the viruses/bacteria in
the water and this is usually at least 30 minutes, though sometimes longer is required (see below).
Residual chlorine levels should be checked daily (using a chlorine comparator is the simplest method)
and chlorine dosing adjusted as necessary. The level of residual chlorine is very easily determined by
use of a simple colour comparator and tablets added to the water.
2. Safety when using chlorine
All forms of chlorine used as water disinfectants can be dangerous if not stored and handled in
the correct manner.
The following simple rules must always be followed and any particular advice and precaution supplied
with a specific product should likewise be followed closely:

Only trained and authorised personnel should be allowed into the chlorine store.

Chlorine is caustic, i.e. can cause burning and must not come into contact with skin or
clothing. Thus protective clothing such as gloves, goggles and overalls or apron should be worn.

Chlorine should be stored under dry and cool dark conditions, preferably raised above ground.
Keep all containers closed and covered with a tight fitting lid when not in use.

Avoid breathing chlorine dust, as it is an irritant to the nose and lungs.

Do not sleep in the same room where chlorine is stored.
3. Bulk chlorination and making a 1% stock solution
There two main options that can be used for the addition of HTH chlorine to water for bulk chlorination:
1. By mixing a 1% solution in a suitable bucket or container and adding to a tank on a batch basis.
2. By use of a constant flow dosing arrangement to drip 1% chlorine solution into outlet flow from a tank.
Handout – W4: Practical chlorination
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WASH Cluster – Water in Emergencies
W4.5
Calculating approximate chlorine dosages
1% solution has =
10 g active chlorine / litre
10,000 mg / litre
10,000 mg / 1,000 ml
10 mg / ml
So each 1 ml of 1% solution has 10 mg of active chlorine
Example: If you need 3 mg/l of chlorine to leave 0.5 mg/l residual of active chlorine
a) If using a 1% solution, 10mg/l
Then for a 20 litre jerry can, you will need:
3 mg / litre x 20 litres = 60 mg chlorine needed
60 mg chlorine = 6 x 1 ml of 1% solution
b)
If using a 3% solution then 10mg x 3 = 30 mg/ml
Then for a 20 litre jerry can and using a 3% solution, you will need:
3 mg / litre x 20 litres = 60 mg chlorine needed
60 mg chlorine = 2 x 1 ml of 3% solution
Ascertaining the required dose for HTH powder
To determine how much chlorine to add, the chlorine demand is measured.
1.
Prepare a 1% chlorine solution1, the quantity depending upon type of chlorine used (see the table
below).
Quantities of chemical required to make 1 Litre of 1% Chlorine Solution2
Source of chlorine
Available
chlorine %
Quantity required
(g)
Approx. measure
High Test Hypochlorite
granules (HTH)
70
14 g
1 heaped tablespoon
Bleaching powder
34
30 g
2 heaped tablespoons
Stabilized tropical
bleach
25
40 g
3 heaped tablespoons
Liquid household
disinfectant
10
100 ml
7 tablespoons
Liquid laundry
disinfectant
5
200 ml
14 tablespoons
Antiseptic solution (e.g.
Milton)
1
1 litre
No need to adjust as it is
a 1% solution
2.
Take 3 or 4 non-metallic containers of known volume (e.g. 20 litre plastic buckets).
3.
Fill the containers with some of the water to be treated and check the pH of the water.
1
2
A 1% solution contains 1% of available chlorine, which is 10g of chlorine per litre, i.e. 10,000 mg/l or 10,000 ppm.
Davis & Lambert, Engineering in Emergencies
Handout – W4: Practical chlorination
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WASH Cluster – Water in Emergencies
W4.5
4.
Add to each bucket a progressively greater dose of 1% solution with a syringe:
1st container: 1ml
2nd container: 2ml
3rd container: 3ml
4th container: 4ml
5.
Stir the water for 10 seconds to mix the chlorine solution throughout the water body.
6.
Wait 30 minutes. (This is essential as this is the minimum contact time for the chlorine to react. If
the pH of the water is high, this minimum time will increase)
7.
Measure the free chlorine residual in each bucket.
8.
Choose the sample that shows a free residual chlorine level between 0.2 and 0.5mg/l.
9.
Extrapolate the 1% dose upto the volume of water to be treated.
10. Pour the solution into the reservoir/dosing vessel, mix well (during filling) and wait 30 minutes
before distributing to consumers.
11. Check chlorine levels at several water distribution points and adjust if required to ensure that
there is free chlorine at all sampling points.
12. Recheck chlorine levels periodically and especially when raw water quality is known to vary to
ensure that the chlorine residual is maintained.
The above method is known as the Modified Horricks’ Method. The point at which the chlorine is added
to the water and a residual starts to be measured is known as the ‘Breakpoint’.
Example: Chlorination of water in a 2,000 litre reservoir
Follow steps 1-5 above.
The free residual chlorine levels of the water in the buckets, measured 30 minutes after adding 1, 1.5,
2 and 2.5ml of 1% chlorine solution respectively, are as follows:
1:
0 mg/l
2:
0.1 mg/1
3:
0.4 mg/1
4:
1 mg/l
The dosing rate chosen therefore will be that in bucket number 3 (result between 0.2 and 0.5mg/l).
Thus if it needs 2ml of 1% solution to chlorinate 20 l of water at the correct dosage, then it needs 100
times as much to chlorinate 2,000l, i.e. 100 x 2ml = 200ml of 1% chlorine solution.
Note:

The taste of chlorine in water is no proof of the presence of free residual chlorine (it could be
combined residual chlorine).

Metal consumes chlorine, so never prepare strong solutions in metal containers (unless they are
enamelled or painted).
Handout – W4: Practical chlorination
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WASH Cluster – Water in Emergencies
W4.5
4. Container based chlorination as an immediate emergency response
As an immediate response, before appropriate water treatment and supply can be set up, for example in a
cholera outbreak, chlorine can be injected directly into the users containers as they collect water.
Workers will need to be supplied with containers of 1% solution and syringes. The 1% solution should then
be injected into the users’ containers depending on the size of the container, see the table below for a
guide. The workers should be clear of the relative sizes of the containers before the exercise. Examples of
the containers could be provided with the amount of 1% solution written on the side as a reminder.
The users should be told to shake or mix the water in the container and to wait for 30 minutes before
drinking the water.
Hygiene promotion should accompany the exercise to explain the reason for the addition of chlorine and to
ensure that the correct process is followed.
Chlorine doses for common sized containers3
Container size
Volume of 1%
solution required
4.5 litres
(1 gallon)
8 drops
20 litres
Half-teaspoon
(2.5 ml of 1% solution)
200 litres drum
(45 gallon)
1 tablespoon + 1 tablespoon
(20 ml of 1% solution)
Guide based on the approximate measures: 1 teaspoon = 5 ml; 1 tablespoon = 15 ml
5. Using floating pot chlorinators
Floating pot chlorinators are for use with large chlorine tablets tricloroisocyanuric acid. The dose rate can be adjusted by use of a
“gate” using the slots on the side, as well as by varying the amount
of tablets used in the pot chlorinators.
Suspend the holder plus chlorine tablets in the water in a position
which gives sufficient contact time to achieve a chlorine residual,
but away from the turbulence caused by the inlet pipe.
Calculate the number of tablets needed for the size of tank and
quantity of water being supplied.
Check residual chlorine levels daily and adjust the number of
tablets if necessary.
It is not recommended to drink water chlorinated with these
tablets for more than three months.
Dose for use of slow dissolving chlorine tablets
Example calculation for the use of slow dissolving chlorine tablets using 200g tablets containing
trichloroisocyanuric acid as the active constituent FI-CHLOR Maxi tabs.
These can be used in the following quantities 3, 4 or 5 tablets (for 45, 70 and 95m3 tank sizes) and
should last for 7-14 days, assuming the tanks are filled once daily, and depending on water quality.
1
3-5
3
200 gramme tablet will treat 36,000
litres/day at 5ppm
200 gramme tablets will treat 70,000
litres/day at 1ppm
2-3
4-7
200 gramme tablets will treat 45,000
litres/day at 1ppm
200 gramme tablets will treat 95,000
litres/day at 1ppm
Davis & Lambert, Engineering in Emergencies
Handout – W4: Practical chlorination
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WASH Cluster – Water in Emergencies
W4.5
6. Chlorination when there is a pH > 8.0, a pH < 6.8, low temperatures or a
turbidity > 5 NTU
Chlorination when there is a pH > 8.0
The ‘contact time’ is normally a minimum of 30 minutes for neutral pH waters. However the length of
contact time required for the active chlorine to be fully effective depends upon many factors, of which
the most important are pH and water temperature. Although most raw water sources have a pH value
within the range 6.5 – 8, as the pH levels rise, the disinfecting properties of chlorine start to become
weaker and at pH 9 there is very little disinfecting power.
The table below shows how higher pH values can be compensated for by either adding additional
chlorine or increasing the contact time.
pH
8.0
8.5
9.0
Required chlorine residual at
20oC (mg/l)
0.5
0.2
0.5
0.8
1.0
1.5
0.2
0.5
0.8
1.0
1.5
Minimum contact time needed for effective
disinfection (min)
30.0
206.0
82.5
52.0
41.0
27.5
412.0
165.0
103.0
82.0
55.0
Chlorination when there is a pH of < 6.8
When there is a pH of less than 6.8, locally purchased lime can be added to raise the pH.
Chlorination at low temperatures
A higher water temperature will enable the chlorine to work faster and conversely a lower
temperature will make the chlorine work slower.
Increase retention time x 1.5 - 3.5 for every 6 oC drop in temperature (below 20 oC)4
Chlorination when there is a turbidity of > 5 NTU
Every effort should be made to reduce the turbidity of the water to less than 1 NTU or at the most 5
NTU by:

Appropriate source selection

Pre-treatment – such as roughing filters, coagulation, flocculation and sedimentation, upflow
clarifiers or other methods.
There may be occasions, particularly at the initial stages of the emergency, when the turbidity is
higher and the options are either chlorinate with higher turbidity or don’t treat the water.
In these circumstances, for short periods of time, the following can be done to try and increase the
effectiveness of the chlorination.
Note, however, that increasing chlorine levels particularly when there is high turbidity, can affect the
taste and may lead users to reject the water and thus increase the risk of them going back to
alternative, potentially less safe, sources.
Any increases in chlorine or expected changes in taste should be discussed with the users through
hygiene promotion activities.
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Buttle & Smith ‘Out in the Cold, Emergency Water Supply and Sanitation for Cold Regions’
Handout – W4: Practical chlorination
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WASH Cluster – Water in Emergencies
W4.5
1. For water from 5 to 20 NTU (or possibly higher) continue to chlorinate the water as usual, using the
method noted in section 3 to determine the amount of chlorine which is required to ensure a
residual of 0.5 mg/l.
The amount of chlorine to be added is likely to be higher than normal as some will be used up by
the material which makes up the particles of turbidity. Make sure the full contact time has been
reached, or even leave contact time some minutes longer before drinking.
2. Chlorinate at a stage of distribution where some settlement has occurred, for example in the
morning after the tank has been filled the night before. Note that the water may have an
additional chlorine demand than when the water is clear.
There will be a need to clean out the sludge in the bottom of the tank regularly.
3. Water tankers with water in them stood overnight can be chlorinated. The sludge at the bottom
should then be flushed out and the water wasted until clearer water with free chlorine residual
comes out of the tank.
4. Filter water through cloths or clothing before adding chlorine.
5. Add a cloth filter at the entrance to tanks. It won’t take out all of the turbidity but may remove
some particles and any larger solids.
The cloth filter will need to be cleaned regularly.
Handout – W4: Practical chlorination
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