Water purification - Drinking water chlorination.pdf - survival(3) - T-e-T

Some water sources contain disease-causing organisms which need to be removed or killed before

the water is safe to drink. If carefully undertaken and monitored, disinfection is an effective means

of removing such organisms. Chlorine is the most widely used disinfectant, and one which is often

the most readily available. This technical brief describes a method of calculating the dose of chlorine

required to disinfect small community water supplies.

Why disinfect?

Water treatment processes such as storage, sedimenta-

tion, and sand filtration will reduce the content of disease-

causing organisms in water, but will not leave it com-

pletely free of such organisms. Disinfection, when ap-

plied and controlled properly, is the most practical and

effective means of removing such organisms.

Chlorine demand and residual

When chlorine is added to a water source, it purifies the

water by damaging the cell structure of bacterial pollut-

ants, thereby destroying them. The amount of chlorine

needed to do this is called the chlorine demand of the

water. The chlorine demand varies with the amount of

impurities in the water. It is important to realise that the

chlorine demand of a water source will vary as the quality

of the water varies.

Methods of disinfection

Boiling water may be effective as a method of disinfection,

but it is not practicable for large quantities. Sunlight can also

act as a natural method of disinfection, but it is difficult to

control and manage. For these reasons, chemical disinfect-

ants (especially chlorine and chlorine compounds) are

used. Iodine may also be used as a disinfectant, but it is

usually more expensive than chlorine compounds.

The aim of chlorination is to satisfy the chlorine demand

of the water source. Once the demand has been satisfied,

any excess chlorine above the level needed to satisfy the

demand remains as a residual of chlorine (chlorine

residual) in the supply.

If a supply is to be adequately disinfected, therefore,

there should be a chlorine residual in the supply, so that

there is the capacity to cope with any subsequent bacte-

rial contamination. The chlorine residual should generally

be in the range 0.3 to 0.5mg of chlorine per litre of treated

water. Any more than this and the supply may taste bad

and be harmful, and people may refuse to use it. Any less,

and there is no guarantee that the supply is adequately

protected. An example is given in Figure 1 below.

Chlorine compounds will destroy disease-causing organ-

isms quickly — usually after 30 minutes. They are widely

used and are relatively inexpensive. If carefully applied,

chlorine has the advantage that a measurable residual of

chlorine in solution can be maintained in the water supply.

This residual provides further potential for disinfection and

is also an important indicator of successful application.

When to use chlorine

Chlorine may be used:

A water supply (from a spring, for example) has a chlorine

demand of 2.0 mg/l.

If exactly 2.0 mg of chlorine is added per litre of water, then the

chlorine demand will just be met and there will be no chlorine

residual.

If 2.5 mg/l of chlorine is added, then the chlorine demand will

be met and exceeded, so that a residual of 0.5 mg/l will be left

in the water when it goes into supply.

when suitable compounds are available and their

application can be strictly controlled;

when there is enough time between the addition of

chlorine to water and the consumption of the water;

where a community has a continuous supply of water,

with storage capacity; and

by individuals to provide additional protection.

When not to use chlorine

Chlorine should not be used:

when a regular supply of chlorine compunds cannot be

guaranteed;

where chlorine may react with other chemicals in the

water creating undesirable or dangerous by-products;

to attempt to kill cysts or viruses; or

Figure 1. An example of chlorine residual

when careful monitoring cannot be provided.

Testing for chlorine residual

The most common test is the dpd (diethyl paraphenylene

diamine) indicator test, using a comparator. This test is

the quickest and simplest method for testing chlorine

residual.

against standard colours on a chart to determine the

chlorine concentration. The stronger the colour, the higher

the concentration of chlorine in the water

Several kits for analysing the chlorine residual in water,

such as the one illustrated in Figure 2, are available

commercially. The kits are small and portable.

With this test, a tablet reagent is added to a sample of

water, colouring it red. The strength of colour is measured

Step 1. Place one tablet in the test chamber (a) and add a few

drops of the chlorinated water supply under test.

Step 2. Crush the tablet, then fill the chamber (a) with the

chlorinated water supply under test.

Step 3. Place more of the same water supply under test

(without a tablet) in the second chamber (b). This is the

blank control for colour comparison.

Step 4. The level of residual chlorine (R) in mg of chlorine per

litre of water (mg/l) is determined by comparing the colour of

the water supply under test in chamber (a) with the tablet

added with the standard colours on the vessel (chamber (b)).

Note: A third chamber (c) would be used if a higher chlorine

residual is to be measured.

Figure 2. Procedure for testing for chlorine residual

Chlorinating water supplies

Chlorine is available in many forms — as chlorine gas and

in compounds such as bleaching powder, high test

hypochlorite (HTH), tablets, granules, and liquid bleach.

each product will reduce over time as the source is

exposed to the atmosphere. All products should be

carefully stored to minimize deterioration.

Each product contains a different amount of usable

chlorine, so different quantities of each will be required for

the same purpose. In addition, the chlorine content of

The best practical method of chlorinating a supply of water

is to use two storage tanks of suitable size alternately, one

filled from the source, while the other is used for supply.

Figure 3. A gravity-feed chlorinator

A chlorination checklist

Chlorine needs at least half an hour contact time with water to disinfect it. The best time to apply it is after any

other treatment process, and before storage and use.

Never apply chlorine before slow sand filtration or any other biological process, as the chlorine will kill off the

bacteria which assist treatment, making the treatment ineffective.

Never add any solid form of chlorine directly to a water supply, as it will not mix and dissolve. Always make

up as a paste first, mixing the chlorine compound with a little water.

Disinfection is only one defence against disease. Every effort should be made to protect water sources from

contamination, and to prevent subsequent contamination during collection and storage.

The correct procedure for applying a disinfectant to water should be strictly adhered to, and water supplies

should be monitored regularly to ensure that they are free from bacteria. Otherwise, people may be misled to

believe that the water is safe to drink when, in fact, it is hazardous to do so.

The optimum chlorine residual in a small, communal water supply is in the range of 0.3 to 0.5mg/l.

The chlorine dose required to disinfect a supply will increase if the water is very turbid. In such circumstances,

it is best to treat the water to reduce turbidity before chlorination.

Caution

All forms of chlorine are harmful to health — avoid skin contact and do not inhale the fumes. Chlorine

should be stored in cool, dark, dry and sealed containers, and out of reach of children.

Modified Horrocks' method of chlorination

With most chlorination methods the operator should make up a solution of known concentration. For the reasons

outlined on page 55, however, it is not usually possible to do this accurately. The modified Horrocks' method

of chlorination can be carried out without prior knowledge of the chlorine content of the chlorine product.

Equipment

• 5 containers (any type — as long as they are all the same size.

Plastic drinks bottles may be used).

• A measuring device to measure out the solid chlorine prod-

uct. An oral rehydration therapy (ORT) spoon would be

suitable. The exact size of the spoon is not critical, but

identical amounts must be measured out each time, so use

a level spoonful, for example. Use a spoon which measures

out about 1g, as this will help in the calculations.

• A device to dispense small quantities of liquid (a small 1ml or

5ml syringe would be suitable).

• Dpd test equipment

6. Transfer 2ml of the liquid from container 1 to container 2, 4 ml

to container 3; 6ml to container 4; and 8ml to container 5.

Container 2 will then have 2mg/l; container 3, 4mg/l; container

4, 6mg/l; and container 5, 8mg/l.

7. Leave containers 2, 3, 4 and 5 to stand for at least 30 minutes

— this is the minimum contact time required for the chlorine

to disinfect the water.

8. Test the water in each container for residual chlorine content

using the Dpd test kit (see page 54).

9. The container with the lowest concentration of chlorine equal

to or more than 0.4mg/l indicates how much chlorine powder

should be added to the water being disinfected.

Method

1. Label the five containers 1 to 5.

2. Place one level spoonful of chlorine product (bleaching

powder or HTH) into the first container. If the spoon has a

capacity of 1g, there is now 1g of chlorine product in the

container.

3. Add a few drops of the water to be chlorinated, and mix to a

paste (dissolving the chlorine-product powder).

4. Dilute the paste with enough water to fill the container. If the

container holds one litre, it now contains one spoonful per

litre, or in this case, one gram per litre. If we take 1ml out of

this container, this will contain 1mg of chlorine product

(1 litre = 1000ml, and 1g = 1000mg).

5. Fill containers 2, 3, 4, and 5 to capacity (1 litre) with the water

to be chlorinated.

Example

A water supply from a spring with a daily flow of 70 m 3 /day needs

chlorinating to make it safe to drink. Tests on the water — using

the modified Horrocks’ method — indicated residual chlorine

concentrations (after 30 minutes) of 0, 0.2, 0.5 and 1.0mg/litre in

containers 2, 3, 4, and 5 respectively.

Therefore, container 4, with a residual concentration of 0.5mg/l,

had the lowest residual chlorine concentration equal to or

exceeding 0.4mg/l.

The concentration of chlorine product added to container 4 was

6mg/l. For a supply of 70m 3 /day, therefore, the amount of

chlorine product to be used is calculated as:

70 x 1000 litres x 6mg/l = 420 000mg = 420g = 0.42kg.

(1m 3 = 1000 litres)

Water purification - Drinking water chlorination.pdf

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