Annex 7 - Chlorine as a Water Disinfectant
Why do we need to disinfect water?
Dirty and polluted water can contain many organisms that are
harmful to humans if they drink it. The disease-causing organisms (pathogens)
include bacteria, bacterial spores, viruses, cysts, protozoa and helminths.
These can cause diseases like cholera, bacillary dysentery, typhoid, infectious
hepatitis and diarrhoea. Disinfection of water aims to kill these pathogens
without leaving any harmful chemical substances in the water.
Water treatments such as sedimentation and filtration can
significantly reduce the number of pathogens in water. However, there is still
likely to be a need to kill the remaining and subsequent pathogens. It is at
this point that chlorine is used as a chemical disinfectant.
Why use chlorine?
Chemical disinfectants for water should have the following
attributes:
- destroy all pathogens present in the water within
an acceptable length of time
- be able to perform within the range of temperatures and
physical conditions encountered
- disinfect without leaving any harmful effects for humans
- permit simple and quick measurement of strength and
concentration in water
- leave sufficient active residual concentration as a safeguard
against contamination that might occur after the water has been collected, e.g.
in containers
- be readily and reliably available at a reasonable
cost
Chlorine is the chemical most widely used as it fulfils the
above criteria and is widely available in one form or another (see section
below)
How does chlorine work?
The precise way in which chlorine kills pathogens is not known.
It is believed that the compounds formed when chlorine is added to water
interfere with the chemical processes which ensure the pathogens' survival.
When a suitable chlorine compound is added to water, only a part
of it becomes effective in killing pathogens. This part is called 'Free Active'
or 'Active' chlorine (AC). AC is very good at invading the cells of pathogens.
It is, therefore, a very efficient killer of pathogens. As a result, only small
amounts of chlorine are required to disinfect polluted water.
What affects chlorine's efficiency?
After it has been added the active chlorine needs a certain
amount of time to kill the pathogens in the water. This is called the 'contact
time'. This amount of time must be allowed after adding chlorine before people
drink the water. How much contact time is required for the active chlorine to be
fully effective depends upon many factors. However, the most important are pH
(level of acidity/alkalinity) and water temperature.
Most raw water sources have a pH value within the range 6.5 - 8.
As pH levels rise the disinfecting properties of chlorine start to become weaker
and at pH 9 there is very little disinfecting power. WHO guidelines recommend
that drinking water should therefore have a pH in the range of 6.5 - 8.5. pH can
have a significant influence on the performance of chlorine in water which we
are likely to be working with for drinking water supplies.
The temperature of the water to be disinfected can have a
significant effect on chlorine efficiency. The time needed for disinfection
becomes longer as the temperature of the water gets lower. There is a noticeable
difference in the killing rate of bacteria between 2 and 20°C.
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. It is, therefore, desirable to clean the water 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.
If iron and manganese are present in the water to be disinfected
a substantial amount of chlorine may combine with them to form compounds which
are insoluble in water. It is, therefore, beneficial to remove the iron and
manganese before chlorination. This may not always be possible, although simple
aeration systems may be appropriate. It is important that the person responsible
for disinfection is aware of the influence that the presence of these metals can
have on chlorine demand.
How long does it take to kill the pathogens?
The disinfecting effect of chlorine is not instantaneous. The
amount of pathogens killed depends upon the 'contact time' between the dosing
and the drinking. For our purposes, a minimum contact time of 30 minutes is
essential. However, when considering this, account must be taken of the pH,
temperature and turbidity of the water. For example, a turbid water with a pH of
7.5 - 8 and a temperature of 10°C will require a longer contact time than a
clear water with a pH of 6.5 - 7 and a temperature of 20°C.
MINIMUM CONTACT TIME MUST ALWAYS BE 30 MINUTES.
Types of chlorine
Chlorine gas and chlorine dioxide are widely used in water
treatment. However, their handling and transport are considered too hazardous
for the sons of projects OXFAM or its partners are likely to be involved in.
CHLORINE IS DANGEROUS THE SAFETY RULES CONCERNING ITS
HANDLING MUST ALWAYS BE FOLLOWED.
Calcium Hypochlorite - Ca (OCl)2
Calcium hypochlorite, also widely known as bleaching powder or
chlorinated lime, comes as a powder containing approximately 33% available
chlorine. It is stored in corrosion-resistant containers. Once the container is
opened, the powder quickly loses its strength. This can be very significant,
e.g. about 5% in 40 days if the container is opened for as little as 10 minutes
per day, or approximately 20% if left open for the whole period.
The powder is not added directly to the water to be disinfected.
The usual method is to make a solution of 1% available chlorine and to add this
to the water.
Solutions of chlorine are more prone to loss of strength than
bleaching powder. Sunlight and high temperatures can speed the amount of active
chlorine lost. To minimise such losses, the solution should be stored in a dark
dry place and at the lowest possible temperature. The solution should be stored
in dark corrosion-resistant containers (glass, plastic, wood, ceramic) which
must be securely closed.
More stable bleaches are available on the market. They are more
expensive to buy but, because they last longer in store, can prove to be more
economical in the long run. High Test Hypochlorite (HTH) is one such stabilised
form of calcium hypochlorite. It contains between 60% and 70% available chlorine
and with suitable storage will maintain its initial strength with little loss.
It is available in tablet or granular form. Other prepared solutions include ICI
Tropical bleach - 34% available chlorine and Stabochlor - 25%.
Sodium Hypochlorite (NaOCL)
Sodium hypochlorite is generally available as a solution
commonly known as bleach. Typical available chlorine contents range from 1 to 5%
but can be as high as 18%. Before using these solutions, the available chlorine
content should be checked. The solutions become less stable as the chlorine
content rises. As with all chlorine disinfecting compounds, extreme care should
be used when handling these solutions.
Buying solutions of sodium hypochlorite is not economic for
large-scale use, as the transport costs associated with it are high, because of
the volume and weight to be transported. It is far better to buy powdered forms
of chlorine and prepare solutions for addition to the water on site.
Slow dissolving Trichloroisocyanuric Acid
This form of chlorine is used extensively to disinfect swimming
pools. The chlorine, which comes in various sized tablets, is supplied by OXFAM
as part of its emergency water supply packs. The compound dissolves very slowly
in water and so is suitable for disinfecting drinking water which is stored in
large capacity tanks as used in an emergency. It is recommended that this form
of chlorine should not be used in drinking water supplies for more than 3 months
in one year. As such, this compound is ideal for use during the first three
months of disinfection or whilst another source of chlorine is being found
locally. It should be noted that the health risks (which are not certain or
proved at the time of writing) associated with prolonged use of the tablets are
much less than the risks ensuing from drinking non-disinfected water.
This form of chlorine is relatively stable and, if stored in
non-humid conditions at temperatures below 25°C, can retain its full
strength for 2 years. OXFAM supplies these tablets with a small plastic basket
which floats inside the reservoir or tank. The basket should be placed near the
inflow of the tank so that the incoming water flows over the tablets. This is
the best way of ensuring good contact between the water and the chlorine. When
using the 45m3 OXFAM storage tank, initially use 3 tablets (4 for the
70m3, 5 for the 95m3). The residual chlorine will need to
be checked daily (see section below) and the number of tablets adjusted
accordingly. The tablets should last between 7 and 14 days.
Use of chlorine
How to make chlorine solutions
As we have already said, the most stable solution is 1%
available chlorine, and it is recommended that this should be the strength of
solution to be prepared. The following tables give an approximate guide to
producing 1% solutions from various chlorine compounds. It should be stressed
that the strength of the solution will be dependent upon the chlorine content of
the chemical used to make the solution.
Table 1 - Quantities of Chemical Required to Make 1 Litre of
1% Chlorine Solution
These quantities of chemicals should be added to 1 litre of
water in the following way. In the case of bleaching powder, the amount of
chemical needed to make a 1% solution is placed in a suitable vessel and
sufficient water is added to make a smooth cream. It is best to use a wooden
stirrer to break up the lumps. When all the lumps have been broken, the cream
should be diluted to the required amount using more water and thorough mixing.
The sediment should be allowed to settle out, and then the clarified liquid can
be taken off to be used as the disinfecting agent in the water to be treated.
For granular forms, such as HTH, adding the required quantity to 1 litre of
water and agitating will be sufficient to ensure good mixing.
The 1% solution is used as the means of disinfecting larger
quantities of water.
When using chlorine to disinfect drinking water the aim is to
kill off all the pathogens and then to leave a small amount of active chlorine
in the water. This remaining chlorine is called the 'residual chlorine'. The
residual chlorine is desirable as it can disinfect further contamination of the
water once it has been collected e.g. dirty water containers. It is desirable to
have a residual chlorine level of 0.3 - 0.5 milligrams per litre (mmg/l). This
can be measured quite simply (see below).
The chlorine demand of water will vary greatly from one location
to another. It is important therefore that the person responsible for the
chlorination process is able to calculate the actual chlorine demand of the
water to be treated.
This is a simple process of trial and error. Specific quantities
of a chlorine solution can be added to litre samples of the water to be treated,
e.g. sufficient to give 3, 4 or 5 mg/l. The residual chlorine can then be tested
after a minimum of 30 minutes contact time. The chlorine demand can then be
determined by deducting the residual from the amount of chlorine added.
When the chlorine demand has been calculated, the desired
residual level can be added arithmetically to give the required chlorine dose
per litre of water, e.g. chlorine demand = 3.5 mg/l, desired residual = 0.5
mg/l, chlorine dose = 4 mg/l. This figure is then used to calculate the amount
of solution to be added to the volume of water to be treated.
Using these figures to give a 5 mg/l dose of chlorine to a
reservoir of 45,000 litres will require 22.5 litres of 1% solution.
It is very important that the residual chlorine is able to be
measured as this can tell the person responsible how effective the chlorination
process has been. A very simple test involves the use of a kit designed for
measuring the chlorine levels in swimming pools. It is called a 'pool test kit'.
A sample of the water to be tested is placed in the kit and a
DPD No. 1 tablet is dropped into it. The chlorine in the water reacts with the
DPD tablet to give a level of coloration in the water.
This colour is compared directly against the colour chart on the
kit. The strength of colour then tells the operator the level of residual
chlorine. The same kit can also measure the pH of the water sample in a similar
comparative manner.
- pre-treatment is important to get water as clean as possible
before chlorination
- do not chlorinate before filtration
- check pH and
temperature to help assess contact time
- ensure that minimum contact time is
always permitted
- always test for residual chlorine levels
- follow the
storage guide for the particular chemical being used
The following simple rules must always be followed and any
particular advice and precautions supplied with a specific product should
likewise be closely followed.
This was prepared as an information note by the Oxfam Public
Health
Team.
