- Is sodium
hypochlorite (SH) the best disinfectant to use?
- What about
using HTH or chlorine tablets instead?
- How do you
ensure adequate shelf life of SH?
- Will adding
NaOH change the effectiveness of the SH?
- Does the
SH inactivate giardia and cryptosporidium?
- What are
disinfection by-products, and are they an issue
in the SWS?
- If you add
SH to water already treated in a municipal water
treatment plant, is there a risk of a chlorine overdose?
- What happens
if a child accidentally drinks the SH?
- Which hypochlorite
generator do you recommend?
- Are there
advantages to having a private company manufacture
- What characteristics
are required for the SH?
- How much
does it cost to manufacture the SH?
- How do we
determine the appropriate SH dose?
- What characteristics
do you recommend for the SH bottle?
- How can
we save money on manufacturing the SH bottle?
- What do
we do if the source water is turbid?
- Why does
free chlorine in treated water decline over time?
- How can
you distribute the SH throughout the country?
1. Is sodium
hypochlorite (SH) the best disinfectant to use?
Our investigations have shown sodium hypochlorite
to be effective and have broad applications. We have
investigated a number of other disinfectants (calcium
hypochlorite, ozone, UV, solar disinfection) and treatment
processes (filters, slow sand filtration) and feel
that sodium hypochlorite offers the best mix of low
cost, ease of use, safety, and effectiveness in areas
where there is enough water to drink and water is
not excessively turbid. We feel that these characteristics
are the reasons why most water treatment systems in
the US and Europe have been using chlorine for disinfecting
drinking water for nearly 100 years. We do, however,
recognize that the other disinfection methods noted
above also effectively disinfect water and are useful
in a number of settings.
What about using HTH or chlorine tablets instead?
Chlorine tablets and/or HTH (also named calcium hypochlorite)
are widely available in some areas. We have found
that a number of potential users of the SWS know that
if they add the tablets to water, it will disinfect
the water. However, we have also found that people
have very different ideas of the appropriate dosing
and the tablets vary in strength considerably. In
Haiti, a small saran wrap bag of approximately 100
HTH pellets is widely available and inexpensive. However,
the pellets are of varying size, the quality of the
pellets is unknown, and depending on impurities in
the manufacturing process they can degrade quickly.
In other countries we have seen very high strength
tablets sold, which, if added to water for disinfection
would impart a strong, unpalatable taste to the water.
Thus, it is extremely important to investigate the
quality and strength of the HTH or chlorine tablets,
and the appropriate dosing strategy before attempting
to use them for drinking water treatment. For these
reasons, we have found that the hypochlorite solution
is a better option.
How do you ensure adequate shelf life of SH?
Sodium hypochlorite is highly reactive and volatile.
At normal pH (6-8) sodium hypochlorite can degrade
substantially within 2-3 weeks. This shelf life is
not adequate for use in the SWS, for the hypochlorite
must remain at high enough concentration to inactivate
disease-causing organisms. By raising the pH of the
hypochlorite solution, you stabilize the solution.
The pH can be raised by the addition of sodium hydroxide,
which is widely available. In order to determine the
amount of sodium hydroxide to add to your sodium hypochlorite
solution, you will need to complete trial-and-error
testing. Add a known volume of sodium hydroxide to
a known volume of sodium hypochlorite, and then measure
the pH with a meter or kit. Because source water quality
is different in each location, there is not one standard
volume of sodium hydroxide to add to ensure pH is
above 11. You will have to start with a known volume
(perhaps 1 tablespoon in 1 gallon, or 5 ml in 1 liter)
and complete iterative trial-and-error testing. The
exact pH is not important in this context - you simply
need to ensure that the pH is above 11.
Will adding NaOH change the effectiveness of the SH?
No, because when the sodium hypochlorite solution
is added to water, the water decreases the pH and
the sodium hypochlorite more active. The chemistry
behind this is: the pH scale is from 0 to 14. Acids
have a pH below 7, bases are above 7, and 7 is neutral.
Most natural water is around pH 6-7. When sodium hypochlorite
is in water, it breaks up into two compounds, with
the concentration of each compound dependent on pH.
One of these compounds is significantly more reactive,
volatile, and more effective at inactivating bacteria
than the other. At high pH (above 11) the majority
of the sodium hypochlorite is in the form of the less-reactive
compound. Thus, when you add sodium hydroxide to the
sodium hypochlorite, you are converting it into the
less-reactive form. However, water is around pH 6-7.
When you add a small amount (5 milliliters) of solution
at pH 11 to a large amount (20 liters) of water at
pH 6-7, the mixture becomes pH 6-7. Thus, when you
add the hypochlorite at pH 11 to your water in the
SWS, you convert the hypochlorite back into the reactive
form, and then it inactivates the disease-causing
Does the SH inactivate giardia and cryptosporidium?
Giardia and cryptosporidium are both protozoa and
are resistant to chlorination because they exist in
water in a cyst form. The hard coat of the cysts protects
giardia and cryptosporidium from being inactivated
by chlorine. Cryptosporidium is more resistant to
chlorine than giardia. See this inactivation table
for more details. Both protozoa, however, are fairly
large. Cryptosporidium is approximately 3-5 times
the size of the bacteria E. coli, and giardia is approximately
5-10 times the size of E. coli. Cryptosporidium and
giardia can thus be removed by filtration. If giardia
or cryptosporidium are a significant health problem
in the project area, a filtration step (through ceramic,
sand, or other filters) can be added before adding
the sodium hypochlorite. The Safe Water System intervention
has been proven to reduce diarrhea in children, and
this intervention does inactivate many of the ones
that cause the most severe disease, like cholera,
dysentery, and typhoid fever.
What are disinfection by-products, and are they an
issue in the SWS?
Disinfection by-products (DBPs) are chemical compounds
formed when chlorine is added to water with organic
material in it. All natural waters have some organic
material in them, and generally waters that are more
turbid (dirty) have more organic material. DBPs are
a concern whenever chlorine is added to drinking water,
whether in the Safe Water System or in a large-scale
water treatment plant in the United States, because
some studies have suggested that ingestion of DBPs
in water over a lifetime may be associated with a
very low risk of cancer. However, this risk is very
small (1 in 100,000 people over 70 years is what the
WHO considers when establishing their drinking water
guidelines (1996)). In areas where many people, and
many children, have diarrheal diseases caused by unsafe
drinking water, the risk of cancer from THMs is miniscule
compared to the risk of death or stunting from diarrheal
diseases. The World Health Organization is very clear
on the importance of microbiologically safe drinking
water (obtained by adding chlorine) as compared to
the risk from DBPs in their Guidelines for drinking-water
quality: "Where local circumstances require that
a choice must be made between meeting either microbiological
guidelines or guidelines for disinfectants or disinfectant
by-products, the microbiological quality must always
take precedence, and where necessary, a chemical guideline
value can be adopted corresponding to a higher level
of risk. Efficient disinfection must never be compromised"
(WHO Guidelines for Drinking Water Quality, Volume
2, 1996). Because the DBP question is often raised,
we have prepared two fact sheets on DBPs: one is a
short one-page document, and the other is a more descriptive
6 page document.
If you add SH to water already treated in a municipal
water treatment plant, is there a risk of a chlorine
This is very unlikely. If sodium hypochlorite is
added to water that is already treated, the water
would most likely still be within an acceptable range
of chlorine residual. Typically, chlorinated urban
water systems have free chlorine levels of around
0.1 to 0.5 parts per million. We calculate our sodium
hypochlorite solution dose to give untreated water
a free chlorine level of around 1 part per million.
So if you add our solution (1 part per million) to
treated urban water (0.1-0.5 parts per million), the
level of the "overtreated" water would still
be in the acceptable range of 0.5-2 parts per million
(which is the range that balances disinfection efficacy
plus reasonable taste).
What happens if a child accidentally drinks the SH?
A review of health effects from accidental and intentional
ingestion of sodium hypochlorite (bleach) in Europe
was published in 1994 by Proctor and Gamble in conjunction
with European poison control centers (Racioppi, et
al. Food and Chemical Toxicity, Volume 32, Number
9, pp 845-861). The results of this review show that
"acute accidental exposure to household bleach
in use or in foreseeable misuse situations results,
in the great majority of the cases, in minor, transient
adverse effects on health". The authors cited
two studies specifically on children: 1) A study in
Chicago showing that of 26 children admitted for accidental
bleach ingestion, only one had a moderate health effect
(irritation of the esophagus, which healed on its
own without intervention), with the remaining children
having only "minor transient irritation effects",
and 2) A study of 23 cases aged 1 - 3 years, with
only one case having "superficial burns in the
esophagus", which disappeared two weeks later.
Suicide attempts in adults have shown that a lethal
dose of sodium hypochlorite varies widely, with lethal
results at 200-500 mL of 3-12% strength. It is important
to remember that the concentration of the SH used
in the SWS is approximately 0.5-1.0 percent. In most
countries, the SH is sold in 250 ml bottles, but in
some 500 ml bottles are used. The hypochlorite ingested
in the majority of the cases mentioned above was full
strength household bleach: 5 percent. Several factors
make it unlikely that the hypochlorite solutions recommended
in the Safe Water System could cause harm. First,
it is unlikely that a child would accidentally drink
250 or 500 milliliters of something that tastes as
bad as the sodium hypochlorite does. Second, based
on the data presented above, it is even less likely,
at the low concentration used in this project, that
anything harmful would occur. However, we highly recommend
that part of the educational materials emphasize the
need to keep the sodium hypochlorite solution stored
somewhere safe (out of sunlight, sealed, away from
children) for health reasons, to protect the sodium
hypochlorite from degradation, and to prevent spills
in households that have limited disposable income
to purchase more solution.
Which hypochlorite generator do you recommend?
A number of companies manufacture hypochlorite generators.
Information on some of them can be found in our small
projects page. There are several advantages in using
a hypochlorite generator. First, local production
of the sodium hypochlorite that minimizes transportation
costs. Second, in the event there is not a reliable
bleach producer in the country, the hypochlorite generator
provides that capacity. Third, revenues from the sale
of the solution can be used to help support operation
and maintenance of the machine and to pay the operator.
Considerations that must be taken into account when
producing bleach in this way include the need for
regular operation and maintenance of the machine,
payment of a reliable person to operate and maintain
the machine, replacement of the cell of the generator
every 5 years, and the need for a reliable electricity
Are there advantages to having a private company manufacture
There are several advantages to having a company
make the solution are: 1) Most likely, all a company
would need to do to make the desired concentration
of hypochlorite is to dilute an existing bleach product.
2) If demand for the solution grows, a company is
better able to expand production. 3) Many companies
have certification from Bureaus of Standards for bleach
products that can often be applied to the new dilute
solution. 4) Most reputable companis have quality
control procedures. In Madagascar, Rwanda, Kenya,
Malawi, India, Afghanistan, and Tanzania, Populations
Services International (a social marketing non-governmental
organization that has successfully implemented a number
of SWS projects) has opted to have private companies
make the bleach for them.
What characteristics are required for the SH?
First, it is important that the concentration is
correct (usually 0.5 to 1.0 mg/l). A concentration
that is too low requires too high a volume to adequately
treat enough water to be practical. A concentration
that is too high is difficult to accurately dose,
raising the risk of too high a dose (which is unpalatable),
or too low a dose (which might not effectively disinfect
the water). Second, it is important that pH of the
solution is at least 11. This increases the shelf
life of the solution.
How much does it cost to manufacture the SH?
In the first year of the country-scale project in
Zambia it cost US$78,000 to manufacture 400,000 bottles
per year. The labor cost was $23,000 and the materials
(salt, vinegar, bottles, and labels) cost was $55,000.
The total production cost was thus US$0.20 per bottle,
and assuming a family usage of one bottle per month,
the production cost for a year's supply for one family
is about US$2.40. After the first year, costs are
expected to decrease. Costs vary by country, depending
on labor, materials, and value added taxes. In small-scale
projects using a local hypochlorite generator and
reusable bottles, the production cost of the hypochlorite
is only the cost of the salt, water, labor, and electricity.
How do we determine the appropriate SH dose?
The hypochlorite dose will depend on the characteristics
of the local water. Usually an amount in the range
of 5 to 10 milliliters added to 20 liters of water
is sufficient to inactivate the disease-causing organisms,
but not leave an unpleasant taste. Once the size of
the cap for your project has been determined, some
simple experiments using the sodium hypochlorite locally
available, source water in your area, and a kit that
measures the amount of free and residual chlorine
can be used to determine the appropriate dose. Please
contact us at firstname.lastname@example.org for more information
on how to complete this testing.
What characteristics do you recommend for the SH bottle?
We recommend the following six characteristics for
the sodium hypochlorite bottle that is kept in the
home: 1) The size of the bottle should be between
250 and 500 milliliters. This is small enough to be
affordable and to ensure that the solution will be
used before it degrades, but large enough that it
will last a family for approximately a month; 2) The
neck of the bottle should be compatible with soda
bottle caps, which tend to be mass-produced, are inexpensive,
typically have the desired volume of 5-10mL for dosing,
and are easily accessed in most locations; 3) The
volume of the cap should be between 5 and 10 ml so
that it can be used to dose the solution; 4) The bottle
should be composed of an opaque plastic, to prevent
exposure of the solution to direct UV radiation from
sunlight, which will decrease the shelf life; 5) A
handle is not necessary. This only increases the cost
and decreases the space available for instructions;
and 6) The neck and cap should have at least four
threads to improve the seal. The cap should have a
raised ring inside to help seal the bottle as well.
How can we save money on manufacturing the SH bottle?
One way to save money when designing the solution
bottle is to design the bottle so that an already
locally available cap will fit it. We recommend a
plastic soft drink bottle cap with a volume of between
5 and 10 milliliters. Using a locally available cap
will save you from having to purchase a mold for the
caps, and soft drink caps are typically mass produced
at very low cost. The existing caps must fit tightly
and securely on the project bottle design, however.
What do we do if the source water is turbid?
Water that looks dirty or cloudy is called turbid
water. Turbidity is a measure of the amount of light
that is scattered as it passes through the water sample.
If more particles are in the water, more light will
be scattered, and the turbidity is thus higher. Water
that looks "dirty" will have a higher turbidity
than water that looks clear. Turbidity itself is not
a problem, however turbidity is often used to represent
the amount of total suspended solids and the amount
of organic matter in the water. There are two issues
with adding chlorine to water that has a high turbidity:
1) Chlorine reacts preferentially with organic matter
before reacting with bacteria. Thus, you need to make
sure to add enough chlorine to inactivate the bacteria,
2) There is a potential of creating more disinfection
by-products if there is a higher concentration of
organic matter in the source water. There are three
strategies that can be used to make turbid water clear:
1) Filtering the water through a cloth filter will
remove some of the organic matter; 2) Letting the
water settle for 12-24 hours so the organic matter
falls to the bottom and then pouring off the clearer
water into a separate vessel will also remove some
of the organic matter; or 3) Increasing the dose of
sodium hypochlorite solution adde d to the water to
make sure there is enough chlorine to inactivate the
disease-causing organisms. Because every community
is different, experiments to determine which is the
most acceptable and appropriate strategy will need
to be conducted in the project community.
Why does free chlorine in treated water decline over
Chlorine is an extremely reactive chemical. Right
after the sodium hypochlorite is added to the water,
chlorine levels decline because the chlorine is reacting
with organic matter and microbes. After those reactions
are complete, chlorine in water will slowly escape
into the air as a gas. This is the reason that free
and total chlorine levels slowly degrade over time
in a covered (but not sealed) container, and also
why we recommend that the pH of the hypochlorite solution
be raised to over 11 to extend the shelf-life before
it is used.
How can you distribute the SH throughout the country?
In many projects, PSI has initiated social marketing
campaigns, which include the activation of networks
of wholesale and retail outlets, facilitating distribution
to communities where vulnerable populations live.
For smaller projects, one idea is to purchase space
on private delivery trucks that are already going
to target locations to deliver goods such as soft
drinks and beer, or to request a donation of space
by the private companies as a charitable activity.