Methods of Water Disinfection
Table of Contents
- Chemical Water
- UV Water Purification
disinfection is routine in developed countries with adequate sanitation
and safe hygiene practices. Depending on the source of the water,
municipal water treatment plants use a variety of physical and chemical
water disinfecting techniques before the water reaches your sink at
home. The most common methods of water purification include heat,
distillation, chemical treatment, UV light and filtration. In
developing countries, where residents don't have access to safe
sanitation and hygiene, alternative methods are often used. In
emergency situations, it is good to familiarize yourself with the
used so that you can disinfect the water yourself, should the need
illness is caused by bacteria, viruses and parasitic cysts - Giardia
and Cryptosporidium - often found in rivers, lakes and other dirty
water sources. Heating the water to boiling point kills these
disease-causing microorganisms and is the surest way to make
contaminated water safe for drinking. The Centers for Disease
(CDC) and the Environmental
Protection Agency (EPA)
both recommend that
you boil water vigorously for one minute, to ensure that all
microorganisms are killed. At altitudes higher than one mile,
boiling point decreases, so you should boil the water for at least
three minutes. Boiling will not rid the water of any unpleasant taste.
However, aeration of the water may improve bad taste. To aerate, pour
the water back and forth from one container to another and allow it to
sit for a few hours, or add a pinch of salt for each liter or quart of
water boiled. In developed countries, boiling is usually the
recommended solution in emergency situations where accidental water
contamination has been reported. However, if boiling is not an option,
residents often resort to drinking bottled water.
is the process of boiling water to produce water vapor which then
condenses onto a cool surface. The solutes do not normally vaporize and
are left in the boiling solution leaving the condensed water vapor
almost completely pure. However, it is possible to get water that is
not completely pure at the end of this process, due to contaminants
with similar boiling points and unvaporized droplets being carried with
the steam. Distillation is a controversial water disinfecting method,
because it de-mineralizes the water, leaving it void of essential
minerals like magnesium and calcium, which prevent nutritional
deficiency. It also increases the risk of toxic poisoning, as it more
readily leaches metals like lead from household piping.
of the most common chemical water disinfecting agents, which may be
used in an emergency situations, are chlorine and iodine
(halogens). Chloramines and Ozone are also used by water
plants. In order for chemical disinfection to be effective, the water
must be filtered and settled first. Chemical disinfection often leaves
an undesirable taste in water, which an activated carbon filter can
remove post-treatment. Otherwise, you can minimize bad taste and odor
by reducing the disinfectant concentration and increasing contact time
before drinking. Another option to remove the taste of chlorine and
iodine from water is to add a pinch of ascorbic acid, which is
available in crystal or power form. This converts chlorine into
chloride and iodine into iodide, both of which have no taste or odor.
may be used in gas, liquid or solid form to disinfect water. Because
chlorine gas is highly toxic and can be dangerous if released into the
atmosphere, this form of disinfection must be done in a very controlled
environment. Otherwise, the danger is avoided by the use of chlorine in
liquid form (sodium hypochlorite) or solid form (calcium hypochlorite).
Household bleach is made up of 3-6 percent sodium hypochlorite and the
EPA recommends using it to disinfect water in emergency situations.
Waste water treatment plants typically use a 12 percent solution.
Calcium hypochlorite water purification tablets are readily available
for emergency situations as well. While chlorine is a highly effective
and widely used method of water disinfection, it reacts with organic
compounds in water, forming trihalomethanes (THMs) and haloacetic
acids, which are carcinogenic in large quantities. The best way to
avoid this is to remove as many organics as possible, prior to
disinfection. If necessary, some commercial water filters also remove
chlorine and its byproduct,s post-treatment.
- Iodine, like chlorine, is available in liquid and tablet
is used by the thyroid and is recommended for emergency use only, in
limited quantities. Iodine water purification is not recommended for
people with who are pregnant, have thyroid disease,
or an iodine allergy.
does not form THMs or haloacetic acid as chlorine does, and, as a
result, is becoming more commonly used as a water disinfectant.
However, nitrates, which impart a bad taste and odor and may be harmful
to humans, are sometimes formed as a byproduct instead. Chloramines are
made when ammonia is added to water containing chlorine, or when water
containing ammonia is chlorinated. They are less effective than
chlorine at killing viruses or protozoa.
is a powerful oxidizing agent that is toxic to most waterborne
organisms. It is widely used in water treatment plants in other parts
of the world, especially in Europe but is fairly new to the U.S. Ozone
is often accompanied by a secondary disinfectant, such as chlorine,
because although it effectively kills existing microorganisms, it
leaves few residuals to prevent the future growth of microorganisms in
the water, post-treatment. The danger of ozone is the production of
carcinogenic bromate as a byproduct. However, bromide ions are required
for this to occur, and there should be few, if any, in treated water.
radiation penetrates the cell walls of microorganisms and disrupts
their DNA, making them unable to reproduce. Though they remain present
in the water, they are inactive, and thus, leave no risk of waterborne
illness. UV light is effective against viruses and bacteria but may not
inactivate parasitic cysts. As with ozone, a secondary method is often
necessary to prevent the regrowth of organisms, post-treatment.
is often the first and last step taken in these common methods of water
disinfection. It is recommended prior to the disinfection of
to remove particles that hinder the disinfecting process. Following
disinfection, filtration may also be used as a secondary method to
remove cysts, along with the bad taste and odor caused by chemical