Transcript Disinfection of Drinking Water

Fresh water microbiology
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Microorganisms in different aquatic zones
1.
A variety of microorganisms live in fresh water.
The region of a water body near the shoreline
(the littoral zone) is well lighted, shallow, and
warmer than other regions of the water.
Photosynthetic algae and bacteria that use light
as energy thrive in this zone.
2.
Further away from the shore is the limnitic
zone. Photosynthetic microbes also live here.
3.
As the water deepens, temperatures become
colder and the oxygen concentration and light in
the water decrease. Now, microbes that require
oxygen do not thrive. Instead, purple and green
sulfur bacteria, which can grow without oxygen,
dominate.
4.
Finally, at the bottom of fresh waters (the
benthic zone), few microbes survive. Bacteria
that can survive in the absence of oxygen and
sunlight, such as methane producing bacteria,
thrive.
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Saltwater presents a different environment to
microorganisms. The higher salt concentration,
higher pH, and lower nutrients, relative to
freshwater, are lethal to many microorganisms.
But, salt loving (halophilic) bacteria abound near the
surface, and some bacteria that also live in
freshwater are plentiful (i.e., Pseudomonas and
Vibrio).
Also, in 2001, researchers demonstrated that the
ancient form of microbial life, known as archaea, is
one of the dominant forms of life in the ocean.
The role of archaebacteria in the ocean food chain
is not yet known, but must be of vital importance.
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Water can also be an ideal means of transporting
microorganisms from one place to another. For
example, the water that is carried in the hulls of
ships to stabilize the vessels during their ocean
voyages is now known to be a means of
transporting microorganisms around the globe.
One of these organisms, a bacterium called Vibrio
cholerae, causes life threatening diarrhea in
humans.
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Which microrganisms in water cause
diseases?
Sometimes microrganisms that cause health effects can be found in
drinking water. However, as drinking water is thoroughly
disinfected today, disease caused by microrganisms is rarely
caused by drinking water.
People that swim in swimming pools will find that the water they swim
in is disinfected with either chlorine, ozone, UV or chlorine dioxide.
But there are people that swim outside in surface water every year.
There are various bacteria and protozoa that can cause disease
when they are present in surface water.
Bacteria are not only known to cause disease when they enter a human
body through food, surface water may also be an important source
of bacterial infection. In table 1 you can see various bacteria that
can be found in surface water, and the diseases they cause when
swallowed in large amounts, along with the symptoms.
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Bacteria
Disease/infection Symptoms
Aeromonas
Enteritis
Very thin, blood- and
mucus-containing diarrhoea
Campylobacter jejuni
Campilobacteriose
Flue, diarrhoea, head- and
stomachaches, fever,
cramps and nausea
Escherichia coli
Urinary tract infections,
neonatal meningitis,
intestinal disease
Watery diarrhoea,
headaches, fever, homiletic
uraemia, kidney damage
Plesiomonas
shigelloides
Plesiomonas-infection
Nausea, stomachaches and
watery diarrhoea,
sometimes fevers,
headaches and vomiting
Vibrio El Tor
(freshwater)
(Light form of) Cholera
Heavy diarrhoea
Salmonella
Typhoid fever
Salmonellosis
Fevers
Sickness, intestinal cramps,
vomiting, diarrhoea and
sometimes light fevers
Streptococcus
(Gastro) intestinal
disease
Stomachaches, diarrhoea
and fevers, sometimes
vomiting
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Protozoa can accumulate in certain body
parts, after they have penetrated a human
body. The accumulations are called cysts.
Because of their parasitic nature, protozoa
can cause various diseases. In table 2 you
can see various protozoa that can be found
in surface water, and the diseases they
cause when swallowed in large amounts,
along with the symptoms.
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Microrganism Disease
Symptoms
Amoeba
Amoebic dysentery
Severe diarrhoea,
headache, abdominal
pain, chills, fever; if not
treated can cause liver
abscess, bowel
perforation and death
Cryptosporidium parvum
Cryptosporidiosis
Feeling of sickness,
watery diarrhoea,
vomiting, lack of appetite
Giardia
Giardiasis
Diarrhoea, abdominal
cramps, flatulence,
belching, fatigue
Toxoplasm gondii
Toxoplasmosis
Flu, swelling of lymph
glands
With pregnant women
subtle abortion and brain
infections
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Drinking water is usually treated to minimize the risk of microbial
contamination. The importance of drinking water treatment has
been known for centuries. For example, in pre-Christian times the
storage of drinking water in jugs made of metal was practiced.
Now, the anti-bacterial effect of some metals is known. Similarly,
the boiling of drinking water, as a means of protection of water has
long been known.
An important aspect of water microbiology, particularly for drinking
water, is the testing of the water to ensure that it is safe to drink.
Turbidity gives an indication of the amount of suspended material in
the water. Typically, if material such as soil is present in the water
then microorganisms will also be present. The presence of particles
even as small as bacteria and viruses can decrease the clarity of
the water. Turbidity is a quick way of indicating if water quality is
deteriorating, and so if action should be taken to correct the water
problem.
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Ancient Water Treatment
The first documented
attempts to treat
drinking water are
recorded in ancient Greek
and Sanskrit writings that
date back to 2000 B.C. At
this time, people were
aware that boiling water
helped to purify it and
that filtration and
straining methods helped
to reduce visible particles
and turbidity in water.
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Because nothing
was known about
microorganisms
or chemical
contaminants,
the motive for
treating water
was to make it
smell and taste
better.
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The Greek scientist Hippocrates, who
invented the first cloth bag filter
around 500 B.C, also believed that if
water tasted and smelled clean, it must
be healthful for the body. His
invention, called the “Hippocratic
sleeve,” was one of the first domestic
water filters (Baker & Taras 1981).
Hippocrates Sleeve:
a conical strainer, made by stitching together
two adjacent sides of a square piece of
cloth, esp. flannel of linen.;
a woollen bag of a square piece of flannel,
having the opposite corners joined, so as to
make it triangular. Used by chemists for
straining syrups, decoctions, etc.
Baker, M.N. and Taras, Michael J. 1981. The Quest for Pure
Water: A History of the Twentieth Century, Volume 1 and
2. Denver: AWWA
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Disinfection of Drinking
Water
Disinfection is the final process to which water is
subjected prior to distribution. All other
treatment processes such as sedimentation,
flocculation, coagulation, filtration cannot give
guarantee for safe water.
Only the disinfection process can assure that the
water is safe. In the absence of all other
processes disinfection alone can be employed as a
single effective treatment process.
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Disinfection is different from sterilization in which
complete destruction of all living organisms occurs
and the effect is permanent. Whereas disinfection
primarily inactivates infectious microorganisms
and is effective for a limited period.
Complete sterilization of a water supply is extremely
difficult to achieve and would not be necessary.
Both chemical and physical methods are used for the
disinfection of water.
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1. Chemical Disinfection
Various chemical agents have been used for
the disinfection of drinking waters. Each
agent has its own advantages and
limitations.
The commonly used agents are chlorine,
chlorine compounds like chlorinated lime,
oxidants like ozone and potassium
permanganate and halogens.
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1.1 Chemical Disinfection by
Chlorine
Chlorine and its compounds are the common chemical
disinfectants. Comparatively they are less harmful
and more effective. They are quick in action. In
the presence of moisture chlorine is highly
corrosive to all metals except silver and lead.
The chlorination process may be of two types. In
pre-chlorination method chlorine is applied prior
to any other treatment, usually for controlling
algae, taste and odor. In post-chlorination method
chlorine is applied after other treatment
processes, especially after filtration.
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The chlorine dosage must be sufficient to leave a
residual of 0.2 to 2.0 mg/l free chlorine. Two
common methods used to measure the residual
chlorine in water are diethyl para-phenylene
diamine method and ortho toluidine method.
Chlorinated lime
It is commonly known as bleaching powder. Before
the discovery of liquid chlorine, chlorinated lime
was widely used for chlorination. It is a loose
combination of slaked lime and chlorine gas.
When added to water it decomposes to give
hypochlorous acid. Chlorinated lime is unstable and
on exposure to air, light and moisture reduces the
chlorine content rapidly.
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1.2 Chemical Disinfection by Ozone
It is a powerful oxidising agent and highly unstable.
It must be manufactured on site by passing dry air
through a high voltage high frequency electrical
discharge.
It has a more rapid effect than chlorine in
destroying viruses and bacteria including spores.
It is also effective in eliminating compounds that
give objectionable taste and color to water.
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The treatment with ozone should leave 1-2 mg/l of
residual ozone. But ozone usually leaves very low
level of residuals and thus there is no protection
against new contamination of the water after
disinfection. The high installation and operation
costs further reduce its use as a disinfectant.
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1.3 Chemical Disinfection by
Potassium Permanganate
It is also a powerful oxidising agent. It has been
found to be effective against cholera pathogen
but not other pathogens. Since it leaves stains in
the container it is not a satisfactory disinfectant.
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1.4 Chemical Disinfection by
Halogens
Bromine is used as a disinfectant for swimming pool
water because of its less irritability to the eyes
than chlorine. Fluorine and iodine are also at times
used as disinfectants.
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http://www.nhmrc.gov.au/publications/synopses/eh19syn.htm
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2. Physical Disinfection by UV
Radiation
Two important physical disinfection methods are
ultraviolet (UV) radiation and boiling of water.
UV radiation
Electromagnetic radiation of ultraviolet range can be
used to destroy microorganisms. This process is
effective in certain small water supplies where the
water is highly polished i.e., filtered and
demineralised.
The process is also used in industries. For example in
breweries, pharmaceutical manufactures, fish
hatcheries and aquariums.
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Actually irradiation must strike the organism to kill
it. In this process some of the radiation energy is
absorbed by the organism and other' constituents
in the medium surrounding the organisms. So if
sufficient dosage of UV reach the organisms
water can be disinfected.
The germicidal effect of ultraviolet energy is
thought to be associated with its absorption by
certain organic components essential for the
functioning of cells. Dissipation of energy by
excitation causes disruption of unsaturated,
bonds, particularly of the purines and pyrimidines.
And this leads to lethal biochemical changes.
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UV treatment does not alter the water chemically.
Only energy is added, which produces heat,
resulting in a temperature rise in the treated
water.
UV rays can penetrate the cell walls of microorganisms. So the only requirement in disinfection
by UV radiation is it must reach the organisms
since the UV radiation absorbed by the organisms
alone can kill them.
This is, one of the main disadvantage of the UV
process. Other disadvantages are lack of a field
test to readily establish the efficiency of the
process inability of UV irradiation to provide a
residual disinfecting power high cost of the
system.
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2.1
Germicidal Efficiency
It has been found out that the
germicidal action of ultraviolet rays is
maximal at the wave length of 250260nm. There is an abrupt decrease
in the efficiency at 290-300nm and
continues upto the visible range.
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At a fixed UV intensity, the contact time required to achieve a
99.9% kill for certain organisms is given below.
E.coli - 60 sec
Shigella - 47 sec
Streptococcus faecalis - 165 sec
Bacillus subtilis - 240 sec
Bacillus spores - 369 sec
B.subtilis spores
UV radiation has also been found to be effective in the
inactivation of viruses. There is no reference to the ability
of UV radiation to destroy cysts. It is the consensus that
UV radiation will not kill any organism which can be seen with
naked eye.
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Monitoring of water quality
In many countries, water microbiology is also the
subject of legislation. Regulations specify how
often water sources are sampled, how the sampling
is done, how the analysis will be performed, what
microbes are detected, and the acceptable limits
for the target microorganisms in the water
sample.
Testing for microbes that cause disease (i.e.,
Salmonella typhymurium and Vibrio cholerae) can
be expensive and, if the bacteria are present in
low numbers, they may escape detection. Instead,
other more numerous bacteria provide an
indication of fecal pollution of the water.
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Escherichia coli has been used as an indicator of
fecal pollution for decades. The bacterium is
present in the intestinal tract in huge numbers,
and is more numerous than the disease-causing
bacteria and viruses. The chances of detecting
E. coli is better than detecting the actual disease
causing microorganisms.
E.coli also had the advantage of not being capable of
growing and reproducing in the water (except in
the warm and food-laden waters of tropical
countries). Thus, the presence of the bacterium in
water is indicative of recent fecal pollution. Finally,
it can be detected easily and inexpensively.
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EPA Microbiological Methods/Online
Publications:
http://www.epa.gov/nerlcwww/online.htm
Australian goverment National Health and
Medical Research Council
http://www.nhmrc.gov.au/publications/
FINNISH STANDARDS ASSOCIATION
SFS 07.100.20 Vesimikrobiologia
http://www.sfs.fi/luettelo/
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SURVIVAL OF EXCRETED
PATHOGENS
Health risks associated with
wastewater use:
http://www.fao.org/docrep/w5367e/w5367e04.htm
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