CHLORINE AND DRINKING WATER TREATMENT

Water is the elixir of life. Sustenance of mankind depends on availability of safe

drinking water. The water drawn from river, pond, lake, springs or ground is
invariably contaminated with killer pathogens. The water is increasingly getting
contaminated day by day as the human population grows due to discharge of
untreated sewages, Industrial pollutants to the surface water and discharge on
land which seeps into the ground contaminating the ground water.

Pathogenic bacteria that have been transmitted by water include coliform group
of bacteria, salmonella and shigella, clostridium welchii, C. botulinum, bacillus
cereus, vibrio cholarae and V. parahaemolyticus. Water borne pathogenic
bacteria salmonella typher causes typhoid, shigella causes dysentery, Vibro
Cholera causes cholera, cryptosporidicem causes diarrhoea, refractory
organisms as Giardia lambillia causes Giavdiasis, Ecoli causes gastroenteritis.
Water for human consumption must be free from these micro-organisms.
Nevertheless, the coliform test has been, and will continue to be, a useful tool for
assessing the quality of water. For recreational purposes the water in swimming
pools also should be free from coliform bacteria and should be microbiologically
safe. The epidemics of cholera, typhoid etc, which are water borne diseases,
were wide spread in India and other countries of the world due to consumption of
untreated contaminated water.

The situation has since been improved due to the realisation that potable water
has to be treated suitably to kill all the pathogenic bacteria's before it can be
consumed by human beings. Agencies engaged in treating & supplying drinking
water are responsible to find suitable method of treatment to make the water
potable. Various methods are adopted to kill pathogenic bacteria and germs in
water and to make it potable. The most effective method is chlorination of water.
The other methods are ozone treatment & UV radiation. The majority of water
treatment plants use chlorination method and very few small installation use
ozone treatment and UV. radiation to disinfect the drinking water. The technology
chosen to treat raw drinking water to produce potable water at a particular site
depends on a number of factors, including the quality of the raw water, whether it
is drawn form surface or underground sources, the volume of water to be treated,
the number of customers served, and the financial and human resources
available to the system operator.

The process of treating drinking water usually involves raw water storage and
sedimentation to remove gross particulates. The process may also entail
aeration to oxidise both inorganic and organic constituents and reduce
objectionable odours. The water is typically treated by coagulation and
sedimentation to remove most of the contained particulate matter and then may
be subjected to a number of additional filtration and other treatment steps to
reduce the concentrations of solids and dissolved compounds still further. Hard
water, which contains high levels of dissolved minerals, may be treated
chemically to soften it by reducing calcium and magnesium contents. The
undesirable elements such as iron and manganese are removed by converting
them to their oxide forms by chemical treatments. Additional treatment steps
may include filtration through beds of sand, membranes, or activated carbon,
and pH adjustment.
These measures can improve greatly the aesthetics of potable water and careful
filtration can reduce the contents of pathogens in the water since many of them
are associated with the particulate matter that is present. However, even a high
degree of pathogen removal by physical means other than nanofiltration may not
be sufficient to completely protect the drinking water supply because the
remaining pathogen population can reestablish itself quickly. E. Coli, for example,
can double its population within one half hour under favorable circumstances. To
prevent regrowth of E. Coli as the water passes through the distribution system
to consumers, complete removal is required at the treatment plant. Effective
removal of viral, bacterial, and protozoan pathogens requires disinfection by
means that destroy them chemically or physically, and different disinfection
practices may be required to treat the different types of pathogens effectively.

Historically, chlorine has been the chemical of choice to disinfect both drinking
water and wastewater. It was first used in United States in 1908, just 25 years
after Pasteur's identification of the significance of microbial activity. The normal
practice is to mix chlorine gas with water being treated at sufficient dosage and
for a sufficient contact time to destroy pathogens. The amount of chlorine and
contact time required depends on degree of destruction required, the mixing
efficiency, the types and amounts of organisms present, and the temperature and
pH of the water being treated. Current EPA regulations require that the
concentration of viruses, in treated water be reduced by a factor of 1 0,000, and
this typically would require a dosage-contact time of 4mg-min/ml for chlorine. In
addition to destroying viruses, these conditions also essentially destroy any
bacteria, present, but would not destroy protozoa to the same degree. Sufficient
chlorine is normally added to insure that the residual chlorine content of the
treated water is in the range of 0.1 to less then 1 mg/I (0.1 to< 1 ppm), which is
sufficient to provide continued or residual disinfection capability.

Benefits of Chlorine Use

Chlorine is having the germicide properties, which is used for drinking water
treatment to kill pathogens and protect from the water borne diseases. The
benefits of chlorine use are:

It is a potent disinfectant and most effective as compared to other

disinfectants.

It leaves residual chlorine after treatment and provides residual protection
till it is delivered in the house hold. No other water disinfectant can
Provide this protection after treatment.

It is most effective for taste and odor control.

It is practical, proven and easy to use. Produces low disinfection by
Products.

It is Economical and affordable. The cost of disinfection with chlorine
is lowest as compared to other disinfectants.

It is not required to be imported. It is available in plenty in the country,
There are 36 chlorine manufacturers in India. Total chlorine production
in India in the year 2008-09 was more than 20 Lac MT.

Expert assistance for use of chlorine is readily available from the near
by chlorine manufacturers.

How Chlorine Works

Chlorine when reacted with water forms Hypochlorous Acid (HOCI) which is the
pathogen killer. Hypochlorous Acid (HOCIs) disrupts cell wall, releases vital
cellular material and end cell function of the pathogen. Liquid chlorine is filled in
ton containers (900 Kg) or in small cylinders is transported from manufacturers to
water works. These are stored at water work for use. The chlorine is drawn from
the chlorine containers in gas form through chlorinators and mixed with the
water. The chlorine requirement is determined on the basis of the chlorine
demand of the water which is proportional to the presence of organic matter &
bacteria. Additional quantity of chlorine is added to maintain residual chlorine in
the water till it is delivered at the home to avoid contamination during
transportation. Pathogens are destroyed depending on the quantity of chlorine
and the residence time. At per WHO guideline for effective disinfection, there
should be residual concentration of free chlorine of 0.5 mg/l after at least 30 min
contact time at PH<8.0. As per Indian standards IS 10500:1991- Specification for
drinking water, the requirement of residual free chlorine is 0.2 mg/l (Min), to be
applicable only when water is chlorinated and tested at consumer end. When
protection against viral infection is required, it should be min 0.5 mg/l.

Source: Everything about water, Nov 2009