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Types of Disinfectants
Posted by Water in Disinfection on 01 27th, 2009 | no responses

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Ozone Treatment:

Ozone gas, 03, is a powerful disinfecting agent that can be used in drinking water applications.Ozone
has been used extensively for disinfection and for taste and odour control in Europe, US and Canada.

Oxygen in the air (02) is composed of two oxygen atoms. Under certain conditions, three oxygen atoms
can be bound together instead, forming ozone (0). Ozone has many advantages as a disinfectant. It kills
all pathogenic organisms by a direct effect on their DNA. Disinfection with ozone occurs 30,000 times
faster than with chlorine, so a prolonged contact time is not necessary. And there is no harmful residual
left in the system. Ozone, being an unstable gas, must be generated onsite and must be distributed into
the water immediately to disinfect it. Ozone is produced by passing a discharge of high voltage
alternating current through dry air or exposing air or oxygen to a high voltage electric arc. A voltage of
between 4000 and 20000 volts is applied to dielectric plates about 6 mm apart or to concentrated tubes,
through which the dry air is blown. The concentration of ozone produced by modern plant is of the order
of 15 to 20g 1m3 of air. The ozone containing air is then introduced into the water either by an injection
system which draws it under reduced pressure or by forcing it under pressure through perforated pipes
or ceramic materials immersed in water. The degree of absorption depends upon the depth of immersion
of the injection apparatus below the water level in the contact tank and the fineness of the air bubbles
introduced, and will vary from 60% to 90%.

Ozone treatment is generally effective in dealing with pathogenic bacteria and cysts. Since no residual
disinfectant is left in water, the water quality is likely to deteriorate in storage.

Advantages of Ozone disinfection are as follows.

1. Complex taste, colour and odour problems are effectively reduced.

Organic impurities are readily oxidized.
2. Effective disinfection is achieved over wide range of temperature and pH.
3. Bactericidal action is rapid (300 to 3000 times faster than chlorine). only short contact period is
required.
4. It reduces chlorine demand and in turn lowers chlorine dosage and so THM formation potential.

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Disadvantages of Ozone disinfection are as follows.

Ozone gas is highly toxic; it does not have the distinct warning smell possessed by chlorine gas and may
not cause immediate discomfort when breathed. Careful safety measure is therefore to be adopted with
its use.

1. The residual does not last long.

2. High electric input and high capital and operating cost (10 to 15 times higher than chlorine are
required).
3. High temperature and humidities may complicate ozone generation.
4. The process is less flexible than those for chlorine in adjusting for flow rate and water quality
variation.

Ultraviolet (UV) Irradiation

Ultraviolet or UV systems expose supply water to intense UV

light, which kill pathogenic bacteria and may remove some
pathogenic cysts. UV rays are found in sunlight, but UV rays can
be artificially produced, by passing electric current through
mercury vapour lamp enclosed in quartz bulb.

Ultraviolet light (UV) destroys microorganisms by changing their

C
genetic information (DNA), but does not produce residual or
S
hazardous by-products, nor does it affect the taste, odor or
W
colour characteristics of the treated water. It is light with very
W
high energy levels and wavelength of 200-400 nanometer (nm).
W
The most effective ultraviolet light for disinfection is UV -C (200-
W
280 nm), specially with a wavelength of 254 nm. (nm is equal to
10 Angstrom units)

The heart of the UV systems is high-performance spectrotherm

lamps (low pressure technology) which provide a stable UV
output through a wider temperature range. This special spectro
thermal lamps show higher degrees of effectiveness and stability
than other conventional lamps. They also exhibit a high UV power output (up to three times more than
competitive low pressure lamps) and long operating live (12,000 h), resulting in decreased overall costs.
The efficient lamps have a high UV emission in the area of the effective wavelengths (254 nm), that
makes it possible to destroy more than 99.99% of all pathogens in water. To further increase efficiency,
reliability and service lifetime, electronic power supplies (ballasts) for the lamps were developed.

The power rating for a UV lamp may be as high as 200 watts. Water must flow very close to the light
source, in a thin layer, and at a uniform, appropriate, flow rate to assure that bacteria are destroyed.
Since any suspended particles (or turbidity) in the water could “shade” bacteria from the direct rays
from the UV source, “live” bacteria could pass through the system. For this reason, all UV systems have
pre-filtration, often including a ceramic filter element, to assure the effectiveness of the UV disinfection
system

The UV treatment, like ozone or mechanical filtering leaves no residual component in the water to insure
its continued disinfection.

Advantages of UV irradiation as a disinfectant are as follows.

1. No chemical is introduced into the water, so the water quality is not significantly affected.

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2. Taste or odours are neither produced nor removed.

3. Exposure time is short.
4. Over exposure does not produce any detrimental effect.

Disadvantages of UV irradiation are as follows.

1. Spores, cysts and viruses are less susceptible than bacteria

Complete turbidity removal is required prior to UV irradiation.
2. There is no residual; therefore, a secondary disinfectant is needed.
Expensive equipment and large amounts of electrical energy are required.
3. Frequent, expensive maintenance of apparatus is necessary.

Iodine

For emergency purposes iodine may be used for treatment of drinking water. Much work at present is
being done to test the effect of iodine in destroying viruses, which are now considered among the
pathogens most resistant to treatment. Tests show that 20 minutes exposure to 8.0 mg/ L of iodine is
adequate to render the water safe. As usual, the dose required varies inversely with contact time. Lower
doses require longer contact time, while higher residuals require shorter contact time. While such test
results are encouraging, they are not enough to assess the physiological effects of iodine in treated
water on the human system. For this reason its use must be considered only on an emergency basis.

Silver

Silver in various forms has been used to inhibit the growth of microorganisms. It is most frequently
found combined with activated carbon in filters. When some bacteria species come into contact with this
silver, they are rendered inactive. There is disagreement among the experts as to the effectiveness of
this process because silver ions in water kill E.coli very well and probably also salmonella, shigella, and
vibro bacteria, but it has found lesser effect on viruses, cysts, and other bacteria species. Silver does not
produce offensive tastes or odors when used in water treatment.

Further, organic matter does not interfere with its effectiveness as is the case with free chlorine. Its high
cost, interferences by chlorides and sulfides, need for long periods of exposure, and incomplete
bactericidal action have hindered its widespread acceptance.

Copper

Copper ions and copper sulphate are used quite frequently to destroy algae in surface waters such as
lakes and reservoirs. But these ions are relatively ineffective in killing bacteria.

Copper-silver ionization is brought about by electrolysis. An electric current is created through copper-
silver, causing positively charged copper and silver ions to form. When copper-silver ionization is
applied, positively charged copper (Cu+ and Cu2+) and silver (Ag+) ions are formed.

The electrodes are placed close together. The water that is disinfected flows past the electrodes. An
electric current is created, causing the outer atoms of the electrodes to lose an electron and become
positively charged. The larger part of the ions flows away through the water, before reaching the
opposite electrode. Generally the amount of silver ions at a copper ion rate of 0.15 to 0.40 ppm lies
between 5 and 50 ppb.

Because of copper-silver ionization, drinking water could be produced safely in space without the use of
chlorine. Electrically charged copper ions (Cu2+) in the water search for particles of opposite polarity,
such as bacteria, viruses and fungi. Positively charged copper ions form electrostatic compounds with
negatively charged cell walls of microorganisms. These compounds disturb cell wall permeability and
cause nutrient uptake to fail. Copper ions penetrate the cell wall and as a result they will create an

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entrance for silver ions (Ag+). These penetrate the core of the microorganism. Silver ions bond to
various parts of the cell, such as the DNA and RNA, celular proteins and respiratory enzymes, causing all
life support systems in the cell to be immobilized. As a result, there is no more celular growth or cell
division, causing bacteria to no longer multiply and eventually die out. The ions remain active until they
are absorbed by a microorganism. In the United States, several drinking water production companies
use copper-silver ionization as an alternative for chlorine disinfection and to prevent the formation of
disinfection byproducts. The standard for trihalomethanes was decreased by EP A from 100 to 80 Ilg/L.

When copper-silver ionization is combined with chlorine disinfection, it is an excellent disinfection

mechanism to deactivate viruses and bacteria.

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