OZONE
Ozone is used extensively in Europe to purify water. Ozone, a molecule composed
Of 3 atoms of oxygen rather than two, is formed by exposing air or oxygen to a
High voltage electric arc. Ozone is much more effective as a disinfectant than chlorine, but no residual levels of disinfectant exist after ozone turns back into 0,.
(One source quotes a half life of only 120 minutes in distilled water at 20 ˚C).
Of 3 atoms of oxygen rather than two, is formed by exposing air or oxygen to a
High voltage electric arc. Ozone is much more effective as a disinfectant than chlorine, but no residual levels of disinfectant exist after ozone turns back into 0,.
(One source quotes a half life of only 120 minutes in distilled water at 20 ˚C).
Ozone is expected to see increased use in the U.S. as a way to avoid the production
And formation of trihalomethanes, and while ozone does break down organic
Molecules, sometimes this can be a disadvantage as ozone treatment can produce
Higher levels of smaller molecules that provide energy source for
Microorganisms. If no residual disinfectant is present (as would happen if ozone
Were used as the only treatment method), these microorganisms will cause the water quality to deteriorate in storage. Ozone also changes the surface charges of
Dissolved organics and colloidially suspended particles. This cause
Microflocculation of the dissolved organics and coagulation of the colloidal
particles.
And formation of trihalomethanes, and while ozone does break down organic
Molecules, sometimes this can be a disadvantage as ozone treatment can produce
Higher levels of smaller molecules that provide energy source for
Microorganisms. If no residual disinfectant is present (as would happen if ozone
Were used as the only treatment method), these microorganisms will cause the water quality to deteriorate in storage. Ozone also changes the surface charges of
Dissolved organics and colloidially suspended particles. This cause
Microflocculation of the dissolved organics and coagulation of the colloidal
particles.
UV LIGHT
Ultraviolet light has been known to kill pathogens for a long time. A low pressure
Mercury bulb emits between 30 to 90 % of its energy at a wave length of 253.7 nm, right in the middle of the UV band. If water is exposed to enough light, pathogens will be killed. The problem is that some pathogens are hundreds of times less sensitive to UV light than others. The least sensitive pathogens to UV are protozoan cysts. Several studies show that Giardia will not be destroyed by many commercial UV treatment units. Fortunately, these are the easiest pathogens to filter out with a mechanical filter. The efficiency of treatment is very dependent on the turbidity of the water. The more opaque the water is, the less light that will be
Transmitted through it.
The treatment units must be run at the designed flow rate to insure sufficient
Exposure, as well as insure turbulent flow rather than plug flow. Another problem
With UV treatment is that the damage done to the pathogens with UV light can be
Reversed if the water is exposed to visible light (specifically 330-500 nm) through
A process known as photoreactivation. UV treatment, like ozone or mechanical
Filtering, leaves no residual component in the water to insure its continued
Disinfection.
The US EPA explored UV light for small scale water treatment plants and found
It compared unfavorably with chlorine due to
1) Higher costs.
2) Lower reliability.
3) Lack of a residual disinfectant.
Mercury bulb emits between 30 to 90 % of its energy at a wave length of 253.7 nm, right in the middle of the UV band. If water is exposed to enough light, pathogens will be killed. The problem is that some pathogens are hundreds of times less sensitive to UV light than others. The least sensitive pathogens to UV are protozoan cysts. Several studies show that Giardia will not be destroyed by many commercial UV treatment units. Fortunately, these are the easiest pathogens to filter out with a mechanical filter. The efficiency of treatment is very dependent on the turbidity of the water. The more opaque the water is, the less light that will be
Transmitted through it.
The treatment units must be run at the designed flow rate to insure sufficient
Exposure, as well as insure turbulent flow rather than plug flow. Another problem
With UV treatment is that the damage done to the pathogens with UV light can be
Reversed if the water is exposed to visible light (specifically 330-500 nm) through
A process known as photoreactivation. UV treatment, like ozone or mechanical
Filtering, leaves no residual component in the water to insure its continued
Disinfection.
The US EPA explored UV light for small scale water treatment plants and found
It compared unfavorably with chlorine due to
1) Higher costs.
2) Lower reliability.
3) Lack of a residual disinfectant.
Source
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