Drinking Water Quality for the 21 st Century
287
4.6 Ultraviolet Irradiation Technology
This technology is most commonly found at small groundwater treatment facilities
(Parrotta and Bekdash 1998) and in point-of-use systems such as those found in a
variety of stores dispensing drinking water for bottling. It involves the use of UV
light as a disinfectant which, when emitted at wavelengths in the range 250 to 270
nm disrupts microorganism DNA (US EPA 1996). In practice, UV dosage is
selected to ensure inactivation of bacteria and viruses and those countries which
permit its use have chosen to set minimum standards ranging from 16 (Norway) to
38 (US) mWscm· 2 • In order to effect inactivation of Giardia cysts, this dosage
would have to be substantially increased, requiring adjusted standards for
application to surface waters where such microbes are more likely to persist. It
remains to be seen whether the technology can protect drinking water from
Crypfosporidium.
There are various types of light sources available. These include excimer
lamps, mercury vapor pressure lamps (classified as high, medium, and low
pressure), xenon lamps, and mercury arc lamps. Vapor lamps are cool burning
lamps which are primarily photochemically active at wavelengths of 254 and 185
nm. Arc lamps are hot burning lamps and have a higher theoretical output in the
UV region. It is also possible to dope vapor lamps so as to shift the spectral
intensity of the lamps and produce more photons in the desired wavelengths. The
lamps are placed inside quartz sleeves and the water to be treated flows in a
narrow region around the sleeves. The presence of significant amounts of
suspended solids or particulates in the water can shield the target microbes from
the incident radiation and decrease the effectiveness of disinfection. Consequently,
the technology is used after filtration in surface water systems. A similar impact
on the amount of effective transmissivity comes from the high temperature
generated, which can cause precipitation of hardness leaving cations and iron on
the exterior of the sleeve. These latter issues, together with documented system
failures due to questionable robustness, have, hitherto, limited the expansion of
this technology. It is anticipated, however, that with the ongoing development of
pulsed UV technology (10 7 mW cm- 2 ms- I ) and the demonstration of effective
inactivation, this cheaper alternative to ozone will provide some major benefits to
the water industry in the new century
4.7 Advanced Oxidation
Advanced oxidation processes (AOPs) generate high levels of hydroxy free
radicals in water which, with their high oxidation potential, react with many
organic species, reducing them to smaller bypro ducts (such as acetic and oxalic
acid) or, in many cases, mineralizing them (Glaze et al. 1987; Aieta et al. 1988).
The lower-molecular-weight organic acids are biodegradable, which proposes
both advantages and disadvantages to such a treatment option. Ifthese by-products
persist into the distribution system, they could, in the absence of a disinfectant
residual, become nutrients providing for bacterial regrowth. On the other hand,
287
4.6 Ultraviolet Irradiation Technology
This technology is most commonly found at small groundwater treatment facilities
(Parrotta and Bekdash 1998) and in point-of-use systems such as those found in a
variety of stores dispensing drinking water for bottling. It involves the use of UV
light as a disinfectant which, when emitted at wavelengths in the range 250 to 270
nm disrupts microorganism DNA (US EPA 1996). In practice, UV dosage is
selected to ensure inactivation of bacteria and viruses and those countries which
permit its use have chosen to set minimum standards ranging from 16 (Norway) to
38 (US) mWscm· 2 • In order to effect inactivation of Giardia cysts, this dosage
would have to be substantially increased, requiring adjusted standards for
application to surface waters where such microbes are more likely to persist. It
remains to be seen whether the technology can protect drinking water from
Crypfosporidium.
There are various types of light sources available. These include excimer
lamps, mercury vapor pressure lamps (classified as high, medium, and low
pressure), xenon lamps, and mercury arc lamps. Vapor lamps are cool burning
lamps which are primarily photochemically active at wavelengths of 254 and 185
nm. Arc lamps are hot burning lamps and have a higher theoretical output in the
UV region. It is also possible to dope vapor lamps so as to shift the spectral
intensity of the lamps and produce more photons in the desired wavelengths. The
lamps are placed inside quartz sleeves and the water to be treated flows in a
narrow region around the sleeves. The presence of significant amounts of
suspended solids or particulates in the water can shield the target microbes from
the incident radiation and decrease the effectiveness of disinfection. Consequently,
the technology is used after filtration in surface water systems. A similar impact
on the amount of effective transmissivity comes from the high temperature
generated, which can cause precipitation of hardness leaving cations and iron on
the exterior of the sleeve. These latter issues, together with documented system
failures due to questionable robustness, have, hitherto, limited the expansion of
this technology. It is anticipated, however, that with the ongoing development of
pulsed UV technology (10 7 mW cm- 2 ms- I ) and the demonstration of effective
inactivation, this cheaper alternative to ozone will provide some major benefits to
the water industry in the new century
4.7 Advanced Oxidation
Advanced oxidation processes (AOPs) generate high levels of hydroxy free
radicals in water which, with their high oxidation potential, react with many
organic species, reducing them to smaller bypro ducts (such as acetic and oxalic
acid) or, in many cases, mineralizing them (Glaze et al. 1987; Aieta et al. 1988).
The lower-molecular-weight organic acids are biodegradable, which proposes
both advantages and disadvantages to such a treatment option. Ifthese by-products
persist into the distribution system, they could, in the absence of a disinfectant
residual, become nutrients providing for bacterial regrowth. On the other hand,