Environmental Engineering Reference
In-Depth Information
Ultraviolet
Ultraviolet (UV) light is electromagnetic radiation just beyond the blue end
of the light spectrum, outside the range of visible light. It has a much higher
energy level than visible light, and in large doses it inactivates both bacte-
ria and viruses. UV energy is absorbed by genetic material in the micro-
organisms, interfering with their ability to reproduce and survive, as long
as the radiation contacts the microorganisms without interference from
turbidity. The big advantage of UV disinfection over chlorine and ozone is
that UV does not involve chemical use. Generally, UV light used for disin-
fecting water is generated by a series of submerged, low-pressure mercury
lamps. Continuing advances in UV germicidal lamp technology are mak-
ing UV disinfection a more reliable and economical option for disinfection
in many plants.
Nonconventional Water Treatment Technologies
Stage 1 of the USEPA's Disinfectants and Disinfection Byproduct Rule
and the new Interim Enhanced Surface Water Treatment Rule, designed
to significantly lower THM byproducts of chlorine disinfection in water,
has driven (along with the regulatory requirements of the PSM standard
and RMP) many water and wastewater treatment utilities to find and use
alternative disinfection methodologies. Although ozonation and ultravio-
let irradiation might be suitable disinfection alternatives, switching from
chlorine to chlorine dioxide (a chemical that has been proven to form fewer
THMs) might also be another viable disinfection alternative. Whichever
disinfection alternative is ultimately selected, remember that the selec-
tion is driven not only by regulatory requirements but also by site-specific
requirements.
The disinfection issues covered to this point are important—the over-
all ramifications of regulatory pressure and environmental impact cannot
be overstated—but other issues besides disinfection must be considered
when deciding which water treatment methodology to employ. Most of the
time, clarification by coagulation, flocculation, sedimentation, and filtra-
tion removes suspended impurities and turbidity from drinking water, and
disinfection (the final step in the process) produces potable water, free of
harmful pathogens. Simply put, the water treatment processes discussed in
the previous sections of this part of the text are usually sufficient to render
most natural surface water (such as a river) potable. In some instances, how-
ever, the water supply may contain materials that are not removed by con-
ventional water-treatment processes, and other treatment processes may be
