Page 513 - Water and wastewater engineering

13-8 WATER AND WASTEWATER ENGINEERING

TABLE 13-2
Spectral ranges of interest in photochemistry
Range name Wavelength range (nm)

Near infrared 700–1,000
Visible 400–700
Ultraviolet
UVA 315–400
UVB 280–315
UVC 200–280
Vacuum ultraviolet (VUV) 100–200

where E energy in each photon, J
34
h Plank ’ s constant, 6.6 10 J · s
c speed of light, m/s

wavelength of radiation, m
In general, the more energy associated with a photon of electromagnetic radiation, the more
dangerous it is to living organisms.
Light photons with wavelengths longer than 1,000 nanometers (nm) have a photon energy
too small to cause chemical change when absorbed, and photons with wavelengths shorter than
100 nm have so much energy that ionization and molecular disruptions characteristic of radiation
chemistry prevail.
Little photochemistry occurs in the near infrared range except in some photosynthetic bac-
teria. The visible range is completely active for photosynthesis in green plants and algae. The
ultraviolet range is divided into three categories connected with the human skin’s sensitivity to
ultraviolet light. The UVA range causes changes to the skin that lead to tanning. The UVB range
can cause skin burning and is prone to induce skin cancer. The UVC range is extremely dangerous
since it is absorbed by proteins and can lead to cell mutations or cell death.
UV electromagnetic energy is typically generated by the flow of electrons from an electrical
source through ionized mercury vapor in a lamp. Several manufacturers have developed systems
to align UV lamps in vessels or channels to provide UV light in the germicidal range for inac-
tivation of bacteria, viruses, and other microorganisms. The UV lamps are similar to household
fluorescent lamps, except that fluorescent lamps are coated with phosphorus, which converts the
UV light to visible light.

Disinfection Byproducts. Chlorine reacts with natural organic matter (NOM) to form a num-
ber of carcinogenic byproducts. These include but are not limited to trihalomethanes (THMs),
haloacetic acids (HAAs), haloacetonitriles, haloketones, haloaldehydes, chloropicrin, cyanogen
chloride, and chlorophenols. The THMs and HAAs occur most frequently and generally repre-
sent the highest concentrations of the organic contaminants.
Chloramines react with NOM to form byproducts similar to those formed by chlorination but
at lower concentrations (U.S. EPA, 1994).