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H. S. Weinberg
such a property also makes them candidates for a controlled bacterial operation,
one in which the compounds are biodegraded to their mineral constituents in the
treatment plants by a captured biomass.
A major consideration when deploying this technology is whether matrix
impurities are likely to scavenge the OH radicals to such a degree that their
concentration would be sufficiently depleted to negate the objectives of their
generation. Nevertheless, various applications of AOPs now exist in both
demonstration treatment plant facilities as well as fullscale. These include
oxidation of synthetic organic contaminants, removal of color, taste and odorcausing compounds, iron, sulfide, and manganese, as well as DBP precursors.
Some of the variants of AOPs are presented below.
4.7.1 UVIPeroxide
When hydrogen peroxide is exposed to UV irradiation, hydroxyl radicals will
form according to Eq. (1):
(1)
Since the molar extinction coefficient for hydrogen peroxide is rather low (19.6
M- I S-I at 254 nm), a high concentration of hydrogen peroxide, leaving a residual in
the range 5 to 20 mg r l , is required in order to produce a significant level of
hydroxyl radicals (Glaze et al. 1987). The quantum yield of the process at 254 nm
is 0.5, i.e. two hydroxyl radicals are produced by the photolytic destruction of 1
hydrogen peroxide molecule. In order to be viable in treatment plant operations,
the process needs to be placed downstream so that the peroxide residual can be
quenched below 0.5 mg rl prior to entering the distribution system. Options for
quenching include chlorine, thiosulfate, sulfite, or GAC.
In addition to the oxidative process of the OH radical, direct photolysis of the
contaminant may also be significant, depending on the wavelength and intensity of
the light source and the transmissivity of the water to be treated. The efficiency
and type of lamp, of which there are several types mentioned earlier, determine
the percentage of useful photons produced. The process may be used to carry
oxidation of the target contaminant to mineralization, but in some cases oxidation
bypro ducts can be more resistant to oxidation than the parent compound. For
example, beyond a certain molar ratio, no improvement in oxidation might be
achieved (Symons and Worley 1995) or the increase might not be large enough to
warrant the extra expense associated with increased peroxide concentration.
Nevertheless, when optimized, 50% TOe reductions have been documented along
with concomitant increases in overall biodegradability of the remaining organic
carbon. Practical advantages of this system include the ability to store hydrogen
peroxide on site and that, since it has infinite solubility in water, mass transfer
considerations, which complicate other oxidative systems such as ozonation, are
unnecessary (Hager 1990). Utilities unable to sufficiently reduce DBP precursors
by enhanced coagulation and who would otherwise be intimidated by the
H. S. Weinberg
such a property also makes them candidates for a controlled bacterial operation,
one in which the compounds are biodegraded to their mineral constituents in the
treatment plants by a captured biomass.
A major consideration when deploying this technology is whether matrix
impurities are likely to scavenge the OH radicals to such a degree that their
concentration would be sufficiently depleted to negate the objectives of their
generation. Nevertheless, various applications of AOPs now exist in both
demonstration treatment plant facilities as well as fullscale. These include
oxidation of synthetic organic contaminants, removal of color, taste and odorcausing compounds, iron, sulfide, and manganese, as well as DBP precursors.
Some of the variants of AOPs are presented below.
4.7.1 UVIPeroxide
When hydrogen peroxide is exposed to UV irradiation, hydroxyl radicals will
form according to Eq. (1):
(1)
Since the molar extinction coefficient for hydrogen peroxide is rather low (19.6
M- I S-I at 254 nm), a high concentration of hydrogen peroxide, leaving a residual in
the range 5 to 20 mg r l , is required in order to produce a significant level of
hydroxyl radicals (Glaze et al. 1987). The quantum yield of the process at 254 nm
is 0.5, i.e. two hydroxyl radicals are produced by the photolytic destruction of 1
hydrogen peroxide molecule. In order to be viable in treatment plant operations,
the process needs to be placed downstream so that the peroxide residual can be
quenched below 0.5 mg rl prior to entering the distribution system. Options for
quenching include chlorine, thiosulfate, sulfite, or GAC.
In addition to the oxidative process of the OH radical, direct photolysis of the
contaminant may also be significant, depending on the wavelength and intensity of
the light source and the transmissivity of the water to be treated. The efficiency
and type of lamp, of which there are several types mentioned earlier, determine
the percentage of useful photons produced. The process may be used to carry
oxidation of the target contaminant to mineralization, but in some cases oxidation
bypro ducts can be more resistant to oxidation than the parent compound. For
example, beyond a certain molar ratio, no improvement in oxidation might be
achieved (Symons and Worley 1995) or the increase might not be large enough to
warrant the extra expense associated with increased peroxide concentration.
Nevertheless, when optimized, 50% TOe reductions have been documented along
with concomitant increases in overall biodegradability of the remaining organic
carbon. Practical advantages of this system include the ability to store hydrogen
peroxide on site and that, since it has infinite solubility in water, mass transfer
considerations, which complicate other oxidative systems such as ozonation, are
unnecessary (Hager 1990). Utilities unable to sufficiently reduce DBP precursors
by enhanced coagulation and who would otherwise be intimidated by the