Drinking Water Quality for the 21 st Century
289
operation of an effective ozonation plant would do well to consider the
UV/peroxide option. It has even been shown to outperform ozonation when
applied to certain waters (Speitel et al. 1999).
4.7.2 Ozone/Peroxide
The use of ozone and hydrogen peroxide in tandem, coined the PEROXONE
process (McGuire and Davis 1988), generates hydroxyl radicals at a level far
greater than by use of ozone alone, and hence is a powerful process for oxidizing
target contaminants. Since molecular ozone is rapidly consumed in this process, it
is used primarily as a chemical oxidative process rather than for disinfection.
Because of this dependency on OH chemistry, this system is particularly
susceptible to matrix impurities which may act as OH scavengers. Additionally, at
high concentrations, hydrogen peroxide may act as an additional radical trap
(Glaze et al. 1987), slowing down the organic degradation process. Hence, the
process needs to be carefully controlled, especially since some comparative
studies (Karimi et al. 1997) show questionable effectiveness of hydrogen peroxide
even if used at or near its stoichiometric optimum dose of 0.3:1 (mg mg- I
HzOz:03). It does appear to effect a greater TOC removal in waters containing
higher levels of humic total organic carbon (Karpel et al. 1996) than in those with
lower levels.
As an example of an attempted application of ozone/peroxide treatment of
contaminated groundwaters, TCE was converted to dichloro- and trichloroacetaldehydes along with di- and trichloroacetic acids (Glaze et al. 1993).
However, the haloacetic acids are known to be quite resistant to further oxidation,
and with their demonstrated toxicity leave the usefulness of this application in
question.
More recent studies have demonstrated that the ozone peroxide process may be
effective in controlling bromate formation when ozone would otherwise engage
ambient bromide in its formation (Speitel et al. 1999).
The process involves the mass transfer of ozone from the gas to the liquid
phase already containing the hydrogen peroxide, which can be the limiting factor
in the case of a fast reaction (Glaze and Kang 1989). Optimization of this mass
transfer process continues to be an engineering challenge in this new era. As an
oxidative technology, rather than a removal technology, the reaction can be
carried to completion, but the cost of such a practice may be inhibitory, depending
on the bypro ducts formed.
4.8 Semiconductor-Mediated UV Photocatalytic Oxidation (Ti02)
An economical alternative to AOP technology has been proposed in the form of
heterogeneous photocatalytic oxidation using a combination of sunlight and
catalyst impregnated or coated adsorbents. Such a process takes advantage of
natural sunlight and eliminates the addition of chemical oxidants. The use of
titanium dioxide as the catalyst in this process originates from the documented
289
operation of an effective ozonation plant would do well to consider the
UV/peroxide option. It has even been shown to outperform ozonation when
applied to certain waters (Speitel et al. 1999).
4.7.2 Ozone/Peroxide
The use of ozone and hydrogen peroxide in tandem, coined the PEROXONE
process (McGuire and Davis 1988), generates hydroxyl radicals at a level far
greater than by use of ozone alone, and hence is a powerful process for oxidizing
target contaminants. Since molecular ozone is rapidly consumed in this process, it
is used primarily as a chemical oxidative process rather than for disinfection.
Because of this dependency on OH chemistry, this system is particularly
susceptible to matrix impurities which may act as OH scavengers. Additionally, at
high concentrations, hydrogen peroxide may act as an additional radical trap
(Glaze et al. 1987), slowing down the organic degradation process. Hence, the
process needs to be carefully controlled, especially since some comparative
studies (Karimi et al. 1997) show questionable effectiveness of hydrogen peroxide
even if used at or near its stoichiometric optimum dose of 0.3:1 (mg mg- I
HzOz:03). It does appear to effect a greater TOC removal in waters containing
higher levels of humic total organic carbon (Karpel et al. 1996) than in those with
lower levels.
As an example of an attempted application of ozone/peroxide treatment of
contaminated groundwaters, TCE was converted to dichloro- and trichloroacetaldehydes along with di- and trichloroacetic acids (Glaze et al. 1993).
However, the haloacetic acids are known to be quite resistant to further oxidation,
and with their demonstrated toxicity leave the usefulness of this application in
question.
More recent studies have demonstrated that the ozone peroxide process may be
effective in controlling bromate formation when ozone would otherwise engage
ambient bromide in its formation (Speitel et al. 1999).
The process involves the mass transfer of ozone from the gas to the liquid
phase already containing the hydrogen peroxide, which can be the limiting factor
in the case of a fast reaction (Glaze and Kang 1989). Optimization of this mass
transfer process continues to be an engineering challenge in this new era. As an
oxidative technology, rather than a removal technology, the reaction can be
carried to completion, but the cost of such a practice may be inhibitory, depending
on the bypro ducts formed.
4.8 Semiconductor-Mediated UV Photocatalytic Oxidation (Ti02)
An economical alternative to AOP technology has been proposed in the form of
heterogeneous photocatalytic oxidation using a combination of sunlight and
catalyst impregnated or coated adsorbents. Such a process takes advantage of
natural sunlight and eliminates the addition of chemical oxidants. The use of
titanium dioxide as the catalyst in this process originates from the documented