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Ozone + Hydrogen Peroxide


An O3/ H2O2 system is an advanced oxidation process (AOP) in which hydrogen peroxide (H2O2) is used in conjunction with ozone (O3) to enhance the formation of hydroxyl radicals (·OH). The hydroxyl radical is a stronger oxidant that molecular ozone alone. AOP is important in drinking water treatment because reactions with some organic molecules (i.e. PCE, TCE, atrazine, taste and odor compounds such as MIB and Geosmin) are not readily oxidized by O3 or may require a higher ozone dose (which may not be cost effective); combining ozone with peroxide can make the process more economical.

See the Ozone and Hydrogen Peroxide overviews for descriptions of those treatment processes. In the O3/H2O2 process, H2O2 is applied ahead of ozone so that the H2O2-treated water is ozonated.

Oxidation with molecular ozone occurs slowly in contrast to oxidation with hydroxyl radicals, which occurs very rapidly. Hydroxyl radical exposure is a function of temperature, pH, alkalinity, natural organic matter (NOM) concentration and composition. Hydroxyl radical exposure increases with increasing temperature, pH and NOM exposure (i.e., NOM decomposes ozone increasing the rate of hydroxyl radical formation), and decreases with increasing alkalinity (alkalinity scavenges ·OH radicals). Some level of oxidation can occur naturally during ozonation (without peroxide addition); however, in O3/ H2O2 systems, hydroxyl radical exposure is significantly higher (100 times greater) than during conventional ozonation.

The most efficient operational use of H2O2/ O3 is to add peroxide during the second stage of operation (e.g., Peroxone process) by injecting it into the second chamber of an ozone contactor. This configuration allows the utility to obtain disinfection credits for ozonation while achieving the benefit of AOP for destruction of micropollutants. Oxidation with AOP is more reactive and faster and is effective in removing taste and odor and organic micropollutants which are difficult to remove with ozonation alone.

In AOP, H2O2 is fed as an aqueous solution, at H2O2 to ozone ratios between 0.2 to 3.0. The optimal ratio for removing most compounds ranges from 0.3 to 0.6. The specific ratio is a function of disinfection requirements, bromide concentration, contaminant concentration, and other water quality parameters. Major by-products formed by AOP processes are expected to be similar to those formed by ozonation alone. Both ozone and AOP processes form bromate in the presence of NOM and bromide. When conventional ozone processes are replaced with AOP, bromate formation may increase if the O3 residual is kept constant (due to higher ·OH and higher O3 dose) and decrease if the O3 dose is reduced.

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