4.1 General-This guide contains information regarding the use of AOPs to oxidize and eventually mineralize hazardous materials that have entered surface and groundwater as the result of a spill. These guidelines will only refer to those units that are currently applied at a field scale level. The user should review applicable state regulations and guidance on the applicability of AOP (see California DTSC 2010, New Jersey DEP 2017, Oklahoma DEQ 2017).

Note 1: Commercialization of AOP for the treatment of wastewater and process water is fairly mature. Several transnational companies offer mobile and large-scale processing units for the treatment of persistent chemicals of concern. Standard Guides D5745, E2081, and E2616 may be useful. Fig. 1 illustrates the general AOP process.

FIG. 1 Schematic Illustration of Hydroxyl Radical's Generation for the Degradation of Organic Pollutants

Source: Amor, Carlos, et al. Application of Advanced Oxidation Processes for the Treatment of Recalcitrant Agro-Industrial Wastewater: A Review. Water 2019, 11(2), 205; https://doi.org/10.3390/w11020205 (open access publication)

Fig. 2 illustrates the range of AOP technologies.

FIG. 2 Examples of Advanced Oxidation Processes

Source: Amor, Carlos, et al. Application of Advanced Oxidation Processes for the Treatment of Recalcitrant Agro-Industrial Wastewater: A Review. Water 2019, 11(2), 205; https://doi.org/10.3390/w11020205 (open access publication)

4.2 Oxidizing Agents:

4.2.1 Hydroxyl Radical (OH)-The OH radical is the most common oxidizing agent employed by this technology due to its powerful oxidizing ability. When compared to other oxidants such as molecular ozone , hydrogen peroxide, or hypochlorite, its rate of attack is commonly much faster. In fact, it is typically one million (10⁶) to one billion (10⁹) times faster than the corresponding attack with molecular ozone (Keller and Reed, 1991 (1)).9 The three most common methods for generating the hydroxyl radical are described in the following equations:

4.2.1.1 Hydrogen peroxide is the preferred oxidant for photolytic oxidation systems since ozone will encourage the air stripping of solutions containing volatile organics (Nyer, 1992 (2) ). Capital and operating costs are also taken into account when a decision on the choice of oxidant is made (see NJ Dept. of Environmental Protection, 2017).

4.2.1.2 Advanced oxidation technology has also been developed based on the anatase form of titanium dioxide. This method by which the photocatalytic process generates hydroxyl radicals is described in the following equations:

4.2.2 Photolysis-Destruction pathways, besides the hydroxyl radical attack, are very important for the more refractory compounds such as chloroform, carbon tetrachloride, trichloroethane, and other chlorinated methane or ethane compounds. A photoreactor's ability to destroy these compounds photochemically will depend on its output level at specific wavelengths (see FRTR Technology Screening Tool).

4.3 AOP Treatment Techniques:

4.3.1 Advanced oxidation processes (AOPs) may be applied alone or in conjunction with other treatment techniques as follows:

4.3.1.1 Following a pretreatment step-The pretreatment process can be either a physical or chemical process for the removal of inorganic or organic scavengers from the contaminated stream prior to AOP destruction.

4.3.1.2 Following a preconcentration step-Due to the increase in likelihood of radical or molecule contact, very dilute solutions can be treated cost effectively using AOPs after being concentrated.

4.4 AOP Treatment Applications-Advanced oxidation processes (AOPs) are most cost effective for those waste streams containing organic compounds at concentrations below 1?% (10?000 ppm). This figure will vary depending upon the nature of the compounds and whether there is competition for the oxidizing agent.