Advanced Oxidation Processes

Advanced Oxidation Processes (AOPs) refer to a set of oxidative water treatments that can be used to treat toxic effluents at industrial level, hospitals and wastewater treatment plants. AOPs are successful to transform toxic organic compounds (e.g. drugs, pesticides, endocrine disruptors etc.) into biodegradable substances. AOPs in general are cheap to install but involve high operating costs due to the input of chemicals and energy required (COMINELIS et al. 2008). To limit the costs, AOPs are often used as pre-treatment combined with biologic treatment (PULGARIN et al. 1999). Advanced oxidation was recently also used as quaternary treatment or a polishing step to remove micro-pollutants from the effluents of municipal wastewater treatment plants and for the disinfection of water. The combination of several AOPs is an efficient way to increase pollutant removal and reduce costs.

Many methods are classified under the broad definition of AOPs. The table shows some of the most studied processes. Advanced oxidation generally uses strong oxidising agents like hydrogen peroxide (H₂O₂) or ozone (O₃), catalysts (iron ions, electrodes, metal oxides) and irradiation (UV light, solar light, ultrasounds) separately or in combination under mild conditions (low temperature and pressure). Among different available AOPs, those driven by light seem to be the most popular technologies for wastewater treatment as shown by the large amount of data available in the literature (STASINAKIS 2008). Solar AOPs are particularly attractive due to the abundance of solar light in regions where water scarcity is high and due to their relatively low costs and high efficiencies.

Examples of methods classified as AOPs. Source: STASINAKIS (2008)

Advanced oxidation involves several steps schematised in the figure below and explained as follows:

• Formation of strong oxidants (e.g. hydroxyl radicals).
• Reaction of these oxidants with organic compounds in the water (KOMMINENI et al. 2008) producing biodegradable intermediates.
• Reaction of biodegradable intermediates with oxidants referred to as mineralisation (i.e. production of water, carbon dioxide and inorganic salts).

(Adapted from COMNINELLIS et al. 2008)

The costs of AOPs are relatively high and directly related to the efficiency and operational time of the processes. It is therefore worth optimising implementation of AOPs at the right place to limit costs. Several strategies were found to achieve this:

• Simultaneous application of different AOPs promotes the rates of organics oxidation. Typical examples include UV/H₂O₂, UV/H₂O₂/TiO₂, UV/Fenton and Ultrasound/UV/TiO₂, among others. These types of combinations may lead to synergetic effects when treatment efficiencies are greater than the sum of efficiencies that could be achieved by the individual treatments alone.
• Sequential application of various AOPs can treat effluents containing a mixture of organics. This approach is useful when the compounds in the mixture present different levels of reactivity towards different AOPs.
• Application of separation treatment prior to AOP treatment to transfer pollutants to another phase so that they can be treated more easily. Such separation treatment includes stripping, coagulation-flocculation, sedimentation, filtration, adsorption etc.
• AOPs can be applied in pre-treatment stage to enhance biodegradability and to reduce toxicity followed by biological post-treatment. This approach is based on the fact that biological treatment is perhaps less costly and more environmentally friendly than other destructive treatments and that complete mineralisation by AOPs incurs excessive treatment costs.

AOPs can be installed at different stages of waste (and also drinking) water treatment plants depending on influent composition and desired effluent quality. AOPs can be installed either as tertiary treatment after the biological (secondary) treatment of wastewater, or as pre-treatment stage in order to enhance the biodegradability of trace organic contaminants (PETROVIC et al. 2011).