Advanced Oxidation

Advanced Oxidation in Wastewater treatment

Advanced Oxidation

One of the most challenging issues of the last decades is the presence of recalcitrant compounds in the effluents of wastewater treatment plants, due to their toxicity on both human health and environment. Although conventional biological processes are usually efficient for the degradation of pollutants occurring in wastewaters, most of these compounds are not effectively removed.

Chemical oxidation processes (COP) or advanced oxidation process (AOP) are transformation processes that may augment current treatment schemes. Oxidation processes may destroy certain compounds and constituents through oxidation and reduction reactions. Advanced oxidation is chemical oxidation with hydroxyl radicals, which are very reactive, and short-lived oxidants. The radicals need to be produced on site, in a reactor where the radicals can contact the organics in the wastewater. Hydroxyl radicals may be produced in Systems using: ultraviolet radiation/hydrogen peroxide, ozone/hydrogen peroxide, ultraviolet radiation/ozone, Fenton’s reagent (ferrous iron and hydrogen peroxide), titanium dioxide/UV radiation, and through other means.

Applicability

Advanced oxidation processes (AOP) have a wide range of applications such as air (odor elimination, purification), soil (remediation) and water decontamination. In water, these processes have the ability to destroy organic pollutants but they can also be adapted to the removal of inorganic metals. Furthermore AOPs are successful to inactivate bacteria, viruses etc. Different kinds of water are therefore suitable for an AOP treatment: for example industrial wastewater containing toxic compounds can be treated by solar photo-Fenton; surface or ground water can be disinfected by means of improved solar water disinfection by adding H2O2 (see also H2O2 and SODIS); both bacteria in drinking water plants or micro-pollutants in sewage systems can be degraded using ozonation; Dissolved arsenic can be removed from water by co-precipitation by means of simple methods  in presence of iron.

Strategies to Implement AOPs

The costs of AOPs are relatively high and directly related to the efficiency and operational time of the processes. It is therefore worth optimizing 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 mineralization by AOPs incurs excessive treatment costs.

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