Wastewater Treatment by Combination of Advanced Oxidation Processes and Conventional Biological Systems

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. In this context, Advanced Oxidation Processes (AOPs), which are oxidation methods relying on the action of highly reactive species such as hydroxyl radicals, are raising great interest for the removal of those organic pollutants not treatable by conventional techniques due to their high chemical stability and/or low biodegradability. As several studies pointed out the effectiveness of AOPs in the degradation of a wide spectrum of both organic and inorganic pollutants, they are considered a highly competitive wastewater treatment technology. However, in order to reduce operating costs associated to the application of AOPs, their proper combination with conventional biological processes should be considered. This work aims to discuss the most common AOPs used as pretreatment of wastewater for its biological processing, in order to highlight the enhancement of wastewater biological treatability supplied by different advanced oxidation methods. To this end, main standard tests and parameters for wastewater biodegradability assessment are also pointed out, thus providing an overview of the most reliable ones. Citation: Cesaro A, Naddeo V, Belgiorno V (2013) Wastewater Treatment by Combination of Advanced Oxidation Processes and Conventional Biological Systems. J Bioremed Biodeg 4: 208. doi:10.4172/2155-6199.1000208 Volume 4 • Issue 8 • 1000208 J Bioremed Biodeg ISSN: 2155-6199 JBRBD, an open access journal Page 2 of 8 Degradation Mechanisms by Advanced Oxidation Processes The efficacy of AOPs in improving biological degradability of recalcitrant compounds in wastewater depends on both chemical and physical properties of contaminants as well as on the generation of reactive free radicals, in most cases hydroxyl radicals [12]. The oxidation reaction between these radicals and the contaminants is the mechanism behind the degradation of the contaminant itself. The generation of these reactive agents can be achieved by means of several processes, including, sonolysis [13], ozone-based processes [14], Fenton-based reactions [15], heterogeneous photocatalysis [16] and various combination of these technologies [17-19]. Each one can be characterized according to the specific method for the production of free radicals. Sonochemical processes imply the application of ultrasound (US), which refers to sound waves with a frequency ranging between 20 kHz and 500 MHz. When ultrasound propagates in a liquid, it promotes the formation of cavitational bubbles, whose collapse is associated to both physical and chemical effects [13]. In particular, at high frequencies, chemical ultrasonic effects are predominant due to the larger formation of free radicals [20]. These radicals move to the liquid-gas interface to react with the organic substrate [21] or, in the case of high concentration, they recombine with each other to form H2O2 [22], which is an oxidative agent as well, thus providing the degradation of contaminants. Sonolysis is a versatile process, which has been widely studied for the degradation of several compounds [23-25] even in combination with other AOPs [26]. Its main disadvantage is related to energy consumption. This item often limits the applicability of the ultrasonic technology to small volumes. Differently, ozonation has shown a very strong oxidizing power with short reaction times, thus allowing the treatment of great amount of wastewaters. The process relies on ozone, which is unstable in an aqueous medium. It decomposes spontaneously by a complex mechanism that involves the generation of hydroxyl free radicals. Therefore, the degradation of pollutants occurs by both ozone itself and radicals [27], although the latter is more powerful than the former, as highlighted in Table 1, reporting the reaction rate constants for both oxidants with reference to several compounds. Although ozonation has already been applied at full-scale, as already pointed out for sonolysis, it is an energy intensive process, characterized by high operating costs, mainly associated to ozone generation. As ozone is an unstable molecule, it should be generated at the point of application. To this end, several methods can be used, but the most common within ozone generation industry is the corona discharge one, which requires a considerable energy input. Ozone technology has also been studied in combination with ultraviolet (UV) radiation, since UV photons are able to activate ozone molecules. In this way, the formation of hydroxyl radicals is promoted [28,29], but any relevant energy saving can be pursued. UV radiation, in the wavelength range between 200 and 280 nm, can also be applied in combination with hydrogen peroxide (H2O2). The major drawback of this process is related to the small molar extinction coefficient of H2O2. Therefore, only a relative small fraction of incident light is exploited, especially when organic substrates will act as inner filters. Moreover, the rate of photolysis of aqueous H2O2 is pH dependent: it was found to increase when more alkaline conditions are used [6]. H2O2 occurs also in Fenton based processes: its reaction with iron in water, under acidic conditions, determines the formation of radicals. The rate constant for the reaction of ferrous ion with hydrogen peroxide is high and Fe(II) oxidizes to Fe(III) in a few seconds to minutes in the presence of excess amounts of hydrogen peroxide, which decomposes by Fe(III) and generates again hydroxyl radicals. The major parameter affecting Fenton processes are: the pH of the solution, the amount of ferrous ions, the concentration of H2O2, the initial concentration of contaminants and the presence of other ions [30]. Moreover, Fenton reagent action can be significantly improved when exposed to UV radiation [31]. Enhancement of reagent yields after light irradiation is the concept on which also photocatalytic processes have been developed. Heterogeneous photocatalysis is a photochemical reaction, accelerated by the action of a catalyst: one of the most widely used and highly effective is TiO2 [32]. The mechanism action is based on the transition of electrons from the valence to the conduction band, which is caused by the light irradiation of the catalyst. In particular, a.