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Over the last two decades, the scientific community and industry have made huge efforts to develop environmental protection technologies. In particular, the scarcity of drinking water has prompted the investigation of several physico-chemical treatments, and synergistic effects have been observed in hyphenated techniques. Herein, we report the first example of water treatment under simultaneous hydrodynamic cavitation and plasma discharge with the intense generation of radicals, UV light, shock waves and charged particles. This highly reactive environment is well suited to the bulk treatment of polluted water (i.e.

E. coli disinfection and organic pollutant degradation). We have developed a new plasma – cavitation reactor and have efficiently applied this hybrid technology to water disinfection. We have also used the technology in the de-gradation of textile dyes.

The results of demographic studies allow us to assume that the world population will reach 10 billion people by 2050, and, even now, more than one billion people are suffering from water scarcity. The risk of epidemics and environmental pollution associated with industrial activity may also increase sharply. Organic contaminants, aromatic compounds and pharmaceuticals in drinking water can cause many diseases, including hormone disruption and cancer. These challenges make finding new water-treatment technologies increasingly urgent. Conventional wastewater methods often do not remove complex organic contaminants. Moreover, techniques for water disinfection either have severe limitations, are associated with the use of toxic substances (like chlorine) or are quite expensive. The use of chlorine, ozone and UV light are well known disinfection techniques. It is commonly known that chlorination is harmful. In addition, some microbes are resistant to chlorine, making the method of limited applicability. Ozonation is an expensive method for water disinfection. While ozone itself is a powerful oxidant that can directly oxidize unsaturated organic compounds, it is also spontaneously converted into a more reactive unselective species, OH, in water. However, ozone is difficult to disperse or dissolve into water, leading to lower gas–liquid mass transfer. For this reason, various mixing technologies must be used to enhance ozonation and increase efficiency, although this leads to the drawback of high costs. Some types of another advanced oxidation processes based on mixing technologies, such as the use of hydrogen peroxide, persulfate and derivative compounds have the same limits. UV-treatment methods are very effective for water disinfection, although careful filtration is needed before irradiation. This limits the economic efficiency of the technique. Recent technological advances in physicochemical transformations can appreciably contribute to the development of more sustainable industrial processes. Powerful cavitation and hot spots that are generated by cavitation in solutions and suspensions can dramatically promote the process. Shock waves and microjets from collapsing cavitation in liquid–solid slurries produce high-velocity interparticle collisions, the impacts of which are sufficient to melt most metals.

Hydrodynamic cavitation has also found important applications in the in-vitiation and enhancement of degradation and catalytic reactions in both homogeneous and heterogeneous systems. We use cavitation in wastewater treatment for the activation of reagents (for example, one of the methods of water treatment based on the use of persulfate and peroxymonosulfate involves cavitation activation) for wastewater treatment, or to enhance ozonation or other types of advanced oxidation processes. Hydrodynamic cavitation has been widely used in environmental remediation as it offers the advantages of process acceleration and higher energy efficiency. It has also been combined with ozonation. Hydrodynamic cavitation releases large amounts of energy (shock waves, microjets, shear forces, turbulences, etc.) in avowing liquid during the extreme implosion of cavitation bubbles, which is caused by a drop and successive rise in local pressure. The mechanisms of the effects caused by hydrodynamic cavitation differ from those of ultrasound. Besides the cavitation number, other hydro-dynamic factors, such as inlet pressure, flow rate, velocities in the constrictions, the ratio between total hole perimeter and the total area of the openings, and the ratio between the total hole area and the cross-sectional area of the pipe, complicate the rationalization. We have recently shown that an cavitation field affects electrical discharge in water, and, in continuous flow, very large volumes of water can be treated. This is due to the fact that the discharge becomes volumetric in the cavitation zone, thus liquid can be treated in a continuous flow. Some physical aspects of plasma formation are reported in. We investigate the disinfection potential of a new hybrid technique that simultaneously combines hydrodynamic cavitation and plasma discharge. Our preliminary findings lead us to believe that this may be an innovative approach that can overcome the limitations of previous methods. We have developed a reactor to treat water in continuous flow with plasma discharge in a cavitation zone that is generated by the hydrodynamic unit. The prototype is an original tool that is well suited to both scale-up and numbering-up. The main goals of our technology are: the disinfection of water and the removal of persistent organic pollutants (POPs) from wastewater. This work deals with the design of a new process for water disinfection. We have developed the following model in order to prove the dis-infection power of the “hydrodynamic cavitation – plasma discharge “hybrid technique. During the growth stage, the radius of cavitation bubbles increases significantly, and the gas pressure inside the bubble may be very low. According to Pashen's rule, an electric discharge occurs at low gas pressures. Consequently, the presence of an electric field can lead to cavitation bubbles becoming lined up in strings. In this case, the discharge develops inside the bubbles and also jumps from bubble to bubble. A so called microchannel is formed between the electrodes, a dynamic effect that continuously forms and disappears in the cavitation and electric fields. We have observe an average glow pattern over the entire volume of the treated liquid. In fact, this is exactly what was observed at the start of plasma discharge in the cavitation zone, together with the generation of hydroxyl radicals, which are produced by water cleavage during cavitation bubble collapse.

If there are organic molecules present in the treated water, there are numerous specific reactions that are capable of producing OH radicals. Hydroxyl radicals have very high reduction potential, and thus radicals are formed both at the liquid/gas interface and within bubbles. Ozone, which is formed in the presence of ultraviolet radiation and discharges inside the bubbles, is another potential source of radicals. As mentioned above, ozone can react with dissolved substances at the gas/liquid interface, as well as initiate the further production of OH radicals.

The reaction products are rapidly spread in the liquid flow, and their distribution is facilitated by cavitation. The free radicals formed can lead to microorganism inactivation and therefore to water disinfection. OH radicals can penetrate bacteria cell walls and membranes, and cause severe damage. This process is based on chemical oxidation, meaning that microorganisms and viruses cannot develop resistance. Of course, the physical effects of cavitation on microorganisms is an important stage of disinfection. The shock waves and high temperatures that occur when cavitation bubbles collapse lead to the weakening of microorganism structure, facilitating the penetration of radicals and UV. It is well known that UV light suppresses the activity of micro-organisms. In addition, UV radiation activates oxidation reactions as it provides additional energy for chemical-bond breakage.

Simultaneous hydrodynamic-cavitation and plasma-discharge treatment cause shockwaves via the action of collapsing bubbles, ultraviolet radiation, hydroxyl radicals and ozone formation. These in-tense effects in the treated liquid cause:- Homogenization- Disinfection by cavitation- Disinfection by radicals and ozone- Disinfection by UV light- Prolonged oxidation after treatment In summary, the proposed hybrid technology for water disinfection causes a synergistic effect to occur between the two energy sources. Although there is a need for further investigation, we can speculate that our technology has impressive potential uses in a wide number of industrial and urban applications.

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