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J. Kosel, et al.
Fig. 8. The e ff ect of the RGHC supercavitation treatment (two-teeth rotor) on the physical parameters of samples isolated from real process waters.
achieved. A strong reduction of 3 logs was also observed for the aerobic bacteria ( μ of − 6.26). Interestingly, even bacterial spores which are highly resistant to mechanical and physical stresses were reduced by 1.3 logs ( μ of − 2.9). The destruction of these groups of microorganisms is particularly important for the paper producing industry especially when an enclosed water recycle system is employed [2,34,46]. Supercavitation treatment decreased COD and increased the dis- solved oxygen content and redox potential (up to 77%) in the RW samples. Decrease in COD indicates that supercavitation signi fi cantly contributed to the degradation of organic contaminants. This could be due to the formation of % OH radicals which act as oxidants for organic molecules [47]. As described in chapter 3.1, supercavitation, which is formed on the presented rotor, also consists of the shedding part where due to individual bubble collapses radical formation is possible. To determine if the COD removal is caused by radicals a scavenger such as methanol could be added to the sample. At similar pH values (pH of 7) to that of the RW samples (pH of 7.6), the % OH radicals exhibit a strong redox potential of + 2.31 V as measured by the normal hydrogen electrode [48]. Therefore, the formation of % OH radicals and the in- crease in dissolved oxygen level consequently elevated the redox po- tential of water [49]. When water jets in hydrodynamic cavitation systems travel through air, they draw substantial quantities of air and the high pressures which are generated during cavitation can dissolve the air into the water [50]. Supercavitation reduced the sediments and the insoluble materials and generally intensi fi ed all the SAC colour values in the RW samples. Because bacteria represent a signi fi cant part of the sediment, the de- struction of cells by supercavitation could cause a reduction in insoluble sediments. Furthermore, Poyato et al. [51] showed that cavitation can break insoluble particles into smaller sized fragments which are termed
bubble cloud shedding and collapse. Moreover, when the two-teeth rotor was spun, almost the entire section behind every tip of both teeth was engulfed within a vapour cavity (Fig. 5D). These two types of hydrodynamic cavitation generated inside the RGHC device were further tested for their antimicrobial potential against the high titers of bacteria B. subtilis . Unsteady developed cavi- tation generated inside the RGHC had a weak impact on the viability of B. subtilis and only slowly reduced its viable count ( μ of − 0.83). However, when supercavitation was applied, the viable count of B. subtilis was reduced by 2.3 logs ( μ of − 5.22). Therefore, for the same treatment times (1 h), the destruction of bacteria B. subtilis was 5.8 times more e ffi cient for the supercavitation in comparison to the un- steady developed cavitation (0.4 logs reduction). Similar trends were repeated for the low initial bacterial titers. Even though for super- cavitation larger disturbances in pressure are uncommon [8] it has al- ready been successfully applied for the destruction of the troublesome bacteria L. pneumophila [28]. The main mechanism by which super- cavitation disrupts bacterial cells is currently unknown, however it might be the result of multiple simultaneous e ff ects such as instant pressure decrease at the entrance of supercavity (transition from liquid to vapour phase) [29] and the generation of very high shear forces (shear rate of 2.6 . 10 4 s − 1 ; which is circumferential velocity/1 mm gap height between rotor and stator). In fact, according to literature, high shear stress can cause extensive cell damage ending with cell hemolysis [45]. Supercavitation treatment was found to reduce the viability of all the major classes of microorganisms present in the RW samples which were isolated from a paper producing plant. This was especially evident for the anaerobic sulphate reducing bacteria ( μ of − 9.70) and for the yeasts ( μ of − 9.00) for which a strong reduction of around 4 logs was
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