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J. Kosel, et al.
Fig. 9. The e ff ect of the RGHC supercavitation treatment (two-teeth rotor) on the destruction of the major classes of microorganisms which were present in real process waters.
The economic analysis showed that for a similar initial bacterial titer, our RGHC, which generated supercavitation, spent 4 times less electrical energy for the reduction of bacteria B. subtilis (67.2 kWh/m 3 / order) in comparison to the Venturi device which was used for the re- duction of E. coli (268.6 kWh/m 3 /order) and was assembled by Arrojo et al. [43]. Moreover, it has to be mentioned that in our experiments the highly resistant Gram-positive B. subtilis was used (wall thickness of 30 nm [55]; may bear a turgor pressure of 2.6 MPa [56]) whereas in the experiments performed by Arrojo et al. [43] the more susceptible Gram negative E. coli was adopted (wall thickness of 2 – 4 nm [57,58]; may bear a turgor pressure of 29 kPa [59]). Furthermore, the e ffi ciency of the RGHC was especially high for the anaerobic sulphate reducing bacteria and for yeasts isolated form the RW samples (3.6 € /m 3 − 3.9 € /m 3 ). This device possesses a number of advantages over previous designs. For example, the RGHC can generate greater shear forces (during supercavitation shear rate was 2.6 . 10 4 s − 1 ; and Rotational Reynolds number was 1.1 . 10 6 [60]) which are caused by the rotation of the rotor and the liquid that is located between the rotor and the stator.
as total suspended solids (TSS), and these are small enough not to settle down and will inde fi nitely remain suspended in the solution which isn ’ t subjected to any form of motion. Colour pollutants in water samples are problematic because they limit the amount of light entering into the water consequently having an inhibiting e ff ect on photosynthesizing organisms and phytoremediation [52]. The increase in colour by su- percavitation is, however, not alarming, because it did not exceed the concentration limits of emission into water determined by the European Norm EN ISO 7887, which are 7 m − 1 for 436 nm (yellow), 5 m − 1 for 525 nm (red), and 3 m − 1 for 620 nm (blue) [53]. In accordance with our results, Lorimer et al. [54] observed that ultrasonic cavitation re- duces the colour removal capability of the electrolytic treatment by disintegrating solid particles present in the samples. The disintegration of larger insoluble particles into many smaller sized particles can con- tribute to the intensi fi cation of colour values. Lastly, in comparison to the RW samples, supercavitation had a signi fi cantly smaller impact on the destruction of microorganisms and on the reduction of COD in the CRW samples. One clear di ff erence between these two types of samples was that only the CRW samples were intensely foaming during cavitation. Due to the foaming the ca- vitation could not result in one stable supercavity, instead large number of smaller cavitation bubbles were formed, which might have reduced the chance of bacteria entering into the area of low pressure. Additionally, the higher amount of smaller bubbles could lead to the cushioning e ff ect which decreases the intensity of bubble collapses and amount of formed radicals and results in lower COD removal.
5. Conclusions
This study evaluates the e ffi ciency of a lab-scale rotation generator of hydrodynamic cavitation for the treatment of a process water iso- lated from an enclosed water recycle system of a paper producing plant. Two set-ups capable of generating di ff erent type of cavitation, namely developed cavitation and supercavitation, were tested. Our results
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