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J. Kosel, et al.
Fig. 4. The schematic presentation of the production section in the board paper mill plant Vipap Videm Kr š ko from which 5 L samples were taken. The fi rst sample was collected from the individual pool (RW1 = RW) and the second sample from the central pool (CRW) of an enclosed water recycle system. The places of sampling on the scheme are presented in bold and are underlined.
observed (Fig. 5 B). In order to generate supercavitation the RGHC was equipped with the two-teeth rotor (r rotor of 0.025 m) which was spun at around 10,000 rpm with a tangential speed of fl uid reaching 26.0 m/s. Because of the chocked cavitation conditions (generated using an added valve installed at the inlet of the RGHC) the entire set-up presented in Fig. 2 B had a fl ow rate of only 0.2 L/min. Filming with the high speed camera revealed the formation of a large and stable vapour cavity which fi lled most of the volume behind every tip of both teeth (Fig. 5D). In detail, two speci fi c features of cavitation were formed on the presented su- percavitation rotor. Cavitation cavity at the outer parts of the rotor resembled a uni fi ed supercavity. The second feature comprised of ca- vitation shedding that was caused because fl uid velocity was dropping towards the centre of the rotor. For the purposes of clarity, we will be using the term supercavitation to describe the simultaneous formation of the uni fi ed supercavity and the shedding part of cavitation.
coe ffi cient (SAC) using absorbance measurements at three wavelengths (436 nm, 525 nm and 620 nm) by UV – visible absorption with a Varian Cary 50UV – Vis spectrophotometer (1 cm cell width, Agilent) after the samples were fi ltered using a 0.45 μm fi lter in accordance with the ISO 7887 [41]. In this way, SAC values were calculated according to the below equation:
100
Abs λ d
( )
×
( ) 1
− SAC m
=
(2) where Abs( λ ) is the absorbance at a given wavelength ( λ ), and d re- presents the measuring cell width (mm).
3. Results
3.1. Hydrodynamic cavitation development
To visualise the cavitation development inside the treatment chamber a series of image sequences were recorded using a high-speed camera Photron SA-Z (Fig. 5). The sequences follow a series of fi ve 0.4 ms long intervals. The rotor was rotating in a counter clockwise fashion. In order to generate developed unsteady cavitation, the RGHC was equipped with the serrated rotor (and stator; r rotor of 0.025 m) which was spun at 9,000 rpm with a tangential speed of fl uid reaching 23.6 m/s. The entire set-up presented in Fig. 2 A had a fl ow rate of 1.8 L/min (Table 1). According to our high-speed camera measure- ments performed in this study, a strong form of developed cavitation was visible behind every gap between the tips of the teeth of the op- posing rotor and stator. Violent shedding and bubble collapsing were
3.2. Validation of cleaning and bacterial attachment
The washing protocol employed for the RGHC device successfully removed all B. subtilis presence between di ff erent cavitation experi- ments. Additionally, we found that colony counts of B. subtilis samples that were taken immediately before and after the fi lling of the RGHC device (with the B. subtilis suspension) di ff ered only slightly (a max- imumdi ff erence of 0.15 log 10 CFUmL − 1 ).
3.3. Hydrodynamic cavitation for the destruction of B. Subtilis
The e ff ect of hydrodynamic cavitation generated inside the RGHC
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