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J. Kosel, et al.
iodine by the cavitation produced H 2 O 2 ) in comparison to the hydro- dynamic cavitation [14]. In speci fi c energy terms, hydrodynamic ca- vitation has a maximum e ffi ciency of about 5 × 10 − 11mol of tri-iodide/ joule of energy compared with the maximum of almost 8 × 10 − 11 mol of tri-iodide/joule for ultrasonic cavitation [15]. Consequently, ultra- sonic cavitation has been successfully applied in various industrial ap- plications [16-19]. These include the synthesis of nanomaterials [16], the generation of highly viscoelastic micelles [20] and the disinfection of wastewater [17]. However, hydrodynamic cavitation can treat larger volumes for a similar energy input and can be easily adopted for large scale continuous fl ow-through industrial applications with signi fi cantly lower equipment costs [21]. Therefore, even though hydrodynamic cavitation produces a lesser amount of reactive oxygen species it is still more e ffi cient considering the relative energy input and the scale of operation [22]. Hydrodynamic cavitation forms due to relative velocity increase between the liquid and the submerged body. Cavitation bubbles form, when the local velocity increases and causes static pressure to drop below the critical vaporization pressure. Most common hydrodynamic cavitation can be seen on hydraulic turbine machinery on rotor ’ s blades passing through the liquid [23,24] or behind the constrictions, where liquid is forced to pass through [25]. Several types of hydrodynamic cavitation can form; speci fi cally, for the present setup, we could observe developed unsteady cavitation and supercavitation. Developed unsteady cavitation is formed when cavi- tation clouds start to shed thus creating pressure pulsations, vibration, erosion, high local temperatures and noise, and can be used for the destruction of bacteria [26,27]. When system pressure is decreased or when fl ow velocity is increased a small cavity will grow and a large single steady vapour fi lled supercavity will develop [8], for which larger disturbances in pressure and temperature are uncommon (noise, vibration and erosion are absent). Therefore, it can be expected that supercavitation does not cause any signi fi cant physical damage to mi- crobial cells. Nonetheless, Š arc et al. [28] observed that supercavitation generated inside the Venturi constriction was e ff ective for the disin- fection of the pathogenic bacteria Legionella pneumophila in tap water, while developed unsteady cavitation removed only 28% of the viable count. They proposed that the disinfection mechanism could be at- tributed to the rapid pressure change between the entrance and exit of the supercavitation cavity. Similarly, Gottlieb et al [29] proposed that a mixture of e ff ects such as instant pressure decrease – pressure shock at the entrance point of the supercavity (transition from the liquid/vapour phase) and instant pressure increase (at the closure of supercavity) play a role in the rupture of bacterial wall. Our aim was to assess the applicability of the rotation generator of hydrodynamic cavitation (RGHC) [30] for the treatment of process waters from a paper producing plant. For this purpose, the starting cavitation experiments were performed on tap water spiked with a Gram-positive bacteria Bacillus subtilis. This sporogenic, bio fi lmforming bacteria was selected because it has a thicker peptidoglycan cell wall (in comparison to Gram-negative bacteria which have a much thinner peptidoglycan layer) and is thus more resistant to mechanical stresses [31], is persistent in paper industry as it can hydrolyse fi bre-bound galactoglucomannans from soft-wood pulp to produce simple sugars [32], can spoil surface sizing materials of paper making [33], and can survive high temperature treatments in paper mills [34]. To examine the e ff ect of both developed unsteady cavitation and supercavitation, two sets of speci fi cally designed rotors and stators were produced for the RGHC machine and were tested on spiked tap water preparations. Finally, to verify our RGHC device, we sampled real process waters from an enclosed recycling system of a paper producing plant and after cavitation treatments the survivability of the major classes of micro- organisms and the chemical and physical changes of samples were as- sessed.
Fig. 1. Rotor-stator design for developed cavitation (left) and supercavitation (right).
2. Methods
2.1. Hydrodynamic cavitation set-up
In this study, the e ff ects of cavitation on di ff erent microorganisms problematic for paper mill industry were investigated using a rotational generator of hydrodynamic cavitation (RGHC) which was fi rst de- scribed by Petkov š ek et al. [30]. The RGHC is based on the centrifugal pump design which has a modi fi ed rotor and a stator added in its housing (Fig. 1). The RGHC is powered by an electric motor of 500 W that propels the modi fi ed rotor. Maximum rotational frequency of the electric motor is 10,000 revolutions per minute (rpm). The stator ’ s position is opposite to the rotor (both 50 mm in diameter; r = 25 mm) and the housed unit of rotor and stator forms the so-called cavitation treatment chamber. The RGHC preserves its fl ow-through pumping function, which makes its installation into the water pipe system simple with no additional pumping required. In our experimental setup, the RGHC was placed in a closed loop experimental water system which is presented in Fig. 2. The experimental setup is made of piping which connects a 2 L reservoir, a heat exchanger, pressure and fl owmeters and the RGHC device. The piping and connections are made of standard household water system materials [35]. Two di ff erent rotor and stator designs were used in order to gen- erate developed cavitation and supercavitation (Fig. 1). The rotor and stator pair used for developed cavitation have a specially designed surface geometry with 12 radial teeth, 3 mm deep and 4 mm wide. The area of each tooth of this serrated rotor disc has been designed in a way that its surface is angled at 8°, giving it a sharp leading edge. When aligned, the space between the angled surface of the individual rotor ’ s tooth and the completely fl at surface of the individual stator ’ s tooth resembles the Venturi nozzle geometry (Fig. 3A). In the case of supercavitation set-up, an additional fl ow regulation valve was installed at the inlet of the RGHC (Fig. 2B) to manipulate the pressure inside the treatment chamber. By closing the valve, the fl ow rate through the RGHC gets severely reduced. The surface geometry of the rotor used for supercavitation consists of two symmetrical teeth (hence its name the two teeth rotor), which resembles the symmetrical Venturi design, fi rst described by Zupanc et al. [36] (Fig. 3C). The di- vergence angle of the teeth ’ s cross section is 10° and the secondary divergence angle is 30° (Fig. 3B). The surface of the added stator is completely fl at and has no teeth with the aim not to induce any addi- tional pressure fl uctuations (this allows for the development of a large steady supercavity behind each rotor ’ s tooth). When using the rotor – - stator con fi guration for supercavitation the electric motor is able to achieve the maximum rotary frequency (10,000 rpms), while in the case of rotor – stator con fi guration with grooves for developed cavita- tion, the rotary frequency is reduced to 9000 rpms due to motor ’ spower limitations. For both the serrated rotor and the two teeth rotor, the gaps be- tween the rotor and the stator were set to be 1 mm (gap length; l ).When in motion, the rotor ’ s teeth force the liquid to move in a radial direction - causing centrifugal forces, which result in a pumping function of the RGHC. The high frequency of rotation also causes the movement of
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