Generation of reactive oxygen and nitrogen species (RONS) in liquids (especially water) has now become a topical field of physical research involving the cavitation phenomenon. The RONS are generated using various physico-chemical mechanisms, involving strong ultrasound fields [1
], reactive gas/liquid admixtures to the liquid (or cavitation cloud), or highly innovative plasma-liquid interactions [2
]. The last, the plasma-chemical approach, brings the benefit of in-situ tailored generation of reactive species, but raises a problem of energy-efficient transport of generated reactive species into the liquid environment. Various liquid discharge technologies were studied intensively for their cross-disciplinary applications in environmental, bio-medical [4
], or agricultural areas [6
]. Water and aqueous solutions (media) treated/activated by atmospheric air plasmas, so-called plasma-activated water (PAW) or plasma-activated media (PAM), were found to have substantial antimicrobial [7
] and antitumor properties [8
]. In agriculture, exposure to PAW can lead to enhanced seed germination and growth of plants [9
] in addition to its strong bactericidal effect [10
]. Further applications of discharge plasmas generated in liquids include chemical synthesis [11
], nanoparticle synthesis [14
], destruction of pollutants in wastewaters [17
], polymer surface treatment [23
], etc. Nevertheless, the main hindrance remains in place even after two decades of intensive research effort: What is an effective method for large-volume liquid treatment?
Direct generation of electrical discharges in liquids is complicated by generally high breakdown strength of liquids (>1 MV/cm). In most cases, the discharge breakdown in a genuinely liquid environment is initiated by applying ultra-fast high voltage pulses to a highly curved discharge electrode(s) [17
]. This approach limits not only the volume of generated plasma, but more importantly, the power input that can be dissipated into the treated medium. The use of a subsidiary gas-phase environment with subsequent mixing of plasma produced species into the liquid is, therefore, more popular today. Introduction of the gas-phase allows reduction of breakdown strength to technically more feasible values of 10 kV/cm. Typical representatives of such an approach include plasma jets submerged in liquid [7
]; discharges generated above [10
] or from the liquid surface [25
], discharges in liquid aerosol [3
]; or discharges initiated inside the gas micro-bubbles introduced [21
] or formed inside the liquid volume [18
]. Still, none of these methods of discharge plasma generation can deliver a sufficiently high and cost-effective throughput of plasma-treated liquid (see Table 1
Recently the authors of this paper applied a novel approach to overcome this technological barrier (see patent [31
]). The method employs an energy-efficient generation of a dense hydrodynamic cavitation cloud (referred to as a cavitation cloud) that proved itself to be a highly suitable environment for the generation of electrical discharges by a pair of axially positioned semi-insulated electrodes. The cavitation cloud comprises an ample number of tiny voids (cavities) with the internal pressure of liquid vapours of a few kPa only. An advantageous low-pressure environment is formed inside the flowing liquid, where even a moderate high-voltage (HV) field of 1 kV/cm can sustain discharge plasma in a considerable volume of liquid, see Figure 1
. The setup is called a hydrodynamic cavitation plasma jet (HCPJ).
Our first experiments with the HCPJ demonstrated its exceptionally high efficacy in direct cyanobacteria remediation (Microcystis aeruginosa
) from contaminated water [28
]. The combined effect of hydrodynamic cavitation and discharge plasma led to a single-pass removal of all cyanobacteria with no increase in microcystin concentration in treated water. In actual numbers, 6 L of cyanobacteria contaminated water (5 × 105
cells/mL) was disinfected in less than 15 s. This volume efficiency is a remarkable improvement over the known methods of underwater discharge generation (Table 1
), even when we compare the HCPJ with a few known works where authors also employed cavitation clouds to sustain the underwater discharge [29
]. Very recently, a plasma-cavitation device for water treatment of throughput similar to ours was presented in [30
], but unfortunately no details were given in the article with respect to discharge ignition or energy consumption, so we can not make a qualified comparison to our previous results in [28
]. In both of these plasma-cavitation device experiments [28
], the high volumetric efficiency (of almost 1 m3
/h) was obtained for direct plasma remediation
of water. However, according to our best knowledge, so far, there are no reports on using such devices for the production of PAW as an active medium
itself, allowing flow rates close to 1 m3
/h to produce PAW with proven remote, ex-plasma
biocidal activity. Reaching this “psychological limit” will significantly push the potential of PAW towards its commercially viable utilization.
In the present paper we demonstrate the efficient production of plasma activated water (PAW) with biocidal activity using HCPJ at the flow rate of 0.55 m3
/h and energy efficiency as high as 1 kWh/m3
of PAW. The biocidal activity of produced PAW is demonstrated on the remediation of algae and cyanobacteria by their delayed exposure to plasma activated water (PAW). Unlike in [28
], where direct
plasma treatment of contaminated medium was studied, here, we used uncontaminated water to produce PAW, and we studied its biocidal effects afterwards. In addition to that, the original HCPJ set-up was modified. The volume of generated plasma was significantly increased by lowering the backpressure at the nozzle output. The PAW was prepared at the volumetric flow rate of 0.55 m3
/h at 400 W power input to plasma; at these conditions, 1 to 5 passes of water through the system were tested. For the sake of reference, the HCPJ-induced biocidal effect of PAW was compared with the effect of ozonisation treatment.
2. Materials and Methods
2.1. Hydrodynamic Cavitation Plasma Jet Device (HCPJ)
The setup of the HCPJ device is given in Figure 2
. The whole device consisted of the HCPJ unit with HV generator, vacuum unit, ozonizer, and electrical/optical diagnostics. For the presented experiments, the HCPJ device [28
] was modified to allow reduction of the backpressure at the water tank below atmospheric pressure, which lead to a significant increase of generated plasma volume. The backpressure of 40 kPa was achieved using a single-stage membrane vacuum pump VM 40 D (LAVAT, Czech Republic), which reduced the gas pressure above the water surface in the water tank. The backpressure was controlled using a gas-tight ball valve and measured with a gas pressure gauge, Leybold Heraeus Nr. 160 40 (Leybold Heraeus, Hanau, Germany). Visual appearance of the discharge ignited using the modified HCPJ unit is shown in Figure 1
The discharge was ignited in the cavitation cloud generated in the reaction chamber made of a transparent polycarbonate tube (12 mm outer diameter, 10 mm inner diameter) enabling the optical diagnostics of the generated HCPJ phenomenon. The cavitation cloud was generated in the water stream that passed through the Venturi nozzle with a minimum inner diameter of 3.5 mm. The cavitation phenomenon was caused by the Bernoulli‘s principle, and the cavitation was characterized by transition between bubble cloud regime and liquid jet surrounded by vapor, which is characteristic of supercavitation. The cavities were saturated with liquid vapours at vapor pressures of several kPa, making the conditions favourable for the generation of the cavity-plasma jet phenomenon. Reducing the backpressure enabled a substantial expansion of the cavitation cloud region resulting in a substantially prolonged active plasma region. The liquid circuit was a closed-loop circuit with a grounded water pump, CALPEDA CT 61/A (Calpeda S.p.A., Montorso Vicentino, Italy, rated power input 100 W, volumetric flow rate approx. 0.55 m3/h at 40 kPa) and a water tank of approximately 2 L of liquid. For the introduction of the ozone gas into the treated liquid, the ozoniser GO-1000H (PROFI OZON, Praha, Czech Republic) was used.
The discharge itself was generated using two electrodes, depicted using a dashed orange line in Figure 2
. They were made of insulated copper wire: 2 mm diameter metal wire and 4 mm diameter with insulation. The wire was cut and submerged co-axially in the liquid stream so that the liquid was in direct contact with only the end surface of the wire. The HV electrode was positioned in the liquid stream in the Venturi nozzle throat, about 6 mm upstream (to the left) from the minimum throat diameter, see Figure 1
. The grounded electrode was placed approximately 16 cm from the HV electrode in the reaction chamber behind the cavitation cloud region. The discharge was energized using a custom-made tunable high voltage generator (HV generator) consisting of a low-voltage tunable generator with adjustable output voltage amplitude and a high-voltage transformer for the up-conversion to HV. The HV generator produced at the output an HV of sinewave waveform. The HV frequency could be tuned to enable HCPJ operation close to the resonance of the HV loop to maximize the energy efficiency of plasma generation. In the present experiments, the frequency of 65 kHz was used. The total average input power measured at the input of the HV generator was 400 W and was kept constant for all presented experiments.
2.2. Treatment Procedures of Contaminated Water
The motivation for the presented research was to find an efficient and industrial-scale method for generation of biologically active water medium, enabling large-scale remediation of contaminated water from, for e.g., algae and cyanobacteria. The decontamination process was studied in two series for algae and cyanobacteria contaminated water, respectively. In both series, tap water was treated either with plasma generated in the cavitation cloud, or by ozonation in the water tank. The treated water was then tested for biocidal activity. Two litre samples of untreated reference water and water treated according to the conditions detailed in Table 2
were tested. The water samples were stored in sterile polyethylene terephthalate (PET) bottles and used for cultivation tests the same day.
2.3. Diagnostical Methods of the Plasma Jet
The discharge parameters of the HCPJ were studied using electrical and optical diagnostics. The voltage applied to the electrodes and the current flowing through the reactor was followed using a digital storage oscilloscope (labelled OSC in Figure 2
), Infiniium DSO-S 204A (Keysight Technologies Inc., Santa Rosa, CA, USA), which enabled continuous acquisition of up to 52 MPts/channel at 2 GHz bandwidth with 10 GSa/s with high-definition resolution of 10-bit. For the voltage and current measurement, the oscilloscope was equipped with an HV probe, Tektronix P6015A, and Pearson 2877 current monitor.
For the optical diagnostics, a fast framing camera in visible (VIS) wave range and two spectrographs operating in ultraviolet (UV) and visible-to-near infrared (VIS-NIR) were used. The Sony CyberShot DSC-RX10 III camera (Sony Corporation, Tokyo, Japan) was used to follow the evolution of the plasma-cavitation phenomenon dynamics. The camera (20 MPix, focal length 24–600 mm, f-number 2.4–4) enabled us to capture the motion in a high-speed framerate of 1000 fps at reduced effective resolution of 1136 × 384 pixels at three colour channels (red, green, blue) simultaneously. This enabled the capturing of slightly averaged spatio-temporal evolutions of the HCPJ, averaging approximately 60 periods of the discharge in a single frame at the estimated (theoretical) spatial resolution of 0.25 mm in the lateral dimension and 0.4 mm in the radial dimension.
Optical emission spectra of the discharge were consecutively recorded in two spectral ranges (UV and UV-VIS-NIR). The emission of the discharge was consecutively acquired from two distinct regions above both ends of the discharge channel. Two quartz optical fibres were used to sample the emission of the discharge (2 m, single fibre 600 µm; Avantes BV, Apeldoorn, The Netherlands). The first fibre was placed above the HV electrode situated inside the Venturi nozzle. The second fibre was placed above the other end of the discharge channel approximately 9 cm away from the first fibre in the direction of the flow of the cavitation cloud. The spots from which the spectra were captured were approximately 1 cm in diameter.
The spectra were recorded without temporal resolution using two fixed grating AvaSpec ULS3648TEC-USB2 spectrometers (Avantes BV). The range of the spectrometer for survey spectra in the broad spectral was from 200 nm to 1100 nm, and it was equipped with a grating UA (200–1100 nm) and entrance slit with the width of 25 µm. The spectral orders that overlapped were suppressed using order-sorting coating with 350 and 600 nm long pass filters. The theoretical resolution of the survey spectrometer configuration was in the order of 1.1 nm. For the acquisition of discharge emission from the UV range with higher spectral resolution, the second spectrometer was used. The second spectrometer was equipped with a grating UE (290–395 nm) and entrance slit with the width of 10 µm and offered a theoretical spectral resolution of the order of 0.1 nm. The recorded spectra were then analysed using a Spectrum Analyzer 1.8 (Masaryk University, Brno, Czech Republic, [34
]) and a massiveOES (Masaryk University, Brno, Czech Republic [35
2.4. Model of Hydrodynamic Cavitation at Reduced Backpressure
The numerical simulation of the cavitating flow without the discharge was performed to better understand the behaviour of the HCPJ at reduced pressure. The backpressure in the model was taken as the outlet pressure of the nozzle. The simulation was performed using the commercial computational fluid dynamics (CFD) software (ANSYS, ANSYS, Inc., Canonsburg, PA, USA; [37
]). The simulation was based on the Schneer–Sauer model [38
]. The model employs a truncated Rayleigh–Plesset equation, and it is relatively robust and sufficiently accurate to gain the necessary insight into the dynamics of the cavitation phenomenon in studied conditions. Details of the model are given in Appendix A
2.5. Algae and Cyanobacteria Culturing Conditions, Growth Inhibition Test
The green alga Raphidocelis subcapitata
(Korshikov) and the cyanobacterium Synechococcus elongatus
(Nägeli) were used as test organisms. Cultures of these species were obtained from the Culture Collection of Autotrophic Organisms (CCALA), Třeboň, Czech Republic. The organisms were cultivated in 100 mL Erlenmayer flasks at 24 ± 1 °C under continuous illumination (90 µmol m2
/s) by fluorescent lamps (Phillips, TLD 36 W/33) in growth media. As a cultivation medium, ZBB medium was used—a 1:1 combination of Z-medium (Zehnder and Staub medium) and BB-medium (Bristol and Bold medium), media preparation and composition can be obtained in https://utex.org/products/bold-basal-medium
The growth inhibition test using the unicellular green algae and cyanobacteria was evaluated using a modified variant of the ISO 8692 test procedure, see https://www.sis.se/api/document/preview/914323/
. The aim of these tests was to determine the effects of treated water on the growth of freshwater microalgae or cyanobacteria. The growing test organisms were exposed to the treated water in 96-well cell culture plates from Thermo Fisher Scientific containing 250 µL of sample per well over a period of 24–72 h. The growth rate in experiments was evaluated by performing in vivo fluorescence measurements using a microplate fluorescence reader SPARK (Tecan, Männedorf, Switzerland).
The growth media and inoculum cultures were prepared and used according to EN ISO 8692:2012 and Culture Collection of Algae and Protozoa (CCAP, https://www.ccap.ac.uk/
). Z-medium was mixed 1:1 with PAW, ozone treated water was mixed 1:1 with ZBB medium, to keep equal nutrient composition in all tests. The initial cell concentrations for the alga and cyanobacterium were 100,000 cells per mL and 400,000 cells per mL, respectively. The plates were covered with a transparent lid and incubated under standard light and temperature conditions without shaking/mixing (see ISO 8692 for cultivation conditions).