Removal of Microcystis aeruginosa through the Combined Effect of Plasma Discharge and Hydrodynamic Cavitation

Improving water productivity in the hydroponics with a plasma-nanobubble hybrid technology

Among new technologies aimed at improving water productivity, nanotechnology have been identified as effective means of enhancing the properties of agricultural water. Building on the synergy of plasma and nanobubbles, this study explored the combination of plasma electric discharge and nanocavitation as a novel approach for soilless cultivation. The study aimed to investigate the impact of this hybrid technology on hydroponics nutrient solution. Additionally, the study further aimed to assess the effect of both technologies individually, as well as various application periods, including 3, 9, and 15 min for hybrid technology use. The study employed the nutrient flow technique to hydroponically cultivate lettuce. The findings showed that the application of each technology individually did not significantly increase yield. However, the hybrid technology treatment for 9 min resulted in a significant yield increase of almost 60 %. This improvement can be attributed to the stability, solubility, and absorption of products resulting from plasma treatment, as well as the antimicrobial and anti-algae effects of both technologies. Additionally, an increase in flavonoid and potassium content and a decrease in iron were observed in plants grown under optimal treatment. Overall, this study demonstrated the potential for synergy between plasma and nanobubble techniques in hydroponic culture.

Simultaneous hydrodynamic cavitation and nanosecond pulse discharge plasma enhanced by oxygen injection

A novel Hydrodynamic Cavitation-Assisted Oxygen Plasma (HCAOP) process, which employs a venturi tube and oxygen injection, has been developed for enhancing the production and utilization of hydroxyl radicals (·OH) in the degradation of organic pollutants. This study has systematically investigated the fluid characteristics and discharge properties of the gas-liquid two-phase body in the venturi tube. The hydraulic cavitation two-phase body discharge is initiated by the bridging of the cavitation cloud between the electrodes. The discharge mode transitions from diffuse to spark to corona as the oxygen flow rate increases. The spark discharge has the highest current and discharge energy. Excessive oxygen results in the change of the flow from bubbly to annular and a subsequent decrease in discharge energy. The effects of cavitation intensity, oxygen flow rate, and power polarity on discharge characteristics and ·OH production were evaluated using terephthalic acid as a fluorescent probe. It was found that injecting 3 standard liter per minute (SLPM) of oxygen increased the ·OH yield by 6 times with only 1.2 times increase in power, whereas<0.5 SLPM of oxygen did not improve the ·OH yield due to lower breakdown voltage. Negative polarity voltage increased the breakdown voltage and ·OH yield due to asymmetric density and pressure distribution in the throat tube. This polarity effect was explained by numerical simulation. Using indigo carmine (E132) as a model pollutant, the HCAOP process degraded 20 mg/L of dye in 5 L water within 2 min following a first-order reaction. The lowest electric energy per order (EEO) was 0.26 (kWh/m3/order). The HCAOP process is a highly efficient flow-type advanced oxidation process with potential industrial applications.

Effect of gas injection on cavitation-assisted plasma treatment efficiency of wastewater

Underwater plasma has been long recognized as a "green" alternative to conventional chemicals in the wastewater treatment processes. However, practical application of underwater plasma is still challenging due to insufficient treatment performance. Recently, we proposed a novel process named ACAP utilizing acoustic cavitation in order to stabilize the plasma generation and to enlarge the plasma processing region. This work continues our investigation regarding the ACAP treatment process focusing on effects of gas injection. Experiments were performed using an ultrasonic installation equipped with a specially designed sonotrode (Diam. 48 mm) operated at a frequency of 20 kHz and acoustic power of 120 W. The results revealed that the degradation efficiency of Rhodamine B, which was used as a model wastewater pollutant, remains almost unchangeable in the case of air injection, but it is doubled when argon is injected into the ACAP reactor. It was found that the argon injection enhances the degradation efficiency significantly even without ultrasound irradiation. Results of additional measurements suggest that the effect of argon is attributed to its ability to yield high temperature during cavitation, comparatively good solubility in water and a better ability to reduce the breakdown voltage in water compared to the air case.

The effects of macrophytes on the growth of bloom-forming cyanobacteria: Systematic review and experiment

Macrophytes have often been considered as a prospective tool for the elimination of cyanobacterial bloom, because they may produce chemical compounds that outcompete bloom-forming cyanobacteria. However, a comprehensive, unbiased overview of evidence to support this is missing. Moreover, studies into the effects of individual macrophyte species have often used different methodologies and, thus, cannot be compared. Herein, we firstly carried out a systematic review of studies into the effects of macrophytes on the growth of bloom-forming cyanobacteria. Secondly, we carried out an experiment into the effects of aqueous and ethanol extracts from 19 macrophyte species on the growth of two of the most common cyanobacteria, Aphanizomenon gracile and Microcystis aeruginosa, using a uniform methodological approach. The systematic review revealed that most of the 69 macrophyte species previously studied have shown a combination of inhibitory, stimulatory, and neutral effects. In our own experiment, an inhibitory effect was exhibited only 15 times out of 532 experimental variants, specifically by Chara globularis, Ceratophyllum submersum, Elodea nuttallii, Hydrilla verticillata, Myriophyllum heterophyllum, M. spicatum, and Vallisneria americana. Put together, these results indicate that the practical application of chemical compounds produced by macrophytes to eliminate cyanobacterial bloom may have lower prospects than previously anticipated.

Flow-mode water treatment under simultaneous hydrodynamic cavitation and plasma

Over the last two decades, the scientific community and industry have made huge efforts to develop environmental protection technologies. In particular, the scarcity of drinking water has prompted the investigation of several physico-chemical treatments, and synergistic effects have been observed in hyphenated techniques. Herein, we report the first example of water treatment under simultaneous hydrodynamic cavitation and plasma discharge with the intense generation of radicals, UV light, shock waves and charged particles. This highly reactive environment is well suited to the bulk treatment of polluted water (i.e. E. coli disinfection and organic pollutant degradation). We have developed a new prototype and have efficiently applied this hybrid technology to water disinfection and the complete degradation of methanol in water with the aim of demonstrating its scalability. We have analyzed the mechanisms of water disinfection under the abovementioned conditions and verified them by measuring cavitation noise spectra and plasma emission spectra. We have also used the degradation of textile dyes and methanol solutions as an indicator for the formation of radicals.

Mass Production of Plasma Activated Water: Case Studies of Its Biocidal Effect on Algae and Cyanobacteria

Efficient treatment of contaminated water in industrially viable volumes is still a challenging task. The hydrodynamic cavitation plasma jet (HCPJ) is a promising plasma source for industrial-scale generation of biologically active environments at high flow rates of several m3/h. The combined effect of a hydro-mechanical phenomenon consisting of hydrodynamic cavitation and electrical discharge in cavitation voids was found to be highly efficient for large-volume generation of reactive oxygen species, ultraviolet (UV) radiation, and electro-mechanical stress in a liquid environment. Here, the persistence of biocidal properties of HCPJ-activated water (i.e., plasma-activated water (PAW)) was tested by the study of algae and cyanobacteria inactivation. Algae and cyanobacteria cultivated in media containing PAW (1:1) were completely inactivated after 72 h from first exposure. The test was performed at a total power input of up to 0.5 kWh/m3 at the treated liquid flow rate of 1 m3/h. A beneficial modification of our previous HCPJ design is described and thoroughly characterized with respect to the changes of hydrodynamic flow conditions as well as discharge performance and its optical characteristics. The modification proved to provide high biocidal activity of the resulting PAW, which confirms a strong potential for further design optimization of this promising water (liquid) plasma source.

Effects of Mixed Allelochemicals on the Growth of Microcystis aeruginosa, Microcystin Production, Extracellular Polymeric Substances, and Water Quality

The inhibition of cyanobacteria growth by allelochemicals, which controls harmful algal blooms has been examined in many studies. The objective of this work was to compare the efficiencies of different allelochemicals and determine a mixing proportion corresponding to the highest algae inhibiting activity and smallest adverse effect. The obtained results demonstrated that artemisinin, nonanoic acid, malonic acid, and ethyl acetate inhibited algal growth more efficiently than D-menthol and lactic acid. Synergies were observed in five groups of allelochemical combinations with inhibition ratios exceeding 80%, and the concentrations of extracellular microcystin-LR in the groups with high algal inhibition ratios were lower than that in the control group on the 7th day. No changes in extracellular polymeric substances compositions were detected after treatment. The permanganate indices of the treated groups were higher than that of the control group; however, this disparity gradually decreased with time. In addition, a sharp decrease in the concentration of dissolved inorganic phosphorus was observed for all treated groups. From the obtained data, the optimal proportion of mixed allelochemicals corresponding to 3.94 mg L−1 of artemisinin, 6.27 mg L−1 of nonanoic acid, 8.2 mg L−1 of malonic acid, and 6.38 mg L−1 of ethyl acetate was suggested.

A Transport-Phenomena Approach to Model Hydrodynamic Cavitation of Organic Pollutants

Hydrodynamic cavitation (HC) has been extensively studied for the Advanced Oxidation of organic compounds in wastewaters since it physically produces an oxidative environment at ambient conditions. This process is simple and economical since it can be realized through a properly designed restriction in a pipeline, even in retrofit solutions. Several experimental works individuated similar values of the optimal operating conditions, especially with regard to the inlet pressure. Up to now, the available modeling works rely on a single-bubble dynamics (SBD) approach and do not consider the actual process configuration and pollutant transport in proximity to the oxidizing environment. This work describes different experimental results (from this research group and others) and applies a novel mathematical model based on a transport-phenomena approach, able to directly simulate the effect of HC on the pollutant degradation. The novel proposed model is able to reproduce well a large number of experimental data obtained in different conditions, with different apparatus and different molecules, and allows to interconnect both SBD, fluid-dynamics, and physio-chemical variables in order to deeply study the interaction between the transport of pollutants and the reactive environment. This paper includes collection and discussion of several experimental results with the related main process parameters, description of the novel model and validation against the cited experimental results (to explain the effect of the operating pressure), sensitivity analysis, and the performance limit of the HC with the proposed modeling approach.