Hydrodynamic cavitation as an efficient water treatment method for various sewage:- A review

Enhancing microalgae cell inactivation through hydrodynamic cavitation: Insights from flow cytometry analysis

Hydrodynamic cavitation (HC) has recently emerged as an effective method for disrupting microalgae cells with lower energy consumption compared to traditional mechanical methods like bead milling and ultrasonication. However, the efficiency and energy utilization of HC can vary depending on the microalgae species and operating conditions. To assess the efficacy of HC and quantify the extent of cellular damage at the cell level, we designed a bench-top cavitation system to investigate the time-dependent responses of the microalgae Dunaliella viridis to multiple HC passes. We evaluated cell disruption efficiency by monitoring cell concentration using a cell counter and analyzing cell size distribution and whole cell counts via flow cytometry. Additionally, we assessed cell viability, metabolic activity, and reactive oxygen species (ROS) levels using the fluorescent probes fluorescein diacetate (FDA), erythrosine B (EB), and 2',7'-dichlorofluorescein diacetate (DCFDA). We further analyzed cell chlorophyll autofluorescence and the kinetics of overall cell inactivation. Our results demonstrated that HC effectively disrupted and inactivated cells, approximating pseudo-first-order kinetics. However, the cell inactivation rate and energy utilization efficiency declined rapidly in the early stages, likely due to the accumulation of cell debris. A P-factor model incorporating two first-order rate constants was then developed to better predict the cell inactivation kinetics and further demonstrated that cavitation number alone was insufficient to characterize the dynamic change in the inactivation rate during cavitation. To maintain high inactivation efficiency, it is recommended to keep the cell debris fraction below 10-20 %. HC was found to inactivate cells by rupturing cell membranes, leading to the rapid release of intracellular contents such as esterase and chlorophyll. HC did not affect intracellular esterase activity or chlorophyll content in cells with intact membranes, but the endogenous ROS levels in viable cells were reduced. The mechanisms of cell damage were discussed in detail. For bioproduct harvesting, cell membrane integrity is suggested as a key physiological endpoint for optimizing HC treatment protocols.

Hydrodynamic Cavitation as a Method of Removing Surfactants from Real Carwash Wastewater

The present work aimed to evaluate whether the use of an innovative method such as hydrodynamic cavitation (HC) is suitable for the simultaneous removal of surfactants of different chemical natures (non-ionic, anionic and cationic) from actual car wash wastewater at different numbers of passes through the cavitation zone and different inlet pressures. An additional novelty was the use of multi-criteria decision support, which enabled the selection of optimal HC conditions that maximized the removal of each group of surfactants and chemical oxygen demand (COD) with minimal energy input. For the optimal HC variants, Fourier transform infrared spectroscopy (FT-IR/ATR) as well as investigations of surface tension, zeta potential, specific conductivity, system viscosity and particle size were carried out. The highest reduction of non-ionic surfactants was found at 5 bar inlet pressure and reached 35.5% after 120 min. The most favourable inlet pressure for the removal of anionic surfactants was 3 bar and the removal efficiency was 77.2% after 120 min, whereas the most favourable inlet pressure for cationic surfactant removal was 3 bar, with the highest removal of 20% after 120 min. The obtained results clearly demonstrate that HC may constitute an effective, fast and cost-efficient method for removing surfactants from real industrial wastewater.

Study on the influence of ultrasound on the kinetic behaviour of hydrogen bubbles produced by proton exchange membrane electrolysis with water

Ultrasonic technology has a significant degassing effect and can increase the efficiency of hydrogen production in the proton exchange membrane electrolysis of water. However, further research is needed to understand its influence mechanism on hydrogen bubbles. In this work, a kinetic analysis is performed to investigate the principle of hydrogen production and the kinetic behaviour of hydrogen bubble evolution by applying the ultrasonic amplification technique under static and flow dynamics in the proton exchange membrane electrolysis cell. The evolution of hydrogen bubbles in the static and in the flow dynamic of the aqueous electrolyte solution under ultrasound was characterised by imaging. The results show that the aqueous electrolyte solution in the flow state reduces the size of hydrogen bubbles and increases the detachment speed compared to the static state, which promotes the process of hydrogen bubble evolution, and that the thermal effect of ultrasound on the temperature of the aqueous electrolyte solution in the flow state is very small compared to the static state and can be ignored. Ultrasound has different effects on the different stages of hydrogen bubble evolution. In the nucleation stage, the ultrasonic cavitation effect increases the highly reactive radicals such as •OH, H•, etc., and the mechanical vibration effect of ultrasound increases the nucleation sites, which are denser and more evenly distributed. In the growth phase, the ultrasonic cavitation effect and the mechanical vibration effect promote the breaking of hydrogen bonds of water molecules and improve mass transport, which promotes the growth of hydrogen bubbles, and the fluctuating energy of positive and negative ultrasound promotes the growth of hydrogen bubbles with the vibration speed. In the detachment phase, the radius of the hydrogen bubbles is influenced by the ultrasound. The radius of the hydrogen bubbles changes with the positive and negative ultrasonic pressure, the radius of the hydrogen bubbles at negative ultrasonic pressure increases, the positive ultrasonic pressure decreases, the changing effect of the radius of the hydrogen bubbles favours the detachment of the hydrogen bubbles. In the polymerisation phase, the ultrasound leads to increased polymerisation of the fine bubble streams. Ultrasound contributes to the hydrogen production effect of proton exchange membrane water electrolysis in actual operation.

Degradation of ammonia nitrogen by an economic combined hydrodynamic cavitation method

Hydrodynamic cavitation (HC) was a kind of advanced oxidation mode. There were defects in the common HC devices, such as high energy consumption, low efficiency, and easy plugging. In order to effectively utilize HC, it was urgent to research new HC devices and used them together with other traditional water treatment methods. Ozone was widely used as a water treatment agent that does not produce harmful by-products. Sodium hypochlorite (NaClO) was efficient and cheap, but too much chlorine will be harmful to water. The combination of ozone and NaClO with the HC device of propeller orifice plate can improve the dissolution and utilization rate of ozone in wastewater, reduce the use of NaClO, and avoid the generation of residual chlorine. The degradation rate reached 99.9% when the mole ratio γ of NaClO to ammonia nitrogen (NH3-N) was 1.5 and the residual chlorine was near zero. As for the degradation rate of NH3-N or COD of actual river water and real wastewater after biological treatment, the ideal mole ratio γ was also 1.5 and the ideal O3 flow rates were 1.0 L/min. The combined method has been preliminarily applied to actual water treatment and was expected to be used in more and more scenarios.