Investigation into cavitational intensity and COD reduction performance of the pinned disc reactor with various rotor-stator arrangements

Effect of protrusion structure on the performance of an advanced hydrodynamic cavitation reactor: An entropy-based analysis

Hydrodynamic cavitation (HC) has emerged as a promising technique for process intensification. Recently developed advanced rotational hydrodynamic cavitation reactors (ARHCRs) have attracted significant attention from both academia and industry due to their notable economic advantages, high processing capacity, and continuous operation in specific applications. However, existing evaluation and optimization criteria for these reactors primarily rely on external parameters, often overlooking the complex micro-scale properties and energy dissipation of internal flow within the cavitation generation unit (CGU) of ARHCRs. To address this, a "simplified flow field" computational flow dynamics (CFD) approach combined with entropy production theory was employed to assess the impact of protrusion installation upstream of the CGU on ARHCR performance. The cavitation volume and total entropy generation were analyzed for protrusions of various shapes, circumferential offset angles (γ), radial positions (r), and side lengths (s). The findings revealed that energy dissipation in ARHCRs is predominantly localized in regions of flow separation and vortex formation within the CGU. Furthermore, an evaluation of multiple design factors identified that a triangular protrusion with a γ of 3.75°, r of 122.5 mm, and s of 1 mm achieved optimal performance. Comparative analysis of the flow field and vortex structures between the triangular protrusion and the baseline model demonstrated that the protrusion modifies downstream vortex dynamics, stabilizes the clearance flow field, and reduces entropy production. Additionally, these flow field modifications expand the low-pressure region, thereby enhancing cavitation performance. In this study, the employed entropy production theory identified the spatial distribution of energy loss and the dominant energy dissipation pathways within the ARHCR, thereby revealing the underlying energy loss mechanism associated with vortex formation and flow separation. These insights contribute to a deeper understanding of energy efficiency in ARHCRs and offer a foundation for optimizing reactor design to minimize energy consumption and enhance process intensification.

Proof-of-concept for removing micropollutants through a combination of sub-atmospheric-pressure non-thermal plasma and hydrodynamic (super)cavitation

The persistence and toxicity of hazardous pollutants present in wastewater effluents require the development of efficient and sustainable treatment methods to protect water resources. In this study, the efficacy and efficiency of a novel combination of two advanced oxidation processes - sub-atmospheric-pressure plasma and hydrodynamic cavitation - were systematically tested for the removal of valsartan (VAL), sulfamethoxazole, trimethoprim, naproxen, diclofenac (DF), tramadol, propyphenazone, carbamazepine, 17β-estradiol (E2) and bisphenol A (BPA). The results show that both sample temperature and plasma power play a role and the highest removal, from 29-99 %, was achieved at 25 ℃ and 53 W of plasma power. E2, BPA, DF, and VAL were removed to the highest degree. These results are particularly important in the case of E2 and BPA, whose harmful environmental effects may start to occur already at sub-ng/L to µg/L levels. The differences in the removals obtained depend strongly on the physicochemical properties, and the compounds with the highest logKOW were removed to the highest extent. The energy yield, in terms of plasma power, was between 1 and 26 mg/kWh under optimal experimental conditions. Our results show that the novel plasma-cavitation treatment shows potential that could prove valuable for upcoming regulatory requirements.

Preliminary analysis: Effect of a rotary generator of hydrodynamic cavitation on rheology and methane yield of wastewater sludge

Slightly acidic (pH 5.1) waste sludge with 4.7 % Total Solids (TS) was treated on a laboratory scale pined disc rotary generator of hydrodynamic cavitation (PD RGHC). Influence of four rotor discs with different number of cavitation generation units (CGUs) was investigated: 8-pins, 12-pins, 16-pins and 8-prism elements. The effect of hydrodynamic cavitation (HC) was investigated by analyzing rheological properties, surface tension, dewaterability, and particle size distribution. After subjecting the sludge to 30 cavitation passes, the dewatering ability of the sludge significantly decreased, resulting in a more than two-fold increase in Capillary Suction Time (CST). All regimes were successful in disintegrating particles to smaller sizes. A slight increase of sludge surface tension was measured post cavitation. Cavitated samples displayed a zero-shear viscosity, in contrast to the untreated sample, where viscosity noticeably increased as shear stress decreased. HC did not improve methane yield. Statistically significant correlations between physio-chemical properties and apparent viscosity at low shear stress were identified. Although there were no discernible statistical differences in sludge characteristics, some trends are visible among investigated CGU designs and warrant further research.

The use of hydrodynamic cavitation for waste-to-energy approach to enhance methane production from waste activated sludge

Anaerobic digestion in wastewater treatment plants converts its unwanted end product - waste activated sludge into biogas. Even if the process is well established, pre-treatment of the sludge can further improve its efficiency. In this study, four treatment regimes for increasing methane production through prior sludge disintegration were investigated using lab-scale cavitation generator and real sludge samples. Three different cavitating (attached cavitation regime, developed cloud shedding cavitation regime and cavitation in a wake regime) and one non-cavitating regime at elevated static pressure were studied in detail for their effectiveness on physical and chemical properties of sludge samples. Volume-weighted mean diameter D[4,3] of sludge's particles decreased by up to 92%, specific surface area increased by up to 611%, while viscosity (at a shear rate of 3.0 s-1) increased by up to 39% in the non-cavitating and decreased by up to 24% in all three cavitating regimes. Chemical changes were more pronounced in cavitating regimes, where released soluble chemical oxygen demand (sCOD) and increase of dissolved organic matter (DOM) compounds by up to 175% and 122% were achieved, respectively. Methane production increased in all four cases, with the highest increase of 70% corresponding to 312 mL CH4 g-1 COD. However, this treatment was not particularly efficient in terms of energy consumption. The best energy balance was found for the regime with a biochemical methane potencial increase of 43%.

Effect of the arrangement of cavitation generation unit on the performance of an advanced rotational hydrodynamic cavitation reactor

Hydrodynamic cavitation (HC) is widely considered a promising process intensification technology. The novel advanced rotational hydrodynamic cavitation reactors (ARHCRs), with considerably higher performance compared with traditional devices, have gained increasing attention of academic and industrial communities. The cavitation generation unit (CGU), located on the rotor and/or stator of an ARHCR, is utilized to generate cavitation and consequently, its geometrical structure is vital for the performance. The present work studied, for the first time, the effect of the arrangement of CGU on the performance of a representative ARHCR by employing computational fluid dynamics based on the "simplified flow field" strategy. The effect of CGU arrangement, which was neglected in the past, was evaluated: radial offset distance (c), intersection angle (ω), number of rows (N), circumferential offset angle (γ), and radial spacing (r). The results indicate that the CGU, with an arrangement of a low ω and moderate c, N, γ, and r, performed the highest cavitation efficiency. The corresponding reasons were analyzed by combining the flow field and cavitation pattern. Moreover, the results also exposed a weakness of the "simplified flow field" strategy which may induce the unfavorable "sidewall effect" and cause false high-pressure region. The findings of this work may provide a reference value to the design of ARHCRs.

A Review on Rotary Generators of Hydrodynamic Cavitation for Wastewater Treatment and Enhancement of Anaerobic Digestion Process

The issue of ever-increasing amounts of waste activated sludge (WAS) produced from biological wastewater treatment plants (WWTPs) is pointed out. WAS can be effectively reduced in the anaerobic digestion (AD) process, where methanogens break down organic matter and simultaneously produce biogas in the absence of oxygen, mainly methane and CO2. Biomethane can then be effectively used in gas turbines to produce electricity and power a part of WWTPs. Hydrodynamic cavitation (HC) has been identified as a potential technique that can improve the AD process and enhance biogas yield. Rotary generators of hydrodynamic cavitation (RGHCs) that have gained considerable popularity due to their promising results and scalability are presented. Operation, their underlying mechanisms, parameters for performance evaluation, and their division based on geometry of cavitation generation units (CGUs) are presented. Their current use in the field of wastewater treatment is presented, with the focus on WAS pre/treatment. In addition, comparison of achieved results with RGHCs relevant to the enhancement of AD process is presented.

Computational analysis of flow conditions in hydrodynamic cavitation generator for water treatment processes

The research on the potential of cavitation exploitation is currently an extremely interesting topic. To reduce the costs and time of the cavitation reactor optimization, nowadays, experimental optimization is supplemented and even replaced using computational fluid dynamics (CFD). One of the approaches towards sustainable water treatment is the use of the cavitation reactor with bluff elements mounted on its stator and rotor. The experimental results show that, besides the rotational speed, the spacing of the rotor pins has the most significant effect on the cavitation intensity and effectiveness, while the pin diameter and the surface roughness are less significant design parameters. The present paper uses a simplified CFD approach to investigate the conditions in the reactor and to select the optimal among a number of geometry variations.

Hydrodynamic Cavitation: A Novel Non-Thermal Liquid Food Processing Technology

Hydrodynamic cavitation (HC), as a novel non-thermal processing technology, has recently shown unique effects on the properties of various liquid foods. The extreme conditions of pressure at ~500 bar, local hotspots with ~5,000 K, and oxidation created by HC can help obtain characteristic products with high quality and special taste. Moreover, compared with other emerging non-thermal approaches, the feature of the HC phenomenon and its generation mechanism helps determine that HC is more suitable for industrial-scale processing. This mini-review summarizes the current knowledge of the recent advances in HC-based liquid food processing. The principle of HC is briefly introduced. The effectiveness of HC on the various physical (e.g., particle size, viscosity, temperature, and stability), chemical (nutrition loss), and biological characteristics (microorganism inactivation) of various liquid foods are evaluated. Finally, several recommendations for future research on the HC technique are provided.