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%.

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.

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.

Integral analysis of hydrodynamic cavitation effects on waste activated sludge characteristics, potentially toxic metals, microorganisms and identification of microplastics.

Wastewater treatment plants, the last barrier between ever-increasing human activities and the environment, produce huge amounts, of unwanted semi-solid by-product - waste activated sludge. Anaerobic digestion can be used to reduce the amount of sludge. However, the process needs extensive modernisation and refinement to realize its full potential. This can be achieved by using efficient pre-treatment processes that result in high sludge disintegration and solubilization. To this end, we investigated the efficiency of a novel pinned disc rotational generator of hydrodynamic cavitation. The results of physical and chemical evaluation showed a reduction in mean particle size up to 88%, an increase in specific surface area up to 300% and an increase in soluble COD, NH4-N, NO3-N, PO4-P up to 155.8, 126.3, 250 and 29.7%, respectively. Microscopic images confirmed flocs disruption and damage to yeast cells and Epistilys species due to mechanical effects of cavitation such as microjets and shear forces. The observed cell ruptures and cracks were sufficient for the release of small soluble biologically relevant dissolved organic molecules into the bulk liquid, but not for the release of microbial DNA. Cavitation treatment also decreased total Pb concentrations by 70%, which was attributed to the reactions triggered by the chemical effects of cavitation. Additionally, the study confirmed the presence of microplastic particles and fibers of polyethylene, polyethylene terephthalate, polypropylene, and nylon 6 in the waste activated sludge.

Performance evaluation of a novel pilot-scale pinned disc rotating generator of hydrodynamic cavitation.

This study investigates hydrodynamic performance of a novel pinned disc rotating generator of hydrodynamic cavitation in comparison with a serrated disc variant on a pilot-scale. Experimental results show that at a given rotational speed and liquid flow rate, the pinned disc generates more intense cavitation (i.e. lower cavitation number, higher volume fraction of vapor and higher amplitude of pressure fluctuations) than the serrated disc, while also consuming less energy per liquid pass (i.e., higher flow rate and pumping pressure difference of water at similar power consumption). Additionally, mechanical and chemical wastewater treatment performance of the novel cavitator was evaluated on an 800 L influent sample from a wastewater treatment plant. Mechanical effects resulted in a reduction of average particle size from 148 to 38 µm and increase of specific surface area, while the oxidation potential was confirmed by reduction of COD, TOC, and BOD up to 27, 23 and 30% in 60 cavitation passes, respectively. At optimal operating conditions and 30 cavitation passes, pinned disc cavitator had a 310% higher COD removal capacity while consuming 65% less energy per kg of COD removed than the serrated disc cavitator. Furthermore, the specific COD-reduction energy consumption of the pinned disc cavitator on the pilot scale is comparable to the best cases of lab-scale orifice and venturi devices operating at much lower wastewater processing capacity.

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

In this study, the hydrodynamic cavitation and wastewater treatment performance of a rotary generator with pin disk for hydrodynamic cavitation are investigated. Various geometrical features and arrangements of rotor and stator pins were evaluated to improve the configuration of the cavitation device. The pilot device used to perform the experiments was upgraded with a transparent cover that allows visualization of the hydrodynamic cavitation in the rotor-stator region with high-speed camera and simultaneous measurement of pressure fluctuations. Based on the hydrodynamic characteristics, three arrangements were selected and evaluated with respect to the chemical effects of cavitation on a 200-liter wastewater influent sample. The experimental results show that the rotational speed and the spacing of the rotor pins have the most significant effect on the cavitation intensity and effectiveness, while the pin diameter and the surface roughness are less significant design parameters. Cavitation intensity increases with pin velocity, but can be inhibited if the pins are arranged too close together. At best configuration, COD was reduced by 31% in 15 liquid passes, consuming 8.2 kWh/kg COD. The number of liquid passes also proved to be an important process parameter for improving the energy efficiency.

The removal of bisphenols and other contaminants of emerging concern by hydrodynamic cavitation: From lab-scale to pilot-scale.

The rapid growth in the variety and quantity of contaminants of emerging concern (CEC) in wastewater indicates the necessity for developing efficient and environmentally friendly methods for their removal. This study investigates the removal efficiency of 46 CEC, including 12 bisphenols, from wastewater using a lab and pilot-scale hydrodynamic cavitation generator alone and in combination with UV illumination (pilot-scale). During lab-scale cavitation, the highest removal efficiencies of bisphenols (15-63%) for this specific design of cavitator were obtained at a rotational frequency (vcav) = 9500 rpm and time (tcav) = 10 min. Temperature and the physicochemical properties (e.g. Kow) of the studied compounds also had a significant effect on removal efficiency. At the pilot-scale, 11 CECs were quantifiable in the wastewater influent, and the generator operated at νcav = 2290 and 2700 rpm. The highest removal efficiencies (15-90%) were obtained at a lower νcav = 2290 rpm while neither an increase in νcav, tcav or the presence of UV-C light increased the removal efficiency. A lower νcav also reduced the hydrodynamic power of the cavitator from 477 W to 377 W, resulting in reduced energy consumption. Overall, the results show the potential of hydrodynamic cavitation for a large-scale application as a pre-treatment technology and pave the way for future improvements in the design of cavitation reactors.