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.

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.

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%.

Water-soluble poly(vinyl alcohol) (PVOH) is widely used in the textile and paper industries and in households as detergent pods. In addition to conventional microplastics, water-soluble PVOH poses an environmental threat because it is usually washed down the drain unnoticed and unobstructed. If not treated during wastewater treatment, it enters the aquatic ecosystem in estimated quantities of several thousand tons annually. The present study aims to address the degradation of PVOH on a laboratory scale by acoustic and hydrodynamic cavitation, assisted or not with an oxidative agent. A hydrodynamic cavitation generator, designed with consideration for real-life application, presents an innovative technology adapted for wastewater treatment. The effects of temperature, addition of external oxidant, and methanol as a hydroxyl radical (^•OH) scavenger to PVOH solutions were systematically studied. At optimal operating conditions, PVOH molar mass averages significantly decreased (from weight average molar mass of 124 to 1.6 kg mol⁻¹in case of 60 min treatment with hydrodynamic cavitation and addition of external oxidant) with concomitant narrowing of molar mass distribution. The SEC/MALS, FTIR, and¹H NMR results show that mechanical degradation of PVOH chains predominates in acoustic cavitation, while chemical effects also play an important role in hydrodynamic cavitation. Findings from this study could serve as model research for the degradation of other carbon-backbone polymers and provide a route to improved ultimate (bio)degradation of functionalized polymers in the environment.

The present study aims to investigate the degradation of HPMC on a laboratory scale by acoustic and hydrodynamic cavitation. The effects of temperature and the addition of an external oxidizing agent on the effectiveness of HPMC degradation were systematically investigated by SEC/MALS-RI, FTIR and ¹H NMR. The results of the experiments without cavitation show that an external oxidizing agent alone reduces the weight-average molar mass at 60 °C in 30 min for 45.1 % (from 335 to 184 kg mol^-1). However, the weight-average molar mass of HPMC decreased significantly more in the cavitation treatment, for 98.8 % (from 335 to 4 kg mol^-1) in 30 min at optimal operating conditions of hydrodynamic cavitation (i.e. addition of external oxidant and 60 °C) with a concomitant narrowing of the molar mass distribution, as shown by the dispersity value, which decreased from 2.24 to 1.31. Compared to acoustic cavitation, hydrodynamic cavitation also proved to be more energy efficient. The FTIR spectra of the cavitated HPMC samples without the addition of H₂O₂ show negligible oxidation of the hydroxyl groups and the glycosidic bonds, confirming that mechanical effects predominate in HPMC degradation in these cases. In contrast, when H₂O₂ was added, FTIR and ¹H NMR show typical signals for cellulose oxidation products, especially when the experiments were performed at 60 °C, confirming that chemical as well as mechanical effects are responsible for the extensive HPMC degradation in these cases. Since treatment methods that lead to lower molar masses and narrower molar mass distributions of the polymers are lacking or require longer treatment times (e.g. 24 h), mechanochemical treatment methods such as cavitation have great potential, as they enable faster polymer degradation (in our case 30 min) through a combination of mechanical and/or chemical degradation mechanisms.