Explore the vast range of titles available.

The atmosphere plays a major role in the dispersion of microplastics in the environment. The atmospheric transport of large microplastics is strongly influenced by their settling behavior, which depends on their physical properties, including size and shape. However, experimental data on the settling behavior of commercially available microplastics with complex, nonspherical shapes in air are rare. Here we present experiments on the gravitational settling velocity of commercially available glitters (nominal diameters between 0.1 and 3 mm) and fibers (lengths between 1.2 and 5 mm). We observed that glitters and fibers settle up to 74 % and 78 % slower compared to volume-equivalent spheres, respectively. The atmospheric transport of fibers has been studied previously; however, there are no studies on the atmospheric transport potential of glitters. Therefore, we used an atmospheric transport model constrained by our experimental results to assess the transport potential of glitters. Our results reveal that glitters exhibit transport distances 12 %–261 % greater than volume-equivalent spheres, highlighting their elevated atmospheric transport potential. As a result, the environmental impact of glitter particles, especially following their use in entertainment events, warrants attention and mitigation.

Flocculation is important in the thickening process to improve the underflow concentration in thickeners for tailing suspensions. Traditional zone settling velocity (ZSV) functions ignore the effect of flocculant dosage on the ZSV and the thickening behavior of thickeners. To investigate the effect of flocculant dosage on the settling flux function, a series of batch settling tests were conducted at various flocculant dosages for unclassified and fine tailings. The correlation among flocculant dosage, solid fraction, and parameters in the ZSV function was revealed. Moreover, a simulation of continuous thickening based on the ZSV function was performed. Results indicated that flocculation influenced settling velocity and floc density. With an increased flocculant dosage, the settling velocity of floc increased, resulting in increased underflow concentration. Conversely, floc density decreased due to a stronger particle–particle interaction, leading to a decreased underflow concentration.

In a seminal article, Maxey (J. Fluid Mech., vol. 174, 1987, pp. 441–465) presented a theoretical analysis showing that enhanced particle settling speeds in turbulence occur through the preferential sweeping mechanism, which depends on the preferential sampling of the fluid velocity gradient field by the inertial particles. However, recent direct numerical simulation (DNS) results in Ireland et al. (J. Fluid Mech., vol. 796, 2016b, pp. 659–711) show that even in a portion of the parameter space where this preferential sampling is absent, the particles nevertheless exhibit enhanced settling velocities. Further, there are several outstanding questions concerning the role of different turbulent flow scales on the enhanced settling, and the role of the Taylor Reynolds number $R_{\\unicode[STIX]{x1D706}}$ . The analysis of Maxey does not explain these issues, partly since it was restricted to particle Stokes numbers $St\\ll 1$ . To address these issues, we have developed a new theoretical result, valid for arbitrary $St$ , that reveals the multiscale nature of the mechanism generating the enhanced settling speeds. In particular, it shows how the range of scales at which the preferential sweeping mechanism operates depends on $St$ . This analysis is complemented by results from DNS where we examine the role of different flow scales on the particle settling speeds by coarse graining the underlying flow. The results show how the flow scales that contribute to the enhanced settling depend on $St$ , and that contrary to previous claims, there can be no single turbulent velocity scale that characterizes the enhanced settling speed. The results explain the dependence of the particle settling speeds on $R_{\\unicode[STIX]{x1D706}}$ , and show how the saturation of this dependence at sufficiently large $R_{\\unicode[STIX]{x1D706}}$ depends upon $St$ . The results also show that as the Stokes settling velocity of the particles is increased, the flow scales of the turbulence responsible for enhancing the particle settling speed become larger. Finally, we explored the multiscale nature of the preferential sweeping mechanism by considering how particles preferentially sample the fluid velocity gradients coarse grained at various scales. The results show that while rapidly settling particles do not preferentially sample the fluid velocity gradients, they do preferentially sample the fluid velocity gradients coarse grained at scales outside of the dissipation range. This explains the findings of Ireland et al., and further illustrates the truly multiscale nature of the mechanism generating enhanced particle settling speeds in turbulence.

Most cold-climate biological nutrient removal facilities experience poor settling mixed liquor during winter, resulting in treatment capacity throughput limitations. The Metro Wastewater Reclamation District in Denver, Colorado, operated two full-scale secondary treatment trains to compare the existing biological nutrient removal configuration (Control) to one that was modified to operate with an anaerobic selector and with hydrocyclone selective wasting (Test) to induce granulation. Results from this evaluation showed that the Test achieved significantly better settling behaviour than the Control. The difference in the mean diluted SVI30 between the Test and Control were statistically significant (P < 0.05), with values of 77 ± 17 and 135 ± 25 mL/g observed for the Test and Control respectively. These settling results were accompanied by differences in the particle size distribution, with notably higher settling velocities commensurate with increasing particle size. The degree of granulation observed in the Test train was between 32 and 56% of the mass greater than ≥250 μm in particle size whereas 16% of the mixed liquor in the Control was ≥250 μm over the entire study period. The improved settling behaviour of the Test configuration may translate into an increase of secondary treatment capacity during winter by 32%.

Dry deposition rates are usually assumed to rely primarily on gravitational settling, with some enhancement due to interactions with surface elements. However, it is known through experiment, theory, and simulation that vertical velocities of particles in turbulent flows can be enhanced by hydrodynamic effects (often referred to as preferential sweeping), which is not explicitly captured by current deposition models. In this work, we leverage scaling analysis of the atmospheric surface layer to develop a coarse mode deposition model that incorporates preferential sweeping. We find that scaling laboratory experiments to field scales points to an increase in the settling velocities of coarse‐mode particles at all heights, though the expected increase is less than limited empirical observations might suggest. We emphasize the likely reasons for this discrepancy and highlight that more research focus should be placed on appropriately modeling the deposition rate of such particle sizes due to these uncertainties.

Particle settling in narrow rough fractures is a common but poorly understood phenomenon during hydraulic fracturing. This study first constructs a large slot with two rough surfaces to simulate rock fractures and employs the particle image velocimetry to measure particle settling. Results show that particle settling in the rough slot is more complex than in the smooth slot. Rough pathways significantly change particle settling characteristics. The rough-walled slot alters the classic settling process by creating preferential pathways, localized trapping, and vortex-driven redistribution. Particles settle along preferential pathways, increasing settling velocity. The wall retardation effect becomes more prominent for large particles, reducing the settling velocity. Particle settling induces vortices throughout the rough surface, affecting particle behavior. Higher particle volume fractions increase settling non-uniformity, leading to unstable fluid flow within fractures, characterized by high vorticity and upward flow. The frequent interplay between particles and particle-walls, and fluid resistance complicates particle trajectories and settling behavior. Fluid viscosity significantly changes settling patterns and promotes particle clusters, forming chain-like and curtain-like clusters in rough fractures. An innovative model is proposed to predict settling velocity in rough fractures.

The motion of thin curved falling particles is ubiquitous in both nature and industry but is not yet widely examined. Here, we describe an experimental study on the dynamics of thin cylindrical shells resembling broken bottle fragments settling through quiescent fluid and homogeneous anisotropic turbulence. The particles have Archimedes numbers based on the mean descent velocity $0.75 \\times 10^{4} \\lesssim Ar \\lesssim 2.75 \\times 10^{4}$. Turbulence reaching a Reynolds number of $Re_\\lambda \\approx 100$ is generated in a water tank using random jet arrays mounted in a coplanar configuration. After the flow becomes statistically stationary, a particle is released and its three-dimensional motion is recorded using two orthogonally positioned high-speed cameras. We propose a simple pendulum model that accurately captures the velocity fluctuations of the particles in still fluid and find that differences in the falling style might be explained by a closer alignment between the particle's pitch angle and its velocity vector. By comparing the trajectories under background turbulence with the quiescent fluid cases, we measure a decrease in the mean descent velocity in turbulence for the conditions tested. We also study the secondary motion of the particles and identify descent events that are unique to turbulence such as ‘long gliding’ and ‘rapid rotation’ events. Lastly, we show an increase in the radial dispersion of the particles under background turbulence and correlate the time scale of descent events with the local settling velocity.

Conventional activated sludge (CAS) and densified sludge obtained using hydro-cyclone selective wasting were compared at a full-scale water resources recovery facility. The densified tested sludge, containing around 30–50% of aerobic granules, showed enhanced settleability with low and stable sludge volume index (SVI) compared to CAS, which suffered recurrent filamentous bulking. Further in-depth batch settling tests were carried out using a 40 cm diameter column fitted with ultrasonic transducers to monitor both sludge blanket height and vertical velocity profiles. Hindered settling and compression parameters were calibrated from the experiment for latter modelling use. Test sludge displayed more than doubled settling velocities compared to CAS, with hindered settling velocities remaining >3 m·h−1 even at high solids concentrations of 6.85 g·L−1. The compression regime was attained at much higher critical concentration for the test sludge. It also displayed enhanced thickening properties, with concentrations obtained after 30 min of settling being 20.9 and 8.5 g·L−1 respectively for test and control sludge. This allows for a substantial reduction of recirculation rates in practice. These results open perspectives in optimizing existing plant operation as well as clarifier design and modelling using densified sludge.

Recent studies have highlighted the importance of the atmosphere in the long-range transport of microplastic fibres. However, their dry deposition in the atmosphere is not fully understood, with the common spherical-shape assumption leading to significant uncertainties in predicting their travel distance and atmospheric residence time. Shapes of microplastic fibres vary greatly, which can be as long as 100 μm and as thin as 2 μm. Shapes of microplastic fibres may greatly affect their dry deposition in the atmosphere. Here we develop a theory-based settling velocity model for simulating atmospheric transport of microplastic fibres in different sizes and shapes. The model predicts a smaller aerodynamic size of microplastic fibres than that estimated by using volumetrically equivalent spherical counterparts. We find that the treatment of flat fibres as cylindrical ones, due to uncertainty in dimensions of sampled microplastic fibres, would cause overestimation of their dry deposition rate. Accounting for fibre thickness in sampled microplastic fibres leads to a mean enhancement of residence time by more than 450% compared to cylindrical ones. The results suggest a much more efficient long-range transport of flat fibres than previously thought.Flat microplastic fibres have much longer residence times and travel further in the atmosphere than previously appreciated, according to simulations of the settling of microplastics with different shapes.

We investigate the behaviour of microscopic heavy particles settling in homogeneous air turbulence. The regimes are relevant to the airborne transport of dust and droplets: the Taylor-microscale Reynolds number is $Re_\\lambda = 289\\text {--}462$, the Kolmogorov-scale Stokes number is $St = 1.2\\text {--}13$ and the Kolmogorov acceleration is comparable to the gravitational acceleration (i.e. the Froude number $Fr = O(1)$). We use high-speed laser imaging to track the particles and simultaneously characterize the air velocity field, resolving all relevant spatio-temporal scales. The role of the flow sampled by the particles is spotlighted. In the present range of parameters, the particle settling velocity is enhanced proportionally to the velocity scale of the turbulence. Both gravity and inertia reduce the velocity fluctuations of the particles compared to the fluid; while they have competing effect on the particle acceleration, through the crossing trajectories and inertial filtering mechanisms, respectively. The preferential sampling of high-strain/low-vorticity regions is measurable, but its impact on the global statistics is moderate. The inertial particles have large relative velocity at small separations, which increases their pair dispersion; however, gravity offsets this effect by causing them to experience fluid velocities that decorrelate faster in time compared to tracers. Based on the observations, we derive an analytical model to predict the particle velocity and acceleration variances for arbitrary $St$, $Fr$ and $Re_\\lambda$. This agrees well with the present observations and previous simulations and captures the respective effects of inertia and gravity, both of which play crucial roles in the transport.