Will it float? Rising and settling velocities of common macroplastic foils

Plastic accumulates in the environment because of insufficient waste handling and the materials' high durability. Better understanding of plastic behaviour in the aquatic environment is needed to estimate transport and accumulation, which can be used for monitoring strategies, prevention measures, and plastic clean-up activities. Plastic transport models benefit from accurate description of particle characteristics, such as rising and settling velocities. For macroplastics, these are however still scarce. In this research, the rising and settling behaviour of three different polymer types (PET, PP, and PE) was investigated, which are the most common in the environment. All of the plastic particles were foils of different surface areas. A new method for releasing rising plastics without interfering the flow and disturbing the column was used. Four models that estimate the velocity based on the characteristics of the plastics are discussed, of which three are from literature, and one is newly derived. These models are validated using the data generated in this research, and data from another study on rising and settling velocities of plastic. From the models that were discussed, the best results are from the newly introduced velocity model for foils (R^2 = 0.96 and 0.58, for both datasets). This model shows potential to estimate the rising and settling velocity of plastics, and should be examined further by using additional data. The results of our paper can be used to further explore the vertical distribution of plastics in rivers, lakes and oceans, which is crucial to optimize future monitoring and cleanup efforts.

Inducing granulation within a full-scale activated sludge system to improve settling

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%.

Settling behaviour of particles in Rayleigh-Benard convection

<p>Our numerical study evaluates the settling rate of solid particles, suspended in a highly <br>vigorous, finite Prandtl number convection of a bottom heated fluid. We explore a broad <br>range of model parameters, covering particle types appearing in various natural systems, <br>and focus in particular on crystals nucleating during the cooling of a magma ocean. The <br>motion of inertial particles within thermal convection is non-trivial, and under idealized <br>conditions of spherical shaped particles with small Reynolds number it follows the <br>Maxey-Riley equation (Maxey and Riley, 1983). Two scaling laws exist for the settling <br>velocities in such system: for particles with small but finite response time, the Stokes' <br>law is typically applied. For particles with a vanishing response time, a theoretical model <br>was developed by Martin and Nokes (1989), who also validated their prediction with analogue <br>experiments. </p><p>We develop a new theoretical model for the settling velocities. Our approach describes <br>sedimentation of particles as a random process with two key constituents: i) transport <br>from convection cells into slow regions of the flow, and ii) the probability of escaping <br>slow regions if a particle enters them. By quantifying the rates of these two processes, <br>we derive a new equation that bridges the gap between the above mentioned scaling laws. <br>Moreover, we identify four distinct regimes of settling behaviour and analyze the lateral <br>distribution of positions where particles reach the bottom boundary. Finally, we apply our <br>results to the freezing of a magma ocean, making inferences about its equilibrium vs <br>fractional crystallization. The numerical experiments are performed in 2D cartesian geometry <br>using the freely available code CH4 (Calzavarini, 2019).</p><p>References:<br>Maxey, M. R. and Riley, J. J.(1983): Equation of motion for a small rigid sphere in a nonuniform flow. <br>Physics of Fluids, 26(4), 883-889.</p><p>Martin, D and Nokes, R (1989): A fluid-dynamic study of crystal settling in convecting magmas. <br>Journal of Petrology, 30(6), 1471-1500.</p><p>Calzavarini, E (2019): Eulerian–Lagrangian fluid dynamics platform: The ch4-project. Software Impacts, 1, 100002.</p>

Effect of salts and pH on the removal of perfluorooctanoic acid (PFOA) from aqueous solutions through precipitation and electroflocculation

This study shows that salt type and pH affect the removal of perfluorooctanoic acid (PFOA) from water through flocculation and electro-enhanced flocculation. PFOA concentrations decreased by approximately 80% with 20 mM FeCl3 and initial pH = 3–10. Electro-enhanced flocculation at pH = 3 reduced PFOA concentrations in water by approximately 70% with CaCl2 and KCl, and by 50% with NaCl. At alkaline pH, PFOA removal was approximately 20% with CaCl2 and KCl. PFOA removal was negligible without salts or with AlCl3, and with NaCl at alkaline pH. Differences in PFOA removal are likely due to the settling behaviour of the cation-PFOA complexes formed.