Abstract
Despite facing many challenges, the exploration of using natural forces and mechanisms besides gravity to enhance particle settling has never ceased. A novel particle separator design, which utilizes multiple vortexes to enhance particle settling, was proposed in this study. The basic principle is using the fluid's energy to generate small swirling currents in a specially designed vortex claw generator. These currents bring suspended particles from the rapid and turbulent inflow to relatively quiet water regions, separating them from the main flows and reducing their travel distance to the wall. To verify the new separator design's performance, comparison studies were carried out in the laboratory using physical models. The results showed that the new design had much higher particle capture rates for the same inflow rates and tested particle sizes. Most importantly, it was able to effectively remove small particles, and particle capture rates were much less affected by fluctuations in inflow rates. Since most existing particle separators failed to perform well under large inflow rates, these characteristics make the new design stand out from other separators. Due to its special structure, its treatment capacity can also be easily increased without changing its horizontal separator size.
HIGHLIGHTS
Enhance solid–liquid separation with a novel method.
Utilizing vortexes generated by inflows to collect suspended particles.
In contrast to most solid–liquid separators, it can remove small particles more effectively at high flow rates.
Particle capture rates are much less affected by fluctuations in inflow rates.
With the same footprint, treatment capacity can be easily increased.
INTRODUCTION
During the rainy season, urban rainwater runoff and combined overflow sewage pose a significant threat to the environment due to their high volume, suspended solid content, and pollution load. Directly discharging these untreated waters into urban water bodies can cause severe pollution. To reduce this pollution load, it is crucial to rapidly remove suspended solids from these large instantaneous flows. However, effectively and economically removing pollutant particles from such large-flow rainwater has always been a challenge in the water treatment industry. Particle separators are widely used due to their low cost, large processing capacity, and small footprint, making them ideal for non-point source pollution treatment. However, one of their greatest weaknesses is that they are incapable of capturing particles in large quantities under large inflow rates.
Many designs for particle separators have been proposed and implemented to address this issue. One of the earliest designs, the Lamellas particle separator, was first suggested by Hazen in 1904 and was further explored by others such as Camp (1946); Culp et al. (1968), and Yao (1973). The basic concept involves inserting inclined plates into the clarifier, which are spaced closely together to increase the clarifier's surface area and enhance the settling of particles. This principle is based on the fact that the flow in the boundary layer is generally weaker than the outer region's flow. Yao's theoretical analysis of various clarifier geometries and laminar flows between plates in 1970 indicated that clarifier flow conditions need to be well-controlled, and settled particles cannot be resuspended under higher-than-expected inflows. Meeting these conditions is challenging, and most high-rate clarifiers use chemical additions to improve their performance. However, there have been few new developments in the Past few decades regarding the creation of new inclined plates.
The hydrodynamic vortex separator (HDVS) is a popular type of particle–liquid separator that mimics natural phenomena such as hurricanes, tornados, and whirlpools. It has been widely employed in stormwater management practices to reduce discharges of solids and pollutants into receiving water. HDVSs come in various formats and operate on the principle of strong vertical pulling forces induced by rotating flows. However, it is important to note that not all horizontal swirling motions can produce strong vertical velocities and forces, HDVSs may not produce high particle removal rates as expected (He et al. 2022).
Numerous evaluations have been conducted on various HDVSs in both field and laboratory settings, with varying results (Tyack et al. 1992; Michelbach & Wohrle 1993; Andoh & Smisson 1994; Averill et al. 1997; Arnett & Gurney 1998; Turner et al. 2000; Okamoto et al 2002; Davidson & Summerfelt 2005). However, determining the true performance of these devices is a subject of debate due to uncertainties in the test results. For instance, the assumption that inflow and outflow rates are consistent and reproducible when using simultaneous sample collection at the inlet and outlet to determine particle removal rates is often not met in practice. Moreover, there is no universal approach to representatively sampling particle concentration in the inflow and outflow, and the sampling method depends on the situation, whether vertical or horizontal, or both. In addition, reliable results require consistent inlet and outlet samples over a long enough period to ensure repeatable testing results. In laboratory tests, obtaining accurate absolute particle removal rates of a particle separator requires a full-size unit, as the gravity force on settling particles cannot be scaled, this results in particle settling speed and travel distances being unable to be scaled in the same way as dimensions and flow rates. Physical scaled models can, however, be used to reliably conduct laboratory comparison tests of different devices under the same testing conditions since the gravitational force effect on the tested devices is the same, and the comparison results obtained do not require gravity to be scaled.
When it comes to gravity settling, turbulence is often seen as a negative influence on particle settling, causing sediment re-suspension, especially in areas far from the boundaries or near sediment-bare beds. However, in the boundary layer, this may not always be the case, as evidenced by experiments. Cuthbertson et al. (1998) discovered that fine non-cohesive particles settling velocities near the bed in a turbulent open channel flow over a rough porous bed were 2.5 times greater than in still water. This was because vortexes generated by the rough bottom increased the transfer of particles from the high-speed outer flow to the near-bed low-speed flow. Research on particle behavior in the wall region of turbulent boundary layers with coherent structures (Marchioli & Soldati 2002) revealed that coherent sweeps and ejections provided efficient transfer mechanisms for particles generated by quasi-streamwise vortexes. Heavy particles tend to migrate towards the wall, and when in the wall layer, they segregate preferentially in regions characterized by streamwise velocity lower than the mean velocity. The relationship between turbulence structure and particle dynamics explains particle behavior in turbulent boundary layers. When a particle is entrained in a sweep, it is expected to continue within the sweep and approach the wall. The local flow structure prevents most particles that have entered the wall layer from being entrained toward the outer flow. In fact, only particles that enter the wall layer with a specific trajectory curvature may be able to be entrained back into the outer flow. Studies on particle deposition in pits in the bottom of an annular flume by Yager et al. (1993) showed that increasing bed roughness changed the flow structure, resulting in increased vertical velocity component of particles and creating sheltered spaces in the gravel bed for particle deposition. Under certain conditions, pits tend to fill in with finer-than-background sediments. All of these research results demonstrated that vertical vortexes near the boundary increase suspended particle flux towards the boundary.
Over the course of several decades, there has been a lack of significant advancements in the technology used to effectively remove smaller contaminated suspended particles under high flow rates without chemical additions (Ferreira & Stenstrom 2013). The primary devices utilized have been the Lamella inclined plates and various HDVSs utilizing vortex technology. Additionally, some designs have incorporated flow baffles to increase flow travel distance or resident time. Although these separators differ in design, they share a common issue: suspended particles travel along with the faster and often turbulent main flow when passing through a separator, resulting in low rates of particle removal, especially for smaller and lighter particles (Ferreira & Stenstrom 2013). Inflow rates also strongly impact the performance of these separators. In this study, an innovative design based on a cone-shaped vortex claw generator was proposed, it uses the inflow-generated small vortexes in the open slots built on the cone wall surface to convey suspended particles away from the main flow. It prompts sedimentation in three ways: (1) by separating suspended particles from the carrier by transferring the particles from fast and strong inflows to a quiet flow region, thereby making the inflow and particles travel through separate channels, (2) by reducing the distances of suspended particles traveling to the boundary, and (3) increasing the settling area by stacking the vortex claw generators vertically together. A physical model of the newly designed particle separator and a general hydrodynamic vortex-based separator of similar size were built to evaluate their performance.
VORTEX PHENOMENON
Simulated flow patterns in narrow slots of the proposed vortex claw generator. Strong swirl flow is generated by the horizontal flow. In graphic: (1) vortex; (2) slot sidewall; (3) horizontal flow.
Simulated flow patterns in narrow slots of the proposed vortex claw generator. Strong swirl flow is generated by the horizontal flow. In graphic: (1) vortex; (2) slot sidewall; (3) horizontal flow.
NEW SEPARATOR DESIGN
The computer model of the newly designed VCPS. It comprises of multiple cone-shaped vortex claw generators and an inflow channel attaches on the outside wall of the tank for removing the floats.
The computer model of the newly designed VCPS. It comprises of multiple cone-shaped vortex claw generators and an inflow channel attaches on the outside wall of the tank for removing the floats.
In the newly designed VCPS, multiple vortex claw generators are stacked vertically. The arrows show the flow moving direction in the VCPS.
In the newly designed VCPS, multiple vortex claw generators are stacked vertically. The arrows show the flow moving direction in the VCPS.
COMPARISON TESTS
To objectively assess the performance of a particle separator, especially for their absolute particle removal rates, is not an easy task due to its complexity and many uncertainties as mentioned in the introduction as well as it is difficult to realistically represent real treated input particles in practice regardless of the type of particle used, in addition, there is not a widely accepted standard testing method. As a result, the absolute particle removal rate of a particle separator is not very meaningful. To avoid all unnecessary and unmeasurable arguments, in this study, physical models were used to assess the performance of the newly designed vortex claw particle separator by comparing it to a general vortex-based separator, rather than determining the absolute removal rates of vortex claw separator.
EXPERIMENTS SETTINGS
Picture of physical models used to compare tests. The left side shows a commonly used hydrodynamic vortex particle separator, while the right side shows a newly developed VCPS.
Picture of physical models used to compare tests. The left side shows a commonly used hydrodynamic vortex particle separator, while the right side shows a newly developed VCPS.
Experimental arrangement for testing the removal of particles, using the commonly used hydrodynamic vortex particle separator as an example.
Experimental arrangement for testing the removal of particles, using the commonly used hydrodynamic vortex particle separator as an example.
In storm runoff, particle sizes, densities, and types vary considerably depending on location, time, and many other factors. Their size can range from 30 μm to a few millimeters. No matter what type of particles are, gravity and vortex will act on them in the same manner. Considering that large and light particles are typically more sensitive to flow conditions than small and heavy ones, in this study, the crushed walnut shell particles with a density of 1.35 g/cm3 and a size range of 90 to 355 mm were used to examine the characteristics of the newly designed VCPS and compare its performance with commonly used HDVSs. Multiple particle sizes (90–125 μm, 125–180 μm,180–250 μm, 250–355 μm) under various flow rates (0.5, 1, 1.5, 2, 2.6 l/s) were adopted in the comparison tests, which corresponded to sand particles of 45 to 175 μm and a density of 2.40 g/cm3 for a comparable settling velocity (determined by Stokes' law). The particle sizes were sorted using a mechanic sieve machine. Although walnut particles can vary slightly in density and size after being wet and then dried, because this was a comparison study and the study procedure was consistent, the results should be comparable. The repeatability testing results as well as our other studies have confirmed this (He et al. 2015; He et al. 2022).
A sample of seed particles was prepared by mixing 200 g of crushed walnuts with 4 l of tap water, the generated suspended particle concentration is similar to that reported in a survey of stormwater best management practices (BMP) studies (Geosyntec Consultants 2008a). Particle samples were fed from the bottom of the container using a peristaltic sampling pump and flexible tubing with a diameter of 5 mm (inside diameter). In order to feed this material into the incoming flow path, the tubing was inserted into the bottom of the vertical feeding pipe approximately 300 mm upstream from the inlet mouth of the settling tank. The particle mixture should be well mixed by the incoming fast turbulent inflows in such an arrangement. A mixer (with a pitched blade impeller) was inserted in the sample container to disturb particles for the 4 min required to empty all particles into the inlet feed. The incoming flow continued for an additional 2 min, allowing all particles to have the opportunity passing through or settling in the separator. Captured particles were drained through a bottom drain (with a thorough separator rinse) of the separator and collected in a 75 mm screen located below the drainpipe exit. The captured particles were oven-dried overnight at 40°C (to prevent burning) and weighed to determine their total weight. The experiments were conducted under well-controlled conditions. For the experiment setting shown in Figure 6 and 1.25 l/s inflow rate, the standard deviation of the obtained particle removal rates (n = 5) was <0.9%. Therefore, no duplicate runs were performed in most particle removal rate tests.
RESULTS AND DISCUSSIONS
A comparison of the particle removal rates for the newly designed VCPS (represented by dotted curves) and a general hydrodynamic vortex particle separator (represented by solid curves).
A comparison of the particle removal rates for the newly designed VCPS (represented by dotted curves) and a general hydrodynamic vortex particle separator (represented by solid curves).
In the top panel, it is illustrated that horizontal flow will create a downward velocity in parallel angled slots. The bottom panel shows that smaller vortexes are likely to form in slots with bent walls.
In the top panel, it is illustrated that horizontal flow will create a downward velocity in parallel angled slots. The bottom panel shows that smaller vortexes are likely to form in slots with bent walls.
The graph displays the particle removal rates of various types of separator structures. These include ones with straight slot walls of vortex claw generators (shown in black), tilted and bent slot walls of vortex claw generators (shown in red), and smooth cones (shown in blue). Each panel shows the test results for different particles.
The graph displays the particle removal rates of various types of separator structures. These include ones with straight slot walls of vortex claw generators (shown in black), tilted and bent slot walls of vortex claw generators (shown in red), and smooth cones (shown in blue). Each panel shows the test results for different particles.
Typically, the location and size of the outlet pipe on particle separators cannot be easily adjusted to meet design and functionality standards, which may raise concerns in some practical applications. To assess this issue for newly designed VCPS, sensitivity tests were conducted on the VCPS by altering the outlet pipe's location and size to evaluate their impact on overall performance. In all panels of Figure 9, the dotted black curves represented the rates of particle removal when the outlet pipe was shifted vertically from the top to the middle of the tank, and when the outlet pipe diameter was increased from 5.08 to 7.62 cm (commercially available 3″ pipe). By comparing the solid black curves (the outlet pipe was located at the top of the tank) to the dotted curves, it was evident that the two groups of curves followed each other closely. This indicated that lowering the outlet pipe location and increasing its size did not alter the flow dynamics within the separator, except for an increase in maximum treatable flow rates. This is a noteworthy advantage as it allows for an easy increase in treatment capacity without changing the size of the footprint, as compared to other particle separators.
CONCLUSIONS
A novel technique for enhancing the separation of liquid and solid substances has been introduced. This innovative method involves using vortexes generated by inflow to extract solids from the primary inflow and transport them to calmer water areas. To investigate the efficiency of this approach, a scaled particle separator was built and evaluated by comparing its particle removal rates with those of a standard vortex particle removal method, as well as various vortex claw generator designs, at different inflow rates and particle sizes. The comparative tests were also performed using traditional smooth lamellas. Based on the results, it has been found that the newly proposed particle enhancing concept for the VCPS has significantly increased the particle removal rates, especially for smaller particles under high inflow rates, with the same footprint. This is a crucial development as it not only helps achieve higher particle removal, but also removes potential contaminations carried by the particles multiple times over. The new VCPS has shown impressive performance, displaying minimal vulnerability to inflow rate changes while exhibiting the ability to easily increase its treatment capacity and alter outlet vertical locations. These capabilities can be attributed to the flow properties around the vortex plate, which include: (a) steady vortexes generated within the slots, which entrain particles into the slots and allow for settling without hindrance from the main flow; (b) enhanced particle settling due to increased contact surface area and reduced travel distance (the vortex plate's surface area is twice that of a smooth lamella); (c) no baffles were present to force treaded flows to make sharp turns when passing through the VCPS, which expects to have less hydraulic resistance and head loss; and (d) increased particle collision frequency within the swirling flow of the slots, which promotes particle flocculation. In the context of this paper, deeper-level proving data is not necessary and cannot be obtained easily. Based on the findings of an exploratory study on the vortex claw concept, it is suggested that further research should be undertaken to investigate the impact of clarifier flow conditions on particle movement along the vortex claw generator. This will aid in the development of an optimal design. However, it is important to note that since the optimal dimensions of the vortex claw generator and the space between individual vortex claw generators are dependent on the rate of inflow, optimal construction designs were not conducted during this study. It would be more meaningful to conduct an optimal study after the main treated inflow rates were determined.
ACKNOWLEDGEMENTS
The present study was financially supported by the Great Lakes Action Plan (GLAP) with funding from Environment and Climate Change Canada. Thanks are offered for the support from the Engineering Branch at the National Water Research Institute for providing very valuable advice and helping to build the physical model.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.