Design-oriented evaluation of the hydrodynamics in a full-scale combined filter-lamella separator for urban stormwater treatment

Due to increasing knowledge of pollutant pathways, new urban stormwater management practices have been developed in recent decades, which are referred to throughout the literature as best management practices (BMP), stormwater control measures (SCM) or low impact development (LID) (Fletcher et al. 2015). These include compact treatment devices integrated into the underground stormwater sewer system, which treat stormwater in a decentralized or semicentralized manner and often combine a preliminary sedimentation with an upward flow filtration in a single unit (Gruening et al. 2011). This combination extends the longevity of a filter by retaining coarse particles in the sedimentation stage (Herr & Sansalone 2015) and is thus also a promising approach for the application on a central scale (Grüning & Schmitz 2018). Overall, the implementation of filter media provides an enhanced removal efficiency with respect to particulate matter (Langeveld et al. 2012) and adsorptive-binding pollutants like hydrocarbons and heavy metals (Vesting et al. 2015; Okaikue-Woodi et al. 2020). Nevertheless, filter media are prone to clogging and therefore require regular maintenance. The service life of a filter until hydraulic failure occurs, i.e. an unacceptable decrease of filter permeability, depends, among other things, on the filter loading rate, the filter grain size and structure, the size of the particles in the stormwater, the influent concentration and filtration rates (Siriwardene et al. 2007; Kandra et al. 2015). The adsorption ability of a filter medium is generally described by the adsorption capacity and the adsorption kinetics, which are usually derived from laboratory-scale batch tests. Removal rates for a specific pollutant may differ depending on filter medium, initial pollutant concentration, composition of the stormwater, filter loading and contact time (Liu et al. 2005; Deng 2020).

In the context of compact treatment devices combining upward flow filtration and sedimentation, not much attention has been paid to the efficient use of the filter area, which depends on the hydrodynamics of the system. To date, explicit research on this topic is rare, except for the study of Pathapati & Sansalone (2009), who indicated a non-uniform filter loading in a radial cartridge filter system. Thereby, the filter surface was only partly utilized by larger particles (75 μm), while smaller particles tended to utilize a larger filter surface. Such circumstances may influence long-term clogging behavior, accelerating the decrease of filter permeability in certain areas. This might also change the underlying flow field of a sedimentation stage and enforce higher filtration rates in specific filter zones, which will diminish the contact time of soluble pollutants in the specific filter areas.

This study aims to showcase these effects and addresses them explicitly by the use of computational fluid dynamics (CFD), which has been applied to a variety of wastewater-related problems (Samstag et al. 2016). Herein, the system under investigation is a compact treatment device, which combines a filtration stage with a lamella separator in a single unit. Lamella separators offer great potential concerning the effective removal of particulate matter in a confined space. However, to take advantage of the increased surface area, a uniform surface loading is a prerequisite, which can be constructively challenging (Fuchs et al. 2014). CFD simulations of lamella separators concerning potable water and stormwater were performed by some authors with different objectives, e.g. Salem et al. (2011); Tarpagkou & Pantokratoras (2014). Tarpagkou & Pantokratoras (2014) followed a macroscopic approach, investigating the overall flow field of a full-scale system with inclined plates. Salem et al. (2011) evaluated the flow distribution in a bench-scale model, comparing simulation results to experimental tracer data. However, a lamella separator combined with a filtration stage has not been investigated.

Throughout this study, the optimal configuration of a combined filter-lamella separator for stormwater treatment is examined via CFD with the software ANSYS Fluent. In a first step, different design configurations are examined by altering the angle of inclination of the inclined plates in the lamella separator. Thereby, the design configuration that produces the most uniform flow distribution along the inclined plates is determined. Afterwards, the distribution of particulate matter along the filter bed is evaluated and analyzed concerning different particle diameters. The last objective of this study is to predict the development of dynamic filter clogging with an increasing volume of stormwater treated. Therefore, a simple nonlinear clogging model based on laboratory data of Kandra et al. (2015) is applied to predict the decrease of hydraulic conductivity along the filter bed. The distribution of particulate matter, filtration rates and flow rates inside the inclined plates are compared before and after loading about half of Germany's annual runoff-efficient precipitation to the system.

The system under investigation is a compact treatment device combining a filtration stage and a lamella separator into a single unit, built by Dr. Pecher AG (Erkrath, Germany). In this study, the lamella stage is represented by an inclined plate system with 47 inclined plates. The geometry of the system is depicted in Figure 1. Herein, stormwater enters via the inlet, advances in the main flow direction and passes upwards through the inclined plates. Afterwards, the stormwater percolates through the filter media. The hydraulic head that forces the water through the filter is generated by a rising free water level in the storage section of the system. The storage section is of rectangular geometry and attached to the actual treatment device by three rectangular openings. There are two free surface water levels: (i) above the filter media and (ii) above the storage section. If the pressure drop of the filter media exceeds the design values of the system, untreated stormwater is discharged from the storage section into the receiving water body. In all other cases, treated stormwater passes through the filter and exits the system over a weir. Throughout this study, three design configurations are evaluated: without filtration and inclined plates at a 60° angle pointing against the main flow direction (configuration A), with filtration and inclined plates at a 60° angle pointing against the main flow direction (configuration B) and with filtration and inclined plates at a 60° reversed angle pointing in the main flow direction (configuration C). For an in-depth analysis, the filter is horizontally divided into 48 equally spaced filter zones with a length of 15 cm. This classification allows us to study the development of heterogeneous clogging along the filter bed. Dimensions of the system and scenario information are given in Table 1.

Modeling was done under steady-state conditions on a 3D geometry; see Figure 1. Regarding the simulation set-up for single-phase flow, the following boundary conditions have been applied: The inlet was defined as a ‘velocity inlet’ with 0.0275 m/s, representing an average rainfall intensity of 5 L/(s·ha). The overflow of the weir was represented by a ‘pressure outlet’ condition. Both free water surfaces were set up as shear-free ‘symmetry’ boundary conditions. All physical walls were defined as ‘no slip’ boundary conditions.

A ‘reflect’ boundary condition was utilized at all physical wall boundaries. Particles reaching the filter media were trapped with a ‘trap’ boundary condition at the downward-facing surface of the filter zones. Therefore, the trajectories of particles that are not removed by sedimentation end at the downward-facing filter surface. For each of the 48 filter zones, mass loading rate and particle size can be individually determined. This approach allows an evaluation of the distribution of particulate matter among the filter bed surface, which gives information about the utilization of the filter area. As particles either were trapped on the filter surface or remained in the system, an ‘escape’ boundary condition was not used. The removal criteria of Pathapati & Sansalone (2009) were adopted for this study: In a first run, the tracking length of neutrally buoyant particles was determined. The tracking length after which at least 95% of the neutrally buoyant particles reached the filter bed was then used as a removal criterion for all particles. Thus, any particle that was still in the system after the tracking length had expired was considered to be removed.

A simple nonlinear clogging model was derived from literature data to account for filter clogging within the 48 subdivisions of the filter bed. In the context of stormwater filtration, several authors have focused on modeling mechanical clogging, e.g. Siriwardene et al. (2007); Wang et al. (2012); Tang et al. (2020). However, the prediction of filter clogging is complex and depends on a set of parameters; therefore, different clogging phenomena (deep bed filtration and surface filtration) are observed within different experiments. Table 2 shows a selection of studies, which were considered as input data for the prediction of the filter clogging. Herein, the type of filtration corresponds to the observed clogging phenomena. However, a clear distinction is sometimes not possible, as mechanisms of deep bed filtration and surface filtration can take place simultaneously. Therefore, only the predominant filtration type is depicted. In this study, suitable data for the prediction of clogging was selected from the literature based on the expected ratio of particle size and filter grain size, which was found to be an important parameter impacting the nature of clogging (Herzig et al. 1970; Kandra et al. 2015).

Convergence criteria was based on scaled residuals dropping below 10⁻³ and velocities, which were monitored at different locations throughout the domain. The solution was considered to be converged when scaled residuals and monitored velocities were stable and did not change during further iterations. In all simulations a second order discretization scheme was used. For pressure-velocity coupling the SIMPLE-algorithm was selected. The influence of grid refinement was investigated using three meshes with increasing number of cells. The grid refinement study was carried out on configuration B. Meshes under investigation had 3,025,762, 5,623,125 and 12,819,821 elements for coarse, medium and fine mesh, respectively. In Figure 4 the mean velocities in the cross-sections of the inclined plates are depicted for different meshes. As the results from the medium sized mesh are in good agreement with the fine mesh, the medium sized mesh was selected for all simulations due to lower computing time.

In configuration A, stormwater is distributed unevenly along the inclined plates; see Figure 5 for flow field and Figure 6 for absolute flow rates. Herein, about 68% of the incoming stormwater passes through the last ten inclined plates. The maximum flow rate was 2.80 L/s in the 46th plate, corresponding to a surface loading of 14.82 m/h with respect to its horizontally projected surface area. Simultaneously, a recirculation zone is established in the vicinity of the inlet, which is accompanied by negative flow rates in the first set of plates, see Figure 5 (configuration A, black dotted box).

In configuration B, stormwater is almost evenly distributed along the inclined plates. Herein, a maximum flow rate of about 1.18 L/s is established in the 46th plate, corresponding to a specific surface loading of 6.24 m/h. Despite the mainly uniform distribution, two zones of slightly higher flow-through form: One in the last set of plates at the end of the building (40th–47th plates), which is already present in configuration A, and a new one, in the vicinity of the inlet (3rd–10th plates); see Figures 5 and 6. A recirculation zone is present up to the fifth inclined plate; see Figure 5 (black dotted box). Above the plates with high flow rates, a redirecting flow is formed; see Figure 5 (red box). In this case, the flow rates exceed the local hydraulic capacity of the filter in this area. Hence, stormwater is forced to enter the filter somewhere else and is redirected into other filter areas.

Configuration C demonstrated that the angle of inclined plates strongly influences the flow field. Herein, high flow rates are established in the 5th–9th plates, transporting about 30% of the incoming stormwater. The maximum flow rate is 2.00 L/s in the 5th plate, corresponding to a surface loading of 10.58 m/h. Above the plates with a high surface loading, strong redirecting flows develop. These are characterized by stormwater that is redirected into neighboring filter zones, which is noticeable by the increased flow velocities in the space between the inclined plates and the filter; see Figure 5 (red boxes). Areas of suppressed flow velocities occur below filter zones that are loaded by stormwater from strong redirecting flows; see Figure 5 (yellow box). However, when the loading by the redirected stormwater ceases, the flow rates in the underlying plates increase again. Likewise, in configuration B, a recirculation zone forms in the first plates (1st–5th plates); see Figure 5 (black dotted box).

Comparing the design configurations A, B and C, an overall improvement is observed by the combination of a filtration stage with a lamella separator. Thereby, the resistance of the filter reduces the maximum observed flow rates in specific plates and promotes a uniform surface loading (configurations A, B and C). However, different flow fields develop by altering the angle of the plates (configurations B and C). In contrast to configuration B, configuration C exhibits a strong shortcutting flow in the first set of plates. This difference may be explained by the increased shear in configuration B in comparison to a reduced shear in configuration C. In configuration C, the plates point in the main flow direction, reducing the overall shear and making the first set of plates directly accessible to stormwater. Thus, the highest flow rates develop in the front of the system. In configuration B, the plates are inclined against the main flow direction, exerting additional shear to the flow, which finally reduces the flow rates in the inclined plates. In both configurations, the interaction of inclined plates and the filter media creates redirecting flows. In configuration C, these pose an additional problem as the redirecting flows suppress the flow rates in the underlying plates, favoring a non-uniform distribution. It is hypothesized that these redirecting flows result from a filter resistance, whose influence does not extend far enough into the flow field of the lamella separator. Thus, a direct vertical connection of plates and filter media might extend this influence, as it prohibits the compensation of pressure gradients along the filter bed by redirecting flows and thereby enforces a uniform surface loading of the inclined plates. Recirculation zones form in all design configurations. However, the presence of a filter media reduces the size of the recirculation zone from 13 to 5 inclined plates from configuration A to B and C, respectively.

The distribution of particulate matter on the filter bed, i.e. the utilization of the filter area by particles, is firstly determined for configurations B and C with a yet unloaded filter. In configuration B a nearly uniform distribution of particles develops along the filter bed. A slightly increased utilization of the filter bed emerges in the rear part (6 m and beyond) and the midsection (3.2–4.8 m). The front of the filter bed (0–1.2 m) is reached by slightly fewer particles. The distribution of particulate matter along the filter bed for configuration B with an unloaded filter (0 mm of stormwater) is depicted in Figure 7 (red curve). In configuration C, a heterogeneous distribution of particulate matter along the filter bed can be observed, elevating an early clogging in the front of the filter bed. Herein, about 36 weight-% of the particles utilize the area from 0.8 m to 2.4 m of the filter bed length, while only 17 weight-% utilize the rear part (5.6 m–7.2 m). In the former case, there will be higher maintenance requirements with respect to the specific area. The distribution of particulate matter along the filter bed for configuration C with an unloaded filter (0 mm of stormwater) is depicted in Figure 7 (yellow curve).

For an in-depth insight, the distribution of particulate matter on the filter bed is studied for different particle diameters (20 μm, 30 μm, 40 μm) for design configuration B. Thereby, it is found that 40 μm particles predominantly hit the filter bed in the rear part, while 20 and 30 μm particles are almost evenly distributed along the filter bed; see Figure 8. Further understanding of the filter utilization can be achieved by plotting the velocity component in the main flow direction from underneath the inclined plates. The velocity component in the main flow direction (x-direction) was sampled from a sampling line 5 cm underneath the plates; see Figure 5. Along the sampling line, the velocity component in x-direction increases rapidly from 0 to 0.02 m/s (0–1.2 m), remains relatively stable (1.2–3.6 m), and decreases again from 0.025 to 0 m/s (3.6–7.2 m); see Figure 8. Simultaneously, the decrease of velocity in x-direction coincides with an increase of particles with a diameter of 40 μm entering the filter bed (3.6 m and beyond). Conversely, the distribution of particles with a diameter of 20 or 30 μm is not significantly impacted. This observation is related to the different particle masses. Particles with high mass experience higher inertial forces when trying to follow the imposed flow of an inclined plate. Thus, they require sufficiently low velocities to leave the main flow and move upward through an inclined plate. An exception is made for areas where the main flow is not yet attached to the inclined plates. Consequently, 40 μm particles do not utilize the front of the filter bed, even if velocities are low. The results indicate that an estimation of filter utilization by particulate matter may be misleading if it is based on single-phase flow or soluble tracer data only.

The development of heterogeneous, dynamic filter clogging is investigated for design configuration B, as it was the best configuration regarding an even surface loading of the inclined plates and the distribution of particulate matter on the filter bed. The aim is to investigate how the flow field and the distribution pattern of particulate matter change with an increasing amount of stormwater treated. Over the course of treating 300 mm of stormwater, 1,710 kg of particles ≤250 μm were injected into the inlet, which is about half of the expected annual material removal from the catchment area. From these, 452 kg of particles ≤66 μm reached the filter bed. The filter load was calculated after each loading step (50 mm, 100 mm, 150 mm, 200 mm, 300 mm) and for each of the 48 filter zones. In between these loading steps, the hydraulic conductivity was updated and the described simulations steps were repeated; see section Simulation procedure for dynamic filter clogging. In Figure 9 the results of the simulation procedure are shown.

After the treatment of 300 mm of stormwater the predicted filter load is on average 42 kg/m²; see Figure 9(a). An overall low filter load is established in the front of the filter bed (0 m–1.2 m), ranging from 30 kg/m² to about 42 kg/m². In the midsection (3.2 m–4.6 m) a slightly higher filter load develops with a maximum of 45 kg/m² at around 3.7 m. In the rear part (6 m–7.2 m) the highest filter load is 60 kg/m² in a single filter zone. Conceivably, the observed filter loads qualitatively match with the observed distribution of particulate matter along the filter bed; see Figures 7 and 9(a). However, a comparison of the distribution patterns after each loading step indicated a change of filter utilization. Thereby, zones with initially high utilization gradually decrease and zones with initially low utilization increase with an increasing volume of stormwater treated; see Figure 7 (red, grey and black curves). This redistribution can be attributed to the formation of hydraulic conductivity gradients in the filter bed. These gradients locally enhance or diminish filtration rates and thereby influence the flow field and the distribution of particulate matter along the filter bed. Nevertheless, the influence of the redistribution is too weak to change the overall pattern of filter load, which does not even out significantly; see Figure 9(a). Over the course of dosing 300 mm of stormwater, the hydraulic conductivity of an unloaded filter gradually decreases from 1.4 × 10⁻² m/s to an average of 5 × 10⁻³ m/s; see Figure 9(b). An overall high hydraulic conductivity develops in the front of the filter bed, ranging from 6.25 to 5 × 10⁻³ m/s. Below-average hydraulic conductivities are observed in the midsection and lower hydraulic conductivities in the rear part, with a minimum of 2.6 × 10⁻³ m/s.

As expected, the gradients of hydraulic conductivity moderate the filtration rates Q_Filter, i.e. the amount of water which flows through the surface area of a specific filter zone. Thus, high filtration rates emerge in filter zones with high and low filtration rates in filter zones with small hydraulic conductivities; see Figure 10(a). Especially in the front (0 m–1.2 m), the midsection (3.2 m–4.6 m) and the rear part (6 m–7.2 m), the filtration rates significantly deviate from filtration rates in an unloaded filter bed (0 mm of stormwater); see Figure 10(a). Remarkably, in the front, filtration rates increase from 1.75 L/(s·m²) to a maximum of 2.18 L/(s·m²), with an overall total increase of +10%.

It is hypothesized that the filtration rates also impact on the flow rates in the underlying inclined plates. Therefore, the relative change of the flow rates in the inclined plates ΔQ/Q₀ is calculated from two different scenarios: (i) configuration B with 300 mm of stormwater treated and (ii) configuration B with 0 mm of stormwater (unloaded filter). Thereby, ΔQ corresponds to the absolute difference in flow rates at 0 mm and 300 m of stormwater treated. Q₀ corresponds to the flow rates in the inclined plates with an unloaded filter. Thus, ΔQ/Q₀ > 0 coincides with an increase and ΔQ/Q₀ < 0 with a decrease of flow rate; see Figure 10(b). Consistently with the high filtration rates, the flow-through in the inclined plates increases in the front of the system. Thereby, the highest relative increase in flow rate is observed in the third inclined plate with about +46%. In total, flow rates in the first six plates increase by +16%. In the midsection and the rear part of the inclined plates the flow-through decreases, which is attributed to the decreased filtration rates in these regions.

In the present study, the hydrodynamics in a combined filter-lamella separator with upward flow filtration was investigated by numerical simulations. Contrary to a lamella separator without filtration stage, the peak flow rate in the inclined plates drops from 2.80 L/s (without filter) to a minimum of 1.18 L/s when a filter is present. In the latter, the best flow situation in terms of flow distribution is achieved by inclined plates pointing at an 60° angle against the main flow direction. Mainly particulate matter in the spectrum ≤63 μm reaches the filter bed and is considered to be involved in the filter clogging. However, larger particles (40 μm) predominantly utilize the rear part of the filter bed, using only 50% of the complete filter area. After the treatment of about half of Germany's runoff-effective precipitation, the hydraulic conductivity of the filter bed decreases from 1.4 × 10⁻² to an average of 5 × 10⁻³ m/s. Thereby, gradients of hydraulic conductivity form along the filter bed, increasing filtration rates in specific areas of the filter from 1.75 L/(s·m²) to a maximum of 2.18 L/(s·m²). In these areas, the retention of pollutants by filter-adsorption processes will likely suffer from an increased load and a simultaneously shortened hydraulic retention time. Thus, the long-term operating behavior of compact treatment devices requires the best possible utilization of the complete filter area. Compact treatment devices with modular filter systems might offer a flexible exchange of clogged filter media, restoring functionality and saving filter substrate. In this case, CFD simulations can provide valuable information regarding optimized design and operational behavior. However, additional future on-site or laboratory investigations of these systems will be necessary to complement the findings from CFD simulations.

This work was supported by the Ministry for Culture and Science of the State of North-Rhine Westphalia (NRW, Germany) through the joint project ‘Future Water’ (project number: 321-8.03-215-116439).

All relevant data are included in the paper or its Supplementary Information.