Abstract
Removal of suspended solids from raw water is an essential process in water treatment plants. Conventional sedimentation tanks in water treatment plants occupy a large area and become expensive in urban areas. The use of plate settlers or tube settlers in sedimentation tanks to increase the efficiency and hence reduce the footprint of sedimentation tanks is an economical solution in water treatment. This study investigated the effectiveness of plate and tube settlers compared to conventional settlers in a water treatment plant. A three-dimensional Computational Fluid Dynamics (CFD) model was set up using ANSYS-CFX 17.2. Seven cases (a conventional settler, three plate settlers and three tube settlers) were analysed to compare the settler performances. The maximum removal efficiencies of all solid classes were approximately equal in plate and tube settlers with the same plate spacing and tube depth: around 100%, 67%, 28% and 9% for the solid classes with particle diameters of 41, 17, 9.5 and 5.0 µm, respectively. The settling efficiency remained unchanged with the increase of the plate settling area beyond 60% of the conventional settler area under the given tank and flow conditions. The tube cross-section shape does not affect the particle removal efficiency of a tube settler.
HIGHLIGHTS
Plate settlers and tube settlers are increasingly used to reduce the water treatment plant footprint with the increasing land prices in urban areas.
The present study compares the efficiency of these settlers in the removal of suspended solids of different size classes by three-dimensional computational modelling using ANSYS CFX.
The model developed here can be used to upgrade existing conventional settling tanks paying due attention to the removal efficiency of different solid classes.
INTRODUCTION
The sedimentation process where suspended solids (SS) are removed from raw water by either chemical or biological processes is one of the essential processes in water treatment plants. The conventional sedimentation process relies on the net downward force due to the difference in density between particles and fluid (Carlsson 1998). The water treatment plants are generally located in urban areas, and the large area occupied by the conventional sedimentation tanks tends to be responsible for up to 30% of the total construction cost of a water treatment plant (Tamayol et al. 2008). Many factors affect the capacity and performance of sedimentation tanks: solids characteristics, fluid characteristics, and design parameters such as tank type, solid loading rate, surface loading rate, hydraulic retention time, weir placement, and solid removal mechanism (Asgharzadeh et al. 2011).
Several efforts have been made by designers to reduce the costs associated with the sedimentation tanks; the conventional settlers are modified by inclined plates or tubes to reduce the particle settling depth and thereby to reduce the detention time and the volume of the settling basins. A plate settler is a sedimentation tank with multiple inclined plates placed parallel to each other, which combine to form a large effective settling area. However, the tube settler has a group of pipes or channels with small cross-sectional area contiguous with each other. These are usually made of lightweight materials (PVC or similar material), with or without a supporting frame. These tubes can be of different shapes including square, circular and hexagonal (Fadel 1985; Gurjar et al. 2017).
The use of tube settlers and plate settlers is the result of economical technological development which helps to increase design surface loads in sedimentation tanks. They increase the capacity of sedimentation tanks and drastically reduce the plant footprint due to the significantly reduced particle settling path and settling time by increasing the effective settling area (Faraji et al. 2013; Al-kizwini 2015; Balwan et al. 2016; Fouad et al. 2016; Al-Dulaimi & Racoviteanu 2018; Bhatia 2018). The superior performance of tube settlers and plate settlers compared to conventional sedimentation tanks is well documented. Computational fluid dynamics (CFD) tools have been applied to optimize the design of contact chambers and conventional sedimentation tanks to minimize short-circuiting and inactive volumes. The determination of the effect of inclination angle, length, and diameter of the tubes on the SS removal efficiency has been the subject of many experimental and numerical studies (Faraji et al. 2013; Tarpagkou & Pantokratoras 2014; Balwan et al. 2016; Al-Dulaimi & Racoviteanu 2018, Sharma & Bhatia 2018, Nguyen et al. 2019a).
Although experimental investigations have compared the performances of tube settlers and plate settlers, their performances have not been systematically evaluated and compared in detail under sediment parameters of raw water such as the particle size distribution of SS. Also, the effect of different cross-sectional shapes of the tubes has not been documented in the previous studies.
The detailed analysis referred to SS of different sizes is essential to understand removal efficiency of different sediment classes with varying sizes of particles in various designs. The application of CFD to simulate the settling process has been widely accepted due to its visualization capabilities and data on the hydraulic regime under different conditions of geometry and flow pattern, density and vortex zone, mass fraction, and settling velocity of particles (Nguyen et al. 2019a).
The objectives of this study are to evaluate the efficiency of tube settlers with tubes of different cross-sectional shapes in use, for removing turbidity and to compare the efficiencies of conventional settlers, plate settlers, and tube settlers for removing turbidity of water within the sedimentation tanks of water treatment plants. In this study, a CFD model setup using ANSYS-CFX 17.2 solves the bulk momentum and continuity equations for the mixture of solids and fluid using the Algebraic Slip Model (ANSYS 2016). The flow field, SS concentration field, and suspended solids removal efficiency (R%) in conventional settlers, plate settlers, and tube settlers are calculated and compared.
MATERIALS AND METHODS
Computational model
The three-dimensional model of sedimentation tanks was set up using the commercial CFD software ANSYS CFX 17.2, which has been extensively used in modelling of complex flows (Tarpagkou & Pantokratoras 2014, Yu et al. 2016; Nguyen et al. 2019b). Flow in the sedimentation tank was defined as a liquid–solid multiphase dispersed flow where sediment particles (dispersed phase component) are mixed with water (continuous phase component) forming a continuous medium mixture with a varying composition and density. This mixture density is affected by the presence of solids in different locations.
Eulerian multi-phase mixture model in which all the phases are considered as continuous was used for the simulations because the overall motion of the particle is of interest rather than tracking individual particles. The mixture model used in ANSYS CFX is the ‘Algebraic Slip Model’ (ASM), which is a simplified version of Ishii (1975). The ASM adopts one fluid with a variable composition where continuous medium with dispersed phase components of different diameters and densities is used.









Model validation
The model was validated using data from the study on settling tanks of the water treatment plant of Acharnes and Athens in Greece by Stamou & Gkesouli (2015). Model validation was done for two scenarios S-1 and S-2 corresponding to inlet mass flow rates of 0.25 and 0.31 m3/s, respectively. The model performance was verified by comparing the streamline patterns, velocity contour plots of S-1, and suspended solid concentration and removal efficiency of S-2 with the observations presented by Stamou & Gkesouli (2015). The solid concentration was calculated using the volume fractions of solids.
Geometry
(a) Sixteen similar rectangular tanks in the Acharnes water treatment plant and (b) plan view of the rectangular tank (Stamou & Gkesouli 2015; Gkesouli 2018).
(a) Sixteen similar rectangular tanks in the Acharnes water treatment plant and (b) plan view of the rectangular tank (Stamou & Gkesouli 2015; Gkesouli 2018).
Computational grid
The main domain and outlet volume shown in Figure 2 were meshed using structured grids with hexahedral elements. The regions of complex geometries including inlet and change of geometry shown in Figure 2 were meshed using unstructured grids. The refined mesh was used at the inlet openings and outlet channels to derive detail flow properties in these regions, which are of more interest in the study. The computational grids consisted of approximately 219,300 elements. To ensure that the results are grid independent, several preliminary computations were carried out before choosing this element size: the grid independency study results are shown in Table S1 in Supplementary Information.
Boundary definition

Inlet flow characteristics of the scenarios used for model verification
Scenario . | Mass flow rate (m3/s) . | Inlet velocity (m/s) . |
---|---|---|
S-1 | 0.25 | 0.119 |
S-2 | 0.31 | 0.149 |
Scenario . | Mass flow rate (m3/s) . | Inlet velocity (m/s) . |
---|---|---|
S-1 | 0.25 | 0.119 |
S-2 | 0.31 | 0.149 |
Details of the inlet suspended solid classes (Stamou & Gkesouli 2015)
Solid class . | Solid diameter (μm) . | Settling velocity (mm/s) . | Scenario S-1 . | Scenario S-2 . | ||
---|---|---|---|---|---|---|
Inlet solid mass fraction MFin . | Inlet solid concentration Sin (mg/l) . | Inlet solid mass fraction MFin . | Inlet solid concentration Sin (mg/l) . | |||
C1 | 41 | 1.6 | 0.45 | 3.15 | 0.45 | 3.15 |
C2 | 17 | 0.27 | 0.17 | 1.19 | 0.36 | 2.52 |
C3 | 9.5 | 0.086 | 0.23 | 1.61 | 0.04 | 0.28 |
C4 | 5 | 0.025 | 0.15 | 1.05 | 0.15 | 1.05 |
Total | 1.00 | 7.00 | 1.000 | 7.00 |
Solid class . | Solid diameter (μm) . | Settling velocity (mm/s) . | Scenario S-1 . | Scenario S-2 . | ||
---|---|---|---|---|---|---|
Inlet solid mass fraction MFin . | Inlet solid concentration Sin (mg/l) . | Inlet solid mass fraction MFin . | Inlet solid concentration Sin (mg/l) . | |||
C1 | 41 | 1.6 | 0.45 | 3.15 | 0.45 | 3.15 |
C2 | 17 | 0.27 | 0.17 | 1.19 | 0.36 | 2.52 |
C3 | 9.5 | 0.086 | 0.23 | 1.61 | 0.04 | 0.28 |
C4 | 5 | 0.025 | 0.15 | 1.05 | 0.15 | 1.05 |
Total | 1.00 | 7.00 | 1.000 | 7.00 |
Model application






Longitudinal section of plate settler configuration of PS-1 (all dimensions are in millimetres).
Longitudinal section of plate settler configuration of PS-1 (all dimensions are in millimetres).
RESULTS AND DISCUSSION
Validation of the model
Streamline pattern in the xy plane at z = −1.9 m: (a) Stamou & Gkesouli (2015) and (b) computed in this study.
Streamline pattern in the xy plane at z = −1.9 m: (a) Stamou & Gkesouli (2015) and (b) computed in this study.
Velocity contour plot in the xz plane at y = 0.7 m: (a) Stamou & Gkesouli (2015) and (b) computed in this study.
Velocity contour plot in the xz plane at y = 0.7 m: (a) Stamou & Gkesouli (2015) and (b) computed in this study.
Comparison of solids removal efficiencies
Solid class . | Measured removal efficiency (Stamou & Gkesouli 2015) . | Removal efficiency (computed in this study) . |
---|---|---|
C1 | 99% | 100% |
C2 | 57% | 55% |
C3 | 25% | 21% |
C4 | 9% | 8% |
Solid class . | Measured removal efficiency (Stamou & Gkesouli 2015) . | Removal efficiency (computed in this study) . |
---|---|---|
C1 | 99% | 100% |
C2 | 57% | 55% |
C3 | 25% | 21% |
C4 | 9% | 8% |
Variation of SS concentrations at x = 63 m along the tank depth (y) for the scenario S-2.
Variation of SS concentrations at x = 63 m along the tank depth (y) for the scenario S-2.
Performance of plate settlers and tube settlers compared to that of conventional settler
Comparison of streamline patterns at the plane z = −1.9 m: (a) conventional settler, (b) PS-1, and (c) TS-1.
Comparison of streamline patterns at the plane z = −1.9 m: (a) conventional settler, (b) PS-1, and (c) TS-1.
SS removal efficiency of plate settlers compared with conventional settler.
SS removal efficiency of tube settlers compared with conventional settler.
Comparison of removal efficiencies of conventional settler and plate settlers.
Comparison of removal efficiencies of plate settler and tube settlers.
Under the studied case, there is no significant difference in the performance of the three different types of settlers for the largest particle size. However, there is an indication of better performance of the plate settlers and tube settlers in removing smaller solid particles. This performance of different types of settlers could be significantly affected by flow fluctuations in settling tanks. This study shows that the CFD model developed can be applied for such detailed comparative studies. No doubt that such studies demand high-end computers.
The CFD modelling can be used to compare design changes in plate and tube settlers to predict and optimize the SS removal efficiency. In process modifications of full-scale units, CFD analysis could be used to avoid possible costly design errors. Furthermore, CFD analysis can also be a useful tool in the design modifications where the capacity increase is required without compromising the SS removal efficiency in a region with land area constraints.
CONCLUSIONS
- (1)
Installing inclined plates and tubes in the conventional settling tank facilitated an increase in settling particles by reducing the recirculating zones and increasing the upward flow.
The solids removal efficiency of conventional settler increased by the installation of plates and tubes to attain low particle settling depth and high effective settling area.
The tube settler cross-sectional shape does not affect the removal efficiency as long as the tubes’ cross-sectional area, length, depth, and number of tubes are kept constant.
- (2)
Under the given tank and flow conditions in the studied case,
- (a)
the removal efficiency of plate settlers remained unchanged with the increase of the settling area beyond 60% of conventional settler settling area,
- (b)
the maximum attainable removal efficiencies of all solid classes were approximately equal in plate settler PS-3 and tube settlers, and they were approximately 100, 67, 28, and 9% for solids with diameters of 41, 17, 9.5, and 5 μm, respectively, and
- (c)
there is no significant difference in the performance of the three different types of settlers with respect to smaller diameter particles, although the removal efficiency was enhanced by plates and tubes. The smallest diameter particle group <5 μm still remained at almost 100% inside the tank, which limited the maximum attainable overall efficiency to 85%.
- (a)
ACKNOWLEDGEMENTS
The authors wish to extend their sincere gratitude to NORAD-NORHED Project ‘LKA-13/0013-WaSo-Asia Project’ for providing financial assistance for conducting this research study.
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.