Research on the CFD numerical simulation and process optimization of countercurrent–cocurrent dissolved air flotation

In the field of water treatment, dissolved air flotation (DAF) has been widely used in the treatment of high algae micro-polluted water and low-temperature and low-turbidity water (Hu 2013; Zhu et al. 2014). The air-floating contact zone is the main place where microbubbles and flocculent particles are mixed, collide, and adhere (Kisner Anderson et al. 2016). The operating effect of the air-floating contact zone directly influences the water purification capacity of the DAF process (Lundh et al. 2002; Wang et al. 2011; Yang et al. 2013). The traditional advection DAF process (Figure 1(a)) is essentially a cocurrent flow DAF process. The microbubbles flow in the same direction as the raw water, and the bubbles and flocs have less chance of contact, and the air bubble adhesion effect is not ideal (Albijanic et al. 2014). To improve the efficiency of bubble-particle adhesion, a countercurrent DAF process has been developed in recent years (Figure 1(b)). During operation, the microbubbles and the raw water flow in the opposite direction, and the bubbles and the flocs fully collide with each other. However, there are several disadvantages of countercurrent air flotation, such as: low impact load, low efficiency of adhesion of foam flocs, unstable water, and deeper tanks (Guo et al. 2002, 2003). To solve the problems of low particle adhesion efficiency, unsatisfactory trapping effect, and unstable adhesion of foam flocs to a single DAF process, countercurrent–cocurrent dissolved air flotation (CCDAF; Figure 1(c)) was developed (Wang et al. 2016a, 2016b). This process combines the advantages of cocurrent and countercurrent flow DAF processes. In the CCDAF process, the raw water flows through the collision contact zone and the adhesion contact zone sequentially, which strengthens the adhesion of the microbubbles and enhances the stability of the air-bubble flocs.

The computational fluid dynamics (CFD) method is used to simulate the CFD flow field in the air-flotation tank of the CCDAF process, and the characteristics of the flow field are analyzed to lay a theoretical foundation for the popularization and application of the CCDAF process (Wang et al. 2016a, 2016b). The key to the CCDAF tank for countercurrent collision and cocurrent flow adhesion lies in its structure, geometry, and process parameters. At present, the design of the air-floating tank relies on the design manual calculation, and the value is within the parameters given by the specification (Lundh et al. 2000a, 2000b). There is no exact method for taking values of parameters, which limits the development of the process (Lundh et al. 2002). Based on the previous research, this paper continues to use CFD numerical simulation technology to simulate the flow field of the structure and operation parameters of a CCDAF floating tank. The post-processing patterns such as the gas phase flow pattern and the nephrogram of gas phase volume distribution are used to visually compare the flow patterns in the tank, optimize the geometric dimensions, and process parameters of CCDAF from the perspective of hydraulics, and provide technical support for the popularization and application of CCDAF processes in engineering.

The CCDAF process is a new type of air floatation process that incorporates countercurrent collision and co-current adhesion processes. The air flotation tank of the CCDAF process includes a countercurrent collision contact zone, a cocurrent adhering contact zone, and an air flotation separation zone. The difference between the CCDAF and the traditional DAF process is that the air-floating contact zone is divided into two stages, namely, a collision contact zone and an adhesion contact zone. Gas-dissolved water is added twice. In the collision contact zone, the microbubbles and raw water countercurrently flow, achieving full collision of the microbubbles with the suspended matter, which become larger with flocculation. The air-bubble flocs then enter the adhesive contact zone as the effluent of the collision contact zone. In the adhesive contact zone, the microbubbles flow in the same direction as the raw water and come into contact. Some of the microbubbles in the collision contact zone enter the adherent contact zone with the water flow. This process increases the concentration of the microbubbles in the adhesion contact zone and also enhances the adhesion of the microbubbles to the suspended matter. Finally, air-bubble flocs are formed and float to the separation zone.

The air flotation tank diagram is shown in Figure 2(a). To facilitate the CFD numerical simulation of the CCDAF process, the CCDAF tank boundary conditions and tank body are simplified. The simplified model is shown in Figure 2(b). The raw water enters contact zone 1 from inlet 1 and the gas-dissolved water flows back to the contact zone through inlet 2 and inlet 3. The recycle ratios of contact zone 1 and contact zone 2 are R₁ and R₂, respectively.

According to the preliminary parameter optimization test, the total recycle ratio (R₁ + R₂) is 10%, and design flow rate is 0.5 m³/h. According to the design manual recommendation, we calculated all the models of 16 size types with the contact zone rising velocity of 10–25 mm/s and the separation zone separation velocity of 1.5–2.5 mm/s. Further screening was carried out according to the design specifications, and the models' contact times less than 60 s were excluded, and finally eight models meeting the specification requirements were obtained. The model parameters are shown in Table 1.

Before using Fluent 14.5 for calculations, we needed to use pre-processing software to build the grid (Kaminsky et al. 2005). The study used ICEM CFD 14.5 for pre-processing and a two-dimensional bilinear over-limit difference method to create a structured grid (Pashchenko 2018). The established mesh model is shown in Figure 3. The number of grids is the same as in Table 1.

All of the above grid determinant quality detection values are 1; Angle quality detection values are all 90, indicating that all grids have an angle of 90° and the grid quality is excellent.

This test does not involve heat transfer and assumes that liquid water is not compressible. Therefore, Fluent only needs to solve the continuity equation, the energy equation, and the momentum equation.

This equation applies to both compressible and incompressible flows. S_m is the source item.

In Equation (3), k_eff is the effective heat transfer coefficient, J_j is the diffusion flow of component j, S_h is the heat of reaction. Other parameters have the same meaning as above.

Fluent 14.5 (Ansys Inc., USA) is the most widely used CFD software (Ye et al. 2006). Fluent offers three multiphase flow models, the Volume of Fluid model, the Mixture model, and the Eulerian model (Kaminsky et al. 2005; Hu 2010; Yu et al. 2011). The mixture model is a simplified multiphase flow model. It is mainly used to simulate two-phase or multi-phase flow. The multiphase flow can be either a fluid or a particle (Han et al. 2011). This experiment mainly studies the characteristics of the flow field in the air flotation tank. The core of the experiment is the status of microbubbles in the flow field, including the movement of microbubbles and the distribution of microbubbles (Chao et al. 2011; Wei & Song 2015; Alizadeh & Khamehchi 2016). The flow in the air flotation tank is turbulent, so the turbulence model adopts the k-ɛ viscosity model.

The model boundary settings are shown in Figure 4. The water inlet to be treated is set to IN1, the two reflux dissolved water inlets are set to IN2 and IN3, the treated water outlet is set to OUT1, the two tank top free surfaces are set to OUT2, OUT3, and finally, the tank wall is set to WALL. According to the preliminary parameter optimization test, the inflow flow rate is 0.5 m³/h, the recycle ratio is 10%, the inlet pipe diameter is 32 mm, the outlet pipe diameter is 50 mm, and the reflux dissolved water inlet pipe diameter is 20 mm. Except for the top free surfaces OUT2 and OUT3, which are defined as pressure outlets, the others are velocity inlets. According to the air/water flow ratio measured by the experiment, the reflux inlet bubble volume fraction was set to 0.0479. Average bubble diameter is 40–50 μm. The WALL uses no sliding wall conditions. The free surface at the top is the water surface, and the simulation is set to the symmetry boundary condition, ensuring that there is no vertical velocity component at the surface and the wall friction is zero. In addition, the UDF (user define function) is applied to the adjacent grid on the surface to increase the quality source term and describe the bubble overflow process. Calculations of the parameters of each velocity entry boundary condition according to the above conditions are shown in Table 2.

The pathlines of gas-phase velocity (Figure 5) show the flow path of gas in gas-dissolved water. In models (a), (b), and (h), the microbubble particle passes through the countercurrent collision zone, cocurrent flow contact zone, and air flotation separation zone in sequence and that trajectory of the model particles is basically the same. Most of the dissolved water entering from inlet 2 enters contact zone 1, the trajectory of the gas-liquid flow point is obvious, and the reverse impact contact effect between bubble and floc is significant. The overall flow state in the air flotation tank is good, and the velocity of the water flow in the contact area is large, while the flow velocity of water in the separation area is significantly reduced and the flow state is stable, which can reduce the fragmentation and desorption of the air-bubble flocs. A favorable environment is formed in these models to facilitate the collision and adhesion of microbubble and flocs, and it is also conducive to the removal of air-bubble flocs. In the models (c), (d), (e), (f), and (g), the dissolved gas water entering from water inlet 2 is mostly influenced by the water flow and enters contact zone 2, and the amount of gas entering contact zone 1 is relatively small, which results in a non-obvious gas-phase flow in contact zone 1 and a relatively weak contact effect between the microbubbles and flocs. In addition, the flow velocity in the separation zone did not decrease significantly, which was not conducive to the floc floating separation.

In the eight models, the flow velocity of the gas-liquid particles at the right baffle of contact zone 2 was fast. After entering the separation zone, the gas is affected by the flow of water in the descending process and there are different degrees of reflux phenomena, and both return to the inlet. The turbulence, backspin, and vortex phenomena of models (e), (f), and (h) are significant and can easily disturb the formed air-bubble flocs and cause them to break as well as affect their adhesion stability, which is detrimental to the air flotation effect. The stability of the flow field in the separation zone is conducive to stable adhesion of the air-bubble flocs, and the desorption phenomenon is less likely to occur. The trajectories of the gas-phase flow lines in models (a), (b), and (c) are smooth and difficult to generate perturbations for, which is beneficial to the floating and removal of the air-bubble flocs.

Based on the above analysis of the characteristics of the gas flow streamlines in the contact zone and the separation zone, we observe that: models (a) and (b) can realize the reverse collision of foam floccules and co-directional adhesion and stable adhesion of the separation zone, which is a more appropriate process size model.

The contour of bubble concentration represents the volume occupied by the gas phase at different positions in the air-flotation tank, and can more directly to indicate the gas-phase volume of fraction at different positions. The contour of bubble concentration of each model is shown in Figure 6.

From Figure 6, it is observed that contact zone 1, contact zone 2, and the separation zone of the CCDAF flotation tank are covered with microbubbles. The number of microbubbles in the contact zone is large, and the bubble concentration in contact zone 2 is higher than that in contact zone 1. The microbubbles gather in the upper part of the separation zone. The degree of disturbance and distribution of microbubbles in each area are influenced by the geometric size of the flotation tank and the flow velocity, and there are large differences.

The gas-phase distributions of contact zone 1 and contact zone 2 in the air-flotation models (a) and (h) are relatively uniform, the flow state is stable, and there are no adverse turbulences, such as an obvious backflow. In models (b), (c), (d), (e), (f), and (g), the gas flow distributions in contact zone 1 and contact zone 2 are not uniform and the disturbance is significant. In addition, the gas phase volume in contact zone 1 in the models (c), (d), (e), (f), and (g) is relatively low, which is not conducive to the collision of microbubbles and flocs. In the separation zone, the distribution of microbubbles in models (a), (c), and (e) is relatively uniform, which facilitates the flotation of the air-bubble flocs in a stable flow state and removal. The air bubbles in the upper part of the separation zone have a high concentration, and a thick layer of microbubbles is gathered. The microbubbles gradually decrease in concentration from the top to bottom and gradually decrease along the depth of the tank. The microbubble layer has a good filtering function. Models (b), (d), (f), (g), and (h) all have different degrees of non-uniform gas-phase distributions, and there is turbulence or a vortex at the inlet of the separation zone, which creates disturbances and easily causes the formed air-bubble flocs to break.

According to the contour of bubble concentration, the model (a) gas distribution in the contact zone is uniform, the flow state is stable, the gas distribution in the separation zone is stepped and evenly distributed, there is no obvious backflow, and the gas-phase distribution is reasonable. Therefore, model (a) is the recommended model for the CCDAF tank design.

Through a comprehensive analysis of the pathlines of gas-phase velocity of each model and the contour of bubble concentration, model (a) is determined to be the best model. The size parameters are: B × L × H = 102 mm × 1,300 mm × 1,350 mm. The ascending flow rate in the contact zone is 10 mm/s, and the separation zone separation velocity is 1.5 mm/s.

The dissolved air pressure and gas-dissolved water recirculation ratio are two important parameters in the operation process of a CCDAF air flotation tank, and the magnitude of its value directly affects the operation effect of the process. The dissolved gas pressure directly affects the gas holdup of the dissolved water, thereby affecting the size and distribution of the microbubbles in the flotation tank. During the experiment, the dissolved gas pressure is maintained at 0.30–0.50 MPa. According to the magnitude of the outgas volume of the dissolved gas device under different pressure conditions, the corresponding gas holdup is shown in Table 3. Taking model (a) as the simulation object, five different gas holdup conditions were selected for simulation. Boundary condition settings are: inlet 1 influent flow is 0.5 m³/h, inlet velocity is 173 mm/s, recycle ratio is 10%; inlet 2 and inlet 3 inlet velocity are 22 mm/s, and average bubble diameter is 40–50 μm. The gas holdups are 2.4%, 4.5%, 6%, 6.8%, and 7.5%, which correspond to the dissolved-air pressures of 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, and 0.45 MPa, respectively. The distribution of microbubbles in the flotation tank at different pressures of dissolved gas are shown in Figure 7.

Figure 7 shows that when the pressure is less than 0.45 MPa, most of the microbubbles in inlets 2 and 3 flow to contact zone 2 under the action of the water flow. As a result, the distribution of the microbubbles in contact zone 1 and contact zone 2 is not uniform. Especially, in contact zone 2, the disturbance of the gas flow is large, resulting in poor contact between the microbubbles and flocs. At the same time, the distribution of the microbubble layer in the air flotation separation zone is also very uneven, which is not conducive to the stable flotation of the air-bubble flocs. When the pressure reaches 0.45 MPa and above, the gas-dissolved water at water inlet 2 rises into contact zone 1 along the wall of the inlet side under pressure and contact zone 1 is filled with a uniform microbubble layer, achieving a collision of bubbles and flocs. In addition, the gas-phase distribution in the air-floating contact zone and separation zone is relatively uniform, which is conducive to the collision of microbubbles as well as the stable flotation of the air-bubble flocs. Therefore, the CCDAF flotation tank is recommended to have a dissolved air pressure of 0.45 MPa.

Under the condition of a dissolved gas pressure of 0.45 MPa, the dissolved water distribution ratio of model (a) was optimized to simulate the gas-phase flow regime in the CCDAF tank under different distribution ratios. The gas phase volume distribution cloud diagrams at different distribution ratios are shown in Figure 8.

It is observed from Figure 8 that when the distribution ratio R₁:R₂ is small, most of the gas-dissolved water entering from water inlet 2 flows horizontally into contact zone 2 under the action of the water flow. The number of microbubbles in contact zone 2 is significantly more than that in contact zone 1. However, the distribution of microbubbles in contact zone 2 is not uniform. Due to the small number of microbubbles in contact zone 1, the reverse impact contact effect is weak. As observed from Figure 8, when R₁:R₂ is 1:5, two vortex areas appear at the inlet end of the separation zone. This phenomenon has a destructive effect on the formed air-bubble flocs. When R₁:R₂ reaches 1:1, the concentration of microbubbles in contact zone 1 and contact zone 2 is basically the same and the distribution is even. The gas-dissolved water entering through water inlet 2 rises near the inlet side wall. Under the action of the water flow, the water to be treated is thoroughly mixed and contacted and then flows down together into contact zone 2. When R₁:R₂ is 2:1 or 3:1, the concentration of microbubbles in contact zone 1 is greater than that in contact zone 2. There is a certain disturbance in the flow state in the separation zone, which increases with the increase of the distribution ratio. At the same time, due to the accumulation of a large number of microbubbles in contact zone 1, the gas phase distribution volume in the separation zone gradually decreases. When R₁:R₂ reaches 4:1 or more, the flow state in the air-floating contact zone 1 and contact zone 2 is complicated and the turbulent flow is intensified, which is not favorable for the adhesion of air-bubble flocs. In summary, it is more appropriate to control the dissolved water distribution ratio to be approximately 1:1.

According to the CFD simulation and optimization results, a 5 m³/h CCDAF pilot plant was established. We performed similar CFD numerical simulation and parameter optimization experiments on a cocurrent DAF tank and countercocurrent DAF tank under the same conditions. The respective optimal model dimensions and operating parameters were obtained, and the corresponding pilot plant was established. Since the flow rate of the pilot plant is 5 m³/h, it is ten times the model design flow. Thus, we keep the length and height unchanged, and expand the width by ten times. Equipment parameters are shown in Table 4. The dissolved gas pressure, recycle ratio, and R1:R2 are consistent with the simulation results, ensuring that rising velocity of the contact zone and separation velocity of the separation zone does not change. The bubble size depends on the dissolved gas pressure and does not change with dimensional changes.

Three pilot plants were stably operated for 20 days under the same coagulation conditions, and the effluent from each flotation tank was taken every day for indicator detection. The 20-day average removal rate of pollutants from different processes is shown in Figure 9.

As observed from Figure 9, the CCDAF process has a better decontamination efficiency than the cocurrent and countercurrent DAF processes. In terms of turbidity removal, the removal rate of the CCDAF process is 4.53% and 5.56% higher than those of the cocurrent and countercurrent DAF processes, respectively. The removal rate of organic matter is basically the same as the removal of turbidity; CCDAF has the best effect, followed by the cocurrent DAF, and the countercurrent DAF is the worst. Compared with the cocurrent DAF processes, the removal effects of CCDAF on COD_Mn, TOC, and UV₂₅₄ were improved by 3.13%%, 2.84%, and 3.49%, respectively. Compared to countercurrent DAF processes, the removal effects of CCDAF on COD_Mn, TOC, and UV₂₅₄ increased by 5.67%, 16.34%, and 5.52%, respectively. The excellent decontamination performance of CCDAF benefits from the interaction of the countercurrent collision contact zone and the cocurrent adhesion contact zone. This improvement increases the contact and adhesion efficiency of the microbubbles with the flocs, and forms air-bubble flocs which are easier to separate, so that the effluent water quality is better than the cocurrent DAF tank and countercocurrent DAF tank. We recommend the use of CCDAF when conditions permit.

Simulation of the flow field and parameter optimization of eight CCDAF tank models were performed. The results show that the water flow in the CCDAF tank is good overall, that the flow state is stable, and that it can realize reverse collision of air-bubble flocs and adhere in the same direction. At a flow rate of 0.5 m³/h, the optimal tank configuration parameters for the CCDAF tank are as follows: B × L × H = 102 mm × 1,300 mm × 1,350 mm; the ascending flow rate in the contact zone is 10 mm/s; the separation velocity of the separation zone is 1.5 mm/s. When the dissolved gas pressure is 0.45 MPa and the R₁:R₂ ratio is approximately 1:1, the gas in the contact zone is evenly distributed. The gas distribution in the separation zone is evenly distributed in a step-like manner. There is no obvious backflow, and the gas phase distribution is reasonable.

Using the simulation and optimization results of three air flotation processes, a 5 m³/h pilot test device was established. The structural dimensions were: B × L × H = 1,020 mm × 1,300 mm × 1,350 mm. The test results show that the CCDAF has a significant pollution removal effect and is obviously superior to the cocurrent DAF processes and countercurrent DAF processes. Compared with the cocurrent DAF processes, the removal effects of CCDAF on turbidity, COD_Mn, TOC, and UV₂₅₄ were improved by 4.53%, 3.13%, 2.84%, and 3.49%, respectively. Compared with countercurrent DAF processes, the removal effects of CCDAF on turbidity, COD_Mn, TOC, and UV₂₅₄ increased by 5.56%, 5.67%, 16.34%, and 5.52%, respectively.

This work was financially supported by the Natural Science Foundation of Shandong Province (ZR2016EEM32), Science and Technology Plans of Ministry of Housing and Urban-Rural Development of the People's Republic of China, and Opening Projects of Beijing Advanced Innovation Center for Future Urban Design, Beijing University of Civil Engineering and Architecture (Research and application of key technologies for dissolved air flotation based on water purification of algae-contaminated lakes, UDC2017031612), and the Doctoral Fund of Shandong Jianzhu University in 2015 (XNBS1511).