Abstract

Countercurrent–cocurrent dissolved air flotation (CCDAF), the popular water purification device, which consists of collision and adhesion contact zones, showed favorable flotation conditions for micro-bubble adhesion and stability. In this study, computational fluid dynamics (CFD) numerical simulation was employed to confirm that the unique CCDAF configuration create reasonable and that the flow field characteristics were good no matter for single phase or gas–liquid two-phase conditions. In addition, the turbulence of the flow field was enhanced with the increasing influent load; the swirling was remarkably reduced with the increase of gas holdup. Meanwhile, a thick micro-bubble filter layer was formed in the separation zone, which favored bubble-flocs agglomerating and rising. The force analysis also showed that the cross section within the tank contribute to the uniformity of the bottom water collection as well as enlargement of the bottom outflow area, therefore improving the overall flotation performance. The simulation results revealed for the CCDAF process can provide technical guidance for engineering design and application.

INTRODUCTION

Dissolved air flotation (DAF), the solid/liquid or liquid/liquid separation process using micro-bubble as carrier, has gained much popularity at water treatment works. The contact zone, as the key counterpart of DAF, is reaction zone where the ‘capture’ of particles by micro-bubbles occurs, which is mainly responsible for the overall clarification efficiency (Yang et al. 2013; Rahman et al. 2014; Maia et al. 2016). In the traditional DAF process, the actual consumption of the pressurized dissolved air water was much higher than the theoretical value calculated according to the concentration of solid particles in water (Misug & Dongheui 2014; Wang et al. 2016b). Therefore, the traditional DAF technology has a larger space for optimization in the contact flocculant zone structure, flow characteristics, adhesion, etc. According to the direction of flow and contact between dissolved gas and raw water, the DAF process can be divided into the cocurrent flow DAF process and countercurrent flow DAF process (Figure 1(a) and 1(b)). In engineering, advection DAF refers to the cocurrent flow DAF process, with a single contact zone, micro-bubbles and raw water flow in the same direction. Bubbles have less contact with the flocs, leading to weak adhesion and low purification efficiency (Wang et al. 2016a). In recent years, to improve the adhesion efficiency of bubbles and particles, countercurrent flow DAF technology was developed that has reverse flow of bubbles with raw water, full bubble collision contact with floc, and microbubble layer playing the filtration role. However, there are some disadvantages, such as low resistance to shock load, low adhesion efficiency, effluent quality instability and deep pool body (Xiong et al. 2015). It is necessary to solve the problems, such as low adhesion efficiency on particles, trapping effect not being ideal, and unstable bubble-floc adhesion. Based on the adhesion mechanism of microbubble and floc particles, combined with the advantages of contact flow and reverse flow DAF process, the contact zone of the DAF process was improved, and the DAF process is developed (countercurrent–cocurrent dissolved air flotation, CCDAF), in which the reverse flow and contact flow are integrated into one (Figure 1(c)). The computational fluid dynamics (CFD) can characterize and analyze the flow field characteristics of the flotation. In this paper, the CFD flow field of the CCDAF process (Figure 1(c)) is simulated using Fluent software. The CCDAF process flow field characteristics were analyzed, and the flow field characteristics of the gas-liquid two-phase fluid float and the distribution of micro-bubble in air float pool were simulated. The feasibility of bubble adhesion was evaluated based on the hydraulics of the CCDAF process, and water load and size of the tank were analyzed. The measures of strengthening the flocculation of the flocs are put forward to evaluate and optimize the hydraulic conditions of the floatation process of the CCDAF process and to provide technical support for the popularization and application of the CCDAF process.

Figure 1

Sketch map of countercocurrent DAF process (a), cocurrent DAF process (b) and CCDAF process (c).

Figure 1

Sketch map of countercocurrent DAF process (a), cocurrent DAF process (b) and CCDAF process (c).

CONSTRUCTION OF THE CCDAF PROCESS AND ESTABLISHMENT OF THE CFD NUMERICAL SIMULATION METHOD

Construction of the CCDAF process

The adhesion process and mechanism of microbubbles and flocs in water primarily follow the mechanism of collision adhesion both for the same flow DAF process and CCDAF process (Zhang et al. 2016). The collision adhesion process is split into two subprocesses: collision and adhesion (Zhang et al. 2013). The CCDAF process is a new air float process that is combined with countercurrent collision and cocurrent flow adhesion (Figure 1(c)).

The CCDAF tank includes a countercurrent impact contact zone, a cocurrent flow adhesion contact zone, and a separation zone (Behin & Bahrami 2012; Wang et al. 2016a). Different from the traditional DAF process, the contact zone of CCDAF is divided into two levels, i.e. the collision contact zone and the adhesion contact zone. Therefore, the dissolved gaswater split (Torres et al. 2017). In the collision contact zone, micro-bubbles and raw water reverse flow complete full collision of the micro-bubbles with the suspended matter, flocculate with the flocs, and enter the adhesion contact zone, where the micro-bubble and raw water cocurrent flow and the contact part of micro-bubbles in collision contact zone enter the adhesion contact zone to increase the concentration of micro-bubbles in the adhesion contact zone. It is easy to complete the adhesion process of micro-bubbles and suspended substances in the adhesion contact zone and to form stable bubble-flocs and surface into the separation zone. Raw water flows through the collision contact zone and adhesion contact zone, which prolongs the collision time of the micro-bubbles and suspended matter, significantly improves the interaction of micro-bubbles with particles, strengthens the adhesion of micro-bubbles, and enhances the stability of bubble-flocs so that the micro-bubble-particle collision efficiency was significantly enhanced. The adaptability of changes in raw water quality is significantly enhanced (Albijanic et al. 2014).

The steps of the CCDAF process are as follows. After the micro-flocculation reaction of the raw water, the raw water enters the collision zone where the micro-bubbles and raw water reverse flow, and the full collision of the micro-bubbles and suspended matter is completed. Since the flocculation process has micro-flocculation, the flocculation continues, and micro-bubbles are involved in the agglomeration and co-copolymerization processes, which initiates the floc formation of bubble-flocculation, which is less than water flak, and their rise to the surface as scum. Flocs and suspended solids cannot be stable-floating under the impingement of water into the adhesion contact zone, where the micro-bubbles and raw water flow contact in the cocurrent way to complete the adhesion process to form a bubble-floc whose density is less than that of water in the flotation separation zone, where the scum was collected via the mechanical slagging system.

The CCDAF process can be based on changes in raw water quality through the choice of opening a different release device to achieve the flexible switch between the cocurrent flow flotation tank, reverse flow floating tank and CCDAF process flotation tank. The CCDAF process has wide application prospects due to adaptability to water quality changes, simple structure, high efficiency, ease of operation, etc. The CCDAF process facilitates the transformation of the traditional flocculation/floating tank and the construction of a new gas floating tank (Yoo & Hsieh 2010).

Establishment of the CFD numerical simulation method for the DAF float flow

Research method of the CFD numerical simulation

The fluent solution process includes nine steps, such as establishment of control equations and determination of the initial conditions, as shown in Figure 2 (Bitog et al. 2011; Hossain et al. 2011). For the two-phase flow numerical simulations, Fluent provides the Eulerian–Eulerian model and the Eulerian–Lagrangian model. The Mixture model in the Eulerian–Eulerian method is mainly used to simulate two-phase or multi-phase flow (Subramaniam 2013). This section mainly studies the characteristics of the flow field in the flotation; the core is the micro-bubble in the flow field, including the volume fraction of the micro-bubbles, the movement of the micro-bubbles and the pressure of the micro-bubbles. Therefore, the Mixture model was used to numerically analyze the flotation tank. The standard k-ɛ model was introduced into the Mixture model. The standard k-ɛ model was used for the gas and liquid phases, and then, the superposition was carried out. Finally, the standard k-ɛ two-equation model was obtained.

Figure 2

Flowchart of the solution obtained using the Fluent software.

Figure 2

Flowchart of the solution obtained using the Fluent software.

Meshing and setting boundary conditions

When the Fluent software is used for numerical simulation, first, a geometric model needs to be built according to the structure of the air flotation tank (Yang et al. 2013). The meshing of the flotation tank geometric model was divided using Gambit software, and the meshing is shown in Figure 3(a).

Figure 3

Physical models, meshes (a) and boundary setting (b) of the flotation tank.

Figure 3

Physical models, meshes (a) and boundary setting (b) of the flotation tank.

The two-dimensional model of the flotation tank has 14,263 faces, including 4903 nodes (Nodes).

The model boundary setting is shown in Figure 3(b). The flocculated raw water enters the flotation tank through inlet 1. The purified water is transferred back to the flotation tank through the releaser after being pressurized to form dissolved water, and the inlet of the dissolved gas is simplified as inlet 2 and inlet 3 of the flotation tank. The effluent is collected through the bottom of the collector pipe, which is set to outlet 1. The flotation tank is in contact with the atmosphere, and the outlet is set to outlet 2.

Test method

Test indicators were tested using the Water and Wastewater Detection and Analysis Method (Fourth Edition Supplement).

CFD NUMERICAL SIMULATION OF THE CCDAF FLOTATION TANK FLOW FIELD CHARACTERISTICS

Flow field characteristics of the flotation tank under different loads

The hydraulic conditions are very important for the flotation tank to achieve optimized adhesion of micro-bubbles and flocs followed by floating up and being removed (Moruzzi & Reali 2014; Wang et al. 2016c). For the established flotation tank, flow patterns has the greatest impact on the hydraulic characteristics and adhesion of micro-bubbles and flocs (Chen et al. 2015). First, the hydraulic characteristics of flotation tank and the feasibility of adhesion of micro-bubbles and flocs were studied on the CCDAF process under the condition of single-phase flow using the CFD numerical simulation of single-phase flow. Secondly, to analyze the size of the flotation tank, we studied the flow field characteristics using CFD numerical simulation under different influent flow rates, and the influent load was reasonably determined (Lakghomi et al. 2015).

To investigate the characteristics of the flotation tank flow field, the influent flow rates were set to 0.4 m3/h, 0.5 m3/h and 0.6 m3/h.

Water inlet 1 and water outlet 1 were used as velocity inlets. The specific parameters are shown in Table 1. The water inlet 2 boundary was used as a pressure inlet. The reflux ratio was 12%, and the relative pressure was 0.4 MPa. The water outlet 2 boundary was used as a pressure outlet, and the relative pressure was 0. Near walls were analyzed using the method of standard wall function.

Table 1

Calculation parameters of each working condition

Influent flow rate and parameters 0.4 (m3/h)
 
0.5 (m3/h)
 
0.6 (m3/h)
 
(m)  (m/s)  (%)  (m)  (m/s)  (%)  (m)  (m/s)  (%) 
Inlet 1 0.032 0.138 5.61 0.032 0.173 5.45 0.032 0.207 5.33 
Outlet 1 0.01 −0.044 7.48 0.01 −0.055 7.27 0.01 −0.066 7.11 
Influent flow rate and parameters 0.4 (m3/h)
 
0.5 (m3/h)
 
0.6 (m3/h)
 
(m)  (m/s)  (%)  (m)  (m/s)  (%)  (m)  (m/s)  (%) 
Inlet 1 0.032 0.138 5.61 0.032 0.173 5.45 0.032 0.207 5.33 
Outlet 1 0.01 −0.044 7.48 0.01 −0.055 7.27 0.01 −0.066 7.11 

Figure 4(a) shows that the numerical simulation results of the air flotation tank is good when the inlet water volume is between 0.4 m3/h and 0.6 m3/h. The water velocity in the collision area is larger, and the flow in the separation area is clearly reduced, the flow is smooth, and the distribution of water along the cross-section is more uniform. The air flotation tank created a good environment for bubble collision and adhesion with flocculant, as well as its upward movement and elimination. After the bubble-flocs’ impact contact and adhesion contact in the contact zone, the bubble-flocs are transferred into the separation zone. In the separation zone, the flow pattern is steady, which creates a hydraulic environment for the floc flotation. This reduces the floc fragmentation and desorption phenomenon. The oblique baffle between the adhesion zone and the separation zone has a favorable effect on the flow state of the flotation separation zone so that the dissolved water flows smoothly from the contact zone to the separation zone, which reduces the interference with the bubble-flocs and promotes the separation of solid and liquid during the flotation. In addition, the bottom of the perforated collector pipe is along the length of the bottom of the tank, which makes the flotation tank effluent water movement uniform and without suction, which is beneficial for separating flocs. With the increase of the influent load, the middle and upper parts of the separation zone have an obvious reflow phenomenon, and the turbulence degree is clearly enhanced. The disturbance of the dissolved water is caused by the recirculation of water in the separation area. The flow velocity and direction fluctuate, which causes bubble-floc desorption and influences the bubble-floc adhesion effect, affecting the quality of the water.

Figure 4

Nephrogram of the flow field velocity (a), vectorgraph of the flow field velocity (b) and orbit diagram of flow field particles (c) in the flotation tank at different influent flows.

Figure 4

Nephrogram of the flow field velocity (a), vectorgraph of the flow field velocity (b) and orbit diagram of flow field particles (c) in the flotation tank at different influent flows.

The extent of disorder can be seen from the vectorgraph of the flow field velocity (Figure 4(b)). The disorder was significantly worse due to the flow rate direction in the contact zone and in the separation zone with increasing amounts of water. This is more obvious in the separation zone. Because the influent load is quite low, the flow pattern and direction of the velocity field is steady in the separation zone, and the flow rate direction into the collector pipe is mainly from top to bottom without the mix phenomenon when the inflow speed is 0.4 m3/h. With the influent load increase, the velocity clearly increases. The extent of disorder in the contact and separation zones is significantly enhanced because water flows rapidly in the contact zone, which results in turbulence, whirls and vortices. The direction of the velocity vector waggles from left to right, when the inflow speed is up to 0.6 m3/h. Two big vortices appear in the central and bottom section of the separation zone, with is a clear contra rotation phenomenon that influences the effect of bubble-flocs floating up, which leads to the bubble-floc breakage and desorption. This is disadvantageous to flotation.

The orbit diagram of particles (Figure 4(c)) shows that water flows through the countercurrent collision zone, cocurrent flow adhesion contact zone and flotation separation zone. The outlet is at the bottom of the collector pipe. The water trajectory is obvious. With the increase of influent load, the particle trajectories are basically the same in the collision contact zone and the cocurrent flow adhesion contact zone. However, the trajectories of the particles in the separation zone are clearly changed. Under the condition of low influent load, the flow of the separation zone is smooth, and the particle trajectory is not obvious. With the increase of the influent load, the particle trajectory gradually appears as an ‘S’ streamline, which gradually increases with the water flow, aspect ratio of the tank and other factors.

From the flow hydraulic characteristics simulation of air flotation of three influent water flows (0.4 m³/h, 0.5 m³/h, and 0.6 m³/h), we can see that when the water flow is high, the separation zone reflux and swing is obvious, and the turbulence is large and easily leads to floc fragmentation and desorption, which affects floc flotation. When the flow rate is low, the surface load is low, the flow velocity of the separation zone is obviously reduced, the flow pattern is more stable, and the effect of water flow on the flocs is reduced. From the point of view of flow field characteristics, low influent load will help adhesion, flotation, and discharge of bubble-flocs. However, economically, the water load is reduced, and the amount of treated water is reduced (Oliveira & Rubio 2012).

From the CFD numerical simulation of the flow field, under the conditions of appropriate influent load, the water flow characteristics of the flotation tank can remain stable, the flow in the collision contact zone and adhesion contact zone are mixed uniformly, and the effect of collision and adhesion of bubble-flocs can be achieved. The flow velocity, vector direction and trajectory of the flow field are in accordance with the characteristics of the flow field of the separation zone, and hydraulic conditions of separation can be achieved. As we can see from the numerical simulation of the single-phase flow, the hydraulic conditions of the CCDAF process can make the micro-bubble and floc collision and adhesion processes occur one after another. The bottom of the air flotation separation zone is arranged with the perforated collector pipe along the length of the bottom so that the flotation tank effluent water movement is uniform, which is favorable for the separation of floc particles during the flotation process. From the aspects of the hydraulic characteristics of the flow field and the economical cost of the treatment load, the inlet water of 0.5 m³/h is suitable for the air flotation test device. The bubble-flocs’ adhesion and separation of the CCDAF process is viable, and the pool structure is feasible.

Flow field characteristics of the flotation tank under different gas holdups

The air flotation process is the bubble-floc adhesion, floc formation and removal process (Verrelli et al. 2011; Wang et al. 2017). The micro-bubbles in the flotation tank account for a certain amount of body volume. Because the difference between the bubble density and water density was large, the gas flow will have a certain impact on the flow field (Simonnet et al. 2008). Fluent software was used to simulate the flow field characteristics of the CCDAF process in the gas–liquid two phase flow environment, and the influence of micro-bubbles on the air floatation flow field was analyzed.

During the experiment, the air-precipitating amount was measured using a self-made device called indirect measurement device for liquid displacement. The measuring equipment is composed of valve, suction flask, cylinder, pipes, etc. The amount of gas released after decompression of dissolved gas water indirectly reflects the actual dissolved gas amount of dissolved gas system. As shown in Table 2, the gas holdup gradually increased with the increase of dissolved gas pressure.

Table 2

Corresponding table of the dissolved gas pressure and gas holdup

Dissolved gas pressure (MPa) 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 
Air precipitating amount (mL) 11 15 24 45 60 68 75 83 91 
Gas holdup (%) 0.1 0.3 0.6 1.1 1.5 2.4 4.5 6.8 7.5 8.3 9.1 
Dissolved gas pressure (MPa) 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 
Air precipitating amount (mL) 11 15 24 45 60 68 75 83 91 
Gas holdup (%) 0.1 0.3 0.6 1.1 1.5 2.4 4.5 6.8 7.5 8.3 9.1 

As seen from Table 2, with the increase of dissolved gas pressure, the gas holdup of water also increases. During the experiment, the dissolved gas pressure is maintained at 0.30–0.50 MPa. According to the relationship between the gas holdup and pressure, six different gas holdup conditions aimed at runback water were selected. The gas–liquid two-phase flow in the flotation tank was simulated numerically under six different gas holdup conditions. The inlet conditions are as follows: inlet 1 influent flow is 0.5 m3/h, inlet velocity is 0.173 m/s, reflux ratio is 12%; inlet 2 influent flow is 0.06 m3/h, and average bubble diameter is 40–50 μm. The gas holdups are 2%, 4%, 5%, 6%, 7% and 8%, which correspond to the dissolved-air pressures of 0.28 MPa, 0.34 MPa, 0.37 MPa, 0.40 MPa, 0.46 MPa and 0.53 MPa, respectively.

In Figure 5(a), the influent load is 0.5 m3/h, and the dissolved water return flow is 0.06 m3/h (reflux ratio of 12%), which is the same influent load. The gas holdup of dissolved air is different. Figure 5(a) shows that there is little change in the water flow in the flotation tank. However, with the increase of the gas holdup, there are slight changes in the flow velocity of the two regions. One of the changes is that the micro-bubbles enter the flotation separation zone from the adhesion contact zone, which is due to the sudden drop of the flow rate. The greater volatility is due to the widening of the flow path, especially to 8%. The second one was in the air flotation separation zone. With the increase of gas holdup, the number of micro-bubbles in water significantly increased, and the buoyancy of flocs and micro-bubbles gradually increased in the separation zone, resulting in the gradual decrease of the downward flow velocity. Meanwhile, the change in the water flow velocity in the separation zone is obvious due to the disturbance of the micro-bubbles. For the same flotation tank height, the water flow rate is quite different, especially when the gas holdup is 8%.

Figure 5

Nephrogram of flow field velocity (a), vectorgraph of flow field velocity (b) and orbit diagram of flow field particles (c) in the flotation tank at different gas content.

Figure 5

Nephrogram of flow field velocity (a), vectorgraph of flow field velocity (b) and orbit diagram of flow field particles (c) in the flotation tank at different gas content.

Compared with the change of air solubility and influent flow, the change in velocity of the flow field is relatively small. The flow field vector is approximately the same as the velocity field, and the direction of flow velocity does not significantly change. However, with the gas holdup increase, the upper part of the contact area has a larger turbulence phenomenon. In the flotation separation zone, due to micro-bubble disturbance, the direction of water flow increases the degree of confusion. At the same air float height, the direction results in a large change.

From the trajectories of particles (Figure 5(c)), the particle trajectory of the fluid mass changes because of different gas holdup. Due to the addition of micro-bubbles, the force of flocs and flow changes. Under the action of micro-bubble buoyancy, the liquid particles provide buoyancy enhancement, resulting in the increase of liquid mass resultant force Fc (Figure 7). Fc reduces the cyclotron phenomenon of liquid particles in water to a certain extent. This phenomenon in the low gas holdup is non-significant. However, at the gas rate of 8%, the liquid particle trajectory significantly changes, the cyclone is significantly weakened, and it is conducive to the floc floating.

It can be seen from Figure 5(a)5(c) that the flow field characteristics of the CCDAF process are stable under different gas holdup conditions. With the increase of gas holdup, the number of micro-bubbles increases. Micro-bubbles cause a slight perturbation of the velocity and flow direction at the inlet of contact zone and separation zone. However, the separation zone was significantly reduced due to the cyclonic phenomenon, which reduces interference on the formed bubble-floc. This is conducive for floc adhesion and floating. Therefore, in the gas–liquid two-phase flow field, the flow of the CCDAF process is more stable.

Distribution characters of micro-bubbles in the flotation tank

The distribution of the volume fraction of gas in the CCDAF flotation tank at different gas holdups was simulated using Fluent. During the test, the dissolved gas pressure was kept at 0.3–0.5 MPa, and six different gas holdups, which were 2%, 4%, 5%, 6%, 7% and 8%, were selected according to different conditions. The distribution of micro-bubbles in the flotation tank was numerically simulated.

The inlet conditions were as follows: for inlet 1, the influent flow was 0.5 m3/h, inlet velocity was 0.173 m/s, and reflux ratio was 12%; for inlet 2, the influent flow was 0.06 m3/h and average bubble diameter was 40–50 μm. The gas holdups were 2%, 4%, 5%, 6%, 7% and 8%, which corresponded to the dissolved-air pressures of 0.28 MPa, 0.34 MPa, 0.37 MPa, 0.40 MPa, 0.46 MPa and 0.53 MPa, respectively.

Figure 6 shows the distribution of micro-bubbles in the flotation tank at different gas holdups. It can be seen from the figure that there is a larger number of micro-bubbles in the contact zone. The Bubble concentration in the cocurrent flow contact zone is greater than that in the countercurrent collision zone. Micro-bubbles accumulate in the upper part of the contact area with the increase of gas holdup. The micro-bubble concentration is in the contact zone and the upper part of the contact zone. The bubble concentration in the separation zone is not very large. With the increase of gas holdup, micro-bubbles gradually diffuse into the separation zone. Because the micro-bubble density is low, micro-bubbles gather in the upper part of the separation zone. As the gas holdup increases, the upper bubble concentration increases, and micro-bubbles aggregate at the top of the separation zone. With the increase of gas holdup, the bubble concentration decreases. Micro-bubbles almost entirely fill the separation zone when the gas holdup is 7–8% (dissolved gas pressure of 0.40–0.50 MPa).

Figure 6

Nephrogram of micro-bubble distribution in flotation tank at different gas holdup.

Figure 6

Nephrogram of micro-bubble distribution in flotation tank at different gas holdup.

The American scholar Edzwald, based on the theoretical view of the trajectory theory, thought that the adhesion process of micro-bubbles and floc particles in the flotation contact zone was similar to the filter process, in which the migration and collision processes of micro-bubbles and particles were not only due to gravity sedimentation, Brownian motion and inertia but also due to the effect of interception and filtration (Edzwald 1995). From the actual observation of the test device (Figure S1, available with the online version of this paper), it can be seen that with the increase of dissolved gas pressure, the micro-bubble layer was clearly moved down, and the upper part of the separation zone gathers a thick layer of the bubble separation area, which was a layer of a thick filter layer that filters the suspended particles or particles after desorption and strengthens the adhesion of bubble flocs. However, in actual operation, the greater the number of micro-bubbles, the greater the dissolved gas pressure or reflux ratio and the higher the energy consumption. However, excess micro-bubbles through the bottom of the water system into the follow-up filter with water will cause the problem of air resistance and other issues. Therefore, from the abovementioned micro-bubble distribution and practical application of two angles, more micro-bubbles were conducive to the effect of flotation, but there was also increased energy consumption that affects the subsequent treatment and other issues. Thus, the appropriate micro-bubble concentration, which was appropriate for the dissolved gas pressure and the reflux ratio, is suitable (Alizadeh & Khamehchi 2016).

Figure 7

Force analysis of bubble-flocs in the separation zone. Vh-horizontal velocity, Vd-downward velocity, Vc-composition of forces.

Figure 7

Force analysis of bubble-flocs in the separation zone. Vh-horizontal velocity, Vd-downward velocity, Vc-composition of forces.

The distribution of micro-bubbles in the flotation tank shows that the collision contact zone and the adhesion contact zone are set in the CCDAF process flotation tank, where there was a large number of micro-bubbles. There were fewer bubbles in the cocurrent flow adhesion contact than in the countercurrent collision zone. The water flow and micro-bubble flow in the collision contact zone were countercurrently collided. The micro-bubbles and water flow were in the same direction to achieve the adhesion contact. In the air flotation separation zone, with the increase in micro-bubbles, the micro-bubble layer gradually moved downward, forming a thick adhesive filter layer, which effectively achieves the re-adhesion of flocs and has a filtering effect. The CFD numerical simulation was consistent with the actual test conditions (Figure S1)). From the micro-bubble distribution characteristics, the CCDAF process was more reasonable and achieved a more effective collision and better adhesion of micro-bubbles and flocs.

Bubble flocs force analysis and tank optimization

Fluent was used to simulate the flow field. The software more intuitively reflects the characteristics of the flow field in the flotation tank and flow field changes under different conditions. In the final analysis, the particle force is changed, and the particle force increases due to the increase of buoyancy of micro-bubbles, unlike the sedimentation tank (Johnson et al. 2009; Yan et al. 2014). In this section, we study the stress of the flocs in the separation zone and discuss the stress conditions in terms of bubble-flocs floating up, the tank structure, and the flow conditions of the flocs and provide the hydraulic conditions and the external environment.

Static force analysis of bubble-flocs

Bubble-flocs float in water and are influenced by gravity, buoyancy and resistance and other external forces in the water (Figure 7(a)) (Murai 2014). The flotation speed of bubble-flocs can be derived from Newton's second law:  
formula
(1)
 
formula
(2)
Vu – particle floating velocity (cm/s), Ff – buoyancy (N), Fr – resistance, Fg – gravity, ρp – particle density (g/cm3), ρw – water density (g/cm3).

Equation (2) shows that the floating velocity Vu of the bubble-flocs is dependent on the density difference between the water and the bubble-flocs, the bubble-floc diameter and the temperature and flow regime of water. If the proportion of bubbles in the floc is larger, then the density of the flocs is smaller, and the floc diameter increases accordingly, both of which increase the flotation rate. In fact, the size of bubble-flocs in water is different. During the air flotation process, the force is constantly changing because of the change of the role of external forces such as hydraulic and collision forces (Subasinghe & Albijanic 2014).

Analysis of the bubble-flocs force in the separated zone

The upward velocity Vu (Figure 7(b)) is obtained via the balance of buoyancy, gravity and resistance (Figure 7(a)). In the flotation separation zone, there are two forces: (1) the horizontal force, which is caused by the diffusion of water, the results in flow velocity Vh; and (2) the uniform flow causes the downward flow force along the bottom, which forms the flow rate Vd, the speed and direction of three flow rates, determining the bubble-flocs with the water outlet (Vc down, Figure 7(c)) or floating to be removed (Vc up, Figure 7(d)). The rising or falling speed parameters depend on the size of the projection on the vertical axis.

Using the abovementioned stress analysis, the following conclusions are obtained: (1) To make the floating effect good, we must try to reduce Vu. This can be achieved by expanding the area of bottom efflux or by improving the uniformity of water. (2) At the end of the pool, Vh ≈ 0, which creates good conditions for the separation of small buoyancy. (3) To achieve the flotation early, Vh should be reduced. This can be achieved by expanding the cross-section of the flotation.

In view of the above two measures to strengthen the air flotation, the perforated collector pipe was set at the bottom of the flotation separation zone and along the length of the bottom of the layout, making the flotation tank effluent water uniformly harder to suction, which is beneficial for separating the flocs. In addition, the cross-section of the flotation tank is set to B = 10 cm and H = 1.45 m. It can be seen from the CFD flow velocity nephrogram that Vh has been smaller at the end of the flotation tank separation zone, which was favorable for the separation of bubble-flocs.

COMPARE THE PERFORMANCE OF THE CCDAF AND CONCURRENT AND REVERSE FLOW DAF

To compare the pollution removal efficiency of CCDAF and concurrent and reverse flow DAF, the CCDAF composite device should be used, and the reflux ratio should be adjusted. When R1/R2 is 1/2, the CCDAF process should operate. The cocurrent flow DAF process operates when R1 is off, and the countercurrent flow DAF process operates when R2 is off.

As seen from Figure 8, the CCDAF process has better pollution removal efficiency compared with the cocurrent flow and countercurrent flow DAF process. In terms of turbidity removal, the CCDAF process increased turbidity removal by 8.8%, as opposed to the cocurrent flow and countercurrent flow DAF processes, which removed turbidity by 5.0%. The law of removing organic matter was basically the same as the removal of turbidity. CCDAF had the best effect, cocurrent was the second best, and countercurrent was the worst. For CODMn, UV254, and dissolved organic carbon (DOC), the CCDAF was enhanced by 4.1%, 5.0% and 0.58%, respectively, with the cocurrent flow DAF and by 6.4%, 8.1% and 2.58%, respectively, with the countercurrent flow DAF.

Figure 8

Three DAF processes to remove the turbidity, CODMn, UV254, and DOC contrast.

Figure 8

Three DAF processes to remove the turbidity, CODMn, UV254, and DOC contrast.

CONCLUSION

  • (1)

    The CCDAF process, which is integrated with cocurrent flow and countercurrent flow, was constructed. The flotation contact zone was divided into the countercurrent collision zone and cocurrent flow adhesion contact zone, with dual dosing of dissolved water. The single-phase flow CFD simulation shows that the structure of the CCDAF flotation tank is reasonable, the collision contact zone and the adhesion contact zone are mixed uniformly, the separation zone flows smoothly, and water is in good condition. With the increase of the influent load, the level of turbulence significantly increased.

  • (2)

    The gas–liquid phase flow CFD simulation shows that the characteristics of the CCDAF process flow field are stable at different gas holdups, the gas holdup increases, and the flow rate of the adhesion contact zone has a slight fluctuation. However, as the number of micro-bubbles increases, the phenomenon of swirling was clearly weakened, which is conducive to the adhesion and buoyancy of the bubble-flocs. As the gas holdup increases, the bubble layer gradually moves down, forming the thick micro-bubble filter layer, which effectively realizes the reattachment of the floccules and filtering. The gas phase increases the characteristics of flow field hydraulics, and the flow field is more stable.

  • (3)

    The analysis of the force of bubble-flocs shows the increase of uniformity of water at the bottom, which enlarges the area at the bottom. The cross section of the tank is helpful for promoting the floating effect of the bubble-flocs.

  • (4)

    The CCDAF process has a better removal effect than the cocurrent flow and countercurrent flow DAF process. The CCDAF process has a better removal rate of turbidity, which was increased by 5.0% and 8.8% for cocurrent and countercurrent flow, respectively. For the organic index, CODMn, UV254, and DOC were increased by 4.1%, 5.0% and 0.58%, respectively, compared with the cocurrent flow DAF, which increased by 6.4%, 8.1% and 2.58%, respectively, compared with the countercurrent flow DAF.

ACKNOWLEDGEMENTS

This work was financially supported by the Natural Science Foundation of the Shandong Province (ZR2016EEM32), Doctoral Fund of Shandong Jianzhu University in 2015 (XNBS1511), Science and Technology Plan of Ministry of Housing and Urban-Rural and opening project of Development Beijing Advanced Innovation Center for Future Urban Design.

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Supplementary data