Feasibility study of high-rate dissolved air flotation process for rapid wastewater treatment

The key technology of high-rate dissolved air flotation (DAF) is achieved by rapid separation of bubble-floc aggregates. According to the specific gravity of the floc in the separation zone, bubble-floc aggregates have a slower rise velocity than microbubbles. Considering the rise velocity of the aggregates, the downward flow rate in the separation zone of the flotation tank dominates the overall efficiency of the DAF process (Edzwald et al. 1999). When designing the high rate DAF, much attention is required because the hydraulic loading rate is faster than the rise velocity of the bubbles. As such, it is highly possible that the treated water is discharged before bubbles and flocs are attached or before the aggregates rise to the water surface. The ideal direction pattern that is maintained in the internal portion of the tank has been suggested as the point that prevents this phenomenon.

The flotation tank consists of a contact zone and separation zone, and the excellence of DAF performance is apparent when the three features of the ideal flow pattern in the separation zone are formed. The three features of the ideal flow pattern include the stratified flow of the surface, the vertical plug flow below the stratified flow, and the horizontal plug flow at the bottom floor. However, this ideal theory for the conventional DAF process with hydraulic load rate (5–15 m/h) does not work in the high-rate DAF process (30 m/h) because the hydraulic load (hydraulic loadings) rate exceeds the rise velocity (Edzwald 2010). The high-speed system can remove free bubbles and the bubble-floc aggregates in the stratified flow area of the separation zone. In particular, research on the hydraulic flow characteristics and stratified flow that occur in the separation zone is required in order to merge the flow pattern in the model. The flow pattern through the separation zone does not follow the simple ideal case of plug flow in the vertical direction. It is influenced by the velocity of water above the baffle (cross-flow velocity), the hydraulic loading, the aspect ratio (length to width ratio), how water is withdrawn at the outlet, and the air bubble suspension in the upper part of the separation zone (Lundh et al. 2002).

The basic research carried out over the last 30 years has laid the groundwork for the development of high-rate DAF technology (Wang et al. 2005). According to previous studies (Haarhoff & Edzwald 2001; Rubio et al. 2007; Oliveira & Rubio 2012), the major trend is to decrease flocculation times, increase DAF loadings, and develop compact (small ‘footprint’ areas) and efficient treatment units. Small aggregates can be easily removed at high DAF (modern design) rates, contradicting the conventional bias that large floc units and bubbles are required for successful separation. In other words, the effective separation of the combined bubble-floc aggregates with small bubbles is determined by the flow rate in the separation zone of the DAF process. While larger separation zones lead to higher separation efficiency of the bubble-floc aggregates, they also create an inefficient situation by increasing the process size. Therefore, technology for the maximum separation of the bubble-floc aggregates through high separation speed in the separation zone while maintaining a minimal physical size is important. Lakghomi et al. (2012) reported that increasing the amount of air improved the removal efficiency and generated a useful stratified horizontal flow pattern.

Because many previous studies report a threshold for high-rate DAF with a hydraulic load rate over 0.4167 m/min (or 25 m/h), modeling and experimental studies are required for water treatment through various methods of flotation separation and measuring the rise velocity of the bubble-floc aggregates for the proper operation and design of high-rate DAF. Thus, the purpose of this study is to understand the characteristics of operating and designing parameters for the DAF process and to evaluate the scope and applicability of the proposed threshold for high-rate DAF with a hydraulic load rate of 0.5 m/min (or 30 m/h) for treating combined sewer overflows (CSOs). In order to successfully design and operate high-rate DAF, standard values of major parameters were determined through model simulation, and laboratory tests were performed to verify the estimated model values in this study. Considering the hydraulic situation in the separation zone and the operational analysis of the bubble-floc aggregates, a basic level of high-rate DAF with a hydraulic loading rate of 30 m/h for design has been determined by simulating the model introducing the population balance concept. The model simulation can describe the formation process of bubble-floc aggregates in the contact zone and calculate the transfer rate of the formed aggregates that transfer into the separation zone. Focusing on the change of the bubble volume fraction in the separation zone, depending upon the hydraulic loading rate, the model simulation was conducted using the population balance turbulence (PBT) model. Laboratory tests were conducted to measure the rise velocity of the bubbles and the bubble-floc aggregates as a function of the run-time change and the depth change of the milky water layer.

Many aggregates formed in the contact zone rise to the top layer. However, a portion of them is moved into the separation zone and rises into the top layer to be separated if the rise velocity in the laminar state is greater than the hydraulic loading rate. Design and performance of the footprint area for removal of free bubbles and of the bubble-floc aggregates applying the Hazen theory of the sedimentation tank have been considered the idealized DAF tank. The vertical plug flow is assumed to occur over the main part of the separation zone (i.e., the clarification section). The horizontal flow below the bubble-floc aggregate layer and above the baffle for separating the contact zone and the separation zone is ignored when evaluating the clarification performance. The free bubbles and the bubble-floc aggregates are removed if the rise velocities, V_b and V_fb, exceed the hydraulic loading rate (V_sz−hl) in the separation zone. As mentioned above, the standard value of rise velocity of the bubbles and the bubble-floc aggregates is approximately 20 m/h. Therefore, these bubbles and the aggregates are removed when the hydraulic loading rate is less than 20 m/h in the separation zone. Edzwald (2010) summarizes that the hydraulic loading rate is conservative enough in order to evaluate the performance of the conventional DAF system. The typical hydraulic loading rate of the conventional DAF system is 5–15 m/h. When considering the footprint area (A_sz) of only the separation zone and having a recycle rate of 10%, the hydraulic loading rate in the separation zone is still 20 m/h or less. Thus, even though the plug flow does not occur, the hydraulic loading rate is used to design the separation zone of the conventional DAF system.

For theoretical experiments, the PBT model written by MATLAB software was used for simulation of size distribution, rising velocity, time, and removal efficiency of bubbles, flocs, and floc-bubble aggregates. The simulation was performed using the following information: floc sizes between 0.1 and 210 μm, bubble size of 100 μm, initial density of flocs 1,100 kg/m³ and 500 kg/m³ (Santana et al. 2012), and hydraulic loading rate V_sz−hl of 0.5 m/min (or 30 m/h) in order to examine the change in the rise velocity of one aggregate formed by one bubble attached on one floc; the initial number concentration of flocs, N_p0, 2,000, the number of bubbles N_bi from 10⁵ bubbles to 10⁶ for estimating the removal efficiency of bubble-floc aggregates; inflow rate of 1.62 m³/h and recycle rate of 10%, bubble diameter d_b of 120 μm, floc diameter d_f of 250 μm, and an initial density of 900 kg/m³ in order to investigate critical values of parameters.

The Labs Test for rise velocity verification of bubble and bubble-floc aggregates was performed in order to measure the rise velocity of the bubbles and the bubble-floc aggregates. In the experiments, two types of flocs were used: the coagulation floc and the flocculation floc. The coagulation floc (Floc Type 1) was formed by rapid mixing at 60 revolutions per minute (rpm) for 30 s, while the flocculation floc (Floc Type 2) was prepared by slow mixing at 30 rpm for 2 min in order to make the floc larger than after the rapid mixing.

According to the hydraulic loading rate increase, the mean velocity of the separation tank is increased and the microbubbles may move to the deep portion of the separation tank. These microbubbles determine the rise velocity as a function of the bubble volume. While many aggregates are separated by rising to the top layer, a portion of them are separated by rising to the top layer when their rise velocities are greater than the hydraulic loading rate in the laminar flow state after moving to the separation zone. If the microbubbles move into the separation zone and do not increase in size, the rise rate is low. However, the bubbles in the separation zone are bigger than those in the contact zone (Haarhoff & Edzwald 2004). The bubbles that are released from the nozzle are distributed at a size of 40 ∼ 60 μm in the contact zone, but they are distributed at a size of 50–150 μm in the separation zone through growth, coalescence, and clustering phenomena (Leppinen & Dalziel 2004).

While the rise velocity of the large floc achieved a high rate, the attachment of many bubbles was required. The rise velocity V_fbi was defined as the proper rise velocity of the proper bubble-floc aggregates when the rise velocity V_fbi became greater than the hydraulic loading rate V_sz−hl of 0.5 m/min (or 30 m/h) and the rise time t_r was faster than the retention time τ_s. In order to check the effect of the number of bubbles attached on the floc, the rise velocity of the aggregates was calculated according to the bubble number attached on a 210 μm floc having an initial floc density of 500 kg/m³. The calculated rise velocity is summarized in Table 1. When one bubble is attached on one floc, N_b1 is defined as the number of bubbles, V_fb1 is indicated as the rise velocity when V_fb1 ≥ V_sz−hl, and t_r ≤ τ_s for meeting flotation condition in the separation zone. Therefore, in the case of a bubble diameter of 100 μm, even though the rise velocity is faster than the hydraulic loading rate, two bubbles are needed to produce a rise time faster than the retention time, as shown in Table 2. When the rise velocity V_fbi is faster than the hydraulic loading rate V_sz−hl and the rise time t_r is faster than the retention time τ_s, N_bi is defined as the proper number of bubbles in order to form the bubble-floc aggregates.

The maximum number of bubbles attached on the 210 μm floc is 1,385 for the 10 μm bubbles and 14 for 100 μm bubbles, that is, the smaller bubbles lead to more bubble attachments (Han 2002). In order to examine the rise velocity of the aggregates according to the attached number of bubbles, the rise velocity when only one bubble is attached needs to be calculated. When the calculated rise velocity was compared with the hydraulic loading rate, the rise velocity of the aggregates attached by a bubble of only 100 μm was greater than the hydraulic loading rate of 0.5 m/min (30 m/h). However, it did not meet the flotation condition in the separation zone. The flotation condition in the separation zone is met when the rise velocity of the aggregates is greater than the hydraulic loading rate (i.e., V_fb ≥ V_sz−hl) and the rise time t_r is faster than the retention time τ_s. To reach t_r ≤ τ_s, the attached number of bubbles, N_bi, was 398 bubbles for the bubble size 10 μm and two bubbles for the bubble size 100 μm. Ranging between 0.59 and 0.68 m/min, the calculated rise velocity of the aggregates was faster than the hydraulic loading rate of 0.5 m/min (or 30 m/h), but the Reynolds number Re was in the transition region between 1 and 50.

As shown in Table 1, the rise velocity changed according to the size of bubbles. As an increase in the hydraulic loading rate V_sz−hl of 0.5 m/min (or 30 m/h) indicates an increase of inflow rate, the recycling flow rate was increased if the recycle ratio was fixed. If the flow rate of the circulating water is not increased, the density of the bubble layer in the separating tank is reduced, and the quality of the treated water worsens. If the flow rate in the contact zone is too rapid, the possibility of collision between bubble and floc becomes smaller and the contact zone will not be sufficiently used.

The rise velocity of the aggregates as a function of the recycle ratio was examined in order to check the degree of dead space to be considered when designing the separation tank. Considering several parameters, including a hydraulic loading rate of 30 m/h, an inflow rate of 1.62 m³/h, and a recycle rate of 10%, the results of the retention time, the rise time, the rise velocity of the aggregates, the Reynolds number, and the dead space are summarized in Table 2.

For flotation in the separation zone during laminar flow, problems arise with a hydraulic loading rate over 0.4167 m/min (25 m/h) because it causes turbulent flow. Thus, a change in the design of the flotation tank is required. When the hydraulic loading rate in the separation zone is 0.4167 m/min to 0.667 m/min (25 to 40 m/h), obtaining sufficient flotation space or maintaining a thicker microbubble cloud layer is required in the separation zone; that is, a thicker bubble layer leads to a higher removal efficiency. The bubble layer can be made much thicker to raise capacity when using the high-rate DAF. In this study, a simulation using a population balance in turbulence (PBT) model concept was conducted and simulation results were verified through laboratory experiments. The change in bubble volume fraction in the high-rate DAF tank using a hydraulic loading rate of 30 m/h was examined through simulation and laboratory experiments.

For the high-rate DAF processes such as CSO treatment, using a hydraulic load rate of 30 m/h to be feasible, these conditions must be satisfied: (1) the flow pattern in the separation zone and stratified flow must be maintained and (2) when dispersion of flocs is very rare, the probability of contact between flocs and bubbles is increased, as many bubbles are supplied. Therefore, the rise velocity of the aggregates is increased when these conditions are satisfied. Especially, the following two things must be considered carefully: (1) if the bubble diameter is less, the collision efficiency between the floc and bubble is increased but the rise velocity of the bubble-floc aggregate becomes slow; and (2) along the flow of the effluent, bubble-floc aggregates smaller than the critical bubble-floc aggregate are discharged with flocs that are not combined with bubbles.

In this study, the effect of the bubble volume fraction in the separation zone of the high-rate DAF on hydraulic loading rate was confirmed. In addition, the hydraulic loading rate may influence the change of flow. The change of flow in the high-rate DAF tank as a function of the hydraulic loading rate may change the tendency of rise along the sloped baffle after inflow water is mixed with recycled water. Computational fluid dynamics (CFD) performance is required to estimate a more accurate change of flow. Therefore, future studies may focus on CFD simulation using information obtained from the results of the current study.

This study is supported by the Korea Ministry of Environment (MOE) as ‘Eco-Innovation (EI) Program’ (E314-00015-0412-1) and the National Research Foundation of Korea (NRF) with grants from the Ministry of Education in Korea, 2014 (NRF-2012R1A1A4A01010342).