Dissolved Air Flotation Performance Research Articles

Simultaneous monitoring of flow patterns, and bubble, and plastics micro-particle characteristics in Dissolved Air Flotation (DAF)

Dissolved air flotation (DAF) is an important water treatment process for removing suspended solids. Understanding the characteristic size and spatial distribution of both bubbles and suspended solids, including their hydrodynamics, within flotation tanks is key to optimising flotation performance. In this study, a continuously operating lab scale DAF tank was constructed to simultaneously monitor size distributions and motions of microbubbles (Ø = 67–86 ± 13–19 µm) and spherical polyethylene microplastic particles (Ø = 69.8 ± 5.3 µm) in unbuffered water at circumneutral pH without salt addition and with 0.5 g L−1 sodium chloride (NaCl) added. Microplastic particles were used for the first time in such a setup. Particle tracking showed different flow regimes in the separation zone, with short-circuiting flow paths varying at different flow rates. Within the range of flowrates employed, the bubble bed that formed was not prominent enough to promote stratified flow. The addition of NaCl greatly reduced microplastic particle counts within the separation zone during flotation. This new multi-factorial approach provides critical insights into the variations in DAF performance.

The influence of physical floc properties on the separation of marine microalgae via alkaline flocculation followed by dissolved air flotation

Flocculation achieved by raising the pH, termed alkaline flocculation is a sustainable way of inducing flocculation in marine microalgae. Flocs formed post-alkaline flocculation have the potential to be harvested via dissolved air flotation (DAF) or sedimentation. While DAF results in faster separation and biomass with a higher solids content compared to sedimentation, it has not been tested in saline environments, particularly when combined with alkaline flocculation. DAF processes, like most separation techniques, are heavily dependent on physical floc properties. In this study, the impact of alkaline floc properties on the performance of DAF versus sedimentation was evaluated to harvest marine Nannochloropsis oculata, benchmarking against the well-studied ferric chloride flocculation. This was followed by the use of the DAF white-water model to illustrate how alkaline and ferric floc properties impact bubble-particle attachment and DAF separation efficiencies. Alkaline flocs were smaller (peakmax < 300 μm; majority flocs 130–470 μm), stronger (~70 % strength factor) and more compact (scattering exponent > 2.30) compared to ferric flocs which were larger (peakmax ~ 1700 μm; >65 % of flocs >1000 μm), relatively weaker (<40 % strength factor), less compact (scattering exponent 1.49–2.30). However, as both flocs were hydrophilic, sedimentation yielded ~15 % greater efficiency than DAF. Stoke's Law suggested that sedimentation benefited due to alkaline floc compactness and large sizes of ferric flocs. Nonetheless, maximum separation efficiencies of ~80–85 % were still obtained via the DAF process. From the white-water model, bubble-floc attachment efficiency of alkaline flocs (0.01–0.001 %) was observed to be at least an order of magnitude greater than ferric flocs (0.001–0.0001 %) despite both methods resulting in comparable DAF separation efficiencies. For alkaline flocs, elevated bubble-floc attachment efficiency compensated low hydrophilicity; for ferric flocs, low bubble-floc attachment was compensated by large floc sizes. Overall, it is suggested that the determination of floc properties post-coagulation-flocculation could be used to optimise separation processes.

Enhancing organic matter removal in desalination pretreatment systems by application of dissolved air flotation

Membrane fouling in reverse osmosis (RO) systems caused by organic matter (OM) remains a significant operational issue during desalination. Dissolved air flotation (DAF) has recently received attention as a pre-treatment option for seawater OM removal; however, only a limited number of studies have been undertaken. This may be because it is difficult to characterise OM in seawater due to the high salt content and low carbon concentration. In this study, DAF pre-treatment experiments were conducted using a model seawater solution, and real seawater and brackish water samples. DAF performance was determined via conventional water quality parameters as well as fluorescence excitation–emission matrix (FEEM) spectroscopy and liquid chromatography with organic carbon detection (LC-OCD). Biopolymers and humic substances were the major organic fractions removed between 38 and 84% and 20–61% depending on the sample, respectively. The optimal normalised coagulant dose (Fe3+ to DOC ratio) was observed to be 0.5–4 at pH5.5 increasing to 4–12 at pH7.5. At pH5.5, the optimum coagulant dose increased with increasing humic character of the feed water. Overall, the OM removal efficiency by DAF observed in this study was higher than reported for other membrane-based processes; a combination of DAF and biofiltration is likely to be complementary.

The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit in the light of a mathematical model.

The influence of floc size and hydraulic detention time on the performance of a dissolved air flotation (DAF) pilot unit was investigated in the light of a known mathematical model. The following design and operational parameters were considered: the hydraulic detention time (tdcz) and hydraulic loading rate in the contact zone, the down-flow loading rate in the clarification zone, the particle size distribution (d F), and the recirculation rate (p). As a reference for DAF performance analysis, the proposed β.td parameter from the above mentioned mathematical model was employed. The results indicated that tdcz is an important factor in DAF performance and that d F and floc size are also determinants of DAF efficiency. Further, β.td was sensitive to both design and operational parameters, which were varied in the DAF pilot plant. The performance of the DAF unit decreases with increasing β.td values because a higher td (considering a fixed β) or a higher β (e.g., higher hydrophobicity of the flocs for a fixed td) would be necessary in the reaction zone to reach desired flotation efficiency.

Optimizing the Air Dissolution Parameters in an Unpacked Dissolved Air Flotation System

Due to the various parameters that influence air solubility and microbubble production in dissolved air flotation (DAF), a multitude of values that cover a large range for these parameters are suggested for field systems. An unpacked saturator and an air quantification unit were designed to specify the effects of power, pressure, temperature, hydraulic retention time, and air flow on the DAF performance. It was determined that a pressure of 621 kPa, hydraulic retention time of 18.2 min, and air flow of 8.5 L/h would be the best controlled parameters for maximum efficiency in this unit. A temperature of 7 °C showed the greatest microbubble production, but temperature control would not be expected in actual application. The maximum microbubble flow from the designed system produced 30 mL of air (±1.5) per L of water under these conditions with immediate startup. The maximum theoretical dissolved air volume of 107 mL (±6) was achieved at a retention time of 2 h and a pressure of 621 kPa. To isolate and have better control over the various DAF operational parameters, the DAF unit was operated without the unsaturated flow stream. This mode of operation led to the formation of large bubbles at peak bubble production rates. In a real-world application, the large bubble formation will be avoided by mixing with raw unsaturated stream and by altering the location of dissolved air output flow.

Application of dissolved air flotation as pretreatment of seawater desalination

The performance of dissolved air flotation (DAF) was evaluated for pretreatment of seawater desalination in this study. For this purpose, DAF was compared with dual medial filtration (DMF) for its performance of particle removal and organic reduction through pilot-scale experiments. A pilot-scale DAF plant with capacity of 3.2 m3/h was installed at southern coast of the East Sea and operated for four months (June–September, 2009). According to this study results, the organic reduction performance of DAF was comparable to that of DMF. Both DAF and DMF removed the same organic fraction. However, DAF could not match DMF in particle removal. The association of DAF and DMF could improve the pretreatment performance and better filtrate quality was obtained in terms of particle removal. The association reduced the clogging head loss and initial turbidity breakthrough.

Dissolved air flotation of surface water for spiral-wound module nanofiltration pre-treatment

Bench-scale dissolved air flotation (DAF) of Tagus River surface water (Valadas, Portugal) was investigated as pre-treatment for spiral-wound module nanofiltration (SWNF). Before the DAF, a coagulation/flocculation of the surface water was performed using aluminium sulphate, ferric chloride and chitosan coagulants and several commercial coagulant aids. The coagulation/flocculation experiments were carried with 2 L of surface water in a jar-test equipment at room temperature. The DAF performance to remove colloidal matter and suspended solids was evaluated through the measurement of the silt density index (SDI) and the modified fouling index (MFI) of the treated water. The air saturation pressure was varied between 300 and 600 kPa and the recycle ratio between 5 and 50%. The DAF without coagulation/flocculation was not effective in the reduction of both the SDI and MFI, even at a high DAF recycle ratio of 50%. The use of coagulants improved the DAF efficacy in reducing the fouling indexes and, for the optimal operating conditions, the MFI of the treated water was reduced by more than one order of magnitude. However, it was not possible to obtain treated water with SDI and MFI values below the recommended ones for SWNF. Further filtration of the DAF treated water with a 5 micron capsule filter was necessary to reduce the SDI below 5%/min and the MFI below 10 s/l 2. The integration of DAF with another process that removes the residual suspended solids and colloidal matter is, therefore, necessary to provide a good pretreatment for the SWNF of Tagus River surface water.

Optimization of Solids Separation in Dissolved Air Flotation

Abstract Sizes of flocs were analyzed to identify characteristics of the particle size distribution optimal for separation by dissolved air flotation (DAF). Optical microscopes and two particle counters were used for floc sizing. A Brightwell Technologies particle counter was found to provide floc size measurements in agreement with improved microscopic methods. The particle counter provided distribution of flocs with sizes down to 1 micron (µm). This allowed for inclusion of flocs with size ranging from 5 to 1 µm, which were excluded from the analyses in the earlier study. Four alum dosages were applied: 15, 25, 40, and 60 mg/L. The turbidity and colour of the DAF effluent at alum dosages of 25, 40, and 60 mg/L were very similar. However, the analysis of the flocs in the treated effluent revealed that, at the alum dose of 60 mg/L, particle removal was the best. Therefore, this dosage was selected as optimal for the solid/liquid separation process. The average size of coagulation flocs at 60 mg/L was approximately 30 µm, and was equal to the estimated size of air bubbles produced by the saturator. Therefore, this study confirms the finding of the earlier work claiming that the optimum DAF performance is attained when the mean floc size and the bubble size are equal. Similar size of floc and bubble indicates that flocs act predominantly as nuclei for bubble formation. This finding contributes to the knowledge of mechanisms of floc air bubble attachment in DAF.

Treatment of drinking water residuals: comparing sedimentation and dissolved air flotation performance with optimal cation ratios

Spent filter backwash water (SFBW) and clarifier sludge generally comprise the majority of the waste residual volume generated and in relative terms, these can be collectively referred to as combined filter backwash water (CFBW). CFBW is essentially a low-solids wastewater with metal hydroxide flocs that are typically light and slow to settle. This study evaluates the impact of adding calcium and magnesium carbonates to CFBW in terms of assessing the impacts on the sedimentation and DAF separation processes. Representative CFBW samples were collected from two surface water treatment plants (WTP): Lake Major WTP (Dartmouth, Nova Scotia, Canada) and Victoria Park WTP (Truro, Nova Scotia, Canada). Bench-scale results indicated that improvements in the CFBW settled water quality could be achieved through the addition of the divalent cations, thereby adjusting the monovalent to divalent (M:D) ratios of the wastewater. In general, the DAF process required slightly higher M:D ratios than the sedimentation process. The optimum M:D ratios for DAF and sedimentation were determined to be 1:1 and 0.33:1, respectively. It was concluded that the optimisation of the cation balance between monovalent cations (e.g., Na +, K +) and added divalent cations (i.e., Ca 2+, Mg 2+) aided in the settling mechanism through charge neutralisation–precipitation. The increase in divalent cation concentrations within the waste residual stream promoted destabilisation of the negatively charged colour molecules within the CFBW, thereby causing the colloidal content to become more hydrophobic.

Fate and removal of Cryptosporidium in a dissolved air flotation water plant with and without recycle of waste filter backwash water

Pilot plant research focused on the removal of Cryptosporidium oocysts by dissolved air flotation (DAF) clarification and by dual media filtration and on the impacts of the recycle of waste filter backwash water containing oocysts. No impacts from recycle of filter backwash (10% rate) were found for turbidity, particle counts (2-15 μm), and UV254 on DAF and filtration performance. DAF achieved Cryptosporidium log removals of 1.6 to 2.2 without or with recycle of filter backwash. No impacts of recycle were found on filtration, and cumulative (DAF plus filtration) log oocyst removals exceeded 4 log. Model predictions show that the fate of Cryptosporidium and the build-up of oocysts in the plant influent depend on: DAF performance, the percent of filtered water production used for backwashing, and the percent of filter backwash recycle flow. A DAF plant using 2.5% of filtered water production for backwashing and achieving 1.6 log removal or greater of oocysts by DAF clarification will not have a build-up of oocysts in the plant influent regardless of the recycle rate.

Removal and fate of Cryptosporidium in dissolved air drinking water treatment plants

In the first part of the paper, data from pilot plant studies are used to evaluate Cryptosporidium removal by dissolved air flotation (DAF) clarification and dual media filters under challenge conditions. Oocyst removals were investigated for design detention times and hydraulic loadings for winter and spring seasons. Coagulation was optimized for turbidity and removal of natural organic matter. DAF performance was better for spring water temperatures achieving 2.5 +/- 0.3 log removal of oocysts compared to 1.7 +/- 0.3 log removal in the winter. Cumulative log removal across DAF and filtration exceeded 5.4, and was not affected by water temperature. Low turbidities and particle counts are indicators of good treatment and good removals of Cryptosporidium. The second part of the paper uses a mathematical model to predict the fate of Cryptosporidium through a DAF plant and the impact of filter backwash recycle on oocyst build-up in the plant influent. Model predictions show that the fate of Cryptosporidium and the build-up of oocysts in the plant influent depend on: DAF performance, the percent of filtered water production used for backwashing, and the percent of filter backwash recycle flow. A DAF plant with 2.5% filtered water production for backwashing and that achieves 1.6 log removal or greater of oocysts by DAF clarification will not have a build-up of oocysts in the plant influent regardless of the recycle rate. This is because the oocysts are concentrated in the DAF floated sludge and not within granular filters.

Original Article

This paper presents the mathematical formulation of a model for bubble–floc agglomeration in the contact zone of dissolved air flotation (DAF). It also contains rising velocity formulae for a bubble–floc agglomerate in the separation zone of DAF. The bubble–floc agglomeration model is based on the population balance of air bubbles, particles, and bubble–particle agglomerates, and the rate of bubble–floc collision and attachment. The model has been designed for two cases: where particles are larger than bubbles and vice versa. The requirements of pre-treatment (coagulation and flocculation) and the amount of air as bubbles are discussed, with the support of a model simulation. The air supply–consumption ratio (Asc) is derived as an important static parameter determining DAF performance, and should be greater than 1. The traditional DAF parameter, the air–solid ratio (As), is proportional to the air supply–consumption ratio. The DAF model parameter evaluation suggested that it should be on the order of 0.01 or more. To ensure a sufficient bubble–floc collision rate, the DAF model simulation revealed, the kinetic parameter (bubble volume concentration times floc diameter, φbd) should be greater than 6.0 × 10−8 m. The performance of DAF in treating a high concentration suspension (>100 mg/L) is dependent mainly on the air–solid ratio (or the air supply–consumption ratio). For low concentration suspensions (≪100 mg/L), φbd instead of the air–solid ratio is a dominant operational parameter in ensuring an acceptable bubble–floc collision frequency. The bubble volume concentration required for treating a low concentration suspension can be reduced with the increase of floc size by pre-flocculation.

Modeling, simulation and operational parameters of dissolved air flotation

This paper presents the mathematical formulation of a model for bubble–floc agglomeration in the contact zone of dissolved air flotation (DAF). It also contains rising velocity formulae for a bubble–floc agglomerate in the separation zone of DAF. The bubble–floc agglomeration model is based on the population balance of air bubbles, particles, and bubble–particle agglomerates, and the rate of bubble–floc collision and attachment. The model has been designed for two cases: where particles are larger than bubbles and vice versa. The requirements of pre-treatment (coagulation and flocculation) and the amount of air as bubbles are discussed, with the support of a model simulation. The air supply–consumption ratio ( A sc ) is derived as an important static parameter determining DAF performance, and should be greater than 1. The traditional DAF parameter, the air–solid ratio ( A s ), is proportional to the air supply–consumption ratio. The DAF model parameter evaluation suggested that it should be on the order of 0.01 or more. To ensure a sufficient bubble–floc collision rate, the DAF model simulation revealed, the kinetic parameter (bubble volume concentration times floc diameter, φ b d ) should be greater than 6.0 × 10 −8 m. The performance of DAF in treating a high concentration suspension (>100 mg/L) is dependent mainly on the air–solid ratio (or the air supply–consumption ratio). For low concentration suspensions (≪100 mg/L), φ b d instead of the air–solid ratio is a dominant operational parameter in ensuring an acceptable bubble–floc collision frequency. The bubble volume concentration required for treating a low concentration suspension can be reduced with the increase of floc size by pre-flocculation.

Particle removal by flotation and filtration: pretreatment effects

Consistent performance of DAF and the quality of DAF effluent—despite considerable variation in flocculation characteristics and flocculated water quality conditions—demonstrate the robust nature of this process.Good dissolved‐air flotation (DAF) and filtration performance were observed for all pretreatment flocculation conditions studied: flocculation times of 5 and 20 min, gate and propeller mixers, two and three stages of flocculation, and low (G of 30 s−1) and high (G of 70 s−1) mixing intensity. The research showed that DAF plants can be designed with flocculation times as low as 5 min and that DAF is a robust particle removal process—i.e., it can produce consistently good water quality for a variety of conditions. Coagulation chemistry (pH and coagulant dosage) has a stronger influence on DAF and filtration performance than physical flocculation design and operating parameters.