Wastewater Treatment Knowledge: Meaning and Function of Dissolved-Air Flotation (DAF)
Dissolved-air flotation (DAF) is a type of flotation technology. It dissolves air into water under pressure to form a supersaturated state, then releases a large number of fine bubbles upon pressure reduction; the bubbles adhere to impurities in the water, forming float bodies that rise to the surface, thereby achieving solid–liquid or liquid–liquid separation [3-4]. According to the pressure at which bubbles are released, DAF is divided into vacuum DAF and pressurized DAF, of which pressurized DAF is the most widely used method in practice [5-6] [12].
Pressurized DAF produces bubbles of about 20–100 μm, characterized by small size, uniformity, and stability, with little disturbance to the tank. This technology is especially suitable for removing pollutants whose density is close to or less than that of water and which are hard to remove by gravity settling, such as grease, colloids, algae, and fibers [4-5].
Flotation was first applied in the mineral-processing industry. In 1905 a U.S. patent published pressurized dissolved-air technology, and in 1907 jet dissolved-air flotation was invented. Thanks to these inventions, DAF found wide application in potable-water treatment, industrial-water treatment, and the treatment of various industrial wastewaters—oil refining, chemical, paper, leather, textile, dyeing, steel, food, pharmaceutical—as well as municipal sewage [7].
Currently, the flotation used in the market is basically partial-reflux dosed DAF, generally with a reflux ratio of 15%–30%. Compared with DAF, cavitation (induced-air) flotation has low efficiency and little research or new-technology application, and is tending to withdraw from the market [6]. In recent years, technology has continued to develop, such as cyclone dissolved-air flotation (CDFUCDAF) and three-phase mixers, new high-efficiency equipment that further improves separation efficiency and scope [8] [15].
Bubbles are generated by changing gas solubility through pressure adjustment, requiring no mechanical aeration device compared with traditional air-entrainment equipment [1]. Under pressurized or negative-pressure conditions, supersaturated gas in the water precipitates to form micron-scale bubbles whose surface has a double-layer structure formed by van der Waals forces:
the outer water film has elastic-mechanical properties
and the inner water molecules show oriented close packing; this structure gives the bubbles a high specific surface area (diameter 10–100 μm) and stable adsorption characteristics [1].
According to the dissolved-air method, there are three technical routes [1]. The full-flow pressurized route pressurizes and dissolves air in all the wastewater to be treated, then sends it through a pressure-release device into the flotation tank for solid–liquid separation; its advantage is a small flotation-tank volume, but power consumption is high since all wastewater must be pressurized. The partial-flow route pressurizes and dissolves air in part of the wastewater, while the rest enters the flotation tank directly and mixes there with the dissolved-air wastewater; its advantage is lower power use, but the dissolution tank needs higher pressure. The reflux-pressurized route (also called partial-reflux DAF) pressurizes and dissolves air in part of the treated effluent (reflux clear water), then sends it after pressure release into the flotation tank to mix and float with the untreated wastewater from the flocculation tank; its advantages are energy savings, full use of flocculant, and stable treatment, especially suitable for high-suspended-solids wastewater [4-5] [12]. Modern flotation theory holds that a 50% reflux ratio gives the best result and is the most commonly used process at home and abroad. According to release pressure, DAF can also be divided into vacuum DAF, where air dissolves in water at atmospheric or pressurized conditions and precipitates under negative (vacuum) pressure; its feature is low air-dissolution pressure, but the flotation tank must be enclosed and structurally complex, suitable for wastewater of low pollutant concentration, and less used in production [5] [12].
A DAF system mainly consists of a pressure-dissolution system, a dissolution-release system, and a flotation-separation system [4-5].
The core equipment of the pressure-dissolution system is the pressure-dissolution tank, with supporting equipment including a pressurized water pump and an air compressor (or an ejector, or a pump-suction-air device) [5]. The tank working pressure is typically 300–500 kPa (0.3–0.5 MPa), and wastewater residence time in the tank is generally 1–4 minutes [12-13].
The key equipment of the dissolution-release system is the dissolved-air releaser (e.g., TS-type) or a pressure-reducing valve. The system requires tiny (20–100 μm), uniform released bubbles [4-5].
The flotation-separation system is the flotation tank, commonly of horizontal-flow or vertical-flow type [4-5]. Design parameters: contact-zone upflow velocity 10–20 mm/s, separation-zone velocity 1–3 mm/s, effective water depth 2.0–2.5 m, surface hydraulic loading 5–10 m³/(m²·h) (up to 12.7 m³/(m²·h)), hydraulic residence time 10–20 min (can be shortened to 10 min) [5] [9] [12].
Other auxiliary equipment includes the sludge scraper, whose scraping speed should be less than 5 m/min [5].
DAF produces tiny bubbles (diameter typically 20–100 μm), uniform and stable, with little water disturbance, especially suitable for separating fine, loose flocs [4-5]; it also has high air solubility, providing sufficient microbubbles and good separation [3] [5]. The process has high surface loading, short hydraulic residence time (usually 10–20 min), small footprint, and low capital cost [5] [14]; the scum has low moisture (generally below 96%) and small volume, facilitating downstream treatment. DAF has a pre-aeration effect, raising the dissolved oxygen of the effluent [14], and works well for low-temperature low-turbidity water and high-algae water [5] [17]; its equipment is relatively simple, highly automated, and easy to maintain [3-4].
The technology also has limitations: relatively high power consumption, 0.02–0.04 kWh more per ton of wastewater than sedimentation [14]. Its performance drops for high-turbidity water (NTU>1000) or particles denser than 1.2 g/cm³ [12], and the releaser clogs easily at high suspended-solids concentration. Operation is relatively complex and demands high management [11-12], and the air compressor is noisy [12].
Key operating parameters of DAF include dissolution pressure, reflux ratio, and coagulant dosage. Dissolution pressure is usually controlled at 0.3–0.5 MPa (300–500 kPa), affecting bubble density and size. The reflux ratio is usually 20%–50% (partial-reflux process), affecting bubble quantity and energy use; studies show the reflux ratio has the most significant effect on flotation [6] [12-13]. Coagulant dosage affects floc formation and removal; studies show the order of influence is: reflux ratio > coagulant dosage > effective residence time. Lower water temperature adversely affects DAF. The tank surface loading is usually 5–12.7 m³/(m²·h), and residence time should not exceed 1.0 h [5] [9] [12]. In addition, influent pH, ion concentration, and bubble stability also affect treatment [13].
Main applications in engineering thermophysics:
Oil-bearing wastewater separation in petrochemicals
Activated-sludge thickening [2]. In water treatment it applies to:
Low-temperature water purification (<4°C)
Cyclone dissolved-air flotation (CDFUCDAF) integrates cyclone centrifugal force with dissolved-air flotation, forming a weak swirl field that enhances oil-droplet–bubble collision and adhesion; bubble size reaches 5–30 μm, the structure is compact, oil-removal efficiency exceeds 90%, and it has been applied on offshore platforms, etc. [15] The three-phase mixer (GEM) flotation technology integrates pressurized dissolved air, chemical mixing, and coagulation–flocculation; microbubbles grow inside and around flocs, forming porous hollow flocs with low moisture that self-float, with better bubble-adhesion effect. [8]
In addition, it is widely used in oil refining, chemical, steel, food processing, dyeing, leather, and wool-washing wastewater to remove grease, suspended solids, and colloids and reduce COD and BOD [5] [7].
In water treatment it applies to low-temperature water purification (<4°C), low-turbidity water (NTU<100), and algae-rich water. DAF is especially suitable for low-temperature low-turbidity and high-algae water, effectively removing algae, turbidity, color, and odor compounds and improving effluent quality [17].
Other fields: in municipal sewage treatment, it can be used for advanced treatment after secondary biological treatment to further improve effluent quality, removing residual suspended solids, phosphorus, and colloidal particles to meet higher discharge or reuse standards [9]. It also has successful applications in offshore-platform produced-liquid treatment and waste-oil refining wastewater [10] [15].
Low-turbidity water (NTU<100) treatment
Studies on flow-pattern and structure optimization of traditional flotation tanks show that optimizing separation-zone height and adding guide baffles can effectively reduce backflow and dead zones and improve space utilization. [16]
Experimental studies on the effects of pressure, hydraulic residence time, gas–water ratio, and coagulant type on pollutant-removal efficiency show that polyaluminum chloride (PAC) works well, and pressurized dissolved-air biological flotation (PA-DAF) can achieve simultaneous nitrification–denitrification. [18]
Studies on special waters such as high-algae and low-temperature low-turbidity show that dissolved-air flotation (DAF) significantly removes algae, turbidity, color, and odor threshold. [17]
