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Classification and Design Principles of Dissolved Air Flotation Abstract: Dissolved air flotation (DAF) is a type of air flotation process that takes advantage of the difference in solubility of water at different pressures. It involves pressurizing and aerating all or part of the water to be treated (or already treated), thereby increasing the amount of air dissolved in the water. This aerated water is then introduced into water to which coagulants have been added, and under normal pressure the air is released; the resulting bubbles attach themselves to the impurity flocs, causing the overall density of these flocs to be less than that of water, so that they rise to the surface and allow for the separation of solids from liquids. Keywords: Dissolved air flotation, DAF, degassing system. Dissolved air flotation (DAF) is a type of air flotation process that takes advantage of the difference in solubility of water at different pressures. It involves pressurizing and aerating all or part of the water to be treated (or already treated), thereby increasing the amount of air dissolved in the water. This aerated water is then introduced into water to which coagulants have been added, and under normal pressure the air is released; the resulting bubbles attach themselves to the impurity flocs, causing the flocs as a whole to have a density lower than that of water and thus rise to the surface, enabling solid-liquid separation. Dissolved air flotation (DAF) is suitable for treating water with low turbidity, high color intensity, high organic content, low oil content, low surfactant content, or high algal load. Compared to other air flotation methods (see Appendix 1 for details), it has advantages such as a high hydraulic load and a compact tank structure. However, its complex manufacturing process, high electricity consumption, and loud noise from the air compressor are among the drawbacks that limit its application. 1 Type: Based on different classification principles, DAF can have various types. 1.1 Based on the pressure at which bubbles emerge from water, it can be divided into vacuum air flotation and pressure dissolved air flotation. The former uses vacuum pumping to dissolve air at normal or increased pressure, and then releases the microbubbles under negative pressure for air flotation ； In the latter method, air is forced to dissolve in water under pressure, and then the pressure is suddenly reduced, causing the dissolved gas to be released from the water. It attaches to the flocs in the form of microbubbles and rises together with them. 1.1.1 Vacuum air flotation tanks, although having low energy consumption, exhibit stable bubble formation as well as adhesion between bubbles and flocs ； But the amount of bubbles released is limited ； Moreover, all equipment components must be sealed inside the air flotation tank ； The structure of the air flotation tank is complex ； It is only suitable for treating wastewater with low pollutant concentrations (not exceeding 300 mg/l), so it is not widely used in practice. 1.1.2 Pressure dissolved air flotation is the most commonly used method both domestically and internationally. The basic processes that can be chosen include full-dissolved air flotation, partial dissolved air flotation, and partial recirculation dissolved air flotation. 1.1.2.1 Full-process dissolved air flotation: The full-process dissolved air flotation method involves pressurizing all the wastewater using a pump; in a dissolved air tank, air is dissolved into the wastewater, and then the wastewater is sent to the flotation tank via a pressure reducing valve. The flowchart is shown in Figure 1. Its features are: ① High gas dissolution capacity, which increases the opportunities for oil particles or suspended particles to come into contact with bubbles ； ②Under the same water treatment volume, it requires a smaller flotation tank than some partial recirculation dissolved air flotation methods. ③All wastewater passes through a pressure pump; the required pressure pump and air dissolution tank are larger than those in the other two processes, resulting in higher investment costs and greater energy consumption for operation. 1.1.2.2 Partial dissolved air flotation: In this method, a portion of the wastewater is pressurized and mixed with dissolved air, while the remaining wastewater enters the flotation tank directly where it mixes with the wastewater containing dissolved air. Its features are: ① It requires a smaller pressure pump than the full-process dissolved air flotation method, thus resulting in lower power consumption ； ②The size of the air flotation tank is the same as that in the full-process dissolved air flotation method, but smaller than that in the partial recirculation dissolved air flotation method. 1.1.2.3 Partial recirculation dissolved air flotation method: In this method, a portion of the treated water is recirculated; this recirculated water is pressurized and dissolved with air, and after depressurization it enters the flotation tank where it mixes with the oil-containing wastewater from the flocculation tank for flotation. The process is shown in Figure 2. Its features are: ① A small amount of water is needed under pressure, resulting in reduced power consumption ； ②It does not promote emulsification during the air flotation process ； ③Good alum flocculation is achieved, with less subsequent flocculation ； ④The volume of the air flotation tank is larger than that of the previous two processes. Modern air flotation theory holds that partial recirculation pressurized dissolved air flotation saves energy, allows for the full utilization of flocculants, and yields better treatment results compared to the fully pressurized dissolved air flotation process. The best treatment effect is achieved when the reflux ratio is 50%; therefore, the pressurized dissolved air flotation process with partial reflux (reflux ratio of 50%) is currently the most commonly used flotation method both domestically and internationally. Figure 2 Schematic diagram of a partial recirculation dissolved air flotation process. 1.2 Based on whether there is a filtering effect from the layer of microbubbles in the flotation tank and the different flow patterns of water, it can be divided into early DAF, conventional DAF, and turbulent DAF. (For specific details, see Appendix 3.) 2 Design Principle: DAF is generally installed before the biological treatment unit and after the physical treatment unit; it is conventionally classified as part of the physical treatment unit. In the case of a two-stage flotation process, for convenience and cost savings, the primary and secondary flotation tanks are often arranged side by side. A liquid level difference of around 500 mm exists between these tanks, which ensures that the wastewater flows from the primary flotation tank to the secondary one; this eliminates the need for lift pumps, thereby achieving energy savings. This is reflected in the vertical layout: it is extremely important, and constitutes one of the key factors, to strictly control the elevations of the scum scraper frame (plate), the adjustable weir, and the top of the slag removal trough during design and construction. Otherwise, it will severely affect the air flotation effect (the foam layer cannot be removed mechanically), which is precisely why an adjustable outlet weir must be used. Figure 2 Process flow diagram of the two-stage flotation tank process. DAF mainly consists of air saturation equipment (also known as a pressure dissolved air system), air release equipment (also known as a dissolved air release system), and a flotation tank (also known as a air flotation separation system). Currently, the design of the dissolved air flotation process and the determination of its optimal operation rely on pilot testing and experience. Below, based on the experience gained from various applications, the design principles of each component are explained separately. 2.1 Pressure dissolved air system (including pressure dissolved air tank, air compressor, water pump, and their auxiliary equipment) 2.1.1 The dissolved air system accounts for 50% of the total energy consumption in the air flotation process, while the cost of the dissolved air tank constitutes 12% of the total capital investment in the factory. Therefore, optimizing the design of the dissolved air system is crucial for reducing the operating costs associated with air flotation. The dissolved air tanks are usually cylindrical in shape and installed vertically. Their volume is calculated based on a wastewater retention time of 25–3 minutes. Dividers or ceramic rings can be installed inside the tanks, or the tanks can also be used without such additions. Since the water and gas phases are mixed inside the air dissolution tank, it is generally designed as a pressure vessel. An automatic exhaust valve is installed on the top of the tank or an automatic pressure relief valve is placed at the bottom to balance the pressure. The pressure inside the tank is usually kept at around 0.45 MPa; based on this value, the parameters for the lift pump, return pump, and air compressor can be determined. In foreign design materials and literature, it is believed that the longer the air-water residence time, the higher the gas dissolution efficiency. This results in a large volume for the air dissolution tank, with residence times sometimes reaching 3–5 minutes. Domestic research has confirmed that the liquid film resistance controls the gas dissolution rate, and the notion that a longer residence time leads to better gas dissolution is not in line with reality. Therefore, the design parameters used in China differ from those abroad; the predetermined gas dissolution efficiency serves as the design criterion, while the liquid phase overflow density and the total mass transfer coefficient of the liquid phase are used as parameters. All studies show that air-stirred tanks with a packing bed are more effective than those without one, with efficiency reaching up to 99%. However, in practical operation, it is often necessary to conduct internal inspections of the air-stirred tank; as a result, systems without a packing bed are commonly used in many air flotation processes. Moreover, most air-stirred tanks without a packing bed… 2.1.2 Pressurized air dissolution method: There are two ways of introducing air, namely air introduction before the pump and air introduction after the pump. The first type is pump-driven air intake; the flow diagram is shown in Figure 3. When the amount of air drawn in is less than the saturation level of air in water at that temperature, a branch pipe is led from the pump’s water delivery pipe back to the suction pipe; a hydraulic ejector is installed on this branch pipe. As the wastewater passes through the hydraulic ejector, a negative pressure is created, which draws in air. This air mixes with the wastewater and is then sent to the air dissolution tank via the suction pipe and the pump. This approach eliminates the need for an air compressor, resulting in good mixing of air and water. However, the water pump must be fed with water through self-priming, and a water head of at least lm is required. Its maximum suction volume cannot exceed 10% of the water pump’s suction capacity; otherwise, the pump will operate unstably, the vacuum level that the pump should maintain will be disrupted, and cavitation will occur. The second type is intake after the pump; the flow diagram is shown in Figure 4. When the amount of air drawn in is greater than the saturation level of air in water at that temperature, the air is forced into the system through the air compressor and into the outlet pipe of the water pump; however, this amount should not exceed 25% of the water pump’s suction capacity. This method ensures stable operation of the water pump, and it does not require operation under positive pressure; however, air supply from an air compressor is needed. To ensure good air dissolution performance, the volume of the air dissolution tank is also relatively large; generally, a more complex filled-type air dissolution tank is required. Figure 3: Schematic diagram of air intake before the pump; Figure 4: Schematic diagram of air intake after the pump. 2.1.3 The amount of air injected is another key factor in flotation operations, and it generally varies depending on the selected dissolved air pressure or reflux ratio. Experiments also show that the effluent quality depends only on the total amount of air introduced into the system (when the bubble sizes are consistent), and is independent of pressure or recirculation ratio alone. It depends on the quality of the wastewater, the type of flocculant (coagulant), and the type of pressure-reduction release device, as determined through repeated practice. 2.1.4 The water level in the air dissolution tank is an important factor affecting the efficiency of air flotation. In Nanning, the area where water and air come into contact has been reduced, resulting in poor dissolved gas efficiency ； If the water level is too low, there is not enough buffer depth, allowing gas to pass through the water layer and enter the air flotation equipment to form large bubbles, resulting in poor air flotation performance. It is recommended to keep the water level at around 1/3 to 1/4 of the tank’s capacity. 2.1.5 The pressure inside the gas dissolution tank is an important factor affecting the gas volume. Under normal circumstances, higher pressure results in more dissolved gas. In the air compression method for filling gas, the pressure inside the dissolved gas tank is determined by both the air pressure from the air compressor and the water pump. During operation, it is first necessary to ensure an adequate water pressure, but the water pressure and air pressure should be roughly equal. In the method of aerating using a water jet pump, the key to maintaining the pressure in the degassing tank is to use an appropriate water pump; generally, the pressure of such a pump should be greater than 0.3 Mpa, while ensuring the required flow rate. The pressure in the degassing tank can be adjusted by controlling the outlet valve of the degassing tank, the outlet valve of the water pump, and the backflow control valve. 2.1.6 In accordance with Article 8.2.7 of the **Standard for Outdoor Drainage Design of the People’s Republic of China**, the design of the dissolved air tank shall meet the following requirements: First, the operating pressure of the dissolved air tank should be in the range of 300–500 kPa (approximately 3–5 kgf/cm2) ； II. The air volume, measured in terms of volume, can be calculated as 5–10% of the wastewater volume ； III. The residence time of wastewater in the dissolved air tank should be determined based on the type of tank; it is generally advisable to keep it between 1 and 4 minutes. Measures should be in place within the tank to promote thorough mixing of air and water ； IV. For dissolved air tanks using partial recirculation, dynamic types should be preferred, and water level control measures must be in place. 2.1.7 Some applications suggest adding a precision air flow stabilizer, whose function is to ensure stable and uniform pressure of the air before it reaches the nozzles in the dissolution tank. The reflux ratio refers to, in the case of partial reflux dissolved air flotation, the ratio of the amount of recycled water that enters the dissolved air tank to be pressurized with dissolved air to the amount of water being treated. The reflux ratio is generally 25% to 50% of the wastewater volume. However, when the quality of the wastewater is poor and the volume of wastewater is low, the recirculation ratio can be increased appropriately to ensure the quality of the effluent. 2.2 Dissolved gas release system (mainly the release head) The releaser is the key component of this system; it has a significant impact on the size and distribution of the bubbles, as well as on the efficiency of air flotation for water purification and the operating costs associated with it. The gas release efficiency of the currently used releaser can reach 99.2%. 2.2.1 Previous studies suggested that the size of the released bubbles is related to the dissolved gas pressure; at low pressures, larger bubbles are formed, which is not conducive to air flotation. Recent domestic research suggests that the pattern of bubble release when dissolved air water loses energy under reduced pressure differs from that of bubbles in still water ； The appearance of large bubbles at low pressure is attributed to a faulty releaser. In addition to releasing a large number of stable tiny bubbles, the key is to prevent clogging. Currently, different types of release valves are used domestically and internationally, including simple valve-type ones, needle valve-type ones, and specialized release valves (patented). There are many patented products for dissolved air release devices, and those with better performance generally have the following feature: an instantaneous pressure drop at the nozzle ； At the inlet of the dispenser, the direction of the water flow changes suddenly (usually by 90°) ； The caliber of the emitter is no more than 2.5 mm, and the residence time of water in the emitter is < 1.5 ms ； The flow speed of the water leaving the release device gradually decreases ； The water leaving the release mechanism will collide with the barrier in front of it. No release mechanism can produce only microbubbles; generally, bubbles with diameters ranging from 40 to 70 μm are formed. The formation of larger bubbles is inevitable, although their presence reduces the efficiency of the system’s operation. 2.2.2 In accordance with Article 8.2.8 of the **Standard for Outdoor Drainage Design of the People’s Republic of China**, the selection of dissolved air release devices shall be determined based on the quality of the oily wastewater, the treatment process, and the performance of the release devices. 2.3 Air flotation separation system (components of the air flotation tank) The function of the air flotation separation system is to provide a sufficient volume for thorough mixing, contact, and adhesion between the microbubbles and the impurities in the water, as well as for the separation of the gas-containing flocs from clean water. 2.3.1 To improve the treatment efficiency of air flotation, coagulants or flotation agents are often added to the wastewater; the dosage varies depending on the quality of the water, and is generally determined through testing. For aluminum flocculants, increasing the stirring intensity can further reduce the turbidity of the effluent. To ensure the coagulation effect of the flotation (coagulation) agent, a static pipe mixer and a reaction chamber should be installed at the water inlet of the flotation tank. The effective volume of the reaction chamber is calculated based on a retention time of 10 minutes for the wastewater (the sum of the inflow volume and the recirculation volume). It is generally divided into three chambers arranged in a labyrinth pattern, with a mixer in each chamber to enhance the coagulation effect; the velocity gradient within each chamber is usually the same. The design of the flocculation tank (i.e., reaction chamber) should preferably provide a plug flow condition (turbulent mixing condition), which can ensure better air flotation efficiency. 2.3.2 The maximum recommended size for a dissolved air flotation tank is 145 m2, with a corresponding water production capacity of 2900–4350 m3/h; the water production capacity per unit area has increased by at least twice. The depth of the dissolved air flotation tank increased from 1.5 m to 5.0 m, and its shape evolved from rectangular to square, with an aspect ratio ranging from (1.2–2):1. Currently, the length of dissolved air flotation tanks that operate well can reach up to 12 m, but the width is limited to 8.5 m, mainly because the maximum span of mechanical scum scrapers is 8.5 m. The residence time of wastewater in the air flotation tank is generally 30–40 minutes, the operating water depth is 15–25 m, the length-to-width ratio is not less than 4, and the surface load is 5–10 m3/m2•h. If the residence time is too short, the impact force of the water flow is high, resulting in a highly turbulent state of the wastewater in the flotation tank. This not only hinders the adhesion of bubbles to flocs, but it also breaks apart some of the flocs that have already attached to the bubbles ； Furthermore, due to turbulence and shorter reaction times, some of the added coagulant does not react completely before being carried away with the effluent, which results in a reduced removal rate of suspended solids in the effluent, and even a tendency toward negative growth. 2.3.3 The air flotation tank is divided into 2 zones: the contact zone and the separation zone. 2.3.3.1 When designing the contact zone, it is necessary to control the upward flow velocity of the flocculated water in order to avoid short-circuiting and deviation in flow; this prevents the bubbles that have adhered to the particles from being stripped away by the water flow during their upward movement, thus ensuring optimal separation results. Generally, it is advisable to keep the upward flow velocity in the contact zone between 10 and 20 mm/s, with a height of 1.5 to 2.0 meters. At such flow velocities and heights, the contact time between the flocs and microbubbles is ensured, while the flocs are not damaged or sink due to excessive floating time. The release devices should be arranged reasonably so that the water released covers the entire area, enabling the flocs beneath the microbubbles to come into contact with it in a sufficient and timely manner. 2.3.3.2 When selecting the separation speed for the separation zone, it should facilitate the floating of gas-loaded flocs. For aerated flocs that are large, low in density, and difficult to break, a higher separation speed is generally used; conversely, a lower value is employed. The flow velocity in the separation zone should be between 1 and 3 mm/s. If the velocity is too low, large flocs will settle due to crowding; if it is too high, the boundary between the flocs containing air and the clear water will extend downward, resulting in the flocs being carried away with the water and a decline in water quality. For situations with high concentrations and a large amount of scum, which lead to crowded floating during solid-liquid separation, the floating speed should be reduced; otherwise, an overly thick scum layer can cause sedimentation, or the separation effect may be impaired due to an insufficient volume in the separation area. When selecting the water collection system, strive to ensure even water collection, so that the small, air-filled floccules that float more slowly do not escape from the tank. To this end, adverse phenomena such as short-circuiting, flow stagnation in fast sections, and backflow due to wall impact should be avoided. When the hydraulic load of the dissolved air flotation tank exceeds 10 m3/m2•h, it is easy for the effluent from flotation to contain bubbles that enter the subsequent filter; these bubbles remain in the upper layer of the filter. Although it has been found that the presence of bubbles in the filter helps to remove particles from the water, it causes a sharp increase in the head loss in the filter, thereby significantly shortening its operating cycle. Therefore, the presence of bubbles in the water fed into the filter should be avoided. Hence, while increasing the hydraulic load on the dissolved air flotation unit significantly, a degassing system must be installed (for details, see Appendix 2) to ensure the proper operation of the process. It features simple installation of a clever scum-skimming device, which prevents disturbance to the scum layer during skimming and thus avoids sedimentation that could affect the quality of the water output. 2.3.4 The design parameters for air flotation tanks vary widely both domestically and internationally. In China, the following parameters are primarily used: In the contact zone, the residence time is > 2.0 minutes, and the surface load rate is 36–72 m3/m2•h. In the separation zone, the surface load rate is 7.2–10.8 m3/m2•h. 2.3.5 According to Article 8.2.9 of the “Design Code for Outdoor Sewerage Systems of the People’s Republic of China”, air flotation tanks can be rectangular or circular in shape. The design of rectangular air flotation tanks shall meet the following requirements: 1. The tank shall be equipped with a reaction section, and the reaction time should be approximately 10–15 minutes ； II. The width of each pool cell should not exceed 4.5 m, and the length-to-width ratio should be 3 to 4 ； III. The effective water depth should be 2.0–2.5 m, and the superheight should not be less than 0.4 m ； IV. The residence time of wastewater in the separation section of the air flotation tank should not exceed 1.0 hour ； V. The horizontal flow velocity of wastewater in the tank should not exceed 10 mm/s ； VI. A scum collection trough should be installed at the end of the air flotation tank ； VII. A scum skimmer should be installed in the tank, with its moving speed preferably ranging from 1 to 5 m/min. 2.3.6 Studies on the air flotation model have led to the emergence of some new concepts, such as the need for flocs the size of a needle tip (tens of micrometers) for the air flotation treatment of drinking water. pH is just as important for floc formation as it is for bubble adhesion. At the optimal pH, the zeta potential of the particles is close to 0 or negative, allowing for a higher overflow flow rate (≤15 m/h). The optimal design for a flotation tank is a plug-flow reactor with a long and narrow contact zone and separation zone, where the residence times are greater than 1.5 minutes and 5 minutes respectively. This results in a much higher surface load compared to conventional designs, along with improved efficiency. Adding a jet mixer ensures that the water exiting the oil separator is thoroughly mixed with the aerated water before entering the flotation tank, allowing the resulting ultra-fine bubbles to be evenly dispersed throughout the tank. Since wastewater generally contains toxic and harmful substances, the structural design of the flotation tank must take into account the installation of anti-corrosion layers and covers; where possible, organized emission of the flotation gases should also be considered. 2.4 Orthogonal experiment analysis showed that, among the three main parameters—recirculation ratio, coagulant dosage, and effective residence time in the flotation tank—the order of their influence on the efficiency of air flotation is: recirculation ratio > coagulant dosage > effective residence time in the flotation tank. The return flow pipeline between the dissolved air tank and the air flotation tank should be short, with minimal pressure loss, in order to prevent air from escaping from the supersaturated water. A decrease in water temperature has an adverse effect on the efficiency of dissolved air flotation. 3 Application drawings (drawing) Appendix 1 Other air flotation methods 1. Dispersed air flotation method. It can be further divided into two methods: the rotor gas-fragmentation method (also known as vortex air flotation or rotary cutting air flotation, see Figure 1) and the micro-pore gas dispersion method. The former draws in air by means of the negative pressure generated by the centrifugal force of the high-speed rotor; after mixing thoroughly with the wastewater that has been lifted to the surface, the gas is broken down into tiny bubbles due to the shear force of the water, and these bubbles disperse throughout the water ； The latter involves passing air through microporous materials or small holes in nozzles, thereby dividing it into tiny bubbles that are dispersed in the water. This method uses simple equipment, but it produces larger bubbles, and large bubbles tend to form in the water. Large bubbles rise at a faster speed in water. The enormous inertial force not only prevents bubbles from adhering well to the flocs, but also causes severe turbulence in the water, thereby breaking apart the flocs. Therefore, in eddy current air flotation, the air intake volume must be strictly controlled. The formation of bubbles depends on the high-speed cutting by the impeller, as well as their natural release in a pressure-free environment. Bubbles with large diameters require more energy to generate, and this makes the flotation treatment of wastewater with high water temperatures less effective. Due to the large size of the bubbles formed, it is more suitable for treating some heavy oil wastewater. Since these large bubbles tend to burst as they rise to the surface, it is recommended that the residence time of the wastewater in the \"separation chamber\" not exceed 20 minutes; longer retention times lead to more bubble ruptures, which may cause the flocs to settle back to the bottom of the tank. The bubbles produced by the dispersed air flotation method are relatively large in diameter, and the micro-porous plates are prone to clogging; however, it is more energy-efficient. It is widely used in mineral flotation, as well as in the preliminary treatment of wastewater containing oils and wools, and for the foam flotation treatment of wastewater with high levels of surfactants. 2. Air flotation method using a dissolved air pump. The dissolved air pump uses either an eddy current pump or a gas-liquid multiphase pump. The principle behind it is that at the inlet of the pump, air enters the pump chamber along with water; the rapidly rotating impeller divides the inhaled air into small bubbles multiple times, and these small bubbles dissolve rapidly in the water under the high pressure inside the pump, forming dissolved air water, which then enters the flotation tank to carry out the flotation process. The bubble diameter produced by the dissolved air pump is generally between 20 and 40 μm. The maximum solubility of air in the pumped water is 100%, while the maximum air content in the dissolved air water is 30%. The performance of the pump remains very stable even when there are changes in flow rate or air volume, thereby providing excellent operating conditions for pump regulation and control of the air flotation process (Figure 2). 3. Electrocoagulation flotation method. This method involves installing multiple sets of electrodes with alternating positive and negative charges in the wastewater; when direct current is passed through, electrolysis, polarization of particles, electrophoresis, redox reactions, as well as interactions between the electrolytic products and the wastewater occur. When electrolysis is carried out using a soluble electrode (usually aluminum-iron) as the anode, the metal of the anode dissolves to release aluminum and iron cations, which then combine with hydroxide ions in water to form highly adsorbent aluminum and iron hydroxides. These substances adsorb and agglomerate impurity particles in water, thereby forming flocs. These flocs adhere to the microbubbles (hydrogen gas) generated at the cathode, thereby enabling air flotation separation. However, the electrolytic coagulation flotation method has issues such as high power consumption, large metal usage, and easy electrode passivation, which makes it difficult to apply in large-scale production. 4. Biological and chemical air flotation methods. Biological air flotation relies on the gases released by microorganisms during their metabolic processes, which adhere to the flocs and cause them to float to the surface ； Chemical flotation involves adding a certain chemical agent to water, and utilizing gases such as oxygen, chlorine, and carbon dioxide generated through chemical reactions to cause the flocs to rise to the surface. Due to various constraints, this air flotation method has a poor level of stability and reliability in treatment, and it is not widely used. Figure 1: Principle of the vortex air flotation process. Figure 2: Principle of air flotation using a dissolved air pump. Appendix 2: Degassing system. The degassing system is divided into two types: internal and external degassing systems. The key to an internal degassing system is to provide a coalescing surface, which is similar in form to inclined tube or inclined plate sedimentation tanks. This surface not only facilitates the merging of excess small bubbles to form larger bubbles with a higher rising speed, but it also induces secondary gas flotation – that is, the re-adhesion of \"free\" bubbles to the remaining flocs – thereby making the buoyancy of the aggregate greater than its weight. Therefore, when the merging surface is designed appropriately, sedimentation of sludge within the internal degassing system can be avoided. There are many types of external degassing systems. The free-fall water weir and retention tank between the air flotation tank and the filter tank are two of the simpler types; a more complex design involves the use of a dedicated bubble stripping column, through which the water from the air flotation process flows in a downward direction, with air being injected at its bottom via diffusers. Studies have found that the efficiency of the three dissolved air flotation processes—without a degassing system, with an external degassing system, and with an internal degassing system—increases in order. At a hydraulic load of 17 m3/m2•h, the effluent turbidity for the three processes was 0.80, 0.65, and 0.60 NUT respectively, while the treatment capacity of the subsequent filters was 360, 380, and 640 m3/m2 respectively ； At a hydraulic load of 44 m3/m2•h, the turbidity of the effluents from the three air flotation processes was 3.80, 1.85, and 1.70 NUT respectively, while the treatment capacity of the subsequent filters was 100, 140, and 180 m3/m2 respectively.