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Classification and design principles of dissolved air flotation 1 Classification (type) According to different classification principles, DAF can have different classifications. 1.1 According to the different pressures when bubbles are separated from water, it can be divided into two types: vacuum air flotation and pressure dissolved air flotation. The former uses vacuuming to dissolve air under normal pressure or pressure, and then releases microbubbles under negative pressure for air flotation. ； The latter is to force the air to dissolve in the water under pressure, and then suddenly reduce the pressure to release the dissolved gas from the water, adhere to the floc in the form of micro bubbles, and float together.  1.1.1 Although the energy consumption of the vacuum air flotation tank is low, the formation of bubbles and the adhesion between bubbles and floc are relatively stable. ； However, the amount of bubble release is limited ； Moreover, all equipment components must be sealed in the flotation tank. ； The structure of the flotation tank is complex ； It is only suitable for treating wastewater with low pollutant concentration (not higher than 300mg/l), so there are not many practical applications. 1.1.2 Pressure dissolved air flotation is currently the most commonly used method at home and abroad. There are three basic processes that can be selected: full process dissolved air flotation, partial dissolved air flotation and partial reflux dissolved air flotation. 1.1.2.1 Full-process dissolved air flotation method The full-process dissolved air flotation method pressurizes all wastewater with a water pump. In the dissolved air tank, air dissolves in the wastewater, and then the wastewater is sent to the flotation tank through the pressure reducing valve. The flow chart is shown in Figure 1. Its characteristics are: ①The large amount of dissolved air increases the chance of contact between oil particles or suspended particles and bubbles ； ②Under the same conditions of processing water volume, it is smaller than the flotation tank required by the partial reflux dissolved air flotation method. ③All wastewater passes through a pressure pump, and the required pressure pump and dissolved gas tank are larger than those of the other two processes, so the investment and operating power consumption are large. 1.1.2.2 Partial dissolved air flotation method Partial dissolved air flotation method is to take part of the wastewater to pressurize and dissolve the air, and the remaining wastewater directly enters the air flotation tank and is mixed with the dissolved air wastewater in the air flotation tank. Its characteristics are: ①Compared with the whole-process dissolved air flotation method, the pressure pump required is small, so the power consumption is low. ； ②The size of the flotation tank is the same as that of the full-process dissolved air flotation method, but smaller than that of the partial reflux dissolved air flotation method.  1.1.2.3 Partial reflux dissolved air flotation method The partial reflux dissolved air flotation method is to take part of the treated water to reflux. The reflux water is pressurized and dissolved air. After decompression, it enters the flotation tank and is mixed with the oily wastewater from the flocculation tank and flotated. The process is shown in Figure 2. Its characteristics are: ①The amount of pressurized water is small and power consumption is saved. ； ②Does not promote emulsification during the flotation process ； ③Alum flowers are formed well and there is less post-flocculation. ； ④The volume of the flotation tank is larger than the first two processes. Modern air flotation theory believes that: Partial reflux pressurized dissolved air flotation saves energy, can fully utilize the flotation (coagulation) agent, and the treatment effect is better than the fully pressurized dissolved air flotation process. The treatment effect is best when the reflux ratio is 50%, so the partial reflux (reflux ratio 50%) pressurized dissolved air flotation process is the most commonly used air flotation method at home and abroad. Figure 2 Partial reflux dissolved air flotation process flow chart 1.2 According to whether the microbubble sludge layer (bed) in the flotation tank has a filtering effect and the different flow patterns of the water, it is divided into: Early DAF, normal DAF and turbulent DAF. (See Appendix 3 for details) 2 Design principal DAF is generally set up before the biological treatment unit and after the physical treatment unit. * It is conventionally classified as a physical processing unit. If it is set to two-stage flotation, for convenience and economy, the first- and second-stage flotation cells are often juxtaposed in the layout. The liquid level difference between the first- and second-stage flotation cells is about 500mm to ensure that sewage flows from the first-stage flotation tank to the second-stage flotation tank, and the lift pump is eliminated to achieve energy saving effects. Reflected in the vertical layout, that is, the elevation of the slag scraper carriage (plate), adjustable weir and top of the slag removal tank must be strictly controlled during design and construction. This is very important and is one of the key factors. Otherwise, the air flotation effect will be seriously affected (the foam layer cannot be removed mechanically), which is why an adjustable outlet weir must be used. DAF is mainly composed of air saturation equipment (also called pressure dissolved air system), air release equipment (also called dissolved air release system) and air flotation tank (also called air flotation separation system). At present, the design of the dissolved air flotation process and the determination of optimal operations require pilot testing and experience. Below, based on the experience summarized in various applications, the design principles of each component are introduced. 2.1 Pressure dissolved air system (including pressure dissolved air tank, air compressor, water pump and its ancillary equipment) 2.1.1 The dissolved air system accounts for 50% of the energy consumption of the entire air flotation process, and the value of the dissolved air tank accounts for 12% of the total infrastructure investment of the factory. Therefore, optimizing the design of the dissolved air system is very important to reduce the cost of air flotation operation. Dissolved gas tanks are mostly cylindrical and arranged vertically. The volume is calculated based on the wastewater retention time of 25 to 3 minutes. The tank can be equipped with partitions, porcelain rings, etc., and empty tanks are also used. Because the water and gas phases are mixed in the dissolved gas 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 reducing valve is installed on the bottom of the tank to balance the pressure. The pressure in the tank is generally controlled at about 0.45MPa. Based on this, the parameters of the lift pump, reflux pump and air compressor can be determined. In foreign design data and literature, it is believed that the longer the gas-water residence time, the higher the gas-dissolving efficiency. This makes the volume of the dissolved gas tank appear huge, and the residence time is sometimes as long as 3 to 5 minutes. Domestic research has confirmed that liquid film resistance controls the dissolved gas rate. It is believed that the longer the residence time, the better the dissolved gas effect is not consistent with reality. Therefore, domestic design parameters are different from foreign countries. The predetermined dissolved gas efficiency is used as the design indicator, and the liquid phase flow density and the total liquid phase mass transfer coefficient are used as parameters. All studies have shown that dissolved air tanks with packed beds are more effective than those without packed beds, and their efficiency can reach up to 99%. However, in actual operation, internal inspections of dissolved air tanks are often required. Therefore, systems without packed beds are often used in many dissolved air flotation processes, and most dissolved air tanks without packed beds are often equipped with internal or external injectors to improve dissolved air efficiency. 2.1.2 The pressurized dissolved air method has two air intake methods, namely air intake before the pump and air intake after the pump. The first is air inlet before the pump. The flow chart is shown in Figure 3. When the amount of air inhaled is less than the saturation of air in water at that temperature, a pipe is led from the water pump pressure pipe back to the water suction pipe, and a hydraulic ejector is installed on the branch pipe. When the wastewater passes through the hydraulic ejector, a negative pressure is created. After the air is sucked in and mixed with the wastewater, it is sent to the dissolved air tank through the water suction pipe and the water pump. This method eliminates the need for an air compressor and has a good air-water mixing effect. However, the water pump must use a self-priming method to inlet water and maintain a water head above lm. Its maximum air suction volume cannot be greater than 10% of the water suction volume of the water pump. Otherwise, the water pump will work unstable, destroy the vacuum degree that the water pump should have, and cause cavitation. The second type is the air intake after the pump. The flow chart is shown in Figure 4. When the air suction volume is greater than the saturation of air in water at that temperature, air is forced into the outlet pipe of the water pump through the air compressor, but it should not be greater than 25% of the water suction volume of the water pump. This method makes the water pump work stably and does not require working under positive pressure, but it does require air to be supplied by an air compressor. In order to ensure a good gas dissolving effect, the volume of the gas dissolving tank is also relatively large, and generally a more complex filled gas dissolving tank needs to be used. Figure 3 Air intake flow chart before the pump Figure 4 Air intake flow chart after the pump 2.1.3 Adjustment of the air injection amount is another key factor in the flotation operation, which generally changes with the selected dissolved air pressure or reflux ratio. Experiments also show that the quality of the effluent only depends on the total amount of air introduced into the system (when the bubble sizes are consistent), and has nothing to do with individual pressure or return ratio. It must be determined through repeated practice based on the quality of sewage, the type of flotation (coagulant) agent and pressure relief releaser. 2.1.4 The water level in the dissolved air tank is an important factor affecting the flotation effect. We are in Nanning City. We have reduced the size of the cellar in the water vapor contact part, and the gas dissolving effect is not good. ； If the water level is too low, there will be no necessary buffering depth, and the gas will pass through the water layer and enter the flotation equipment to form large bubbles, and the flotation effect will be poor. It is recommended that the water level be controlled at about 1/3 to 1/4 of the tank. 2.1.5 The pressure in the dissolved gas tank is an important factor affecting the gas volume. Generally speaking, when the pressure is high, there is more dissolved air. In the air compressor filling method, the pressure in the dissolved air tank is determined by the air pressure of the air compressor and the water pump. During normal operation, sufficient water pressure must first be ensured, but the water pressure and air pressure must be basically equal. When using a water ejector to add gas, the key to ensuring the pressure of the dissolved air tank is to use a suitable water pump. Generally, the water pump pressure should be greater than 0.3Mpa while ensuring the rated flow rate. The pressure of the dissolved air tank can be adjusted by adjusting the water outlet valve of the dissolved air tank, the water pump outlet valve, and the backflow control valve. 2.1.6 According to the "People's Republic of China * * Article 8.2.7 of the "Standard Outdoor Drainage Design Code" The design of the dissolved gas tank shall meet the following requirements: 1. The working pressure of the dissolved gas tank should be 300~500kPa (about 3~5kgf/cm2) ； 2. The air volume is measured by volume and can be calculated based on 5 to 10% of the sewage volume. ； 3. The residence time of sewage in the gas-dissolving tank should be determined according to the type of tank, generally 1 to 4 minutes. There should be measures to promote full mixing of gas and water in the tank. ； 4. The dissolved gas tank using partial reflux should be of dynamic type and should have water level control measures. 2.1.7 It is mentioned in some applications that a precision air flow stabilizer is added. Its function is to ensure smooth and uniform pressure before the air enters the nozzle of the dissolved air tank. The reflux ratio refers to the ratio of the amount of reflux water that enters the dissolved air tank to pressurize dissolved air and the amount of treated water when partial reflux dissolved air flotation is used. The reflux ratio is generally 25% to 50% of the wastewater. However, when the sewage quality is poor and the sewage volume is not large, the return ratio can be appropriately increased to ensure the effluent quality. 2.2 The release of the dissolved air release system (mainly the release head) is the key device of the system. It has a significant impact on the size and distribution of bubble formation, as well as on the flotation water purification effect and operating costs. The release efficiency of the currently used release device can reach 99.2%. 2.2.1 Previous research believed that the size of the released bubbles is related to the pressure of dissolved air. Most large bubbles are formed at low pressure, which is not conducive to air flotation. The latest domestic research believes that: The release pattern of bubbles when dissolved gas water is decompressed and energy dissipated is different from the situation of bubbles in still water. ； The appearance of large bubbles at low pressure is attributed to a defective releaser. In addition to releasing a large number of stable tiny bubbles, the key is how to prevent clogging. Currently, different types of releasers are used at home and abroad, including simple valve type, needle valve type and special release (patented). There are many patented products for dissolved gas releasers, among which the ones with better effects generally have the following characteristics:: There is a momentary pressure drop at the nozzle ； The direction of the water flow will suddenly change at the inlet of the release device (usually 90°) ； The diameter of the releaser does not exceed 2.5mm, and the residence time of water in the releaser is <1.5ms. ； The velocity of the water leaving the release gradually decreases ； Water leaving the release will collide with a baffle in front of it. It is impossible for any releaser to produce only microbubbles, but generally bubbles with a diameter between 40 and 70μm. The generation of some large bubbles is inevitable, although the existence of these large bubbles will reduce the operating efficiency of the system.  2.2.2 According to the "People's Republic of China * * Article 8.2.8 of "Standard Outdoor Drainage Design Code" The selection of dissolved air releaser should be determined based on the oily sewage water quality, treatment process and releaser performance. 2.3 Air flotation separation system (air flotation tank component) The function of the air flotation separation system is to ensure a certain volume to complete the full mixing, contact and adhesion of microbubbles and impurities in the water, as well as the separation of air-laden flocs from clean water. 2.3.1 In order to improve the treatment effect of air flotation, coagulant or flotation agent is often added to the wastewater. The dosage varies depending on the water quality and is generally determined by testing. For aluminum flocculants, the effluent turbidity can be further reduced by increasing the stirring intensity. In order to ensure the coagulation effect of the flotation (coagulant) agent, a static pipeline mixer and a reaction chamber should be installed at the water inlet end of the flotation tank. The effective volume of the reaction chamber is calculated based on the residence time of wastewater (the sum of the water inlet volume and the return volume) for 10 minutes. It is generally divided into three rooms, arranged in a labyrinth style, and a mixer is installed in each room to improve the coagulation effect. The velocity gradient in each room is often the same. The design of the flocculation tank (that is, the reaction chamber) is best to provide a plug flow state (turbulent flow state), which can ensure a better air flotation effect. 2.3.2 The maximum recommended size of the dissolved air flotation tank can reach 145m2, and the corresponding water production capacity is 2900~4350m3/h. The water production capacity per unit area is at least doubled. The depth of the dissolved air flotation pool increases from 1.5m to 5.0m, and the pool shape develops from rectangular to square, with an aspect ratio of (1.2~2): between 1. Currently, the length of a well-operated dissolved air flotation tank can be up to 12m, but the width is limited to 8.5m, mainly because the maximum span of the mechanical slag scraper is 8.5m. The residence time of sewage in the flotation tank is generally 30 to 40 minutes, the working water depth is 15 to 25m, the aspect ratio is not less than 4, and the surface load is 5 to 10m3/m2·h. If the residence time is too short, the impact of the water flow will be large, and the sewage in the flotation tank will be in a strong turbulent state. This will not only be detrimental to the adhesion of bubbles and flocs, but will also break up some of the flocs that have adhered to the bubbles. ； In addition, due to turbulent flow and short reaction time, part of the added coagulant flows out with the effluent before complete reaction, resulting in a reduction in the removal rate of suspended solids in the effluent, and even a negative growth trend. 2.3.3 The air flotation tank is divided into two areas: contact area and separation area. 2.3.3.1 When designing the contact area, attention should be paid to controlling the upward flow rate of the flocculated water to avoid short flow and biased flow, so that the adhered bubbles will not be sheared off by the water flow during the floating process and affect the subsequent separation effect. Under normal circumstances, the rising flow rate of the contact zone is preferably controlled at 10-20mm/s, and the height is preferably 1.5-2.0m. At this flow rate and height, the contact time between flocs and microbubbles is ensured, and the flocs will not be damaged or sink due to too long floating time. Reasonably arrange the release device so that the scope of the released water covers the entire area and can fully and timely contact the flocs under the microbubbles. 2.3.3.2 When selecting the separation speed in the separation zone, it should be conducive to the floating of air-laden flocs. For air-laden flocs with large flocs, low density and difficult to break, a larger separation speed is generally adopted, otherwise a smaller value is used. The flow rate in the separation zone should be 1 to 3 mm/s. If the flow rate is too small, large flocs will settle due to crowding. If the flow rate is too high, the interface between the air-laden flocs and clean water will extend downward, causing the flocs to flow out with the water and the water quality to decline. If the concentration is high and there is a lot of scum, resulting in crowded floating phenomenon during solid-liquid separation, the floating speed should be reduced. Otherwise, the scum layer will be too thick, which will cause slag falling, or the separation effect will be affected because the volume of the separation zone is too small. When selecting a water collection system, try to collect water as evenly as possible to prevent small air-laden flocs that float slowly from flowing out of the pool. For this reason, undesirable phenomena such as short flow, fast part stagnation, and wall backflow should be avoided. When the hydraulic load of the dissolved air flotation tank is >10 m3/m2·h, it is easy for the flotation water to carry bubbles into the subsequent filter, and the bubbles will exist in the upper layer of the filter. Although some people have found that the presence of bubbles in the filter is beneficial to the removal of particles in the water, it will lead to a sharp increase in the filter head loss, thereby significantly shortening the filter operation cycle. Therefore, the presence of bubbles in the filter inlet water should be avoided. Therefore, while greatly increasing the hydraulic load of the dissolved air flotation tank, a degassing system must be installed (see Appendix 2 for details) to ensure the normal operation of the process. The slag scraping equipment is easy to install and smart, so that it will not disturb the scum layer and cause slag falling, which will affect the quality of the effluent water. 2.3.4 The design parameters of air flotation tanks at home and abroad vary widely. my country mainly adopts the following parameters:: contact zone: Residence time > 2.0min Surface loading rate 36～72 m3/m2·h Separation zone: Surface loading rate 7.2～10.8 m3/m2·h 2.3.5 According to "The People's Republic of China * * Article 8.2.9 of "Standard Outdoor Drainage Design Code" The air flotation tank can be rectangular or circular. The design of the rectangular air flotation tank should meet the following requirements: 1. The air flotation tank should be equipped with a reaction section, and the reaction time should be 10 to 15 minutes. ； 2. The width of each grid should not be greater than 4.5m, and the length-to-width ratio should be 3 to 4 ； 3. The effective water depth should be 2.0~2.5m, and the super height should not be less than 0.4m ； 4. The residence time of sewage in the separation section of the air flotation tank should not be greater than 1.0h ； 5. The horizontal flow rate of sewage in the pool should not be greater than 10mm/s ； 6. A foam collecting tank should be installed at the end of the flotation tank. ； 7. A foam scraper should be installed in the pool, and the moving speed of the foam scraper should be 1 to 5m/min. 2.3.6 The research on air flotation model has led to some new concepts, such as the air flotation treatment of drinking water requires needle tip size (tens of microns) flocs. pH is as important for floc formation as it is for bubble adhesion. At the optimal pH, the ζ potential of the particles is close to 0 or negative, and a higher overflow flow rate (≤15m/h) can be used. The optimal design of the flotation tank is a plug flow reactor with a long and narrow shape in the contact zone and separation zone, with residence times greater than 1.5 min and 5 min respectively. In this way, the surface load is much higher than the traditional value, and the efficiency is higher. Adding a jet mixer can ensure that the effluent from the grease trap and the dissolved gas water are fully mixed before entering the flotation tank, so that the formed ultrafine bubbles are evenly distributed in the tank. Because sewage generally contains toxic and harmful substances, the structural design of the flotation tank must consider the installation of an anti-corrosion layer and a top cover. When conditions permit, the organized discharge of flotation gas can be considered. 2.4 Obtained from orthogonal test analysis: The primary and secondary relationship between the three main parameters of the reflux ratio, the coagulant dosage and the effective residence time of the flotation tank (pool) on the air flotation effect is:: Reflux ratio > coagulant dosage > effective residence time of the flotation tank (pool). The return water delivery pipeline between the dissolved air tank and the flotation tank should be short and the pressure loss should be small to prevent air from escaping from the supersaturated water. Lowering the water temperature has a negative impact on the dissolved air flotation effect. 3 Application drawings (drawing) Appendix 1 Other air flotation methods 1. Dispersed air flotation method. It can be divided into two types: rotor air flotation method (also called vortex concave air flotation or rotary shear air flotation, see Figure 1) and micropore air distribution method. The former relies on the negative pressure caused by the centrifugal force of the high-speed rotor to suck in the air, and after it is fully mixed with the raised wastewater, under the action of the shear force of the water, the gas breaks into microbubbles and diffuses in the water. ； The latter is to make the air pass through the microporous material or the small holes in the nozzle to be divided into small bubbles and distributed in the water. The equipment of this method is simple, but the bubbles produced are larger, and large bubbles are easy to occur in the water. Large bubbles rise faster in water. The huge inertial force not only cannot make the bubbles adhere well to the flocs, but will instead cause severe turbulence in the water body and crush the flocs. Therefore, the air intake volume must be strictly controlled for vortex concave air flotation. The generation of bubbles relies on the high-speed cutting of the impeller and the natural release in a pressureless system. The bubble diameter is large and the power consumption is high. Especially for the air flotation treatment of high water temperature sewage, the treatment effect is unsatisfactory. Due to the large bubbles generated, it is more suitable for treating some heavy oil wastewater. Since large bubbles are easy to burst during the floating process, it is recommended that the residence time of the wastewater in the "separation chamber" should not exceed 20 minutes during the design. The longer the time, the more bubbles will burst, which may cause the floc to settle to the bottom of the pool again. The bubbles produced by the dispersed air flotation method are larger in diameter, and the micropore plate is also susceptible to blockage, but it is more economical in terms of energy consumption. It is mostly used for mineral flotation and primary treatment of wastewater containing grease, wool and other wastewater, as well as foam flotation treatment of wastewater containing a large amount of surfactant.  2. Dissolved air pump flotation method. The dissolved air pump uses a vortex pump or a gas-liquid multiphase pump. Its principle is that air and water enter the pump casing together at the inlet of the pump. The high-speed rotating impeller cuts the inhaled air into small bubbles multiple times. The small bubbles quickly dissolve in the water under the high-pressure environment in the pump to form dissolved air water and then enter the air flotation tank to complete the air flotation process. The diameter of the bubbles generated by the dissolved air pump is generally 20 to 40 μm, the maximum solubility of the inhaled air reaches 100%, and the maximum air content in the dissolved air water reaches 30%. The performance of the pump is very stable when the flow rate changes and the air volume fluctuates, providing excellent operating conditions for pump adjustment and control of the air flotation process (Figure 2). 3. Electrolytic condensation flotation method. This method is to place multiple sets of positive and negative electrodes in wastewater. When direct current is passed through, electrolysis, particle polarization, electrophoresis, oxidation and reduction, and interactions between electrolytic products and wastewater will occur. When a soluble electrode (generally aluminum-iron) is used as the anode for electrolysis, the metal of the anode will dissolve the cations of aluminum and iron and combine with the hydroxide ions in the water to form highly adsorbent aluminum and iron hydroxides to adsorb and condense impurity particles in the water, thereby forming flocs. This kind of floc adheres to the microbubbles (hydrogen gas) generated on the cathode, allowing air flotation separation to be achieved. However, the electrolytic condensation flotation method has problems such as high power consumption, large metal consumption, and easy passivation of electrodes. Therefore, it is difficult to be suitable for large-scale production. 4. Biological and chemical flotation. Bioflotation relies on the gases released by microorganisms during their metabolic process to adhere to floc and float to the surface of the water. ； The chemical flotation method is to add certain chemicals to the water to make the flocs float with the help of oxygen, chlorine, carbon dioxide and other gases generated by chemical reactions. This air flotation method is limited by various conditions, so the stability and reliability of the treatment are poor, and there are not many applications. Degassing systems are divided into two types: internal and external degassing systems. The key to an internal degassing system is to provide a coalescing surface in the form of a sloping tube or sloping plate sedimentation tank. This surface can not only promote the merger of excess small bubbles and produce large bubbles with a large rising speed, but can also cause secondary air flotation, that is, the re-adhesion of "free" bubbles and residual floc, making the buoyancy of the aggregate greater than gravity. Therefore, when the merging surface is properly designed, the deposition of sludge in the internal degassing system can be avoided. There are many forms of external degassing systems. The free-fall weir and retention tank between the flotation tank and the filter are the simpler ones. The more complicated one is to set up a special bubble blow-off column, and the air floated water passes through the column in a downflow form. At the same time, air is injected at the bottom through a diffuser. Some studies have found: The effects of the three dissolved air flotation processes without degassing system, with external degassing system and with internal degassing system are gradually enhanced. When the hydraulic load is 17 m3/m2·h, the effluent turbidities of the three processes are 0.80, 0.65 and 0.60NUT respectively, and the treatment capacities of the subsequent filters are 360, 380 and 640 m3/m2 respectively. ； When the hydraulic load is 44 m3/m2·h, the turbidities of the effluent from the three air flotation processes are 3.80, 1.85 and 1.70 NUT respectively, and the treatment capacities of the subsequent filters are 100, 140 and 180 m3/m2 respectively. This post was last edited by johncom on 2009-2-24 10:48 ]

In addition, air flotation equipment is also a relatively mature equipment. The key to judging its quality is whether it can meet the design requirements, such as the removal rate of SS or oil.； I can’t say how good the brand is. Most of the products in China are produced in Jiangsu. In fact, you can also process them yourself. In addition, currently only Wuxi Hudong Company has the production technology and good quality for shallow air flotation in China.

So, what are the major brands of flotation pools in the world?

Air flotation tanks are generally designed and produced by sewage treatment companies themselves. Only foam scrapers and dissolved air equipment are finalized products.

The flotation tank is suitable for substances with relatively light specific gravity, such as oil, pulp black and white liquor, organic matter, etc. If it contains substances that are easy to settle, it must first pass through the sedimentation tank, or use a sinking and flotation tank.

The aerators in the flotation tank are generally imported brands, and the flotation tank is independently designed by the engineering company.