Views: 218 Author: Site Editor Publish Time: 2024-06-29 Origin: Site
If you need to know about flotation and flotation equipment, just read this article.
With the rapid development of industry and agriculture, global environmental problems are becoming more and more serious, posing a serious threat to human survival and development. Environmental pollution is reflected in three major aspects: water, atmosphere, and solid waste, among which water pollution is the most prominent.
Sewage discharge has seriously affected the basic living space of human beings, making the already scarce water resources even worse.
Therefore, sewage treatment has become an important task for global environmental resource protection.
As a long-standing and efficient solid-liquid separation technology, flotation was originally used for mineral processing. Its principle is to introduce a large number of tiny bubbles into sewage, and through the interaction of surface tension and particulate matter, through complex processes such as adsorption, flocculation and hydrodynamics, it adheres to suspended particles to form floccules with a specific gravity of less than 1. According to the principle of flotation, it floats to the water surface to achieve solid-liquid separation and purify sewage.
This article covers multiple dimensions such as flotation principle, application, design, operation and development trend, to help everyone gain a deep understanding of this key water treatment technology.
Table of contents（Click to go to where you want to see）
1.2 Kinetic theory
2. Types of flotation technology
2.1.1 Jet flotation method
2.2.2 Microporous cloth air flotation method
2.2.3 Impeller aeration flotation
2.3 Dissolved Air Flotation
2.3.1 Dissolved air pump flotation method
2.3.2 Dissolved air vacuum flotation
2.3.3 Pressurized dissolved air flotation
2.3.2.1 Fully dissolved gas process
2.3.2.2 Partially dissolved gas process
2.3.2.3 Partial reflux pressure dissolved air flotation method
3.2 The 50s
3.3 1970s and 1980s
3.4 Since the 1990s
4. Dissolved air flotation equipment components
4.2.1 The relationship between the releaser and microbubbles
4.3 Flotation Tank
4.3.1 Horizontal flotation tank
4.3.2 Vertical flow flotation tank
FAQ
1. Factors affecting the dissolved gas volume of the pressurized dissolved gas tank
2. Factors Affecting the Effect of Pressurized Dissolved Air Flotation on Water Purification
3. How does air dissolve in an air dissolving tank?
4. How are microbubbles formed?
5. Application scope of air flotation technology
Air flotation technology can achieve solid and liquid-liquid separation, mainly relying on the adhesion between bubbles and flocs and separating air-entrained flocs from water.
First, relying on Henry's law, dissolved air water can be stable and exist in large quantities in water at a specific temperature;
Dissolved air water can remain in water for a long time because there is a stable phase interface between the gas and liquid in contact with each other. A layer of stagnant air film and liquid film is at the interface. There is a relatively stable equilibrium state on both sides of them. The air and liquid film's outside is a continuously flowing gas-liquid fluid. The mass transfer resistance of each solute phase remains in the stagnant movie, that is, the double membrane theory.
During the water treatment process, bubbles can collide and adhere to floc particles because both microbubbles and floc particles in water have abundant surface free energy and belong to a thermodynamically unstable system.
The adhesion between the two is a thermodynamic spontaneous process, and the free energy gradually weakens to promote the adhesion process between microbubbles and particles.
When microbubbles and particles adhere to each other, there is a contact angle of gas-liquid-solid three-phase equilibrium between the two. The adhesion between microbubbles and solid particles increases with the increase of the equilibrium contact angle. The interfacial tension between the gas and liquid phases causes the mutual adhesion between bubbles and particles.
By studying the effect of Zeta potential on the equilibrium contact angle of air-entrained flocs, it is found that when the Zeta potential is 17mV, the equilibrium contact angle reaches the maximum value. At this time, the collision adhesion between the flocs and the microbubbles is more solid, and the air-entrained flocs are more stable.
The larger the equilibrium contact angle, the greater the capillary force, and the stronger the hydrophobicity of the surface of the three-phase body; that is, there are hydrophobic groups on the surface of the three-phase body.
The hydrophobic colloid uses the charge of its potential ions to maintain stability. Suppose the charge of the potential ions of the hydrophobic colloid is destroyed. In that case, the colloid will be destabilized and adsorbed on the hydrophobic groups of the network structure formed by aluminum or iron-containing coagulants, forming air-entrained flocs with loose surfaces and high water content.
The process of collision and adhesion between microbubbles and floccules
In addition to the free energy on the surface of microbubbles and floc particles, there are intermolecular forces.
(1) Adsorption of microbubbles.
Micro-nanobubbles have a large specific surface area, which manifests in surface tension.
The surface film molecules of microbubbles have a blocking effect on the microbubbles themselves because their surface is wrapped with a layer of transparent water film. The water molecules in the water film are stable under surface tension, van der Waals attraction, and hydrogen bonds.
A hydration film will form between floc particles and bubbles. This process has both permanent dipoles between polar molecules and the interaction of hydrophobic forces. Liquid film and colloidal forces have an essential influence on the adsorption between bubbles and particles.
Microbubbles can adsorb floc particles because they tend to reduce surface-free energy. This is also why bubbles adhere to each other, which satisfies the thermodynamic theory.
(2) Between particles and bubbles.
In water, there are not only electrostatic forces and van der Waals forces between floc particles and bubbles but also interface
polarity.
The DLVO theory holds that in the flotation process, the mutual adhesion between floc particles and microbubbles is the sum of the potential energy of repulsion and attraction.
Through experimental research, it is found that the interaction potential energy between particles and bubbles shows a specific change pattern as the distance between the two shortens. When the distance between the two is 4~17nm, the total interaction potential energy is positive, manifesting as repulsion. When the distance between the two is less than 4nm or greater than 17nm, the total interaction potential energy is negative, manifesting as attraction.
Therefore, in the process of bubble and particle collision and adhesion, as long as the two are within a specific range of action, the advantage of the interaction potential energy as attraction can be used to promote the adhesion between the two.
(3) Surfactants and microbubbles.
The surface tension of the clean bubble surface is enormous, and the intermolecular force is also greater. The bubble surface lacks the wrapping of the amphiphilic molecule adsorption layer and is very easy to break.
Surfactants have a long chain structure and amphiphilicity, effectively reducing the interfacial tension between the two phases and enabling the gas to exist stably in the liquid.
Therefore, floc particles and microbubbles can form air-entrained flocs and float to the water surface, thanks to the netting, sweeping, and adsorption bridging between flocs and modified bubbles. In addition, bubbles and flocs are combined to form copolymers, which grow together as bubbles rise - copolymerization. When introduced into water, bubbles collide and adhere with the flocculated flocs. Due to the existence of copolymerization, the negatively charged bubbles not only play the role of surface hydrophobicity to make positively charged particles firmly adsorbed and electrically neutralized but also play the role of bubble aggregation. After the scum is formed during the floating process, they adhere tightly together and will not sink quickly.
The surface of microbubbles in the liquid is negatively charged, and the surface of other colloidal particles, such as algae cells, is also negatively charged. The bubble surface needs to be modified to promote the adhesion between bubbles and particles.
Microbubble modification technology adds a modifier to the water during the bubble generation process so that the modifier adheres to the bubble surface with the generation of bubbles so that the bubble surface is positively charged, thereby improving the adhesion efficiency of microbubbles and colloidal particles in water.
There are hydrophobic groups in the surface modifier. When the modifier is close to the bubble, the surface tension is slight, and the modifier is more likely to be adsorbed on the bubble surface by compressing the hydration film. This not only overcomes the defect of the hydration film on the bubble surface being unstable due to intermolecular van der Waals forces but also provides more adsorption sites for the adhesion of flocs, which helps to form larger air-entrained flocs.
Generally, cationic polymers in surface modifiers are selected to play a role. Cationic polymers contain long chain structures and can form larger flocs using the bridging effect between chains.Improve removal efficiency.
The collision and adhesion of bubbles and flocs is the premise for removing pollutants, and floating air-entrained flocs to the water surface for scraping is the fundamental way to remove impurities.
Air-entrained flocs are subject to the interaction of gravity, buoyancy, and resistance in water. The overall density of air-entrained flocs increases with the increase of bubble content in the flocs, the volume of air-entrained flocs will become relatively more significant, the rising speed will also increase, and it will be easier to float to the water surface.
There are many ways to classify flotation technology. According to the different ways of generating bubbles, flotation technology can be divided into the following types:
The air flotation method uses mechanical shear force to crush the air mixed in water into fine bubbles and then flotate. There are mainly jet flotation, diffuser plate aeration flotation, and impeller air flotation.
The structure of the ejector is shown in Figure 1-1. The high-speed airflow ejected from the nozzle forms a vacuum in the suction chamber, causing the straw to suck in air. The air-water mixture undergoes intense energy exchange in the throat, and the air is crushed into tiny bubbles.
After entering the diffusion section, the kinetic energy is converted into potential energy, further compressing the air, increasing the solubility of the air, and then entering the flotation tank to release the bubbles.
Figure 1-1 Jet flotation process diagram
1-pressure water; 2-nozzle: 3-suction chamber; 4-contraction section; 5-throat: 6-suction and dispersion section: 7-air-water mixture: 8-suction pipe
After compressed air enters from the bottom of the pool, it is cut into tiny bubbles by the microporous diffusion plate. The main principle is to allow air to enter the water through tiny bubbles through the diffusion device with acceptable gaps for flotation.
Figure 1-2 Difuser aeration flotation
1- Inflow chamber: 2- Air inlet: 3- Separation column: 4- Microporous ceramic diffusion plate: 5- Scum: 6- Outflow liquid
The advantages of this method are that it is simple and easy to operate, and it is relatively energy-saving; however, the diffusion micropores are easily blocked, and the bubbles produced by the microporous plate are relatively large (diameter 1-10mm), which greatly limits its scope of use.
This method is mostly used for mineral flotation and primary treatment of wastewater containing oil, wool, etc., and foam flotation treatment of wastewater containing a large amount of surfactants, which can greatly reduce energy consumption.
The principle of aeration flotation of the diffusion plate is shown in Figure 1-2.
The impeller aeration flotation is aerated by the negative pressure formed under the fixed cover plate when the impeller rotates at high speed, and the air is sucked from the air pipe.
The air entering the wastewater and the circulating water flow is thoroughly mixed by the impeller, forming fine bubbles thrown out outside the impeller. The bubbles rise vertically and float after the rectifier plate stabilizes the flow. The formed foam is scraped out of the tank by the slow-moving scraper.
The process equipment is simple, but the bubbles generated are significant, and giant bubbles are easily generated in the water. It is suitable for treating wastewater with small water volumes and high pollutant concentrations.
The structure of the impeller flotation equipment is shown in Figure 1-3.
Figure 1-3 Structure schematic diagram of impeller air-flotation equipment
1-Impeller; 2-Cover; 3-Rotating shaft: 4-Sleeve: 5-Bearing: 6-Inlet pipe: 7-Water inlet trough: 8-Water outlet trough: 9-Foam trough: 10-Scraper plate: 11-Rectifier plate
The electrolytic flotation method is to insert multiple sets of insoluble anode and cathode electrodes with positive and negative alternations into the wastewater, pass direct current, produce electrolysis, electrophoresis, polarization of particles, oxidation-reduction, interaction between electrolysis products, etc., and directly electrolyze the wastewater.
The anode and cathode produce fine bubbles of hydrogen and oxygen, which adhere to the pollutant particles in the wastewater or the floccules formed by coagulation treatment and float to the water surface, creating a foam layer, and then scrape off the foam to separate and remove the pollutants.
The flotation device of the electrolytic flotation method is shown in Figure 1-4.
The electrolytic flotation equipment is simple, easy to manage, easy to control the operating conditions, compact, and practical. Still, problems include high power consumption and easy scaling of the electrode plates.
This method is mainly used in small-scale industrial wastewater treatment and sludge concentration.
Figure 1-4 Vertical-flow electrolytic flotation tank
1-inflow chamber: 2-rectifier grid: 3-electrode group: 4-outflow hole: 5-separation chamber: 6-water collection hole: 7-outlet pipe: 8-sludge discharge pipe: 9-slag scraper: 10-water level regulator: 11-outlet: 12-inlet: 13-sludge discharge
Dissolved air flotation dissolves air in water under a certain pressure, reaches a supersaturated state, and then suddenly reduces the pressure of the dissolved air water. At this time, the air dissolved in the water escapes from the water in the form of tiny bubbles.
Since this operation process can also artificially control the contact time between wastewater and bubbles, the separation effect is better than the dispersed air method and is widely used.
Dissolved air pump flotation is one of the most studied dissolved air flotation methods. The dissolved air pump flotation process is shown in Figure 1-5.
Figure 1-5 Dissolved air pump flotation process diagram
1-wastewater; 2-stirring device; 3-flocculation tank; 4-release pipe; 5-contact zone; 6-separation zone; 7-slag outlet; 8-slag scraper; 9-slag discharge; 10-clean water; 11-return water; 12-gas-liquid separation tank; 13-dissolved air pump; 14-overflow tank
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 pump's inlet, and the high-speed rotating impeller cuts the inhaled air into tiny bubbles many times. The little bubbles quickly dissolve in the water under the high-pressure environment in the pump to form dissolved air water and then enter the flotation tank to complete the flotation process.
The diameter of the bubbles generated by the dissolved air pump is generally 20-40μm. The pump's performance is very stable when the flow rate changes and the gas volume fluctuates, which provides better operating conditions for the pump adjustment and control of the flotation process.
The waste gas is aerated under normal pressure to dissolve the gas entirely. Then, under vacuum conditions, the dissolved gas in the wastewater is precipitated to form fine bubbles. The adhered particulate impurities float on the water surface to form foam scum and are removed. The amount of air precipitated depends on the dissolved air and the vacuum degree.
The advantage of this method is that the dissolved air pressure is lower than that of the pressurized dissolved air method. The bubbles' formation, the bubbles' adhesion to particles, and the floating of flocs are all in a stable environment. The flocs are rarely destroyed, and the energy consumption of the flotation process is relatively small.
However, its most significant disadvantage is that the flotation operates under negative pressure, and the scraper and other equipment must be in a sealed flotation tank. Therefore, the structure of the flotation tank is complex and challenging to maintain and operate. Therefore, this method is less used.
Under pressurized conditions, air is dissolved in water to form an air supersaturation state. Then, the pressure is reduced to normal, allowing air to precipitate and release into the water as tiny bubbles to achieve flotation. This method forms small bubbles, about 20 to 100 μm, with sound treatment effects and wide applications.
Pressurized dissolved air flotation processes can be divided into three types: complete dissolved air process, partial dissolved air process, and reflux pressurized dissolved air process.
The fully dissolved gas process is used to pressurize and dissolve all wastewater encountered in a flotation tank through the pressure relief device for solid-liquid separation.
Its advantages are large dissolved gas volume and increased contact opportunities between bubbles, oil particles, and suspended particles; under the same water treatment conditions, it is smaller than the flotation tank required by the partial reflux dissolved gas flotation method, thereby reducing infrastructure investment.
However, since all wastewater passes through the pressure pump, the emulsification degree of oily wastewater is increased, and the required pressure pump and dissolved gas tank are more significant than the other two, so the investment and operating power consumption are more significant.
The fully dissolved gas process is shown in Figure 1-6.
Figure 1-6 Flow chart of pressure dissolved air floatation of full-dissolved-air mode
1- Raw water inlet: 2- Pressure pump: 3- Air addition: 4- Pressure vessel tank (including packing layer): 5- Pressure reducing valve: 6- Air flotation tank: 7- Air release valve: 8- Scraper: 9- Water collection system; 10- Chemical reagent: 11- Pressure gauge: 12- Scum: 13- Water outlet
② Partially dissolved gas process
The partially dissolved air process pressurizes and dissolves part of the wastewater. The rest of the wastewater directly enters the flotation tank, which saves electricity compared to the complete dissolved air process. Only part of the water passes through the dissolved air tank, and the volume of the dissolved air tank is small.
However, since the amount of gas provided by the pressurized dissolved air of the wastewater is small, the pressure of the dissolved air tank needs to be increased.
Its characteristics are: the pressure pump required by the full-process dissolved air flotation method is smaller, so the power consumption is low; the amount of emulsified oil caused by the pressure pump is lower than that of the full-process dissolved air flotation method;
The 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 flotation method. The partially dissolved air process is shown in Figure 1-7
Figure 1-7 Flow chart of pressure dissolved air floatation of part-dissolved-air mode
1- Raw water: 2- Pressure pump: 3- Air addition: 4- Pressure dissolved air tank (including packing layer): 5- Pressure reducing valve: 6- Air flotation tank: 7- Air release valve: 8- Scraper: 9- Water collection system: 10- Chemical reagents: 11- Pressure gauge: 12- Scum: 13- Outlet water
Partial reflux pressure dissolved air flotation is water treatment's most commonly used flotation method.
Its basic process is to separate a part of the water from the flotation tank into the dissolved air tank. After this part of the water dissolves the air from the air compressor, it is released into the inlet water of the flotation tank under normal pressure through the dissolved air releaser at the inlet of the flotation tank. The air dissolved under pressure is also released as microbubbles, which adhere to the flocs entering the flotation tank and float to the water surface for removal.
Its characteristics are less pressurized water and low power consumption; no emulsification is promoted during the flotation process; good alum floc formation; and a larger volume of the flotation tank than the previous two processes.
Figure 1-8 Flow chart of pressure dissolved air floatation of reflux mode
1- Raw water inlet: 2- Pressure pump: 3- Air inlet: 4- Pressure dissolved air tank (including packing layer): 5- Pressure reducing valve: 6- Air flotation tank: 7- Air release valve: 8- Scraper: 9- Water collection pipe and return clean water pipe: 10- Water outlet
The two methods of full-process dissolved air flotation and partially dissolved air flotation are directly used to pressurize and dissolve air in raw water, so the raw water suspended matter easily blocks the dissolved air releaser. In addition, both methods release gas only after the flocs are formed, so the flocs are easily destroyed.
Compared with the full-process dissolved air flotation, the water volume used for pressurized dissolved air in the two processes of partial dissolved air flotation and partial reflux dissolved air flotation only accounts for 30% to 35% and 10% to 20% of the total water volume respectively.
Therefore, under the same energy consumption, the dissolved air pressure can be significantly increased, the bubbles formed are more petite and uniform, and the flocs are not destroyed.
Regardless of the dissolved air flotation process, its structure mainly includes a pressure-dissolved air system, a dissolved air release system, a reaction system, and a separation system.
Compared with electrolytic flotation and dispersed air flotation, dissolved air flotation has the following characteristics: the air solubility in water is significant, which can provide enough microbubbles to meet the solid-liquid separation requirements of different requirements and ensure the removal effect;
The bubble particle size generated after decompression is small (20-100 μm), the length is uniform, the microbubbles rise very slowly in the flotation tank, and the disturbance to the flotation tank is minor, which is particularly suitable for the separation of loose flocs and fine solids;
The equipment and process are relatively simple, and maintenance and management are convenient.
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Design and manufacture of dissolved air flotation systems for industry! Wastewater treatment.The following are the high-efficiency dissolved air flotation systems we produce.
we can customize dissolved air flotation for various wastewater treatment application scenarios and strictly meet the requirements of wastewater treatment effluent indicators.
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The flotation method first appeared in mining, using bubbles to extract valuable components from ores, and it is called flotation.
As early as 1860, when William Haynes obtained a patent, the flotation process had a particular scale of industrial application in mineral processing（History of Flotation Patents）. It is still widely used and has made significant progress, especially for ores with different compositions; the characteristics of various flotation agents can be used to carry out selective flotation separation smoothly.
In the early 20th century, there were several patents for flotation technology using gas as a flotation medium. In 1904, Elmore proposed electrolysis to generate bubbles, which later developed into electrolytic flotation in water treatment.
In January 1902, Charles V. Potter, an Australian, obtained a British patent for the flotation of sulfides in a hot acid solution. He used a stirrer and claimed that the solution would "react on the soluble sulfides present to form bubbles of sulphurated hydrogen on the ore particles and thereby raise them to the surface."
The Potter Flotation Apparatus
In the same year, Elmore also used a vacuum to generate bubbles, which later developed into vacuum-dissolved gas flotation. In 1906, Sulman and others invented a stirrer or impeller to vigorously stir the slurry to aerate the slurry, which evolved into the impeller dispersed gas flotation method in water treatment.
1914 Callow used an underwater porous disperser to introduce bubbles into the water. This method is called microporous diffused air flotation in the water treatment field.
The above two methods are called diffused air flotation in the water treatment industry.
In 1905, Sulman pressurized and oxygenated the water and released it under reduced pressure. This method became the first patent for pressurized dissolved air flotation.
Apparatus for separating metallic particles in ore
Hoover ore concentration apparatus
In the field of water treatment, as early as 1920, C.L. Peck considered using air flotation to treat sewage.
In 1924, Peterson et al. dissolved air into water under high pressure and then released it under normal pressure to produce fine bubbles, which were applied to white water in the paper industry to recover fibers.
In 1943, C.A. Hansan and H.B. Goraas published an article on air flotation treatment of sewage in the drainage magazine "Sewage Works Journal" and concluded that air flotation can remove almost all suspended solids. It was concluded that it has no significant effect on the BOD formed by dissolved solids.
In 1945, an article titled "Water purification by flotation" written by S.H. Hoppe was published. The article was very short and concluded that the air flotation method required less time than the traditional method. It was recommended that further attention should be paid to this method. This was about The earliest report on the use of air flotation in water supply.
In the 1950s, there were already industrial flotation machines. Still, the water purification effect was poor due to the poor microbubble generation technology. Flotation water purification technology development was relatively slow, and there were few reports on its research and application.
In the 1960s, the partial return pressure dissolved air flotation method appeared. This flotation method has a good water purification effect and dramatically improved economic efficiency, thus expanding its scope of application. Therefore, it was applied to industrial wastewater in the early 1960s and was promoted to drinking water purification in the mid-1960s.
1961 Sweden built the first wastewater treatment plant using partial return pressure flotation. In water treatment, the "flotation filter" manufactured by Purac, Sweden, was operated in the mid-1960s.
In South Africa, pilot studies on flotation began in the 1960s. In 1969, a treatment plant for removing algae from the effluent of mature ponds was built using dissolved air flotation technology, and the effluent was reused for drinking water for residents. South African researchers subsequently applied dissolved air flotation technology to sludge thickening and industrial wastewater treatment.
It was not until the late 1970s that this technology gained regional opportunities to treat eutrophic water bodies[6]. By the 1990s, dissolved air flotation had been widely used.
It is reported that as early as 1965, a factory in Finland used the dissolved air flotation process to treat drinking water. Since then, many factories have adopted this process, and the treatment effect has been perfect, proving that dissolved air flotation technology is a suitable treatment process for low-temperature, humus-rich water bodies.
The Soviet Union conducted extensive research on flotation to treat various industrial wastewater. It published a monograph on "Flotation Treatment of Wastewater" in 1976, but its content was biased towards medium and small-scale experiments, and there were not many projects with actual applications.
Dr. Krofta invented flotation equipment in the 1970s in the United States and established the Krofta Engineering Company. The popularity of the product made Krofta a synonym for the equipment. Subsequently, the flotation equipment was widely used in the water treatment industry and introduced to my country in the 1980s for white water treatment in the papermaking industry, with good results.
In the United Kingdom, the country's water treatment company introduced dissolved air flotation devices on a commercial scale in the 1970s. It installed the first dissolved air flotation complete set at a filtration station near Aberdeen Port in 1975.
The initial research on dissolved air flotation in the Netherlands began in the 1970s. In 1979, the Netherlands' first dissolved air flotation plant began to be designed and built.
As of 1995, 7 water supply companies in the Netherlands have adopted dissolved air flotation technology, mainly to remove algae.
According to reports from the United States and South Africa, dissolved air flotation technology has been widely used in many countries and regions, such as South Africa, Northern Europe, the Netherlands, the United Kingdom, the former Soviet Union, Japan, and India.
An important reason for the rapid development of flotation water purification technology in the 1970s was the improvement of microbubble generation technology. It is to release pressurized air dissolved in water into microbubbles under specific controlled conditions through a special releaser.
Since this special releaser significantly impacts water treatment effect and water production costs, it is listed as a patent abroad, such as the AKA patent releaser developed by the Swedish AKA company.
In the 1980s, flotation separation technology was listed as one of the top ten new chemical technologies in the United States. It promoted the rapid development of flotation technology and further applied it.
M. Hiraide et al. introduced flotation technology from the analytical chemistry perspective, especially the precipitation flotation of organic matter and inorganic ions and ion flotation, which further promoted the development of flotation technology.
In 1985, the American HydroCal Environmental Protection Company invented a new flotation technology - induced air flotation (abbreviated as THK).
THK is a new mechanical air flotation technology. The environmental protection company also invented the CAF vortex flotation system in California, USA, which uses a unique aerator design to replace the traditional nozzle and nozzle dissolving method. The process was introduced to my country in 1997.
After the 1990s, with the development of water pump technology, gas-liquid mixing pumps were gradually applied to flotation processes.
The pump sucks air and water at the same time and achieves gas-liquid mixing through pressurized mixing in the pump. In addition, the theoretical research on flotation technology has become more in-depth. Through careful research on dynamics, thermodynamics, fluid mechanics, etc., two dynamic models have been summarized: group equilibrium theory model and trajectory theory model.
In terms of fluid mechanics, the development of flotation has been divided into three generations based on the difference in load and flow state, realizing the transition of flow state from laminar flow to turbulent flow, and turbulent flotation has appeared. Theoretical research provides an in-depth theoretical basis for developing flotation technologies such as dissolved air flotation, vortex flotation, and jet flotation and promotes the widespread application of flotation technology.
Since 2001, M-IEpcon AS has successfully launched the EPCON compact flotation device, which combines rotary centrifugal separation technology and degassing flotation separation technology. It has now reached the commercial operation stage and was rated as one of the top ten innovative technologies in 2004 by the well-known Offshore Oil Engineering Technology Conference (OTC).
The pressurized dissolved air flotation equipment comprises a pressure-dissolved air system, an air release system, a flotation tank, and auxiliary equipment.The pressure water pump delivers water and air to the gas tank at a certain pressure, allowing the water and air to contact fully.
There are many types of dissolved air tanks, as shown in Figure 1. Among them, the dissolved air tank filled with fillers has the highest efficiency, about 30% higher than the dissolved air tank without fillers. It is recommended to use a spray-type filled tank with low energy consumption and high dissolved air efficiency, as well as air supplied by an air compressor.
1.Direct Flow
2. Longitudinal partition type
3. Casing type
4. Diaphragm type
5. Filling type
6. Flower plate type
7. Water jet type
8. Tumbling type
9. Spray type
10. Vortex type
The air release system comprises a dissolved air releaser and a dissolved air water pipeline.
The dissolved air releaser reduces the pressure of the water in the pressure vessel so that the gas in the dissolved air water is released as microbubbles and can quickly and evenly adhere to the particulate matter.
China's most commonly used dissolved air releasers are the TS-type dissolved air release, which won the National Invention Award. Its improved TJ-type dissolved air release and TV-type dissolved air releaser(Comprehensive Knowledge about The Release Device of Dissolved Air Flotation).
TS type dissolved air releaser
TJ type dissolved air releaser
TV type average distribution dissolved air releaser
The TS-70 low-pressure dissolved air releaser is the first special releaser in China. It can release a large number of microbubbles that meet the requirements of flotation water purification under low pressure. For this reason, the dissolved air releaser won the National Invention Award in 1980.
With the continuous promotion of domestic flotation water purification technology, the TJ type releaser has been improved on the basis of the TS type. It retains the excellent release performance of the TS type releaser, increases the water output, and adds a water ejector vacuum device. When blocked, it can be flushed in situ without disassembling the releaser, but it has the disadvantages of uneven water distribution at the nozzle and the addition of a vacuum device.
The TV type uniformly distributed vibration dissolved air releaser is the third generation dissolved air releaser newly developed after the TS type and TJ type dissolved air releasers. It is successfully developed based on the exploration of the basic principle of dissolved air release and combined with the vibration principle.
It not only absorbs the excellent performance of TS and TJ dissolved air releasers, but also improves the uniformity of the distribution of water released by the releaser, increases the probability of collision and adhesion between microbubbles and impurities in the water to be treated, and further improves the flotation water purification effect.
In addition, once the TV type releaser is blocked, as long as the vent valve is opened outside the flotation tank and the compressed air source is connected, the blockage in the releaser can be flushed clean with pressurized dissolved air water. This not only overcomes the disadvantage of the TS type dissolved air releaser being easily blocked, but also saves the vacuum device compared to the TJ type dissolved air releaser.
At present, the commonly used releasers in China are the TS, TJ and TV type releasers developed by Tongji University, and the research on releasers is also focused on these three types of releasers. This type of releaser has developed to the fifth generation of products, but has not changed much.
Their common characteristics are: perfect gas release, when the dissolved gas pressure is above 0.15MPa, about 99% of the dissolved gas can be released; they can work under lower pressure, when the dissolved gas pressure is above 0.2MP, they can achieve good water purification effect and save electricity; the released bubbles are fine, with an average bubble diameter of 20~40μm, the bubbles are dense, and the adhesion performance is good.
Image of the water tank before the release of dissolved air water
Image of the water tank after the release of dissolved air water
(1) The diameter of the generated bubbles is closely related to the dissolved gas pressure and the releaser inclination angle. The dissolved gas pressure must be increased to create tiny, average-diameter bubbles. However, when the dissolved gas pressure is more significant than 0.25MPa, the effect of the increase in pressure on the average diameter of the bubbles becomes smaller and smaller.
(2) As the releaser inclination angle changes (from vertical to inclined), the larger the inclination angle, the larger the generated bubble diameter will be, and the average diameter of the bubbles will increase with the increase in the rising height. However, it was found in the experiment that when the dissolved gas pressure is at a high value, as the bubbles rise, the change in the releaser inclination angle has less and less effect on the average diameter of the bubbles;
(3) The inclination angle of the releaser will also affect the number of bubbles it generates. The inclined placement of the releaser will lead to uneven distribution of the gas phase inside it, affecting the number of bubbles released from the small hole at the end of the releaser and thus affecting the flotation efficiency.
After the later calculation of the experiment, it can be seen that the number of bubbles will decrease with the increase in height, and the number of bubbles generated when the releaser is placed vertically is much greater than the number of bubbles generated when placed obliquely.
Schematic diagram of releaser inclination angle change
Read more about: Dissolved Air Flotation Releaser
Microbubbles and suspended particles in the water are thoroughly mixed, contacted, and adhered in the flotation tank, and the gas-carrying particles are separated from the water.
There are many ways to arrange the flotation tank. According to the water quality characteristics, treatment requirements, and various specific conditions to be treated, many flotation tanks have been built, According to the type of use of the flotation tank, it can be divided into a horizontal flow flotation tank, vertical flow flotation tank, shallow high-efficiency flotation tank, co-flotation tank, ion flotation tank, circular flotation tank, counter-current and co-current flow flotation tank, and other forms.among which the horizontal flow type and the vertical flow type are the most commonly used.
The advection flotation tank is the most widely used, and the reaction tank and the flotation tank are often combined, as shown in Figure 1-9.
The water flow is directed to the bottom after the wastewater enters the reaction tank and completes the reaction. It enters the flotation contact chamber from the bottom to extend the contact time between the flocs and the bubbles. The scum on the pool surface is scraped into the scum collecting tank, and the clean water is collected by the bottom water collecting pipe.
The advantages of the advection flotation tank are shallow pool depth, low cost, simple structure, and easy operation. The disadvantage is that the volume utilization rate of the separation part is not high.
Figure 1-9 Horizontal flow flotation tank
①Reaction tank ②Contact chamber ③Flotation tank
The basic process parameters of the vertical flow flotation tank are the same as those of the horizontal flow flotation tank, as shown in Figure 10. Its advantage is that the contact chamber is in the center of the tank, the water flows to the surroundings, and the hydraulic conditions are good.
The disadvantages are that connecting with the reaction tank and the low volume utilization rate is difficult. Experience shows that a vertical flow flotation tank should be used when the treated water volume is more significant than 150~200m3/h, and there are more sinkable substances in the wastewater.
Figure 10 Vertical flow flotation tank
1. Reaction tank 2. Contact chamber 3. Flotation tank diagram
The dissolved air flotation auxiliary equipment consists of a scraper, a reagent dosing device (flocculant), etc.
In the flotation system, there are many types of scrapers, the most common of which are bridge, traveling, and chain.
For the flotation system, no matter what type of scraper, there will be sludge that cannot be drained cleanly at the end of the flotation tank or in the surface floating mud collection tank during actual operation.
And because the solid content of the surface sludge produced by flotation is about 2%, some sludge will flow back into the flotation tank after being scraped to the end of the flotation tank by the scraper. At the same time, there will be sludge that cannot be drained cleanly in the mud discharge tank of the flotation tank. In this case, it is necessary to add a spray system to flush the sludge; otherwise, it will be more difficult to clean after the sludge solidifies for a long time.
When designing, the track of the scraper can also be included in the spray range. The required spray water can be kept to a minimum, and the design and position of the spray nozzle can be selected accordingly. The duration and frequency of flushing should be set to be adjustable. A remotely controlled regulating valve can be designed and installed for water plants with high automation requirements. At the same time, a valve for manually adjusting the flushing water flow can be considered.
Automatic Polymer preparation unit is animportant dosing machine in wastewater treatment.There are different kinds of dosing machine which ischoosen according to the nature of agent, dosage,client's requirments and budget. In terms of materi-als,carbon steel,stainless steel, PE,PP, U-PVCand other materials are also available.
① Pressure value in the dissolved air tank
The pressure in the dissolved gas tank directly affects the amount of dissolved gas. The greater the pressure value, the greater the theoretical value of the dissolved gas volume.
② Temperature in the gas tank
The temperature in the dissolved gas tank also directly affects the amount of dissolved gas, in the formula P*A=ExA (, the Henry coefficient decreases with increasing temperature, which means that the amount of dissolved gas will decrease with increasing temperature.
③ Water flow status
In 1923, Lewis and Whitman proposed the famous "double membrane theory" to explain how air is transferred from the gas to the liquid phase.
The "double membrane theory" believes that laminar gas films and liquid films are on both sides of the contact surface between the gas and liquid phases. The mass transfer resistance of air from the gas to the liquid phase in the molecular state is mainly concentrated on these two membranes, especially in the fluid film in the flow state.
Read more about: 《Factors affecting the dissolved gas volume of the pressurized dissolved gas tank》
① Bubble size
The size of microbubbles is one of the most critical factors affecting flotation efficiency. During dissolved air flotation, when the bubble diameter is less than 50μm, the removal rate is relatively high; when bubbles larger than 80μm are generated, there is almost no water purification effect.
② dissolved air releaser
Through the analysis of bubble size, it can be seen that the bubble size dramatically influences the flotation water purification effect, and the component that affects the bubble size is the pressure dissolved air releaser.
③ Dissolved gas pressure
The dissolved gas pressure affects the air that can be dissolved in a specific volume of water. At the same temperature, the higher the pressure, the greater the amount of gas dissolved in water. This will affect the size and number of bubbles generated and directly impact the flotation effect.
④ Releaser placement angle
The releaser is arranged in the contact area of the flotation tank. Its arrangement can be divided into (1) reverse contact type, (2) co-directional push flow type, and (3) vertical radiation type according to the installation direction of the releaser.
Each releaser placement method affects the release angle of dissolved air water. It can be seen that the release angle of dissolved air water is also an essential factor influencing the flotation water purification effect.
Read more about:《Factors Affecting the Effect of Pressurized Dissolved Air Flotation on Water Purification》
The two most abundant gases in the air are N2 and O2, both of which are non-polar gases. For such non-polar molecules with very low solubility, they must exist in the gaps between water molecules, so gap filling is a way for air to dissolve in water.....
Read more about:《How does air dissolve in an air-dissolving tank?》
The dissolved air water in the dissolved air tank is in a high-pressure state after being pressurized by the air compressor. The oxygen and nitrogen in the air are dissolved in the water by gap filling and hydration to form supersaturated dissolved air water.
After the releaser releases the high-pressure supersaturated dissolved air water, it changes from a high-pressure state to normal pressure. Due to the sudden pressure drop, the solubility of the gas in the water also decreases significantly. At this time, many bubbles are precipitated from the water. It generally takes three processes for the gas to precipitate from the water to form stable microbubbles......
Read more about:《How are microbubbles formed?》
5. Application scope of air flotation technology
Separate fine suspended matter, algae, and micro-flocculation in water;
Recover useful substances from wastewater, such as pulp fibers from papermaking wastewater;
Concentrated excess activated sludge, especially suitable for biochemical treatment processes that are prone to sludge bulking;
Separate and recover metal ions in wastewater, suspended oil, and emulsified oil in oily wastewater.
