Comparison of four air flotation equipment

Air flotation methods can be divided into diffuse air flotation, dissolved air flotation (including vacuum air flotation) and electrolytic air flotation. It is currently used in water supply, industrial wastewater and urban sewage treatment. Compared with other solid-liquid separation equipment, air flotation equipment has the characteristics of less investment, minimal floor space, high degree of automation, and convenient operation and management. In practice, it should be selected and used according to the characteristics of wastewater treatment process, water quality and quantity of wastewater.
　　
Electrolytic air flotation equipment
　　
The electrolytic air flotation device uses indiscerptible anodes and cathodes to directly electrolyze wastewater. The tiny bubbles of hydrogen and oxygen generated by electrolysis float the flocculated suspended matter to the water surface. To achieve the purpose of solid-liquid separation.
　　
The size of the bubbles produced by the electrolysis method is much smaller than that produced by dissolved air flotation and diffused air flotation, and there is no turbulent flow. The equipment removes a wide range of pollutants. In addition to reducing BOD, it can also oxidize, decolorize and sterilize organic waste water. It has strong adaptability to changes in waste water load, generates less sludge, occupies less land, and does not generate noise. It has developed rapidly in recent years. Electrolytic air flotation equipment still has problems such as electrolysis energy consumption, large plate loss, and high operating costs, which limit the popularization and use of this type of equipment.
　　
Air flotation equipment
　　
Air-dispersing air flotation equipment sucks air into the water under the negative pressure caused by the centrifugal force of the high-speed rotating impeller, and becomes fine air bubbles and diffuses in the water. The air bubbles rise from the bottom of the pool to the water surface and are brought to the water surface together with the suspended solids in the water. To achieve the purpose of solid-liquid separation. The formed scum is continuously scraped out of the tank by the slowly rotating scum scraper. The mechanical shear force of the water flow and the bubbles produced by the diffusion plate are relatively large (about 1mm in diameter), which are not easy to adsorb with fine particles and flocs, and the force are easy to break the flocs. Therefore, air-dispersing air flotation is not suitable for treating wastewater with fine particles and flocs. The air flotation time of the diffused air flotation equipment is about 30 minutes, and the dissolved air volume reaches 0.51m3/m3 (air/water).
　　
The peripheral linear velocity of the rotating impeller is about 12.5m/s. The scope of application of the equipment includes paint, leather, oil refining, printing and dyeing, chemical, dairy processing, fiber production, paper making, food and beverage, slaughtering house, textile, mechanical processing, municipal sewage and other small sewage treatment projects.
　　
Dissolved air vacuum flotation equipment
　　
The supersaturated air in the water can be released in the form of fine bubbles during decompression, so that the impurity particles in the water are adhered and floated up. To achieve the purpose of solid-liquid separation. If the air is first pressurized to dissolve in water to form an air supersaturated solution, and then reduced to normal pressure to release the air, it is called pressurized dissolved air flotation; if the wastewater is aerated under normal pressure and then the air is induced to escape under vacuum conditions, the device is called vacuum air flotation.
　　
Dissolved air vacuum flotation equipment is an air flotation equipment that dissolves air in water under normal or increased pressure and precipitates under negative pressure. The advantage of vacuum air flotation equipment is that the formation of air bubbles, its adhesion to particles, and the floating of air bubbles and particle flocs are all carried out in a stable environment, the possibility of floc damage is small, and the energy consumption required for the entire air flotation process is low. Its disadvantage is that the amount of dissolved air in water is limited, and it is not suitable for wastewater containing suspended solids with a concentration greater than 250-300mg/L; another disadvantage is that a sealed container is required, and a slag scraping machine is also required in the container, which has a complicated structure. It is rarely used in projects. The possible air volume of this equipment is affected by the vacuum degree that can be achieved (generally operating vacuum degree is 40kPa), the amount of fine bubbles released is very limited, and the structure is complicated, and the operation and maintenance are inconvenient, so it has been phased out.

Pressurized dissolved air flotation equipment
　　
Pressurized dissolved air flotation equipment is to pressurize clean water to (3-4) × 105Pa, and add air at the same time to dissolve the air in the water, and then suddenly reduce it to normal pressure. The air dissolved in the water is in the form of tiny bubbles (bubble diameter About 20-100μm), it releases from the water and floats the suspended particles in the water on the water surface. Thereby solid-liquid separation is achieved. Pressurized dissolved air flotation equipment is a kind of air flotation equipment with a wide range of applications at present. The equipment can be widely used in various types of wastewater treatment (especially oily wastewater treatment), sludge concentration and water supply treatment.
　　
Pressurized dissolved air flotation equipment mainly consists of three parts: air saturation equipment, equipment for releasing air and mixing with wastewater, and equipment for solid-liquid or liquid-liquid separation. According to the type, nature and treatment efficiency of suspended matter contained in raw water, it can be divided into three types: full pressurized dissolved air flotation, partial pressurized dissolved air flotation and reflux pressurized dissolved air flotation.
　　
At present, the pressurized dissolved air flotation method is the most widely used. Compared with other air flotation equipment, it has the following characteristics:
　　
i. Under pressurized conditions, the solubility of air is large, and the number of air bubbles for air flotation is large, which can ensure the effect of air flotation;
　　
ii. The dissolved gas is released by sudden decompression, and the generated bubbles are not only fine, uniform in particle size, and dense, but also stable in floating, and have little disturbance to the liquid, so it is especially suitable for solid-liquid separation of loose flocs and fine particles;
　　
iii. The process and equipment are relatively simple, easy to manage and maintain;
　　
iv. Especially the partial reflux type, the treatment effect is remarkable and stable, and can greatly save energy consumption.