(1) The basic concept of air flotation

The air flotation treatment method is to pass air into the wastewater, and precipitate out of the water in the form of tiny bubbles as a carrier, so that the emulsified oil, tiny suspended particles and other pollutants in the wastewater adhere to the bubbles, and float to the water surface together with the bubbles. Forming foam - a three-phase mixture of gas, water and particles (oil), the purpose of separating impurities and purifying wastewater is achieved by collecting foam or scum. Flotation is mainly used to treat emulsified oil or tiny suspended particles with a relative density close to 1 that are difficult to remove by natural sedimentation or floating in wastewater.

(2) The basic principle of air flotation

1. The relationship between the floating with air flocs and the surface load of air floating

When the flocs adhering to the bubbles float in the water, they will be affected by external forces such as gravity G and buoyancy F on the macroscopic level. The floating speed of the aerated flocs can be derived from Newton's second law, and the floating speed depends on the density difference between the water and the aerated flocs, the diameter (or characteristic diameter) of the aerated flocs, and the temperature and flow state of the water. If the proportion of air bubbles in the air-filled flocs is larger, the density of the air-filled flocs will be smaller; while its characteristic diameter will increase accordingly, the change of the two can greatly improve the floating speed.

However, in the actual water flow, the size of the air-bearing flocs is different, and the resistance caused by it is also changing. The specific floating speed can be determined according to the experiment. The surface load of the air flotation can be determined according to the measured floating speed value. The determination of the floating speed must be determined according to the requirements of the water.

2. Adhesion of flocs to air bubbles in water

As mentioned above, the main separation objects of pollutants in water by air flotation treatment are generally two types, flocs and particle monomers of coagulation reaction. In the process of air flotation, there are three ways for the combination of bubbles to flocculation and particle monomers, namely bubble support, bubble entrainment and air particle adsorption. Obviously, the strength of the entrainment and adhesion between them, that is, the firmness of the combination of air and particles (including flocs), is not only related to the shape of particles and flocs, but also to the water, air, and flocculation. Influence of the properties of the three-phase interface between grains. The content of the active agent in the water, the hardness of the water, and the concentration of the suspended solids are closely related to the viscous and flotation strength of the bubbles. The quality of air flotation operation is fundamentally related to this. In practical applications, the water quality must be adjusted.

3. Formation and characteristics of bubbles in water

The size and strength of the bubbles formed depends on the various application conditions and the surface tension of the water when the air is released. (Surface tension is a pair of forces that are equal in magnitude and opposite in direction, respectively acting on the contacting parts of the surface layers, and its acting direction is always tangent to the liquid surface.)

(1) The smaller the bubble radius, the greater the additional pressure in the bubble, and the more and more violent the collision probability of the air molecules in the bubble to the bubble film. Therefore, to obtain stable microbubbles, the strength of the bubble film must be guaranteed.

(2) The bubbles are small, the floating speed is fast, the disturbance to the water body is small, and the flocs will not be broken. And it can increase the collision probability of bubbles and flocs. However, it is not that the finer the bubbles, the better. If the bubbles are too fine, the floating speed will be affected, so the size of the air flotation tank and the project cost. In addition, adding a certain amount of surfactant can effectively reduce the surface tension coefficient of water, strengthen the fastness of bubble film, and reduce r.

(3)Adding highly soluble inorganic salts to the water can weaken the fastness of the bubble film and make the bubbles easily burst or become larger.

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