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DAF is a kind of air floatation. It uses the different solubility characteristics of water under different pressures to pressurize and aerate all or part of the water to be treated (or treated) to increase the air solubility of water. It is released under normal pressure through the addition of the water with concrete, and the air is separated to form small bubbles, which adhere to the impurity floc particles, resulting that the overall density of floc particles is less than water and began to rise to separate the solid from the liquid.
DAF is suitable for the treatment of water with low turbidity, high chroma, high organic content, low oil content, low surfactant content or rich algae. Compared with other air flotation methods (see Appendix 1 for details), it has the advantages of high hydraulic load and compact tank body. However, its application is limited by its complex process, large power consumption and large noise of air compressor.
1 type according to different classification principles, DAF can be classified differently.
1.1 According to the different pressure at which bubbles are separated from water, they can be divided into two kinds: vacuum air flotation and pressure dissolved air flotation. The former uses the method of vacuumizing to dissolve air under normal pressure or pressure, and then releases microbubbles under negative pressure for air supply floating use; the latter uses the method of forcing air to dissolve in water under pressure, and then suddenly depressurizes to release the dissolved gas from water, adheres to flocs in the form of microbubbles, and floats 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 flocs are relatively stable; however, the amount of bubbles released is limited; moreover, all equipment components should be sealed in the air flotation tank; the structure of the air flotation tank is complex; it is only suitable for the treatment of wastewater with low pollutant concentration (no more than 300mg / L), so the actual application is not much.
1.1.2 The pressure dissolved air floatation is a commonly used method at home and abroad. There are three basic flow options: full flow dissolved air floatation, partial dissolved air floatation and partial reflux dissolved air floatation.
1.1.2.1 The whole process dissolved air flotation The whole process dissolved air flotation is to pressurize all waste water with a water pump, dissolve air in the waste water in the dissolved air tank, and then send the waste water to the air flotation tank through a pressure reducing valve. See Figure 1 for flow chart. Its characteristics are as follows: ① large dissolved air volume increases the contact opportunity between oil particles or suspended particles and bubbles; ② under the same treatment water volume, it is smaller than the air flotation pool required by partial reflux dissolved air flotation. ③ all waste water passes through the pressure pump, and the pressure pump and air dissolving tank required are larger than the other two processes, so the investment and operation power consumption are larger.
1.1.2.2 The partial dissolved air floatation The partial dissolved air floatation is to take part of the waste water for pressurization and dissolved air, and the rest of the waste water directly enters the air floatation tank and mixes with the dissolved gas waste water in the air floatation tank. Its characteristics are as follows: ① the pressure pump required for the whole process is small, so the power consumption is low; ② the size of the air flotation tank is the same as that of the whole process, but smaller than that of the partial reflux solution.
1.1.2.3 The partial reflux dissolved gas floatation The partial reflux dissolved gas floatation is to take part of the treated water for reflux, pressurize the return water and dissolve the gas, enter the air floatation tank after decompression, mix and air floatation with the oily wastewater from the flocculation tank. See Figure 2 for the flow chart. Its characteristics are as follows: ① less pressurized water, less power consumption; ② no promotion of emulsification in the process of air flotation; ③less flocculation after the formation of alum,; ④ larger volume of air flotation tank than the first two processes. According to modern air flotation theory, partial reflux pressurized dissolved air flotation can save energy, make full use of flotation (coagulation) agent, and the treatment effect is better than that of full pressurized dissolved air flotation process. At the same time, when the reflux ratio is 50%, the partial reflux (reflux ratio is 50%) pressurized dissolved air flotation process is commonly used at home and abroad.
1.2 According to whether the microbubble sludge layer (bed) in the air flotation tank has filtration effect and different flow patterns of water, it can be divided into early DAF, ordinary DAF and turbulent DAF.
2. Design principal of DAF is generally set in front of biological processing unit and after physical processing unit, and it is usually classified as physical processing unit. If it is set as two-stage flotation, in order to facilitate saving, the first and second stage flotation cells are often arranged in parallel. The first and second stage flotation cells have a liquid level difference of about 500mm to ensure that sewage flows from the first stage flotation cell to the second stage flotation cell, and the lifting pump is canceled to achieve energy saving effect. It is reflected in the vertical layout, that is to say, during design and construction, it is necessary to strictly control the height of the scraper Trailer (plate), the adjustable weir and the top of the slag removal trough, which is very important and is one of the key factors, otherwise it will seriously affect the floatation effect (the foam layer can not be removed by mechanical means). This is the reason why the adjustable effluent weir must be adopted.
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