Types And Precautions For Using Dissolved Air Flotation (DAF) Machines

Dissolved air flotation (DAF) refers to the process of using highly dispersed microbubbles to adhere to pollutants in wastewater, forming a floating body with a density less than that of water, thereby achieving solid-liquid and liquid-liquid separation. It is suitable for removing suspended solids, oils, and fats with a density less than 1 t/m³ in water. DAF machines can be used for both pretreatment and advanced treatment of wastewater. These machines are widely used in wastewater treatment across industries such as oil refining, papermaking, chemicals, coking, washing, and food processing.

The pressure dissolved air flotation (DAF) machine mainly consists of a dissolved air pump, release device, sludge scraper, air compressor, dosing system, and power distribution system. It is suitable for treating water with low turbidity, high color, high organic matter content, low oil content, low surfactant content, or high algae content. It is widely used in industrial wastewater treatment in industries like papermaking, printing and dyeing, electroplating, chemicals, food, and oil refining. It is most effective for wastewater treatment with a flow rate of less than 500 m³/d.

Compared to other dissolved air flotation methods, this machine has advantages such as a high hydraulic load and a compact tank structure. However, its application is limited by disadvantages such as a complex process, high power consumption, and loud air compressor noise. The dissolved air flotation process is divided into the following sections: mixing/flocculation zone, dissolved air release zone, sedimentation zone, dissolved air water return zone, sludge tank, and clear water tank.

The cavitation air flotation unit is a new type of wastewater treatment equipment designed primarily for removing grease, colloids, and suspended solids from industrial or municipal wastewater. The system consists of an aeration device, a sludge scraping device, and a sludge discharge device. The aeration device mainly uses a patented vortex-type aerator, and the sludge scraping device consists primarily of a scraper and a traction chain.

The working principle is as follows: The dissolved air flotation unit is driven by a motor to rotate at high speed (usually between 1000 and 3000 r/min). The high-speed rotation of the bottom diffuser impeller (whose blades are hollow) creates a negative pressure zone in the water, causing air from the liquid surface to enter the diffuser impeller through the hollow tube of the "vortex head" and be released into the water. The air is then sheared into small bubbles by the high-speed rotation of the blades. As these small bubbles rise, they adhere to the flocs, forming new low-density flocs. The buoyancy of the water carries the suspended solids to the surface, where the sludge is then removed by the sludge scraping device.

The process flow is as follows: Pre-treated wastewater flows into a small aeration section equipped with a vortex aerator. The high-speed rotation of the diffuser impeller at the bottom of the vortex aerator creates a vacuum zone in the water, drawing air from the surface through a ventilation pipe. The three-pronged shearing action generated by the high-speed rotation of the impeller breaks the air into microbubbles and dissolves oxygen into the water. Solid suspended matter adheres to the microbubbles and floats to the surface, where it is carried by the radial airflow to the scraper. The scraper, driven by a motor-gear transmission device mounted on one side of the tank, moves along the width of the tank, scraping the floating solid suspended matter onto an inclined metal plate. The solids are then pushed from the inlet of the flotation tank to the sludge discharge pipe at the outlet. A horizontally installed screw conveyor in the sludge discharge pipe transports the collected sludge to a sludge collection tank; this screw conveyor is usually also driven by the motor of the scraper. The purified wastewater flows into the overflow tank through the outlet below the metal plate. The overflow weir controls the water level of the entire flotation tank to ensure that the liquid in the tank does not flow into the sludge discharge pipe.

An open return pipe extends from the aeration section along the bottom of the flotation tank. While generating microbubbles, the vortex aerator also creates a negative pressure zone at the bottom of the tank, which draws the wastewater from the bottom of the tank back to the aeration section and returns it to the flotation section. This process ensures that about 30% to 50% of the wastewater is returned, allowing the system to continue operating without an external water intake.