Principle and Process of Dissolved Air Flotation Unit

Solid Liquid Separator January 29, 2024 Dissolved Air Flotation

The Dissolved Air Flotation (DAF) unit is a machine designed to remove suspended solids, plant oils, and various colloidal substances from industrial production and municipal engineering wastewater. This equipment is widely used in the treatment of industrial wastewater from industries such as refining, chemical, brewing, slaughtering, electroplating, and dyeing, as well as municipal wastewater. According to the aeration method, it is divided into: air-inflated DAF, dissolved air flotation, and electrochemical DAF.

I. Working Principle of DAF Unit

Efficient mixing of gases that are difficult to dissolve in water or two or more different liquids (producing fine bubbles with a particle size of 20-50 micrometers). Using tiny bubbles as carriers, impurity particles in water adhere, and the particles, trapped by the bubbles, float until separated from the water surface, achieving solid-liquid separation.

II. Wastewater Treatment Process of DAF Unit

Raw water enters the mixing reactor, and chemicals (degreasing agent or coagulant) are added to form separable flocs.
Pre-treated wastewater enters the DAF device, and tiny bubbles (bubble diameter 30-50 μm) released from the influent chamber wastewater mix with the gas-liquid mixture. These tiny bubbles attach to the flocs in the wastewater, forming gas buoyant bodies with a density smaller than water. These gas buoyant bodies rise to the water surface, condense into floating oil (or floating scum), and are scraped into the oil (scum) tank by an oil (scum) scraper.
Solid particles in the influent chamber settle here and are discharged through the sand discharge valve. The system requires periodically opening the sand discharge valve to keep the influent chamber clean.
The wastewater enters the water distribution area of the DAF device, where fast-rising particles float to the water surface. Slower-rising particles are separated in the corrugated inclined plate. Once a particle contacts the corrugated inclined plate, it can rise against the direction of water flow due to the buoyancy.
All heavy particles will sink, and the sunken particles will be collected by the bottom sludge scraper and discharged by periodically opening the sludge discharge valve.

III. Tubular Mixing Reactor

The efficient tubular dosing reactor consists of three specially designed mixing tubes. Chemicals, coagulants, and dissolved bubbles are introduced. By designing to control the mixing energy and mixing time of each tube section, the optimal coagulation effect is achieved.

IV. Features of Efficient Tubular Dosing Reactor (PFR):

The mixing energy and time in the pipeline are easy to control, and the coagulation and flocculation reactions are stable, producing uniform flocs.
Due to the dissolved bubbles in the pipeline, the bubbles are closely combined with the inside of the flocs.
Since the dosing points are in the middle of the pipeline, the consumption of water treatment chemicals can be minimized.
There is no reverse mixing in the pipeline, avoiding short circuits and shorts.

V. Inclined Plate DAF Unit Working Principle

The ADNF DAF separation system uses inclined plates and pipes for separation, widely used in sedimentation tanks. Based on the theory of settling in a still pond, multiple inclined plates are placed in the tank at an angle to the horizontal plane. Water flows through the inclined plate, heavy solids settle at the bottom of the inclined plate, and light solids float to the top of the inclined plate.

VI. Achieving Solid-Liquid Separation

Because the fluid between the plates maintains laminar flow, particles are not disturbed and can reach the nearest plate. Once a particle touches the plate, it will start flowing upstream. Due to the speed distribution of the laminar pattern, the reverse flow of particles is not hindered, given that the water flow velocity on the plate is 0.