What is Dissolved air flotation or DAF?

Dissolved air flotation (DAF) is one of the most versatile and effective process in water, wastewater treatment and desalination

Dissolved air flotation (DAF) is one of the most important and effective process in the treatment of all types of water -industrial and urban wastewater treatment, water processing, water reuse and desalination processes-. It can be installed in different points of water treatment line due to multiple design possibilities of dissolved air flotation units.

In several ocassions, dissolved air flotation system is installed at the beginning of the water treatment process, immediately after the pretreatment to remove fat and grease. It also can be implanted at the inlet of biological treatment for reducing pollutant load. In other many times it is placed at the final of water treatment line to clarify water before being discharged. In desalination -in which the development of fiber glass is starting to consolidate as construction material- it works to protect the membranes from the presence of algae in the water too. Furthermore it can be installed at the top of the sludge treatment line as sludge thickener to reduce the volume.

Dissolved air flotation is based on a physico-chemical process which separates solid and liquid particles by the addition of very small air bubbles. These bubbles stick on solid particles and give them a high rate of ascent. Once in the surface, the solids -the sludge- are pulled mechanically to the hopper. Dissolved air flotation gives a rate of ascent 5 times higher than settling speed in a conventional settling tank, thereby saving time and space.

Flocculation and coagulation

Waste water is constantly pumped to the flocculator pipe -a zigzag shaped tube to make easier the mix of the water with the chemical reagents-. In there, the coagulant is added. This chemical product can destabilize suspended particles or colloids to facilitate the subsequent solid-liquid separation. Some of the most used coagulants in water treatment are aluminum sulfate, ferrous sulfate, ferric chloride or aluminium polychloride.

Then, in the same pipe, the flocculant is added. This is a polymer with opposing charge for attracting smaller flocs or colloids to form bigger flocs. In this way air micro-bubbles are bonded to the bigger flocs to create particles with less density than water and forming making possible the flotation.

Sludge on one side, water on the other one

After the solid-liquid separation, a part of treated water -approximately 30%- is recirculated to be mixed with waste water. A developed injection system distributes the pressure of saturated water in the flotation unit. On this way, the generated bubbles can get the ideal size to favor the flotation of the flocs.

Finally, in the surface of the flotation unit a scraper system pulls the generated sludge to the hopper for collecting it and the clarified water is discharged in a spillway on the opposite side. In the meanwhile, heavier particles that could not reach a lower density than water are kept in the sedimentation chamber to be recirculated at the beginning of the water treatment line to receive the appropriate treatment.