Dissolved air flotation(DAF) definition

Dissolved Air Flotation (DAF) is a proven and effective water treatment process to clean wastewater by removing total suspended solids (TSS), oil and grease, BOD, COD and metals as an alternative to clarification processes.

Dissolved Air Flotation (DAF) system removes contaminants using a dissolved water-in-air solution created by injecting air under pressure into a recirculating stream of clarified DAF effluent. This recirculating stream is then mixed and blended with the incoming wastewater in an internal contact chamber where the dissolved air exits the solution as micron-sized bubbles that attach to the contaminants. The bubbles and contaminants rise to the surface and form a floating bed of material, which is removed by a surface skimmer into the internal hopper for further treatment. To improve solids removal, coagulants/flocculants are added to induce aggregation of suspended solids and colloidal particles.

Thanks to the design and technology of DAF clarifiers, they can serve many industries. Dissolved air flotation technology can be integrated into custom clarifiers to accommodate different streams from low to high impurity levels. In general, they are well suited for wastewater streams containing floating or suspended oil, grease and impurities.

Uses of dissolved air flotation

Dissolved air flotation systems are commonly used in the following industries and production processes.

  • Petroleum, petrochemical and refineries
  • Natural gas processing plants
  • Pharmaceutical and cosmetics
  • Mining, quarrying and aggregate production
  • Water and wastewater treatment
  • Food processing
  • Commercial and industrial laundry
  • Pulp and paper production
  • Algae removal and desalination pretreatment
  • Pretreatment for downstream biological treatment systems

Dissolved air flotation advantages

Some of the major advantages of DAF include better performance in removing light particles that are difficult to settle, such as algae. DAF can also typically achieve lower effluent turbidity than settling, typically below 0.5 NTU. it is less temperature sensitive, especially at the low temperatures common in sedimentation, and has a very short start-up time of about 30 minutes. In addition, DAF does not require the production of heavy flocs for settling, so lower coagulant dosages and shorter flocculation times can be used. The process can also operate at much higher surface loading rates (SLR) than settling, especially in high-speed DAF processes up to 20 gpm/ft2.

By mechanically removing the sludge, the DAF sludge can be very thick (2% to 5% solids), thus eliminating the need for thickening prior to dewatering. The polymer does not need to be stable due to air attachment, whereas the high-speed settling process must use polymer to attach the particles to the ballast. If the clarification process is used for membrane filtration pretreatment, the use of polymers is a major concern. However, DAF is not suitable for raw water with high density solids or turbidity above 100 NTU because the required pumping recovery percentage is higher, making the process more energy intensive and less economical than competing technologies. In general, DAF uses more energy than sedimentation due to the requirements of recirculating pumps and air compression.

Types of dissolved air flotation

Dissolved air flotation method can be divided into 4 types: full flow pressurized, reflux type, partial raw water type and pressurized air type.

  1. The full-flow pressurized dissolved air flotation method has a large dissolved air volume and requires a small flotation tank volume, which is widely used in oilfield wastewater treatment.
  2. Return flow type dissolved air flotation method is part of the purified water back to the dissolved gas tank pressure dissolved gas, and then with the incoming liquid into the flotation cell, so it can be used in the case of raw water need to pre-coagulation and raw water oil content is relatively high.
  3. The partial raw water type dissolved air flotation method is similar to the full flow pressurized dissolved air flotation method and is more suitable for treating oilfield wastewater with low oil content.
  4. The pressurized dissolved air flotation method is used to press the gas into the liquid through a porous disc, porous plate or a special nozzle, which has a shorter residence time than several other dissolved air flotation processes.