What is a DAF?
Dissolved air flotation (DAF) is a water treatment process that clarifies wastewater by removing suspended solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it is then removed by a skimming device. Chemicals can be added to the feed water to improve solids removal.
Dissolved Air Flotation (DAF) systems are designed to remove suspended solids (TSS), biochemical oxygen demand (BOD5), and oils and greases (O&G) from a wastewater stream. Contaminants are removed through the use of a dissolved air-in-water solution produced by injecting air under pressure into a recycle stream of clarified DAF effluent. This recycle stream is then combined and mixed with incoming wastewater in an internal contact chamber where the dissolved air comes out of solution in the form of micron-sized bubbles that attach to the contaminants. The bubbles and contaminants rise to the surface and form a floating bed of material that is removed by a surface skimmer into an internal hopper for further handling.
Dissolved air flotation (DAF) has proven to be very effective for the removal of biological solids from wastewater streams. Some of the advantages over traditional, gravity clarification include:
- Smaller footprint and more rapid implementation
- Higher sludge solids content (4%TS vs. 1%TS) resulting in a much lower sludge volume
- Ability to handle bulking solids that tend to float
- More reliable effluent quality
Some of the more common biological applications include:
- Clarification of aerobic biomass in an activated sludge process
- Polishing effluent from aerobic ponds
- Clarification of anaerobic solids in an anaerobic contact process
- Polishing effluent from anaerobic lagoons
- Removal of algae from ponds
- Polishing effluent from high-rate anaerobic processes
One of the most common DAF applications is for the pretreatment of wastewater to remove suspended solids and oils and greases prior to discharge to a municipal sewer or a biological treatment system. For example:
- A Pretreatment DAF can be used by an industrial discharger to meet specific limits for oil and grease and/or suspended solids set by a municipality (e.g., to meet a discharge limit of 100 mg/L O&G, etc.).
- An industry may need to use a Pretreatment DAF to remove contaminants (e.g., product solids, oils and greases, heavy metals, etc.) that would negatively impact a biological wastewater treatment system downstream.
In most cases, the wastewater contaminants must be chemically flocculated in order for the floatation system to remove them. Pretreatment DAF is used in about every industry class.
Where and Why are DAFs used?
DAFs are widely used in treating industrial wastewater effluents from oil refineries, chemical plants and paper mills to the food & beverage industries. DAFs are used to removed suspended solids, such as Total Suspended Solids (TSS), Fats, Oil & Grease (FOG) and other pollutants from wastewater so that the water is suitable for re-use or discharge to a wastewater treatment facility.
How does a DAF work?
Wastewater is fed into a DAF system and hit with a stream of “whitewater”, which is recirculated clarified water from the DAF that’s super saturated with dissolved air. As these two mixtures blend together, microscopic bubbles attach to solid particulates, giving them enough buoyancy to surface in the DAF tank. As solids accumulate in a floating layer on the top of the DAF tank, a skimmer gently nudges the sludge toward a discharge hopper.
Any solids that don’t float will sink to the “V” bottom of the DAF tank. Settled solids are concentrated and discharged by an automatically controlled pneumatic drain valve. The clarified water flows out via an under-over weir on either side of the DAF unit. Some of this water is used in the recirculation loop while the rest flows out of the vessel.