How does a DAF work?
Dissolved air flotation (DAF) systems are utilised to eliminate suspended solids, fats, oils, and greases from a variety of wastewaters in a variety of processing applications such as food and dairy, meat, fish, and poultry rendering, and waste vegetable oils. Dissolved air flotation (DAF) thickens sludge’s (i.e. makes the sludge thicker) by encouraging solids to float to the surface rather than sinking as in gravity thickening.
The formation of small diameter bubbles from air released from solution after being pressurised to 40 to 60 psi is required for dissolved air flotation. Because the solubility of air increases with pressure, large amounts of air can be dissolved.
In current flotation practise, two general approaches to pressurisation are used:
(1) Air charging and pressurisation of recycled clarified effluent or another dilution flow, followed by addition to the feed sludge; and
(2) Air charging and pressurisation of the combined dilution liquid and feed sludge.
DAF is used to increase the concentration of sludge solids when these solids are neutrally buoyant, they neither sink nor float and thus cannot be easily eliminated by settling. This is typically true for waste activated sludge (WAS), which requires a relatively low solids loading rate (SLR) of 2030 kg dry solids per m2 tank area per day (kgDS/(m2 d)) if treated by gravity thickening, compared to nearly five times this rate for DAF. Polymer dosing at 25 g polymer per kg DS (g/kgDS) raises the SLR value even more by generating larger, more buoyant flocs.
To achieve saturation concentrations, the DAF process dissolves air under pressure in water. When the pressure is released, air precipitates in the form of small bubbles that rise through the vessel. The bubbles collide with the sludge particles, sweeping them to the surface and forming scum. A scraper is then used to remove the scum. The clarified effluent is used in part to create the air-saturated water (recycle) stream.
The primary variables for flotation thickening:
(1) Type and quality of sludge,
(2) Solids and hydraulic loading rates,
(3) Pressure,
(4) Recycle ratio,
(5) Feed solids concentration,
(6) Detention period,
(7) Air-to-solids ratio, and
(8) Use of chemical aids.
Factors determining the process's effectiveness:
· The sludge origin
· The recycle ratio, and
· The thickness of the sludge blanket
Because of the higher particle density, primary sludge’s are less effectively treated by flotation than secondary sludge’s. They also have a significant grit content, which settles at the bottom of the DAF unit.
DAF thickeners, like gravity thickeners, are equipped with scrapers to collect sediment. The mass of air used per unit mass of sludge solids is referred to as the air to solids ratio. This ratio is typically in the 0.02006 kg air/kg DS range for sludge thickening applications.
Even though gas concentration increases with pressure, the concentration of air in the recycle stream is primarily determined by pressure. The recycle ratio, which is the ratio of the recycle flow rate to the influent sludge, determines the amount of air available for flotation.
The speed of the surface sludge scrapers can be used to control the thickness of the floating sludge blanket, with increasing scraper speed resulting in a thinner blanket. The sludge blanket must be thin enough to reduce the possibility of solids contaminating the clarified effluent.
For more information, contact Netsol Water.
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