Dissolved Air Flotation Unit
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Effluent dosed with AlumPolyelectrolyte from flocculation tank is fed into coagulation tube (feed well) situated at the centre of DAF unit. Coagulation tube is a vertical cylinder suspended in the centre of the tank where the flocculated effluent enters tangentially to ensure intimate mixing of the flocculated solids with the micro bubbles which are generated due to depressurisation of recycled effluent (which is also fed tangentially).
When the mixture of air & solid reach top of coagulation tube, the centrifugal force tends to throw airsolids horizontally towards periphery of the tank & move upwards & outwards to form a blanket of skimming (froth). This accumulated skimming which floats on the top is continuously scraped from the surface by the scraper arm into the scum box.
DAF separation principle is based on the fact that air is soluble is liquid in direct proportion to the pressure applied. The treated effluent is recycled to the saturation vessel by means of high pressure recycle pump where plant air is injected which ensures saturation of this liquid with air. This saturated liquid is depressurised through a valve located on the discharge line of the saturation vessel. Which is connected to the coagulation tube. When the pressure is released the air dissolved under pressure comes out of the solution in the form of minute bubbles. These minute bubbles get attached to the flocculated effluent & thus forming solids with combined specific gravity less than the effluent, which finally floats to the surface.
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