Daftech Engineers Private Limited

Daftech Engineers Private Limited

DAF PROCESS (Dissolved Air Flotation Process)

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DAFTECH ENGINEERS PVT. LTD
Specialists in wastewater treatment by flotation, DAFTECH is a company promoted by people with more than two decades of experience in Dissolved Air Flotation (DAF). DAFTECH designs and manufacturers a complete line of circular Dissolved Air Flotation Clarifiers and Dissolved Air Flotation Units.

Flotation has been widely used for industrial wastewater treatment. The typical industrial wastes treated by flotation are paper mill backwater, oily wastewater as well effluents from sugar, dairy, textile industries and tanneries. Although flotation has been used extensively for activated sludge thickening, applications where dissolved air flotation (DAF) is used for secondary clarification are rather recent.


WHY DAF?
The fact that dissolved air flotation clarifiers or dissolved air flotation units (with 3 - 5 minutes of detention time) can replace sedimentation (with 2 - 3 hours of detention time) for clarification, has been overlooked by environmental engineers for many decades. Modern high rate DAF clarifiers have advanced to such an extent that they could overshadow conventional settling clarifiers in wastewater treatment.

DAF hydraulic loading has increased from 1 L/m2.s to 2.5 L/m2.s and for a triple stacked unit to 7.5 L/m2.s. The detention time has decreased from 30 minutes to 3 minutes. Air dissolving is improved and now requires only 10 seconds retention time in the air mixing tube instead of the previous 60 seconds. The smaller size and weight of a modern DAF unit allow for inexpensive construction and flexibility of erection using prefabricated units. The availability of excellent flocculating
chemicals gives a high stability of operation and high clarification degree.


FLOTATION PROCESS
DAFTECH Floatcells are circular in shape, ranging in diameter from 1.2 to 21.3 meters and a liquid depth of only 0.65 meter. The complete set up consists of a flotation tank, feed pump, pressure pump, compressor, air mixing tube, chemical feed system and a spiral scoop for collection and removal of floated sludge.

The inlet, outlet and sludge removal mechanisms are contained in the central rotating section. This section and the spiral scoop rotate around the tank at a speed synchronized with the flow. The system is operated in the recycle flow pressurization mode where a portion of clarified effluent (25 - 30 percent) is continuously recycled from the collection tank to the air mixing tube at about 6.5 bar pressure. After pressure release, the aerated water is mixed with the influent flow just before the inlet to the distribution duct that moves with the same velocity, but in opposite direction to the incoming flow thus creating a quiescent state in the flotation chamber. Fine bubbles generated in this manner attach to the suspended particles and make them float to the surface.

The spiral scoop takes up the floated sludge, pouring it into the stationary center section where it is discharged by gravity. Clarified water is removed by extraction pipes, which are attached to the moving center section and discharged. Wiper blades attached to the moving distribution duct scrape the bottom and the sides of the tank and discharge settled sludge into the built-in sump for periodic purging. The variable speed gear motor drives the rotating elements and scoop. Electrical current for the gear motor is fed from a rotary contact mounted on the central shaft.


AIR MIXING
The separation efficiency of Total Suspended Solids in a DAF cell depends substantially on the generation of uniformly sized microscopic air bubbles.

A part of the clarified water, generally 25 to 30% of the feed rate, is pressurized through a centrifugal pump to about 65 meter head and let into a cylindrical Air Mixing Tube through a nozzle at one end. The nozzle is so oriented inside the tank that the jet of water churns spirally along the inside wall of the tube forming a vortex along the central axis of the tube.

Into this tube is brought compressed air which passes through a microscopically porous medium placed adjacent to the inside wall of the tube.

The tube produces a saturated air-water mixture because the rate of absorption of any gas in a liquid is directly proportional to the pressure in which the dissolution takes place.

The air-water mixture that comes out of the other end of the tube passes through a pressure release valve where the pressure is reduced to the level of the incoming wastewater feed. At this stage, air bubbles of 30 - 50 microns are produced profusely which get released into the wastewater thereby giving the necessary buoyancy
to the suspended solids to rise to the surface of the water in the flotation tank.


ZERO VELOCITY
A quiescent state is an absolute necessity for good flotation. This is achieved by ensuring zero velocity of the wastewater when it enters the flotation tank.

The feed of wastewater premixed with depressurized recycle stream is led into the main tank through a distribution tube with adjustable nozzles. It is then discharged into the tank through baffles and then a turbulence arrester. The rotational velocity of the incoming wastewater into the tank is counterbalanced by the mechanical movement of the distribution tube in the opposite direction but at the same speed. This ensures quiescent condition and minimum turbulence.

DAFTECH Floatcells are designed with a retention time of 3 minutes. The equipment therefore is much more compact than rectangular DAF clarifiers of 15 to 30 minutes of retention and occupies only half of the floor space.


FLOATCELL DISSOLVED AIR FLOTATION CLARIFIER

floatcell dissolved air flotation clarifier


DAF FLOATCELL FLOW DIAGRAM


LEGENDS

1 Wastewater Inlet 7 Floatcell Wall 13 Wastewater Pump
2 Clarified Water Outlet 8 Tank Wall Support 14 Air Compressor
3 Floated Sludge Outlet 9 Rotating Tank Wall 15 Flocculant Feed Point
4 Clarified Water Recycle Outlet 10 Level Control Weir 16 Pressure Release Valve
5 Rotary Joint 11 Spiral Scoop 17 Purge Valve
6 Spiral Scoop Shaft 12 Recycle Pump 18 Sample Point










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