Dissolved Air Flotation

Shaju PeterManaging DirectorDaftech Engineers Pvt. Ltd.

What Is It?•A liquid solid separation process achieved by

microscopic air bubbles attaching themselves to solids suspended in a liquid and causing these solids to float to the surface.

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Why Should You Use It?• Smaller space requirement Example: 1 MGD – 18’ dia DAF

clarifier• DAF recovered solids are 3 to 4 times

thicker than gravity sedimentation• Faster processing times – 3 minutes

compared to 2 to 3 hours in conventional clarifiers

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How Does It Work?

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A stream of treated water (recycle) is taken from the DAF unit and is pumped into an Air Mixing Tube. Air and water are then ULTRA MIXED under pressure in the Tube and then is released at a point that allows it to mix thoroughly with the raw waste stream entering the cell.

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As the air comes out of solution millions of bubbles are attached to the material that is going to be floated to the surface, causing the solids to agglomerate and rise to the surface of the tank in the form of a sludge blanket on top of the clear water.

This sludge blanket is then allowed to thicken and de-water prior to removal by a rotating scoop.

How Does It Work?

How Efficient Is the DAF System?

•90 to 98 % capture of TSS with chemical addition (typically a polymer)

•2 to 3.5 % sludge consistency•85 to 90 % air dissolution capability•80 to 90 % oil and grease removal•50 to 80 % BOD reductions

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Where Can You Use the DAF Unit?

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Industry Anticipated Performance

Pulp and Paper

Fibre recovery 90 to 98 % recovery

Where Can You Use the DAF Unit?Food Processing

CanningPrepared foodsSnack foodsSeafood

85 to 95 % TSS Removal85 to 95 % O&G Removal

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Oily Wastes

Oil refineriesCar wash operationsPaint shops

85 to 95 % TSS Removal90 to 95% O&G Removal

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Where Can You Use the DAF Unit?

Food Providers

Chicken ProcessingCattle slaughteringMilk and Dairy

85 to 97 % TSS Removal95 to 99 % O&G Removal80 to 85 % BOD Removal

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Where Can You Use the DAF Unit?

MiscellaneousTanneriesAlgae removalPharmaceuticalTextile effluentsSewage treatment

90 to 95 % TSS Removal

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Where Can You Use the DAF Unit?

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Recycle and Partial Pressurization

• Raw influent is introduced directly into the flotation cell. A portion (generally 25% of raw flow) of the system effluent is pressurized and recycled to the flotation cell where it is blended with the raw wastewater flow.

Air Mixing System

• The pressurization system uses compressed air that is forced into solution by a controlled air-water interface and mixing components located inside an Air Mixing Tube. The Tube is fitted with an automatic air bleed off valve and pressure safety relief valve.

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Sludge Removal System

• The flotation cell is fitted with a spiral scoop mechanism to assure effective removal of the floating material. The surface skimmer,centre distribution tube, and lower sludge scraper arm are all driven by a stepless variable motor

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Sizing Criteria

• Hydraulic loading (m3/hr./M2)• Solids loading (kg./M2/hr.)• Air-to-solids ratio (kg. of air/kg. of

solids)

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Hydraulic Loading

• The relationship of the surface area of the flotation cell(m2) versus the influent rate into the flotation cell (m3/hr.) For full pressurization systems, the influent rate is the capacity of the pressure pumps. For recycle pressurization systems, the total influent is the raw influent flow plus the recycle pump rate. It is possible to obtain up to 9.6m3/hr/m2 depending on the application.

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Solids Load

• The relationship of the surface area of the flotation cell (M2) versus the amount of solids entering the system per hour. Design points for solid loadings range from 10 to 16 kg./M2/hr. Depending upon the application and the type of solids involved.

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Air-to-Solid Ratio• The ratio of the number of kg of air to

the number of kg. Of solids. Generally, air is injected in a range of 2 to 8 % by volume of raw wastewater intake. Depending upon the type of solids and application, the air-to-solids ratio ranges from 0.02 to 0.1.

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Bastanka Cheese PlantUkraine• The unit treats 68 m3/hr of dairy wastewater in the

primary mode.

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Pollutant Inlet Outlet

TSSBOD

900 ppm1500 ppm

50 ppm600 ppm

United Foods CompanyUAE

• The DAF unit of just 6 feet diameter treats 200m3/day of effluents.

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Pollutant Inlet Outlet

TSS 1000 ppm 50 ppm

O&G 800 ppm 15 ppm

BOD 400 ppm 200 ppm

SPIC Pharma, Cuddalore

• The DAF unit treats effluent containing very high TSS of 8000 ppm from Penicillin production. The flow of 10m3/hr has O&G of over 6000 ppm. The outlet TSS of 100 ppm and O&G of 50-100 ppm is achieved by a very high rate of recycle – as high as 200%.

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City of BogotaColombia

• The DAF unit treats city sewage in the primary mode.

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Pollutant Inlet Reduction

TSS 500 ppm 90%

BOD 350 ppm 50%

Delmo ChickenSri Lanka

• The DAF unit handles 26 m3/hr of their wastewater stream containing 3500 ppm of fine particles of meat which they were otherwise wasting. The system recovers the suspensions which are used as input for another food product. The unit is fabricated in SS 316.

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Developments

• Retention time progressively reduced from 20 minutes to 3 minutes.

• In circular DAF units overflow weir internalized to reduce space.

• Large saturation vessels replaced by air mixing tube of just 10 seconds retention.

• Trials are in progress with DAF pump to eliminate compressed air usage.

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DAFTECH ENGINEERS PVT. LTD.DAFTECH ENGINEERS PVT. LTD.SCO 1A, Sector-7C, Madhya Marg, SCO 1A, Sector-7C, Madhya Marg, Chandigarh, India- 160019 Chandigarh, India- 160019 Call Us : +(91)-172 5018121Call Us : +(91)-172 5018121mail@daftechindia.commail@daftechindia.comwww.daftechindia.comwww.daftechindia.com